Table of contents

We present a new hybrid code for large-volume, high-resolution simulations of cosmic reionization, which utilizes an N-body algorithm for dark matter, physically motivated prescriptions for baryons and star formation, and an adaptive ray-tracing algorithm for radiative transfer of ionizing photons. Two test simulations, each with 3 billion particles and 400 million rays in a 50 Mpc h^-1 box, have been run to give initial results. Halos are resolved down to virial temperatures of 10⁴ K for the redshift range of interest in order to robustly model star formation and clumping factors. This is essential to correctly account for ionization and recombination processes. We find that the halos and sources are strongly biased with respect to the underlying dark matter, re-enforcing the requirement of large simulation boxes to minimize cosmic variance and to obtain a qualitatively correct picture of reionization. We model the stellar initial mass function (IMF) by following the spatially dependent gas metallicity evolution, and distinguish between the first generation, Population III (PopIII) stars and the second generation, Population II (PopII) stars. The PopIII stars with a top-heavy IMF produce an order of magnitude more ionizing photons at high redshifts z ≳ 10, resulting in a more extended reionization. In our simulations, complete overlap of H II regions occurs at z ≈ 6.5, and the computed mass- and volume-weighted residual H I fractions at 5 ≲ z ≲ 6.5 are both in good agreement with high-redshift quasar absorption measurements from the Sloan Digital Sky Survey (SDSS). The values for the Thomson optical depth are consistent within 1 - σ of the current best-fit value from third-year Wilkinson Microwave Anisotropy Probe (WMAP) results.

The Sunyaev-Zel'dovich (SZ) signal is highly non-Gaussian, so the SZ power spectrum (along with the mean y-parameter) does not provide a complete description of the SZ effect. Therefore, SZ-based constraints on cosmological parameters and on cluster gastrophysics which assume Gaussianity will be biased. We derive an analytic expression for the n-point joint PDF of the SZ power spectrum. Our derivation, which is based on the halo model, has several advantages: it is expressed in an integral form which allows quick computation; it is applicable to any given survey and any given angular scale; and it is straightforward to incorporate many of the complexities which arise when modeling the SZ signal. To illustrate, we use our expression to estimate p(C_ℓ), the one-point PDF of the SZ power spectrum. For small sky coverage (applicable to BIMA/CBI and the Sunyaev Zel'dovich Array experiments), our analysis shows that p(C_ℓ) on the several arcminute scale is expected to be strongly skewed, peaking at a value well below the mean and with a long tail which extends to high C_ℓ values. In the limit of large sky coverage (applicable to the South Pole Telescope and Planck), p(C_ℓ) approaches a Gaussian form. However, even in this limit, the variance of the power spectrum is very different from the naive Gaussian-based estimate, because different ℓ models are strongly correlated, making the cosmic variance of the SZ bandpower much larger than the naive estimate. Our analysis should also be useful for modeling the PDF of the power spectrum induced by gravitational lensing at large ℓ.

We present the first results from a new generation of simulated large sky coverage (~100 deg²) Sunyaev-Zel'dovich effect (SZE) cluster surveys using the cosmological adaptive mesh refinement N-body/hydro code Enzo. We have simulated a very large (512³h^-3 Mpc³) volume with unprecedented dynamic range. We have generated simulated light cones to match the resolution and sensitivity of current and future SZE instruments. Unlike many previous studies of this type, our simulation includes unbound gas, where an appreciable fraction of the baryons in the universe reside. We have found that cluster line-of-sight overlap may be a significant issue in upcoming single-dish SZE surveys. Smaller beam surveys (~1') have more than one massive cluster within a beam diameter 5%-10% of the time, and a larger beam experiment like Planck has multiple clusters per beam 60% of the time. We explore the contribution of unresolved halos and unbound gas to the SZE signature at the maximum decrement. We find that there is a contribution from gas outside clusters of ~16% per object, on average, for upcoming surveys. This adds both bias and scatter to the deduced value of the integrated SZE, increasing difficulty in accurately calibrating a cluster Y-M relationship. Finally, we find that in images where objects with M > 5 × 10¹³M_☉ have had their SZE signatures removed, roughly a third of the total SZE flux remains. This gas exists at least partially in the warm-hot intergalactic medium (WHIM) and will possibly be detectable with the upcoming generation of SZE surveys.

We detect a dip of 20%-45% in the surface brightness and number counts of NRAO VLA Sky Survey (NVSS) sources smoothed to a few degrees at the location of the WMAP cold spot. The dip has structure on scales of ~1° to 10°. Together with independent all-sky wavelet analyses, our results suggest that the dip in extragalactic brightness and number counts and the WMAP cold spot are physically related, i.e., that the coincidence is neither a statistical anomaly nor a WMAP foreground-correction problem. If the cold spot does originate from structures at modest redshifts, as we suggest, then there is no remaining need for non-Gaussian processes at the last scattering surface of the cosmic microwave background (CMB) to explain the cold spot. The late integrated Sachs-Wolfe effect, already seen statistically for NVSS source counts, can now be seen to operate on a single region. To create the magnitude and angular size of the WMAP cold spot requires a ~140 Mpc radius completely empty void at z ≤ 1 along this line of sight. This is far outside the current expectations of the concordance cosmology, and adds to the anomalies seen in the CMB.

We study weak gravitational lensing of galaxy clusters in terms of the MOND (modified Newtonian dynamics) theory. We calculate shears and convergences of background galaxies for three clusters (A1689, CL 0024+1654, and CL 1358+6245) and the mean profile of 42 SDSS (Sloan Digital Sky Survey) clusters and compare them with observational data. The mass profile is modeled as a sum of X-ray gas, galaxies, and dark halo. For the shear as a function of the angular radius, MOND predicts a shallower slope than the data irrespective of the critical acceleration parameter g₀. The dark halo is necessary to explain the data for any g₀ and for three interpolation functions. If the dark halo is composed of massive neutrinos, its mass should be heavier than 2 eV. However, the constraint still depends on the dark halo model and there are systematic uncertainties, and hence a more careful study is necessary for a more stringent constraint.

In galactic nuclei with sufficiently short relaxation times, binary supermassive black holes can evolve beyond their stalling radii via continued interaction with stars. We study this "collisional" evolutionary regime using both fully self-consistent N-body integrations and approximate Fokker-Planck models. The N-body integrations employ particle numbers up to 0.26 × 10⁶ and a direct-summation potential solver; close interactions involving the binary are treated using a new implementation of the Mikkola-Aarseth regularization algorithm. Even at these large values of N, two-body scattering occurs at high enough rates in the simulations that they cannot be simply scaled to the large-N regime of real galaxies. The Fokker-Planck model is used to bridge this gap; it includes, for the first time, binary-induced changes in the stellar density and potential. The Fokker-Planck model is shown to accurately reproduce the results of the N-body integrations and is then extended to the much larger N-regime of real galaxies. Analytic expressions are derived that accurately reproduce the time dependence of the binary semimajor axis as predicted by the Fokker-Planck model. Gravitational wave coalescence is shown to occur in ≤10 Gyr in nuclei with velocity dispersions below ~80 km s^-1. Formation of a core results from a competition between ejection of stars by the binary and resupply of depleted orbits via two-body scattering. Mass deficits as large as ~4 times the binary mass are produced before coalescence. After the two black holes coalesce, a Bahcall-Wolf cusp appears around the single hole in one relaxation time, resulting in a nuclear density profile consisting of a flat core with an inner, compact cluster, similar to what is observed at the centers of low-luminosity elliptical galaxies.

We explore millimeter line diagnostics of an obscuring molecular torus modeled by a hydrodynamic simulation with three-dimensional non-local thermodynamic equilibrium radiative transfer calculations. Based on the results of a high-resolution hydrodynamic simulation of the molecular torus around an active galactic nucleus, we calculate the intensities of the HCN and HCO⁺ rotational lines as two representative high-density tracers. Three-dimensional radiative transfer calculations shed light on a complicated excitation state in the inhomogeneous torus, even though a spatially uniform chemical structure is assumed. We find that similar transition coefficients for the HCN and HCO⁺ rotational lines lead to a natural concordance of the level population distributions of these molecules and a line ratio R_HCN/HCO+ ≲ 1 for the same molecular abundance value over 2 orders of magnitude. Our results suggest that HCN must be much more abundant than HCO⁺ (y_HCN ≳ 10y) in order to obtain the high ratio (R_HCN/HCO+ ~ 2) observed in some nearby galaxies. There is a remarkable dispersion in the relation between integrated intensity and column density, indicative of possible shortcomings of the HCN(1-0) and HCO⁺(1-0) lines as high-density tracers. The internal structures of inhomogeneous molecular tori down to subparsec scales in external galaxies will be revealed by the forthcoming Atacama Large Millimeter/submillimeter Array. Three-dimensional radiative transfer calculations of molecular lines with a high-resolution hydrodynamic simulation prove to be a powerful tool to provide a physical basis for molecular-line diagnostics of the central regions of external galaxies.

Although the giant radio galaxy M87 harbors many distinct regions of broadband nonthermal emission, the recently reported fast variability of TeV γ-rays from M87, on a timescale of days, strongly constrains the range of speculations concerning the possible sites and scenarios of particle acceleration responsible for the observed TeV emission. A natural production site of this radiation is the immediate vicinity of the central supermassive black hole (BH). Because of its low bolometric luminosity, the nucleus of M87 can be effectively transparent for γ-rays up to an energy of 10 TeV, which makes this source an ideal laboratory for the study of particle acceleration processes close to the BH event horizon. We critically analyze different possible radiation mechanisms in this region and argue that the observed very high energy γ-ray emission can be explained as the inverse Compton emission of ultrarelativistic electron-positron pairs produced through the development of an electromagnetic cascade in the BH magnetosphere. We demonstrate, through detailed numerical calculations of acceleration and radiation of electrons in the magnetospheric vacuum gap, that this "pulsar magnetosphere-like" scenario can satisfactorily explain the main properties of the TeV γ-ray emission from M87.

The TeV blazar PKS 2155-304 was the subject of an intensive 2 week optical and near-infrared observing campaign in 2004 August with the CTIO 0.9 m telescope. During this time, simultaneous X-ray data from RXTE were also obtained. We compare the results of our observations to the results from two previous simultaneous multiwavelength campaigns on PKS 2155-304. We conclude that the correlation between the X-ray and UV/optical variability is strongest and the time lag is shortest (only a few hours) when the object is brightest. As the object becomes fainter, the correlations are weaker and the lags longer, increasing to a few days. Based on the results of four campaigns, we find evidence for a linear relationship between the mean optical brightness and lag time of X-ray and UV/optical events. Furthermore, we assert that this behavior, along with the different multiwavelength flare lag times across different flux states, is consistent with a highly relativistic shock propagating down the jet producing the flares observed during a high state. In a quiescent state, the variability is likely to be due to a number of factors including both the jet and contributions outside of the jet, such as the accretion disk.

In order to contribute to the general effort aiming at the improvement of our knowledge about the physical conditions within the broad-line regions (BLRs) of active galactic nuclei (AGNs), here we present the results achieved by our analysis of the spectral properties of a sample of 90 broad-line-emitting sources, collected from the Sloan Digital Sky Survey database. By focusing our attention mainly on the Balmer series of hydrogen emission lines, which is the dominant feature in the optical wavelength range of many BLR spectra, we extracted several flux and profile measurements, which we related to other source properties, such as optical continuum luminosity, inferred black hole mass, and accretion rate. Using the Boltzmann plot method to investigate the Balmer-line flux ratios as a function of the line profiles, we found that AGNs that emit broader lines typically have larger Hα/Hβ and smaller Hγ/Hβ and Hδ/Hβ line ratios. With the help of some recent investigations, we model the structure of the BLR, and we study the influence of the accretion process on the properties of BLR plasma.

A new model is presented that explains the origin of the broad emission lines observed in the LINER/Seyfert nucleus of M81 in terms of a steady state spherically symmetric inflow, amounting to ~1 × 10^-5M_☉ yr^-1, which is sufficient to explain the luminosity of the active galactic nucleus (AGN). The emitting volume has an outer radius of ~1 pc, making it the largest broad-line region (BLR) yet to be measured, and it contains a total mass of ~5 × 10^-2M_☉ of dense, ~10⁸ cm^-3, ionized gas, leading to a very low filling factor of ~5 × 10^-9. The fact that the BLR in M81 is so large may explain why the AGN is unable to sustain the ionization seen there. Thus, the AGN in M81 is not simply a scaled-down quasar.

We present Spitzer mid-infrared spectra of 12 Seyfert 1.8 and 1.9 galaxies over the 5-38 μm region. We compare the spectral characteristics of this sample to those of 58 Seyfert 1 and 2 galaxies from the Spitzer archives. An analysis of the spectral shapes, the silicate 10 μm feature and the emission-line fluxes have enabled us to characterize the mid-IR properties of Seyfert 1.8/1.9s. We find that the EWs of the 10 μm silicate feature are generally weak in all Seyfert galaxies, as previously reported by several studies. The few Seyfert galaxies in this sample that show deep 10 μm silicate absorption features are highly inclined and/or merging galaxies. It is likely that these absorption features originate primarily in the dusty interstellar medium of the host galaxy rather than in a dusty torus on parsec scales close to the central engine. We find that the EW of the PAH band at 6.2 μm correlates strongly with the 20-30 μm spectral index. Either of these quantities is a good indicator of the amount of starburst contribution to the mid-IR spectra. The spectra of Seyfert 1.8s and 1.9s are dominated by these starburst features, similar to most Seyfert 2s. They show strong PAH bands and a strong red continuum toward 30 μm. The strengths of the high-ionization forbidden narrow emission lines [O IV] 25.89 μm, [Ne III] 15.56 μm, and [Ne V] 14.32 μm relative to [Ne II] 12.81 μm are weaker in Seyfert 1.8/1.9s and Seyfert 2s than in Seyfert 1s. The weakness of high-ionization lines in Seyfert 1.8-1.9s is suggestive of intrinsically weak AGN continua and/or stronger star formation activity leading to enhanced [Ne II]. We discuss the implications of these observational results in the context of the unified model of AGNs.

Energetic feedback processes during the formation of galaxy clusters may have heated and ionized the majority of the intergalactic gas in protocluster regions. When such a highly ionized superbubble falls along the sight line to a background quasar, it would be seen as a large void with little or no absorption in the Lyα forest. We examine the spectra of 137 quasars in the Sloan Digital Sky Survey to search for such voids and find no clear evidence of their existence. The size distribution of voids in the range 5 Å ≲ Δλ ≲ 70 Å (corresponding to physical sizes of 3 h^-1 ≲ L ≲ 35 h^-1 comoving Mpc) is consistent with the standard model for the Lyα forest without additional ionized bubbles. We adapt a physical model for H II bubble growth during cosmological reionization to describe the expected size distribution of ionized superbubbles at z ~ 3. This model incorporates the conjoining of bubbles around individual neighboring galaxies. Using the nondetection of voids, we find that models in which the volume filling factor of ionized bubbles exceeds ~20% at z ~ 3 can be ruled out, primarily because they overproduce the number of large (40-50 Å) voids. We conclude that any preheating mechanism that explains galaxy cluster observations must avoid heating the low-density gas in the protocluster regions, either by operating relatively recently (z ≲ 3) or by increasing entropy primarily in high-density regions.

We have used the Hubble STIS and FUSE archives of ultraviolet spectra of bright AGNs to identify intergalactic Lyα absorbers in nearby (z ≤ 0.1) voids. From a parent sample of 651 Lyα absorbers, we identified 61 "void absorbers" located >1.4 h Mpc from the nearest L* or brighter galaxy. Searching for metal absorption in high-quality (S/N > 10) spectra at the location of three diagnostic metal lines (O VI λ1032, C IV λ1548, Si III λ1206), we detected no metal lines in any individual absorber, or in any group of absorbers using pixel co-addition techniques. The best limits on metal-line absorption in voids were set using four strong Lyα absorbers with N > 10¹⁴ cm^-2, with 3 σ equivalent-width limits ranging from 8 mÅ (O VI) to 7-15 mÅ (C IV) and 4-10 mÅ (Si III). Photoionization modeling yields metallicity limits Z < 10^-1.8±0.4Z_☉ from nondetections of C IV and O VI, some ~6 times lower than those seen in Lyα/O VI absorbers at z < 0.1. Although the void Lyα absorbers could be pristine material, considerably deeper spectra are required to rule out a universal metallicity floor produced by bursts of early star formation, with no subsequent star formation in the voids. The most consistent conclusion derived from these low-z results and similar searches at z = 3-5 is that galaxy filaments have increased their mean IGM metallicity by factors of 30-100 since z ~ 3.

We use a modified version of the halo-based group finder developed by Yang et al. to select galaxy groups from the Sloan Digital Sky Survey (SDSS DR4). In the first step, a combination of two methods is used to identify the centers of potential groups and to estimate their characteristic luminosity. Using an iterative approach, the adaptive group finder then uses the average mass-to-light ratios of groups, obtained from the previous iteration, to assign a tentative mass to each group. This mass is then used to estimate the size and velocity dispersion of the underlying halo that hosts the group, which in turn is used to determine group membership in redshift space. Finally, each individual group is assigned two different halo masses: one based on its characteristic luminosity and the other based on its characteristic stellar mass. Applying the group finder to the SDSS DR4, we obtain 301,237 groups in a broad dynamic range, including systems of isolated galaxies. We use detailed mock galaxy catalogs constructed for the SDSS DR4 to test the performance of our group finder in terms of completeness of true members, contamination by interlopers, and accuracy of the assigned masses. This paper is the first in a series and focuses on the selection procedure, tests of the reliability of the group finder, and the basic properties of the group catalog (e.g., the mass-to-light ratios, the halo mass-to-stellar mass ratios). The group catalogs including the membership of the groups are available on request.

Jets from active galactic nuclei (AGNs) in the cores of galaxy clusters have the potential to be a major contributor to the energy budget of the intracluster medium (ICM). To study the dependence of the interaction between the AGN jets and the ICM on the parameters of the jets themselves, we present a parameter survey of two-dimensional (axisymmetric) ideal hydrodynamic models of back-to-back jets injected into a cluster atmosphere (with varying Mach numbers and kinetic luminosities). We follow the passive evolution of the resulting structures for several times longer than the active lifetime of the jet. The simulations fall into roughly two classes, cocoon-bounded and non-cocoon bounded sources. We suggest a correspondence between these two classes and the Faranoff-Riley types. We find that the cocoon-bounded sources inject significantly more entropy into the core regions of the ICM atmosphere, even though the efficiency with which energy is thermalized is independent of the morphological class. In all cases, a large fraction (50%-80%) of the energy injected by the jet ends up as gravitational potential energy due to the expansion of the atmosphere.

Sharp edges in X-ray surface brightness with continuous gas pressure called cold fronts have often been found in relaxed galaxy clusters such as Abell 496. Models that explain cold fronts as surviving cores of head-on subcluster mergers do not work well for these clusters, and competing models involving gas sloshing have been recently proposed. Here, we test some concrete predictions of these models in a combined analysis of density, temperature, metal abundances, and abundance ratios in a deep Chandra exposure of Abell 496. We confirm that the chemical discontinuities found in this cluster are not consistent with a core merger remnant scenario. However, we find chemical gradients across a spiral "arm" discovered at 73 kpc north of the cluster center and coincident with the sharp edge of the main cold front in the cluster. Despite the overall SN Ia iron mass fraction dominance found within the cooling radius of this cluster, the metal enrichment along the arm, determined from silicon and iron abundances, is consistent with a lower SN Ia iron mass fraction (51% ± 14%) than that measured in the surrounding regions (85% ± 14%). The "arm" is also significantly colder than the surroundings by 0.5-1.6 keV. The arm extends from a boxy colder region surrounding the center of the cluster, where two other cold fronts are found. This cold arm is a prediction of current high resolution numerical simulations as a result of an off-center encounter with a less massive pure dark matter halo, and we suggest that the cold fronts in A496 provide the first clear corroboration of such model, where the closest encounter happened ~0.5 Gyr ago. We also argue for a possible candidate dark matter halo responsible for the cold fronts in the outskirts of A496.

We present the discovery of a 40 kpc Hα tail and at least 29 emission-line objects downstream of a star-forming galaxy, ESO 137-001, in the rich, nearby cluster A3627. The galaxy is known to possess a dramatic 70 kpc X-ray tail. The detected Hα tail coincides positionally with the X-ray tail. The Hα emission in the galaxy is sharply truncated on the front and the sides near the nucleus, indicating significant ram pressure stripping. ESO 137-001 is thus the first cluster late-type galaxy known unambiguously to have both an X-ray tail and an Hα tail. The emission-line objects are all distributed downstream of the galaxy, with projected distances of up to 39 kpc from the galaxy. From the analysis on the Hα_off frame and the estimate of the background emission-line objects, we conclude that it is very likely that all 29 emission-line objects are H II regions in A3627. The high surface number density and luminosities of these H II regions (up to 10⁴⁰ ergs s^-1) dwarf the previously known examples of isolated H II regions in clusters. We suggest that star formation may proceed in the stripped ISM in both the galactic halo and intracluster space. The total mass of formed stars in the stripped ISM of ESO 137-001 may approach several times 10⁷M_☉. Therefore, stripping of the ISM not only contributes to the ICM, but also adds to the intracluster stellar light through subsequent star formation. The data also imply that ESO 137-001 is in an active stage of transformation accompanied by the buildup of a central bulge and depletion of the ISM.

We construct a large data set of global structural parameters for 1300 field and cluster spiral galaxies and explore the joint distribution of luminosity L, optical rotation velocity V, and disk size R at I and 2MASS K bands. The I- and K-band velocity-luminosity (VL) relations have log slopes of 0.29 and 0.27, respectively, with σ_ln(VL) ~ 0.13, and show a small dependence on color and morphological type in the sense that redder, earlier type disk galaxies rotate faster than bluer, later type disk galaxies for most luminosities. The VL relation at I and K bands is independent of surface brightness, size, and light concentration. The log slope of the I- and K-band size-luminosity (RL) relations is a strong function of morphology and varies from 0.25 to 0.5, with a mean of 0.32 for all Hubble types. At most luminosities, early-type disk galaxies have shorter scale lengths than later type ones. The average dispersion σ_ln(RL) decreases from 0.33 at I band to 0.29 at K, likely due to the 2MASS selection bias against lower surface brightness galaxies. The VL and RL residuals are largely uncorrelated with each other with a correlation coefficient r = -0.16 and Δ log V|L/Δ log R|L = -0.07 ± 0.01; the RV - RL residuals show a weak positive correlation with r = 0.53. These correlations suggest that scatter in luminosity is not a significant source of the scatter in the VL and RL relations. We discuss in two Appendices various pitfalls of standard analytical derivations of galaxy scaling relations, including the Tully-Fisher relation with different slopes. Our galaxy database is available at http://www.astro.queensu.ca/~courteau/data/VRL2007.dat.

We investigate formation and evolution of galactic disks immersed in assembling live DM halos. Models have been evolved from cosmological initial conditions and represent the collapse of an isolated density perturbation. The baryons include gas participating in star formation (SF) and stars with the energy feedback onto the ISM. We find that (1) the triaxial halo figure tumbling is insignificant and the angular momentum (J) is channeled into the internal circulation, while the baryonic collapse is stopped by the centrifugal barrier; (2) density response of the (disk) baryons is out of phase with DM, thus washing out the inner halo ellipticity; (3) the total J is neatly conserved, even in models accounting for stellar feedback; (4) the specific J for DM is nearly constant, while that for baryons is decreasing; (5) early stage of disk formation resembles the cat's cradle—a small amorphous disk fueled via radial string patterns—followed by growing oval disk whose shape varies with its orientation to the halo major axis; (6) the disk gas layer thins when the SF rate drops below ~5 M_☉ yr^-1; (7) about half of the baryons remain outside the disk SF region or in the halo as a hot gas; (8) rotation curves appear to be flat and account for the observed disk/halo contributions; (9) a range of bulge-dominated to bulgeless disks was obtained, depending on the stellar feedback parameter, _SF: smaller _SF leads to a larger and earlier bulge; lower density threshold for SF leads to a smaller, thicker disk; gas gravitational softening mimics a number of intrinsic processes within the ISM; (10) models are characterized by an extensive bar-forming activity; (11) nested bars form in response to the gas inflow along the primary bars, as shown by Heller, Shlosman, and Athanassoula.

Although the spectrum of a prototypical early-type galaxy is assumed to lack emission lines, a substantial fraction (likely as high as 30%) of nearby red sequence galaxy spectra contain emission lines with line ratios characteristic of LINERs. We use spectra of ~6000 galaxies from the SDSS in a narrow redshift slice (0.06 < z < 0.08) to compare the stellar populations of red sequence galaxies with and without LINER-like emission. The spectra are binned by internal velocity dispersion (σ) and by emission properties to produce high-S/N stacked spectra. The recent stellar population models of R. Schiavon make it possible to measure ages, [Fe/H], and individual elemental abundance ratios [Mg/Fe], [C/Fe], [N/Fe], and [Ca/Fe] for each of the stacked spectra. We find that red sequence galaxies with strong LINER-like emission are systematically 2-3.5 Gyr (10%-40%) younger than their emission-free counterparts at the same σ. This suggests a connection between the mechanism powering the emission (whether AGN, post-AGB stars, shocks, or cooling flows) and more recent star formation in the galaxy. We find that mean stellar age and [Fe/H] increase with σ for all galaxies. Elemental abundance [Mg/Fe] increases modestly with σ in agreement with previous results, and [C/Fe] and [N/Fe] increase more strongly with σ than does [Mg/Fe]. [Ca/Fe] appears to be roughly solar for all galaxies. At fixed σ galaxies with fainter r-band luminosities have lower [Fe/H] and older ages but similar abundance ratios compared to brighter galaxies.

We present deep 3.6 μm observations of three z ~ 5 GRB host galaxies with the Spitzer Space Telescope. The host of GRB 060510B, at z = 4.942, is detected with a flux density of 0.23 ± 0.04 μJy, corresponding to a rest-frame V-band luminosity of 1.3 × 10¹⁰L_☉, or ≈0.15 L_*,V,z=3. We do not detect the hosts of GRBs 060223A and 060522 and constrain their rest-frame V-band luminosity to <0.1 L_*,V,z=3. Our observations reveal that z ~ 5 GRB host galaxies are a factor of ~3 less luminous than the median luminosity of spectroscopically confirmed z ~ 5 galaxies in the Great Observatories Origins Deep Survey and the Hubble Ultra Deep Field. The strong connection between GRBs and massive star formation implies that not all star-forming galaxies at these redshifts are currently being accounted for in deep surveys and GRBs provide a unique way to measure the contribution to the star formation rate density from galaxies at the faint end of the galaxy luminosity function. By correlating the comoving star formation rate density with comoving GRB rates at lower redshifts, we estimate a lower limit to the star formation rate density of 0.12 ± 0.09 and 0.09 ± 0.05 M_☉ yr^-1 Mpc^-3 at z ~ 4.5 and z ~ 6, respectively. This is in excellent agreement with extinction-corrected estimates from Lyman break galaxy samples. Finally, our observations provide initial evidence that the metallicity of star-forming galaxies evolves more slowly than the stellar mass density between z ~ 5 and z ~ 0, probably indicative of the loss of a significant fraction of metals to the intergalactic medium, especially in low-mass galaxies.

We studied the clustering properties and multiwavelength spectral energy distributions of a complete sample of 162 Lyα-emitting (LAE) galaxies at z ≃ 3.1 discovered in deep narrowband MUSYC imaging of the Extended Chandra Deep Field-South. LAEs were selected to have observed frame equivalent widths >80 Å and emission line fluxes >1.5 × 10^-17 ergs cm^-2 s^-1. Only 1% of our LAE sample appears to host AGNs. The LAEs exhibit a moderate spatial correlation length of r₀ = 3.6 Mpc, corresponding to a bias factor b = 1.7, which implies median dark matter halo masses of log₁₀M_med = 10.9M_☉. Comparing the number density of LAEs, 1.5 ± 0.3 × 10^-3 Mpc^-3, with the number density of these halos finds a mean halo occupation ~1%-10%. The evolution of galaxy bias with redshift implies that most z = 3.1 LAEs evolve into present-day galaxies with L < 2.5L*, whereas other z > 3 galaxy populations typically evolve into more massive galaxies. Halo merger trees show that z = 0 descendants occupy halos with a wide range of masses, with a median descendant mass close to that of L*. Only 30% of LAEs have sufficient stellar mass (>~3 × 10⁹M_☉) to yield detections in deep Spitzer IRAC imaging. A two-population SED fit to the stacked UBVRIzJK+[3.6, 4.5, 5.6, 8.0] μm fluxes of the IRAC-undetected objects finds that the typical LAE has low stellar mass (1.0 × 10⁹M_☉), moderate star formation rate (2 ± 1 M_☉ yr^-1), a young component age of 20 Myr, and little dust (A_V < 0.2). The best-fit model has 20% of the mass in the young stellar component, but models without evolved stars are also allowed.

We present HST NICMOS+ACS and Spitzer IRAC+MIPS observations of 41 galaxies at 2 < z < 3.5 in the FIRES MS 1054 field with red and blue rest-frame optical colors. About half of the galaxies are very compact (effective radii r_e < 1 kpc) at rest-frame optical wavelengths; the others are extended (1 kpc < r_e < 10 kpc). For reference, 1 kpc corresponds to 0.12'' at z = 2.5 in the adopted cosmology. We separate actively star-forming galaxies from quiescent galaxies by modeling their rest-frame UV-NIR SEDs. The star-forming galaxies span the full range of sizes, while the quiescent galaxies all have r_e < 2 kpc. In the redshift range where MIPS 24 μm imaging is a sensitive probe of reradiated dust emission (z < 2.5), the 24 μm fluxes confirm that the light of the small quiescent galaxies is dominated by old stars, rather than dust-enshrouded star formation or AGN activity. The inferred surface mass densities and velocity dispersions for the quiescent galaxies are very high compared to those in local galaxies. The galaxies follow a Kormendy relation (between surface brightness and size) with approximately the same slope as locally, but shifted to brighter surface brightnesses, consistent with a mean stellar formation redshift of z_f ~ 5. This paper demonstrates a direct relation between star formation activity and size at z ~ 2.5 and the existence of a significant population of massive, extremely dense, old stellar systems without readily identifiable counterparts in the local universe.

We present the first comparison of the dynamical properties of different samples of z ~ 1.4-3.4 star-forming galaxies from spatially resolved imaging spectroscopy from SINFONI/VLT integral field spectroscopy and IRAM CO millimeter interferometry. Our samples include 16 rest-frame UV-selected, 16 rest-frame optically selected, and 13 submillimeter galaxies (SMGs). We find that rest-frame UV and optically bright (K < 20) z ~ 2 star forming galaxies are dynamically similar, and follow the same velocity-size relation as disk galaxies at z ~ 0. In the theoretical framework of rotating disks forming from dissipative collapse in dark matter halos, the two samples require a spin parameter ranging from 0.06 to 0.2. In contrast, bright SMGs (S_{850 μm} ⩾ 5 mJy) have larger velocity widths and are much more compact. Hence, SMGs have lower angular momenta and higher matter densities than either the UV or optically selected populations. This indicates that dissipative major mergers may dominate the SMGs population, resulting in early spheroids, and that a significant fraction of the UV/optically bright galaxies have evolved less violently, either in a series of minor mergers, or in rapid dissipative collapse from the halo, given that either process may leads to the formation of early disks. These early disks may later evolve into spheroids via disk instabilities or mergers. Because of their small sizes and large densities, SMGs lie at the high surface density end of a universal (out to z = 2.5) "Schmidt-Kennicutt" relation between gas surface density and star formation rate surface density. The best-fit relation suggests that the star formation rate per unit area scales as the surface gas density to a power of ~1.7, and that the star formation efficiency increases by a factor of 4 between non-starbursts and strong starbursts.

We present the results of a Very Large Array (VLA) and Owens Valley Radio Observatory Millimeter Wavelength Array (OVRO MMA) follow-up to our single-dish surveys of the neutral atomic and molecular gas in a sample of nearby luminous compact blue galaxies (LCBGs). These luminous, blue, high surface brightness, starbursting galaxies were selected using criteria similar to those used to define LCBGs at higher redshifts. The surveys were undertaken to study the nature and evolutionary possibilities of LCBGs, using dynamical masses and gas depletion timescales as constraints. Here we present nearly resolved VLA H I maps of four LCBGs, as well as results from the literature for a fifth LCBG. In addition, we present OVRO MMA maps of CO(J = 1-0) in two of these LCBGs. We have used the resolved H I maps to separate the H I emission from target galaxies and their companions to improve the accuracy of our gas and dynamical mass estimates. For this subsample of LCBGs, we find that the dynamical masses measured with the single-dish telescope and interferometer are in agreement. However, we find that we have overestimated the mass of H I in two galaxies by a significant amount, possibly as much as 75%, when compared to the single-dish estimates. These two galaxies have companions within a few arcminutes; we find that our single-dish and interferometric measurements of H I masses are in reasonable agreement for galaxies with more distant companions. Our CO(J = 1-0) maps, despite long integration times, were faint and barely resolved, making analysis difficult, except to verify the central concentration of the molecular gas. The H I velocity fields indicate that all five galaxies are clearly rotating, yet distorted, likely due to recent interactions. Our measurements of the gas and dynamical masses of LCBGs point toward evolution into low-mass galaxies such as dwarf ellipticals, irregulars, and low-mass spirals, consistent with studies of LCBGs at higher redshifts.

A complete flux-limited sample of 50 galaxies is presented having f_ν(24 μm) > 10 mJy, chosen from a survey with MIPS on Spitzer of 8.2 deg² in the NOAO Deep Wide-Field Survey (NDWFS) region in Bootes. Spectra obtained with the low-resolution modules of IRS on Spitzer are described for 36 galaxies in this sample; 25 show strong PAH emission features characteristic of starbursts, and 11 show silicate absorption or emission, emission lines, or featureless spectra characteristic of AGNs. Infrared or optical spectral classifications are available for 48 of the entire sample of 50; 33 galaxies are classified as starbursts and 15 as AGNs. [There are an additional 19 Galactic stars with f_ν(24 μm) > 10 mJy in the survey area.] Using a relation between 7.7 μm PAH luminosity and SFR derived from previous IRS observations of starbursts, the SFR per unit volume of the local universe (SFRD) is determined from the complete sample and is found to be 0.008 M_☉ yr^-1 Mpc^-3. This provides an extinction-free measurement of SFRD independent of optical properties and provides a parameter that can be used for direct comparison to high-redshift starbursts being discovered with Spitzer. Individual sources in the sample have SFRs from 0.14 to 160 M_☉ yr^-1. The derived value for the local SFRD is about half that of the local SFRD deduced from bolometric luminosities of the IRAS 60 μm Bright Galaxy Sample, with the deficiency being at lower luminosities and arising primarily from the small number of low-luminosity sources in the 10 mJy sample. The agreement for higher luminosities confirms the validity of using the 7.7 μm PAH feature as a measure of SFRD in the high-redshift universe, where this is often the only indicator available for faint sources.

We have studied the relationship between the star formation rate (SFR), surface density, and gas surface density in the spiral galaxy M51a (NGC 5194), using multiwavelength data obtained as part of the Spitzer Infrared Nearby Galaxies Survey (SINGS). We introduce a new SFR index based on a linear combination of Hα emission-line and 24 μm continuum luminosities, which provides reliable extinction-corrected ionizing fluxes and SFR densities over a wide range of dust attenuations. The combination of these extinction-corrected SFR densities with aperture synthesis H I and CO maps has allowed us to probe the form of the spatially resolved star formation law on scales of 0.5-2 kpc. We find that the resolved SFR versus gas surface density relation is well represented by a Schmidt power law, which is similar in form and dispersion to the disk-averaged Schmidt law. We observe a comparably strong correlation of the SFR surface density with the molecular gas surface density, but no significant correlation with the surface density of atomic gas. The best-fitting slope of the Schmidt law varies from N = 1.37 to 1.56, with zero point and slope that change systematically with the spatial sampling scale. We tentatively attribute these variations to the effects of areal sampling and averaging of a nonlinear intrinsic star formation law. Our data can also be fitted by an alternative parameterization of the SFR surface density in terms of the ratio of gas surface density to local dynamical time, but with a considerable dispersion.

We report the identification of a recurrent ultraluminous X-ray source (ULX), a highly absorbed X-ray source (possibly a background AGN), and a young supernova remnant near the center of the starburst galaxy M82. From a series of Chandra observations taken from 1999 to 2005, we found that the transient ULX first appeared in 1999 October. The source turned off in 2000 January, but later reappeared and has been active since then. The X-ray luminosity of this source varies from below the detection level (~ 2.5 × 10³⁸ ergs s^-1) to its active state between ~ 7 × 10³⁹ and 1.3 × 10⁴⁰ ergs s^-1 (in the 0.5-10 keV energy band) and shows unusual spectral changes. The X-ray spectra of some Chandra observations are best fitted with an absorbed power-law model with photon index ranging from 1.3 to 1.7. These spectra are similar to those of Galactic black hole binary candidates seen in the low/hard state, except that a very hard spectrum was seen in one of the observations. By comparing with near-infrared images taken with the Hubble Space Telescope, the ULX is found to be located within a young star cluster. Radio imaging indicates that it is associated with a H II region. We suggest that the ULX is likely to be a >100 M_☉ intermediate-mass black hole in the low/hard state. In addition to the transient ULX, we also found a highly absorbed hard X-ray source which is likely to be an AGN and an ultraluminous X-ray-emitting young supernova remnant which may be related to a 100 yr old gamma-ray burst event, within 2'' of the transient ULX.

We present optical HST STIS observations made with two slits crossing four of the optically brightest starburst clumps near the nucleus of M82. These provide Hα kinematics, extinction, electron density, and emission measures. From the radial velocity curves derived from both slits we confirm the presence of a stellar bar. We derive a new model for the orientation of the bar and disk with respect to the main starburst clumps and the cluster M82-A1. We propose that clump A has formed within the bar region as a result of gas interactions between the bar orbits, whereas region C lies at the edge of the bar and regions D and E are located farther out from the nucleus but heavily obscured. We derive extremely high interstellar densities of 500-900 cm^-3, corresponding to ISM pressures of P/k ≈ (0.5-1.0) × 10⁷ cm^-3 K, and discuss the implications of the measured gas properties on the production and evolution of the galactic wind. Despite varying pressures, the ionization parameter is uniform down to parsec scales, and we discuss why this might be so. Where the signal-to-noise ratios of our spectra are high enough, we identify multiple emission-line components. Through detailed Gaussian line fitting, we identify a ubiquitous broad (200-300 km s^-1) underlying component to the bright Hα line and discuss the physical mechanism(s) that could be responsible for such widths. We conclude that evaporation and/or ablation of material from interstellar gas clouds caused by the impact of high-energy photons and fast flowing cluster winds produce a highly turbulent layer on the surface of the clouds from which the emission arises.

Large-scale star formation in disk galaxies is hypothesized to be driven by global gravitational instability. The observed gas surface density is commonly used to compute the strength of gravitational instability, but according to this criterion, star formation often appears to occur in gravitationally stable regions. One possible reason is that the stellar contribution to the instability has been neglected. We have examined the gravitational instability of the Large Magellanic Cloud considering the gas alone, and considering the combination of collisional gas and collisionless stars. We compare the gravitationally unstable regions with the ongoing star formation revealed by Spitzer observations of young stellar objects. Although only 62% of the massive young stellar object candidates are in regions where the gas alone is unstable, some 85% lie in regions unstable due to the combination of gas and stars. The combined stability analysis better describes where star formation occurs. In agreement with other observations and numerical models, a small fraction of the star formation occurs in regions with gravitational stability parameter Q > 1. We further measure the dependence of the star formation timescale on the strength of gravitational instability, and quantitatively compare it to the exponential dependence expected from numerical simulations.

We present the results of a deep Hubble Space Telescope (HST) exposure of the nearby globular cluster NGC 6397, focussing attention on the cluster's white dwarf cooling sequence. This sequence is shown to extend over 5 mag in depth, with an apparent cutoff at magnitude F814W ~ 27.6. We demonstrate, using both artificial star tests and the detectability of background galaxies at fainter magnitudes, that the cutoff is real and represents the truncation of the white dwarf luminosity function in this cluster. We perform a detailed comparison between cooling models and the observed distribution of white dwarfs in color and magnitude, taking into account uncertainties in distance, extinction, white dwarf mass, progenitor lifetimes, binarity, and cooling model uncertainties. After marginalizing over these variables, we obtain values for the cluster distance modulus and age of μ₀ = 12.02 ± 0.06 and T_c = 11.47 ± 0.47 Gyr (95% confidence limits). Our inferred distance and white dwarf initial-final mass relations are in good agreement with other independent determinations, and the cluster age is consistent with, but more precise than, prior determinations made using the main-sequence turnoff method. In particular, within the context of the currently accepted ΛCDM cosmological model, this age places the formation of NGC 6397 at a redshift z ~ 3, at a time when the cosmological star formation rate was approaching its peak.

We present a homogeneous photometric and spectroscopic analysis of 18 stars along the evolutionary sequence of the metal-poor globular cluster NGC 6397 ([Fe/H] ≈ -2), from the main-sequence turnoff point to red giants below the bump. The spectroscopic stellar parameters, in particular stellar parameter differences between groups of stars, are in good agreement with broadband and Strömgren photometry calibrated on the infrared flux method. The spectroscopic abundance analysis reveals, for the first time, systematic trends of iron abundance with evolutionary stage. Iron is found to be 30% less abundant in the turnoff point stars than in the red giants. An abundance difference in lithium is seen between the turnoff point and warm subgiant stars. The impact of potential systematic errors on these abundance trends (stellar parameters, the hydrostatic and LTE approximations) is quantitatively evaluated and found not to alter our conclusions significantly. Trends for various elements (Li, Mg, Ca, Ti, and Fe) are compared with stellar structure models including the effects of atomic diffusion and radiative acceleration. Such models are found to describe the observed element-specific trends well, if extra (turbulent) mixing just below the convection zone is introduced. It is concluded that atomic diffusion and turbulent mixing are largely responsible for the subprimordial stellar lithium abundances of warm halo stars. Other consequences of atomic diffusion in old metal-poor stars are also discussed.

We present a direct detection of the gravitational lens that caused the microlensing event MACHO-95-BLG-37. This is the first fully resolved microlensing system involving a source in the Galactic bulge, and the second such system in general. The lens and source are clearly resolved in images taken with the High Resolution Channel of the Advanced Camera for Surveys on board the Hubble Space Telescope (HST) ~9 yr after the microlensing event. The currently available data are not sufficient for the final, unambiguous identification of the gravitational lens and the microlensed source. While the light-curve models combined with the high-resolution photometry for individual objects indicate that the source is red and the lens is blue, the color-magnitude diagram for the line of sight and the observed proper motions strongly support the opposite case. The first scenario points to a metal-poor lens with mass M ≈ 0.6 M_☉ at the distance D_l ≈ 4 kpc. In the second scenario, the lens could be a main-sequence star with M = 0.8-0.9 M_☉ about halfway to the Galactic bulge or in the foreground disk, depending on the extinction.

We use atomic hydrogen (H I) data from the Southern Galactic Plane Survey to study the kinematics of the fourth quadrant of the Milky Way. By measuring the terminal velocity as a function of longitude throughout the fourth Galactic quadrant we have derived the most densely sampled rotation curve available for the Milky Way between 3 kpc ≤ R ≤ 8 kpc. We determine a new joint rotation curve fit for the first and fourth quadrants, which can be used for kinematic distances interior to the solar circle. From our data we place new limits on the peak-to-peak variation of streaming motions in the fourth quadrant to be ~10 km s^-1. We show that the shape of the average H I profile beyond the terminal velocity is consistent with gas of three velocity dispersions, a cold component with Δv = 6.3 km s^-1, a warmer component with Δv = 12.3 km s^-1, and a fast component with Δv = 25.9 km s^-1. By examining the widths with Galactic radius we find that the narrowest two components show little variation with radius and their small-scale fluctuations track each other very well, suggesting that they share the same cloud-to-cloud motions. The width of the widest component is constant until R < 4 kpc, where it increases sharply.

Energetic electrons are widely observed in vicinity of planetary bow shocks, interplanetary shocks, and coronal mass ejection driven shocks. In this paper the electron energization is numerically studied by tracing exact test particle electron trajectories in the time-dependent electromagnetic fields typical for the Earth's bow shock. The time-dependent re-forming shock profiles are self-consistently generated by one-dimensional multiscale hybrid simulations. It is shown that energetic electron bursts occur cyclically at the shock re-formation period. The upstream electron distributions show time-varying loss-cone, beam, and ring-beam features in velocity space. The beam density, beam speed, average beam kinetic energy, velocity spread, and associated wave growth rate in the upstream region of the shock cyclically change with time by a factor of ~2-4. These characteristics are qualitatively very different from the continuous beam expected for a time-stationary shock and are important for the plasma instabilities/radio emission phenomena near the shocks.

Spatial associations have been found between interstellar neutral hydrogen (H I) emission morphology and small-scale structure observed by the Wilkinson Microwave Anisotropy Probe (WMAP) in an area bounded by l = 60°, 180° and b = 30°, 70°, which was the primary target for this study. This area is marked by the presence of highly disturbed local H I and a preponderance of intermediate- and high-velocity gas. The H I distribution toward the brightest peaks in the WMAP Internal Linear Combination (ILC) map for this area is examined, and by comparing with a second area on the sky it is demonstrated that the associations do not appear to be the result of chance coincidence. Close examination of several of the associations reveals important new properties of diffuse interstellar neutral hydrogen structure. In the case of the high-velocity cloud MI, the H I and WMAP ILC morphologies are similar, and an excess of soft X-ray emission and Hα emission have been reported for this feature. It is suggested that the small angular scale, high-frequency continuum emission observed by WMAP may be produced at the surfaces of H I features interacting one another, or at the interface between moving H I structures and regions of enhanced plasma density in the surrounding interstellar medium. It is possible that dust grains play a role in producing the emission. However, the primary purpose of this report is to draw attention to these apparent associations, without offering an unambiguous explanation as to the relevant emission mechanism(s).

The electronic and vibrational spectroscopic properties and ionization energies of diamondoids (nano-diamonds, microdiamonds) are computed using density functional theory (DFT). Spectra of both the neutral and cationic forms of diamondoids, ranging in size from C₁₀H₁₆ to C₃₈H₄₂, and the IR spectrum of a diamondoid-PAH hybrid molecule are presented. For the 23 neutral species, the C-H stretching bands fall near 3.47 μm and are the strongest in the spectra. Diamondoid ionization energies (IEs) are found to be quite large (about 8 eV). The electronic excitation energies of the neutral species are of the same order as the IEs and have very small oscillator strengths (f-values). For the cations, the C-H stretching peak positions differ somewhat from the neutrals and their average integrated band strengths are about half those of the neutrals. The computed electronic excitation energies for the cations are much smaller than those of the neutral species, with f-values that are very small. The neutral diamondoids will absorb most strongly near 3.47 μm, which is very close to the position of an absorption band associated with dense clouds that has been tentatively attributed to the tertiary C-H stretch of diamond-like carbon. The spectroscopic properties described here imply that 3 μm emission from highly vibrationally excited diamondoid cations and neutral species should be most intense in regions with strong radiation fields. These results, in conjunction with the observation that the 3.5 μm emission feature originates very close to the exciting star, strongly supports its assignment in HD 97048 and Elias 1 to diamondoid species. While previous work makes an excellent case for large neutral diamondoids, our work shows that some cation contribution cannot be excluded. The very small f-values explain why so few sources show this emission.

A new, unbiased Spitzer MIPS imaging survey (~1.8 deg²) of the young stellar content of the Vela Molecular Ridge-D (VMR-D) is presented. The survey is complete down to 5 and 250 mJy at 24 and 70 μm, respectively. A total of 849 sources are detected at 24 μm, and 52 of them also have a 70 μm counterpart. The VMR-D region is one that we have already partially mapped in dust and gas millimeter emission, and we discuss the correlation between the Spitzer compact sources and the millimeter contours. About half of the 24 μm sources are located inside the region delimited by the ¹²CO(1-0) contours, corresponding to only one-third of the full area mapped with MIPS. Therefore, the 24 μm source density increases by about 100% moving from outside to inside the CO contours. For the 70 μm sources, the corresponding density increase is a factor of 4. About 400 sources of these have a 2MASS counterpart, and we have used this to construct a K_s versus K_s - [24] diagram and to identify the protostellar population inside the cloud. We find an excess of Class I sources in VMR-D in comparison with other star-forming regions. This result could be reasonably biased by the sensitivity limits at 2.2 and 24 μm or, alternatively, may reflect a very short lifetime (≲10⁶ yr) for the protostellar content in this molecular cloud. The MIPS images have identified embedded cool objects in most of the previously identified starless cores in the region; in addition, there are six very young, possibly Class 0 objects identified. Finally, we report finding the driving sources for a set of five out of six very compact protostellar jets that had been previously discovered in near-IR images of VMR-D.

Observations of outflows associated with pre-main-sequence stars reveal details about morphology, binarity, and evolutionary states of young stellar objects. We present molecular line data from the Berkeley-Illinois-Maryland Association array and Five Colleges Radio Astronomical Observatory toward the regions containing the Herbig Ae/Be stars LkHα 198 and LkHα 225S. Single-dish observations of ¹²CO J = 1 - 0, ¹³CO J = 1 - 0, N₂H⁺J = 1 - 0, and CS J = 2 - 1 were made over a field of 4.3' × 4.3' for each species. ¹²CO J = 1 - 0 data from FCRAO were combined with high-resolution BIMA array data to achieve a naturally weighted synthesized beam of 6.75'' × 5.5'' toward LkHα 198 and 5.7'' × 3.95'' toward LkHα 225S, representing resolution improvements of factors of approximately 10 and 5 over existing data. By using uniform weighting, we achieved another factor of 2 improvement. The outflow around LkHα 198 resolves into at least four outflows, none of which are centered on LkHα 198-IR, but even at our resolution we cannot exclude the possibility of an outflow associated with this source. In the LkHα 225S region we find evidence for two outflows associated with LkHα 225S itself, and a third outflow is likely driven by this source. Identification of the driving sources is still resolution limited and is also complicated by the presence of three clouds along the line of sight toward the Cygnus molecular cloud. ¹³CO J = 1 - 0 is present in the environments of both stars along with cold, dense gas as traced by CS J = 2 - 1 and (in LkHα 225S) N₂H⁺J = 1 - 0. No 2.6 mm continuum is detected in either region in relatively shallow maps compared to existing continuum observations.

This paper reexamines the problem of ambipolar diffusion as a mechanism for the production and runaway evolution of centrally condensed molecular cloud cores, a process that has been termed the gravomagneto catastrophe. Our calculation applies in the geometric limit of a highly flattened core and allows for a semianalytic treatment of the full problem, although physical fixes are required to resolve a poor representation of the central region. A noteworthy feature of the overall formulation is that the solutions for the ambipolar diffusion portion of the evolution for negative times (t < 0) match smoothly onto the collapse solutions for positive times (t > 0). The treatment shows that the resulting cores display nonzero, but submagnetosonic, inward velocities at the end of the diffusion epoch, in agreement with current observations. Another important result is the derivation of an analytic relationship between the dimensionless mass-to-flux ratio λ₀ ≡ f of the central regions produced by runaway core condensation and the dimensionless measure of the rate of ambipolar diffusion . In conjunction with previous work showing that ambipolar diffusion takes place more quickly in the presence of turbulent fluctuations, i.e., that the effective value of can be enhanced by turbulence, the resultant theory provides a viable working hypothesis for the formation of isolated molecular cloud cores and their subsequent collapse to form stars and planetary systems.

We present a method for simulating the evolution of H II regions driven by point sources of ionizing radiation in magnetohydrodynamic media, implemented in the three-dimensional Athena MHD code. We compare simulations using our algorithm to analytic solutions and show that the method passes rigorous tests of accuracy and convergence. The tests reveal several conditions that an ionizing radiation hydrodynamic code must satisfy to reproduce analytic solutions. As a demonstration of our new method, we present the first three-dimensional, global simulation of an H II region expanding into a magnetized gas. The simulation shows that magnetic fields suppress sweeping up of gas perpendicular to magnetic field lines, leading to small density contrasts and extremely weak shocks at the leading edge of the H II region's expanding shell.

HST NICMOS narrowband images of the shocked molecular hydrogen emission in OMC-1 are analyzed to reveal new information on the BN/KL outflow. The outstanding morphological feature of this region is the array of molecular hydrogen "fingers" emanating from the general vicinity of IRc2 and the presence of several Herbig-Haro objects. The NICMOS images appear to resolve individual shock fronts. This work is a more quantitative and detailed analysis of our data from a previous paper. Line strengths for the H₂ 1-0 S(4) plus 2-1 S(6) lines at 1.89 μm are estimated from measurements with the Paschen-α continuum filter F190N at 1.90 μm and continuum measurements at 1.66 and 2.15 μm. We compare the observed H₂ line strengths and ratios of the 1.89 and 2.12 μm 1-0 S(1) lines with models for molecular cloud shock waves. Most of the data cannot be fit by J-shocks but are well matched by C-shocks with shock velocities in the range of 20-45 km s^-1 and preshock densities of 10⁴-10⁶ cm^-3, similar to values obtained in larger beam studies which averaged over many shocks. There is also some evidence that shocks with higher densities have lower velocities.

In this article, we present the results of a series of 12 3.6 cm radio continuum observations of T Tau Sb, one of the companions of the famous young stellar object T Tauri. The data were collected roughly every 2 months between 2003 September and 2005 July with the Very Long Baseline Array (VLBA). Thanks to the remarkably accurate astrometry delivered by the VLBA, the absolute position of T Tau Sb could be measured with a precision typically better than about 100 μas at each of the 12 observed epochs. The trajectory of T Tau Sb on the plane of the sky could therefore be traced very precisely and was modeled as the superposition of the trigonometric parallax of the source and an accelerated proper motion. The best fit yields a distance to T Tau Sb of 147.6 ± 0.6 pc. The observed positions of T Tau Sb are in good agreement with recent infrared measurements, but they seem to favor a somewhat longer orbital period than that recently reported by Duchêne and coworkers for the T Tau Sa/T Tau Sb system.

We present the observational results of Galactic H II region Sh 2-294, using optical photometry, narrowband imaging, and radio continuum mapping at 1280 MHz, together with archival data from the 2MASS, MSX, and IRAS surveys. The stellar surface density profile indicates that the radius of the cluster associated with the Sh 2-294 region is ~2.3'. We estimate the minimum reddening E(B - V) = 1.35 mag and a distance of 4.8 ± 0.2 kpc to the region. The ratio of the total to the selective extinction (R_V) is found to be 3.8 ± 0.1, indicates an anomalous reddening law for the dust inside the cluster region. We identified the ionizing source of the H II region, and spectral type estimates are consistent with a star of spectral type ~B0 V. The 2MASS JHK_s images reveal a partially embedded cluster associated with the ionizing source along with a small cluster towards the eastern border of Sh 2-294. The radio continuum and Hα images show the ionization front along the direction of the small cluster. An arc-shaped diffuse molecular hydrogen emission and a half-ring of dust emission are also seen in the direction of the ionization front at 2.12 and 8 μm (MSX), respectively. Self-consistent radiative transfer model of mid- to far-infrared continuum emission detected near small cluster is in good agreement with the observed spectral energy distribution of a B1.5 ZAMS star. The morphological correlation between the ionized and molecular gas, along with probable timescale involved between the ionizing star, evolution of H II region, and small cluster, indicates that the star formation activity observed at the border is probably triggered by the expansion of H II region.

We have imaged the outflow from the luminous young stellar object IRAS 20126+4104 (I20126) with the Submillimeter Array in CO (3-2), HCN (4-3), and SiO (5-4) at 1''-2'' resolutions within a radius of ~20'' from the central driving source. Our observations reveal at least three different components of the outflowing gas: (1) A compact (~4000 AU) bipolar outflow toward the central young stellar object. With a dynamical timescale of ~120 yr, this component represents a very new jet/outflow activity in I20126. (2) A collimated outflow with an extent of ~0.2 pc previously detected in SiO (2-1). Both morphology and kinematics favor this component being a jet-driven bow shock system. (3) An S-shaped CO outflow with an extent of ~0.4 pc. This component records the precession history very well. Its kinematic feature, where the velocity increases with distance from the YSO, indicates, independently of other evidence, that the outflow axis is moving toward the plane of the sky. The three outflow components record the history of the primary jet precession over scales ranging from a few hundred AU to approximately 0.4 pc. Our results indicate that CO (3-2) emission is a good tracer to probe the primary jet. The gas densities and SiO relative abundances in I20126 shocks are estimated using the large velocity gradient calculations. The inferred SiO abundances of (1-5) × 10^-8 in I20126 outflow lobes are comparable to the expected enhancement at shocked regions.

Nonradial p-mode oscillation spectra, computed from a dense grid of pre-main-sequence models, are fit to observed oscillation spectra of several stars in the young cluster NGC 6530. The five stars we consider, all previously identified as pulsating pre-main-sequence stars, each have from two to nine observed oscillation frequencies. For those stars with a more complete set of frequencies we are able to constrain the models using the oscillation spectra alone and confirm that the stars are in their pre-main-sequence and not post-main-sequence phase of evolution. For the stars with only two observed frequencies we are able to reduce the solution space of possible models. Comparing our model fits to the surface temperatures and luminosities derived from the observed colors and parallaxes, we find that the model fits are consistent with the cluster's distance, i.e., the luminosities agree, but we discover that all of our models are systematically too cool. We attribute some of the discrepancy in the surface temperature to uncertainties in the surface boundary conditions of our models, but argue that most of the difference is a direct consequence of applying a single average color-dependent dereddening correction to all the stars when, in fact, it appears that the stars we selected are embedded in varying degrees of gas and dust. For one of the stars we identify a rotationally split l = 1 p-mode from which we derive a rotation period of 18 days.

We present high-resolution X-ray spectra of the nearby multiple T Tauri star (TTS) system Hen 3-600, obtained with the High Energy Transmission Grating Spectrograph (HETGS) aboard the Chandra X-Ray Observatory. Both principal binary components of Hen 3-600 (A and B, separation 1.4'') were detected in the zeroth-order Chandra HETGS X-ray image. Hen 3-600-A, the component with a large mid-infrared excess, is a factor of ~2-3 fainter in X-rays than Hen 3-600-B, due to a large flare at Hen 3-600-B during our observation. The dispersed X-ray spectra of the two primary components overlap significantly, so spectral analysis was performed primarily on the first-order spectrum of the combined A + B system, with analysis of the individual dispersed spectra limited to regions where the contributions of A and B can be disentangled via cross-dispersion profile fitting. This analysis results in two lines of evidence indicating that the X-ray emission from Hen 3-600 A + B is derived, in part, from accretion processes. (1) The line ratios of He-like O VII in the spectrum of Hen 3-600 A + B indicate that the characteristic density of its X-ray-emitting plasma is significantly larger than those of coronally active main-sequence and pre-main-sequence stars. (2) A significant component of low-temperature (2-3 MK) plasma is present in the Hen 3-600 A + B spectrum; this "soft excess" appears somewhat stronger in component A. These results for Hen 3-600 A + B are consistent with, although less pronounced than, results obtained from X-ray grating spectroscopy of more rapidly accreting TTS systems. Indeed, all of the emission signatures of Hen 3-600 A + B that are potential diagnostics of accretion activity—from its high-resolution X-ray spectrum, through its UV excess and Hα emission-line strengths, to its weak near-infrared excess—suggest that its components (and component A in particular) represent a transition phase between rapidly accreting, classical T Tauri stars and nonaccreting, weak-lined T Tauri stars.

Observations of pre-main-sequence star rotation periods reveal slow rotators in young clusters of various ages, indicating that angular momentum is somehow removed from these rotating masses. The mechanism by which spin-up is regulated as young stars contract has been one of the longest standing problems in star formation. Attempts to observationally confirm the prevailing theory that magnetic interaction between the star and its circumstellar disk regulates these rotation periods have produced mixed results. In this paper, we use the unprecedented disk identification capability of the Spitzer Space Telescope to test the star-disk interaction paradigm in two young clusters, NGC 2264 and the Orion Nebula Cluster (ONC). We show that once mass effects and sensitivity biases are removed, a clear increase in the disk fraction with period can be observed in both clusters across the entire period range populated by cluster members. We also show that the long-period peak (P ~ 8 days) of the bimodal distribution observed for high-mass stars in the ONC is dominated by a population of stars possessing a disk, while the short-period peak (P ~ 2 days) is dominated by a population of stars without a disk. Our results represent the strongest evidence to date that star-disk interaction regulates the angular momentum of these young stars. This study will make possible quantitative comparisons between the observed period distributions of stars with and without a disk and numerical models of the angular momentum evolution of young stars.

Most Vega-like stars have far-infrared excess (60 μm or longward in IRAS, ISO, or Spitzer MIPS bands) and contain cold dust (≲150 K) analogous to the Sun's Kuiper Belt region. However, dust in a region more akin to our asteroid belt and thus relevant to the terrestrial planet building process is warm and produces excess emission in mid-infrared wavelengths. By cross-correlating Hipparcos dwarfs with the MSX catalog, we found that EF Cha, a member of the recently identified, ~10 Myr old, "Cha-Near" moving group, possesses prominent mid-infrared excess. N-band spectroscopy reveals a strong emission feature characterized by a mixture of small, warm, amorphous, and possibly crystalline silicate grains. Survival time of warm dust grains around this A9 star is ≲10⁵ yr, much less than the age of the star. Thus, grains in this extrasolar terrestrial planetary zone must be of a "second generation" and not a remnant of primordial dust and are suggestive of substantial planet formation activity. Such second generation warm excess occurs around ~13% of the early-type stars in nearby young stellar associations.

We report on the statistics of strong (W_r > 0.15 Å) C IV absorbers at z = 1.5-3.5 toward high-redshift gamma-ray bursts (GRBs). In contrast with a recent survey for strong Mg II absorption systems at z < 2, we find that the number of C IV absorbers per unit redshift dN/dz does not show a significant deviation from previous surveys using quasi-stellar objects (QSOs) as background sources. We find that the number density of C IV toward GRBs is dN/dz|_GRB(z ~ 1.5) = 2.2, dN/dz|_GRB(z ~ 2.5) = 2.3 and dN/dz|_GRB(z ~ 3.5) = 1.1. These numbers are consistent with previous C IV surveys using QSO spectra. Binning the entire data set, we set a 95% c.l. upper limit to the excess of C IV absorbers along GRB sight lines at twice the incidence observed along QSO sight lines. Furthermore, the distribution of equivalent widths of the GRB and QSO samples are consistent with being drawn from the same parent population. Although the results for Mg II and C IV absorbers along GRB sight lines appear to contradict one another, we note that the surveys are nearly disjoint: the C IV survey corresponds to higher redshift and more highly ionized gas than the Mg II survey. Nevertheless, analysis on larger statistical samples may constrain properties of the galaxies hosting these metals (e.g., mass, dust content) and/or the coherence-length of the gas giving rise to the metal-line absorption.

We present early Very Large Telescope UV-Visual Echelle Spectrograph (VLT UVES) high-resolution spectra of the afterglow of GRB 030329 at redshift z = 0.16867 ± 0.00001. In contrast to other spectra from this burst, both emission and absorption lines were detected. None of them showed any temporal evolution. From the emission lines, we determine the properties of the host galaxy, which has a star formation rate (SFR) of 0.198 M_☉ yr^-1 and a low metallicity of 0.17 Z_☉. Given the low total stellar host mass log M_⋆ = 7.75 ± 0.15 M_☉ and an absolute luminosity m_B = -16.29, we derive specific SFRs (SSFR) of log SFR = -8.5 yr^-1 and SFR = 15.1 M_☉ yr^-1L. This fits well into the picture of GRB hosts as being low-mass, low-metallicity, actively star-forming galaxies. The Mg II and Mg I absorption lines from the host show multiple narrow (Doppler width b = 5-12 km s^-1) components spanning a range of v ~ 230 km s^-1, mainly blueshifted compared to the redshift from the emission lines. These components are likely probing outflowing material of the host galaxy, which could arise from former galactic superwinds, driven by supernovae from star-forming regions. Similar features have been observed in QSO spectra. The outflowing material has high column densities of log N_{Mg II} = 13.99 ± 0.04 cm^-2 and log N_{Mg I} = 12.39 ± 0.04 cm^-2 and the nonvariability of the column densities implies a distance of at least 560 pc from the burst, further supporting an outflow scenario.

The widely existing shallow decay phase of the X-ray afterglows of gamma-ray bursts (GRBs) is generally accepted to be due to long-lasting energy injection. The outflows carrying the injecting energy, based on the component that is dominant in energy, fall into two possible types: baryon-dominated and lepton-dominated ones. The former type of outflow could be ejecta that is ejected during the prompt phase of a GRB and consists of a series of baryonic shells with a distribution of Lorentz factors, and the latter type could be an electron-positron pair wind that is driven by the postburst central engine. We here provide a unified description for the dynamics of fireballs based on these two types of energy injection and calculate the corresponding high-energy photon emission by considering synchrotron radiation and inverse Compton scattering (including synchrotron self-Compton and combined inverse Compton) of electrons. We find that, in the two energy-injection models, there is a plateau (even a hump) in high-energy light curves during the X-ray shallow decay phase. In particular, a considerable fraction of the injecting energy in the lepton-dominated model can be shared by the long-lasting reverse shock since it is relativistic. Furthermore, almost all of the energy of the reverse shock is carried by leptons, and thus, the inverse Compton emission is enhanced dramatically. Therefore, this model predicts more significant high-energy afterglow emission than the baryon-dominated model. We argue that these observational signatures would be used to discriminate between different energy-injection models in the upcoming Gamma-Ray Large Area Space Telescope (GLAST) era.

In the framework of the internal shock scenario, we model the broadband prompt emission of gamma-ray bursts (GRBs) with emphasis on the GeV-TeV bands, utilizing Monte Carlo simulations that include various processes associated with electrons and protons accelerated to high energies. While inverse Compton emission from primary electrons is often dominant, different proton-induced mechanisms can also give rise to distinct high-energy components, such as synchrotron emission from protons, muons, or secondary electrons/positrons injected via photomeson interactions. In some cases, they give rise to double spectral breaks that can serve as unique signatures of ultra-high-energy protons. We discuss the conditions favorable for such emission, and how they are related to the production of ultra-high-energy cosmic rays and neutrinos in internal shocks. Ongoing and upcoming observations by the Gamma-Ray Large Area Space Telescope (GLAST), atmospheric Cerenkov telescopes, and other facilities will test these expectations, and provide important information on the physical conditions in GRB outflows.

We calculate durations and spectral parameters for 218 Swift bursts detected by the BAT instrument between and including gamma-ray bursts (GRBs) 041220 and 070509, including 77 events with measured redshifts. Incorporating prior knowledge into the spectral fits, we are able to measure the characteristic νF_ν spectral peak energy E_{pk, obs} and the isotropic equivalent energy E_iso (1-10⁴ keV) for all events. This complete and rather extensive catalog, analyzed with a unified methodology, allows us to address the persistence and origin of high-energy correlations suggested in pre-Swift observations. We find that the E_{pk, obs}-E_iso correlation is present in the Swift sample; however, the best-fit power-law relation is inconsistent with the best-fit pre-Swift relation at >5 σ significance. It has a factor ≳2 larger intrinsic scatter, after accounting for large errors on E_{pk, obs}. A large fraction of the Swift events are hard and subluminous relative to (and inconsistent with) the pre-Swift relation, in agreement with indications from BATSE GRBs without redshift. Moreover, we determine an experimental threshold for the BAT detector and show how the E_{pk, obs}-E_iso correlation arises artificially due to partial correlation with the threshold. We show that pre-Swift correlations found by Amati et al., Yonetoku et al., and Firmani et al., and independently by others are likely unrelated to the physical properties of GRBs and are likely useless for tests of cosmology. Also, an explanation for these correlations in terms of a detector threshold provides a natural and quantitative explanation for why short-duration GRBs and events at low redshift tend to be outliers to the correlations.

A model is developed for the confinement and collimation of a baryon-poor outflow by its surrounding medium. Both confinement by kinetic pressure of a static corona and confinement by the ram pressure of a supersonic wind emanating from a disk surrounding the inner source are considered. Solutions are presented for the structure of the shocked layers of a deflected baryon-poor jet (BPJ) and exterior wind. The dependence of the opening angle of the BPJ on the parameters of the confining medium are carefully examined. It is found that the BPJ shock may either converge to the symmetry axis or diverge away from it, depending on the opening angle of the BPJ injection cone. In the latter case, the inner flow exhibits a nonuniform structure, consisting of an ultrarelativistic core containing the unshocked BPJ enveloped by the slower, shocked BPJ layer. The implications of our results to the prompt GRB emission are briefly discussed.

We present the results of observations of the radio emission from SN 2001gd in NGC5033 from 2002 February 8 through 2006 September 25. The data were obtained using the VLA at wavelengths of 1.3 cm (22.4 GHz), 2 cm (14.9 GHz), 3.6 cm (8.4 GHz), 6 cm (4.9 GHz), and 20 cm (1.5 GHz), with one upper limit at 90 cm (0.3 GHz). In addition, one detection has been provided by the GMRT at 21 cm (1.4 GHz). SN 2001gd was discovered in the optical well past maximum light, so that it was not possible to obtain many of the early radio "turn-on" measurements that are important for estimating the local CSM properties. Only at 20 cm were turn-on data available. However, our analysis and fitting of the radio light curves, along with the assumption that the Type IIb SN 2001gd resembles the much better studied Type IIb SN 1993J, enables us to describe the radio evolution as being very regular through day ~550 and consistent with a nonthermal-emitting model with a thermal absorbing CSM. The presence of SSA at early times is implied by the data, but determination of the exact relationship between the SSA component from the emitting region and the free-free absorption component from the CSM is not possible, as there are insufficient early measurements to distinguish between models. After day ~550, the radio emission exhibits a dramatically steeper decline rate, which, assuming similarity to SN 1993J, can be described as an exponential decrease with an e-folding time of 500 days. We interpret this abrupt change in the radio flux density decline rate as implying a transition of the shock front into a more tenuous region of circumstellar material. A similar change in radio evolution has been seen earlier in other SNe, such as SN 1988Z, SN 1980K, and SN 1993J.

The condensation of matter from a corona to a cool, optically thick inner disk is investigated for black hole X-ray transient systems in the low/hard state. A description of a simple model for the exchange of energy and mass between corona and disk originating from thermal conduction is presented, taking into account the effect of Compton cooling of the corona by photons from the underlying disk. It is found that a weak, condensation-fed inner disk can be present in the low/hard state of black hole transient systems for a range of luminosities that depends on the magnitude of the viscosity parameter. For α ~ 0.1-0.4, an inner disk can exist for luminosities in the range ~(0.001-0.02)L_Edd. The model is applied to the X-ray observations of the black hole candidate sources GX 339-4 and SWIFT J1753.5-0127 in their low/hard state. It is found that Compton cooling is important in the condensation process, leading to the maintenance of cool inner disks in both systems. As the results of the evaporation/condensation model are independent of the black hole mass, it is suggested that such inner cool disks may contribute to the optical and ultraviolet emission of low-luminosity active galactic nuclei.

We present the results of simultaneous radio (VLA) and X-ray (RXTE) observations of the Z-type neutron star X-ray binary GX 17+2. The aim is to assess the coupling between X-ray and radio properties throughout its three rapidly variable X-ray states and during the time-resolved transitions. These observations allow us, for the first time, to investigate quantitatively the possible relations between the radio emission and the presence of hard X-ray tails and the X-ray state of the source. The observations reveal (1) a coupling between the radio jet emission and the X-ray state of the source, that is, the position in the X-ray hardness-intensity diagram (HID); (2) a coupling between the presence of a hard X-ray tail and the position in the HID, qualitatively similar to that found for the radio emission; (3) an indication of a quantitative positive correlation between the radio flux density and the X-ray flux in the hard tail power-law component; (4) evidence for the formation of a radio jet associated with the flaring branch-to-normal branch X-ray state transition; and (5) that the radio flux density of the newly formed jet stabilizes when the normal-branch oscillation (NBO) in the X-ray power spectrum stabilizes its characteristic frequency, suggesting a possible relation between X-ray variability associated with the NBO and jet formation. We discuss our results in the context of jet models.

We present results of a population synthesis of millisecond pulsars from the Galactic disk. Excluding globular clusters, we model the spatial distribution of millisecond pulsars by assuming their birth in the Galactic disk with a random kick velocity and evolve them to the present within the Galactic potential. We assume that normal and millisecond pulsars are standard candles described with a common radio luminosity model that invokes a new relationship between radio core and cone emission suggested by recent studies. In modeling the radio emission beams, we explore the relativistic effects of time delay, aberration, and sweep-back of the open field lines. While these effects are essential to understanding pulse profiles, the phase-averaged flux is adequately described without a relativistic model. We use a polar cap acceleration model for the γ-ray emission. We present the preliminary results of our recent study and the implications for observing millisecond pulsars with GLAST and AGILE.

We report on the detailed modeling of the X-ray spectra of three likely neutron stars. The neutron stars, observed with XMM-Newton, are found in three quiescent X-ray binaries in the globular clusters: ω Cen, M13, and NGC 2808. Whether they are accreting at very low rates or radiating energy from an accretion-heated core, their X-ray spectra are expected to be those of a hydrogen atmosphere. We use and compare publicly available hydrogen atmosphere models with constant and varying surface gravities to constrain the masses and radii of the neutron stars. Thanks to the high XMM-Newton throughput and the accurate distances available for these clusters, using the latest science analysis software release and calibration of the XMM-Newton EPIC cameras, we derive the most stringent constraints on the masses and radii of the neutron stars obtained to date from these systems. A comparison of the models indicates that previously used hydrogen atmosphere models (assuming constant surface gravity) tend to underestimate the mass and overestimate the radius of neutron stars. Our data constrain the allowed equations of state to those which concern normal nucleonic matter and one possible strange quark matter model, thus constraining radii to be from 8 km and masses up to 2.4 M_☉.

We present the first mid-infrared (5.5-14.5 μm) spectrum of a highly magnetic cataclysmic variable, EF Eridani, obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. The spectrum displays a relatively flat, featureless continuum. A spectral energy distribution model consisting of a 9500 K white dwarf, an L5 secondary star, cyclotron emission corresponding to a B ≈ 13 MG white dwarf magnetic field, and an optically thin circumbinary dust disk is in reasonable agreement with the extant Two Micron All Sky Survey (2MASS), Infrared Array Camera (IRAC), and IRS observations of EF Eri. Cyclotron emission is ruled out as a dominant contributor to the infrared flux density at wavelengths ≳3 μm. The spectral energy distribution longward of ~5 μm is dominated by dust emission. Even longer wavelength observations would test the model's prediction of a continuing gradual decline in the circumbinary disk-dominated region of the spectral energy distribution.

We describe Chandra High Energy Transmission Grating Spectrometer observations of RT Cru, the first of a new subclass of symbiotic stars that appear to contain white dwarfs (WDs) capable of producing hard X-ray emission out to greater than 50 keV. The production of such hard X-ray emission from the objects in this subclass (which also includes CD -57 3057, T CrB, and CH Cyg) challenges our understanding of accreting WDs. We find that the 0.3-8.0 keV X-ray spectrum of RT Cru emanates from an isobaric cooling flow, as in the optically thin accretion disk boundary layers of some dwarf novae. The parameters of the spectral fit confirm that the compact accretor is a WD, and they are consistent with the WD being massive. We detect rapid, stochastic variability from the X-ray emission below 4 keV. The combination of flickering variability and a cooling flow spectrum indicates that RT Cru is likely powered by accretion through a disk. Whereas the cataclysmic variable stars with the hardest X-ray emission are typically magnetic accretors with X-ray flux modulated at the WD spin period, we find that the X-ray emission from RT Cru is not pulsed. RT Cru therefore shows no evidence for magnetically channeled accretion, consistent with our interpretation that the Chandra spectrum arises from an accretion disk boundary layer.

We present the first detailed study of the properties (temperatures, gravities, and masses) of the NGC 6791 white dwarf population. This unique stellar system is both one of the oldest (8 Gyr) and most metal-rich ([Fe/H] ~ +0.4) open clusters in our Galaxy and has a color-magnitude diagram (CMD) that exhibits both a red giant clump and a much hotter extreme horizontal branch. Fitting the Balmer lines of the white dwarfs in the cluster using Keck/LRIS spectra suggests that most of these stars are undermassive, ⟨M⟩ = 0.43 ± 0.06 M_☉, and therefore could not have formed from canonical stellar evolution involving the helium flash at the tip of the red giant branch. We show that at least 40% of NGC 6791's evolved stars must have lost enough mass on the red giant branch to avoid the flash and therefore did not convert helium into carbon-oxygen in their core. Such increased mass loss in the evolution of the progenitors of these stars is consistent with the presence of the extreme horizontal branch in the CMD. This unique stellar evolutionary channel also naturally explains the recent finding of a very young age (2.4 Gyr) for NGC 6791 from white dwarf cooling theory; helium-core white dwarfs in this cluster will cool ~3 times slower than carbon-oxygen-core stars, and therefore the corrected white dwarf cooling age is in fact ≳7 Gyr, consistent with the well-measured main-sequence turnoff age. These results provide direct empirical evidence that mass loss is much more efficient in high-metallicity environments and therefore may be critical in interpreting the ultraviolet upturn in elliptical galaxies.

Low-mass helium-core white dwarfs (M < 0.45 M_☉) can be produced from interacting binary systems, and traditionally all of them have been attributed to this channel. However, a low-mass white dwarf could also result from a single star that experiences severe mass loss on the first ascent giant branch. A large population of low-mass He-core white dwarfs has been discovered in the old metal-rich cluster NGC 6791. There is therefore a mechanism in clusters to produce low-mass white dwarfs without requiring binary star interactions, and we search for evidence of a similar population in field white dwarfs. We argue that there is a significant field population (of order half of the detected systems) that arises from old metal-rich stars which truncate their evolution prior to the helium flash from severe mass loss. There is a consistent absence of evidence for nearby companions in a large fraction of low-mass white dwarfs. The number of old metal-rich field dwarfs is also comparable with the apparently single low-mass white dwarf population, and our revised estimate for the space density of low-mass white dwarfs produced from binary interactions is also compatible with theoretical expectations. This indicates that this channel of stellar evolution, hitherto thought hypothetical only, has been in operation in our own Galaxy for many billions of years. One strong implication of our model is that single low-mass white dwarfs should be good targets for planet searches because they are likely to arise from metal-rich progenitors. We also discuss other observational tests and implications, including the potential impact on SNIa rates and the frequency of planetary nebulae.

The origin of brown dwarfs (BDs) is still an unsolved mystery. While the standard model describes the formation of BDs and stars in a similar way recent data on the multiplicity properties of stars and BDs show them to have different binary distribution functions. Here we show that proper treatment of these uncovers a discontinuity of the multiplicity-corrected mass distribution in the very low mass star (VLMS) and BD mass regime. A continuous IMF can be discarded with extremely high confidence. This suggests that VLMSs and BDs on the one hand, and stars on the other, are two correlated but disjoint populations with different dynamical histories. The analysis presented here suggests that about one BD forms per five stars and that the BD-star binary fraction is about 2%-3% among stellar systems.

We report on the unprecedented red supergiant (RSG) population of a massive young cluster, located at the base of the Scutum-Crux Galactic arm. We identify candidate cluster RSGs based on 2MASS photometry and medium-resolution spectroscopy. With follow-up high-resolution spectroscopy, we use CO band-head equivalent width and high-precision radial velocity measurements to identify a core grouping of 26 physically associated RSGs—the largest such cluster known to date. Using the stars' velocity dispersion and their inferred luminosities in conjunction with evolutionary models, we argue that the cluster has an initial mass of ~40,000 M_☉ and is therefore among the most massive in the galaxy. Further, the cluster is only a few hundred parsecs away from the cluster of 14 RSGs recently reported by Figer at al.. These two RSG clusters represent 20% of all known RSGs in the Galaxy, and now offer the unique opportunity to study the presupernova evolution of massive stars, and the blue- to red-supergiant ratio at uniform metallicity. We use GLIMPSE, MIPSGAL, and MAGPIS survey data to identify several objects in the field of the larger cluster which seem to be indicative of recent regionwide starburst activity at the point where the Scutum-Crux arm intercepts the Galactic bulge. Future abundance studies of these clusters will therefore permit the study of the chemical evolution and metallicity gradient of the Galaxy in the region where the disk meets the bulge.

It is known that there must be some weak form of transport (called cool bottom processing, or CBP) acting in low-mass red giant branch (RGB) and asymptotic giant branch (AGB) stars, adding nuclei, newly produced near the hydrogen-burning shell, to the convective envelope. We assume that this extra mixing originates in a stellar dynamo operated by the differential rotation below the envelope, maintaining toroidal magnetic fields near the hydrogen-burning shell. We use a phenomenological approach to the buoyancy of magnetic flux tubes, assuming that they induce matter circulation as needed by CBP models. This establishes requirements on the fields necessary to transport material from zones where some nuclear burning takes place, through the radiative layer and into the convective envelope. Magnetic field strengths are determined by the transport rates needed by CBP for the model stellar structure of a star of initially 1.5 M_☉, in both the AGB and RGB phases. The field required for the AGB star in the processing zone is B₀ ~ 5 × 10⁶ G; at the base of the convective envelope this yields an intensity B_E ≲ 10⁴ G. For the RGB case, B₀ ~ 5 × 10⁴–4 × 10⁵ G, and the corresponding B_E are ~450-3500 G. These results are consistent with existing observations on AGB stars. They also hint at the basis for high field sources in some planetary nebulae, and the very large fields found in some white dwarfs. It is concluded that transport by magnetic buoyancy should be considered as a possible mechanism for extra mixing through the radiative zone, as is required by both stellar observations and the extensive isotopic data on circumstellar condensates found in meteorites.

CCD photometric observations of AD Cancri obtained from 2000 March 7 to 2004 December 20 are presented. Variations of the light levels at the primary minimum and both maxima are found. Uniform solutions of four sets of photometric data were derived by using the Wilson-Devinney method. The solutions suggest that AD Cancri is a shallow W-type contact binary (f = 8.3% ± 1.3%) with a high mass ratio of 1/q = 0.770 ± 0.002. The long-term variation of the light curve is explained by variable dark-spot models of the more massive component star with a possible 17 yr cycle. Our 13 times of light minimum over 5 years, including others collected from the literature, have been used for the period study. The complex period changes can be sorted into a long-term period increase at rate of dP/dt = +(4.94 ± 0.16) × 10^-7 days yr^-1, a 16.2 yr periodic component (A₃ = 0.0155 days), and a very small amplitude period oscillation (A₄ = 0.0051 days, P₄ = 6.6 yr). The existence of third light may indicate that there is a tertiary component in the binary system. Solving the four-band light curves of Samec & Bookmyer, it is found that the contribution of the tertiary component to the total light of the triple system increases with wavelength, which suggests that it is very cool and may be a very red main-sequence star. The small-amplitude period oscillation may be caused by the light-time effect of the cool tertiary component (M₃ ~ 0.41 M_☉). The 16.2 yr periodic component in the orbital period and the 17 yr cyclic activity of the dark spot on the more massive component both may reveal that the more massive component displays solar-type magnetic activity with a cycle length of about 16 yr.

We explore the dependence of presupernova evolution and supernova nucleosynthesis yields on the uncertainties in helium-burning reaction rates. Using the revised solar abundances of Lodders for the initial stellar composition, instead of those of Anders and Grevesse, changes the supernova yields and limits the constraints that those yields place on the ¹²C(α,γ)¹⁶O reaction rate. The production factors of medium-weight elements (A = 16-40) were found to be in reasonable agreement with observed solar ratios within the current experimental uncertainties in the triple-α reaction rate. Simultaneous variations by the same amount in both reaction rates or in either of them separately, however, can induce significant changes in the central ¹²C abundance at core carbon ignition and in the mass of the supernova remnant. It therefore remains important to have experimental determinations of the helium-burning rates so that their ratio and absolute values are known with an accuracy of 10% or better.

FS CMa stars form a group of objects with the B[e] phenomenon that were previously known as unclassified B[e] stars or B[e] stars with warm dust (B[e]WD) until recently. They exhibit strong emission-line spectra and strong IR excesses, most likely due to recently formed circumstellar dust. These properties have been suggested to be due to ongoing or recent rapid mass exchange in binary systems with hot primaries and various types of secondaries. The first paper of this series reported an analysis of the available information about previously known Galactic objects with the B[e] phenomenon, the initial selection of the FS CMa group objects, and a qualitative explanation of their properties. This paper reports the results of our new search for more FS CMa objects in the IRAS Point Source Catalog. We present new photometric criteria for identifying FS CMa stars as well as the first results of our observations of nine new FS CMa group members. With this addition, the FS CMa group has now 40 members, becoming the largest among the dust-forming hot star groups. We also present nine objects with no evidence for the B[e] phenomenon, but with newly discovered spectral line emission and/or strong IR excesses.

We have obtained a series of high-resolution optical spectra for 2MASSW J1207334-393254 (2M 1207). Two consecutive observing nights at the ESO Very Large Telescope with the Ultraviolet and Visual Echelle Spectrograph yielded a time series with a resolution of ~12 minutes. Additional high-resolution optical spectra were obtained months apart at the Magellan Clay telescope using the Magellan Inamori Kyocera Echelle (MIKE) instrument. Combined with previously published results, these data allow us to investigate changes in the emission line spectrum of 2M 1207 on timescales of hours to years. Most of the emission line profiles of 2M 1207 are broad, in particular that of Hα, indicating that the dominant fraction of the emission must be attributed to disk accretion rather than to magnetic activity. From the Hα 10% width, we deduce a relatively stable accretion rate between 10^-10.1 and 10^-9.8M_☉ yr^-1 for two nights of consecutive observations. Therefore, either the accretion stream is nearly homogeneous over (sub)stellar longitude, or the system is seen face-on. Small but significant variations are evident throughout our near-continuous observation, and they reach a maximum after ~8 hr, roughly the timescale on which maximum variability is expected across the rotation cycle. Together with past measurements, we confirm that the accretion rate of 2M 1207 varies by more than 1 order of magnitude on timescales of months to years. Such variable mass accretion yields a plausible explanation for the observed spread in the -M diagram. The magnetic field required to drive the funnel flow is on the order of a few hundred G. Despite the obvious presence of a magnetic field, neither radio nor X-ray emission has been reported for 2M 1207. It is possible that strong accretion suppresses magnetic activity in brown dwarfs, similar to the findings for higher mass T Tauri stars.

The energy densities of matter and the vacuum are currently observed to be of the same order of magnitude: (Ω ≈ 0.3) ~ (Ω ≈ 0.7). The cosmological window of time during which this occurs is relatively narrow. Thus, we are presented with the cosmological coincidence problem: why, just now, do these energy densities happen to be of the same order? Here we show that this apparent coincidence can be explained as a temporal selection effect produced by the age distribution of terrestrial planets in the universe. We find a large (~68%) probability that observations made from terrestrial planets will result in finding Ω_m at least as close to Ω_Λ as we observe today. Hence, we, and any observers in the universe who have evolved on terrestrial planets, should not be surprised to find Ω ~ Ω. This result is relatively robust if the time it takes an observer to evolve on a terrestrial planet is less than ~10 Gyr.

We identify two classes of transiting planet, based on their equilibrium temperatures and Safronov numbers. We examine various possible explanations for the dichotomy. It may reflect the influence of planet or planetesimal scattering in determining when planetary migration stops. Another possibility is that some planets lose more mass to evaporation than others. If this evaporation process preferentially removes helium from the planet, the consequent reduction in the mean molecular weight may explain why some planets have anomalously large radii.

We report the relative abundances of 17 elements in the atmosphere of the white dwarf star GD 362, material that, very probably, was contained previously in a large asteroid or asteroids with composition similar to the Earth-Moon system. The asteroid may have once been part of a larger parent body not unlike one of the terrestrial planets of our solar system.

The surface density profile Σ(r) of the solar nebula protoplanetary disk is a fundamental input to all models of disk processes and evolution. Traditionally it is estimated by spreading out the augmented masses of the planets over the annuli in which the planets orbit today, the so-called minimum-mass solar nebula. Doing so implicitly assumes that the planets completely accreted all planetesimals in their feeding zones, but this assumption has not been tested. Indeed, models of the growth of Uranus and Neptune predict that these planets could not have grown to ~10 M_⊕ within the lifetime of the disk, even though they must have, to accrete H/He atmospheres. In this paper we adopt the starting positions of the planets in the "Nice" model of planetary dynamics (Tsiganis and coworkers), in which the solar system started in a much more compact configuration. We derive a surface density profile that is well approximated by the power law Σ(r) = 343(f_p/0.5)^-1(r/10 AU)^-2.168 g cm^-2, where f_p is the fraction of the solid mass in the form of planetesimals. We show that this profile is inconsistent with a steady state accretion disk but is consistent with a steady state decretion disk that is being photoevaporated. We calculate the growth of planets in the context of this disk model and demonstrate for the first time that all of the giant planets can achieve their isolation masses and begin to accrete H/He atmospheres within the lifetime of the disk. The fit of our inferred Σ(r) to the augmented masses of the planets is excellent (<10%), but only if Uranus and Neptune swtiched places early in the solar system's evolution, a possibility predicted by the Nice model.

We have developed a Monte Carlo simulation code to study the effects of refraction due to spatial variation of the solar wind density and scattering due to random density fluctuations on directivities, time profiles, and sizes and positions of the apparent sources of the interplanetary type II and type III radio bursts excited at the fundamental (F) and second harmonic (H) of the electron plasma frequency, f_pe. We have focused on the 120 kHz fundamental and harmonic emissions with sources at the heliocentric distances of 0.2097 AU (~115 kHz plasma level), and 0.3875 AU (~60 kHz plasma level), respectively, and computed the distributions of trajectories of traced rays in a refracting, as well as a refracting and scattering, medium. These distributions show that (1) the scattering by random density fluctuations extends the visibilities of F and H components from ~18° to ~90°, and from ~80° to ~150°, respectively; (2) the time profiles constructed using the dispersion of the arrival times of the scattered rays at any given frequency may contain two peaks corresponding to F and H emissions, or a single peak consisting of both F and H components (fundamental followed by the harmonic), or a single H peak depending on the speed of the electron beam and the observer's location; and (3) the scattering broadens the sizes of the fundamental and harmonic point sources to ~25° and ~37°, and by elevating their apparent radial distances to f/3 and f/2 levels, respectively. We also present simultaneous observations of a type II and a couple of type III radio bursts by Ulysses and Wind spacecraft separated by more than 100° and show that the widely visible radio bursts correspond either to a scattered fundamental, to a refracted or scattered harmonic, or to a mixture of scattered fundamental and harmonic emissions.

Variations in the solar wind density introduce variable delays into pulsar timing observations. Current pulsar timing analysis programs only implement simple models of the solar wind, which not only limit the timing accuracy, but can also affect measurements of pulsar rotational, astrometric, and orbital parameters. We describe a new model of the solar wind electron density content which uses observations from the Wilcox Solar Observatory of the solar magnetic field. We have implemented this model into the TEMPO2 pulsar timing package. We show that this model is more accurate than previous models and that these corrections are necessary for high-precision pulsar timing applications.

In this paper the plasma properties of three streamers observed in 2003 by the Ultraviolet Coronagraph Spectrometer (UVCS) are presented for five heights from 1.75 to 5.0 R_☉. The kinetic temperatures for protons (T_k,p) and the O⁵⁺ ions (T_k,O) are derived as a function of height with preferential heating of O⁵⁺ over protons recorded. By examining how T_k,p varies with latitude at each height, an idea of the magnetic field morphology is found. At 1.75 R_☉ the elemental abundances (O, S, Ar, and Fe), electron temperature, and electron density are derived from the UV spectral data. All three streamers were quiescent with typical abundance values; however, no depleted cores were found. The first ionization potential (FIP) effect was detected for all three streamers with a bias of ~4. This is consistent with slow solar wind in situ measurements, thereby supporting the hypothetical connection between the two. All three streamers had a higher than expected electron temperature. The electron densities above 1.75 R_☉ are derived from the Large Angle Spectroscopic Coronagraph (LASCO) C2 polarized brightness data. Estimates for the O⁵⁺ outflow velocities are obtained using the O VI λ1032 over λ1037 intensity ratios and the estimated electron densities. All three streamers showed evidence of significant outflows at 4.0 and 5.0 R_☉.

We previously described coronal events that expand gradually outward over an interval of 1-2 days and then suddenly tear apart in the coronagraph's 2-6 R_☉ field of view to form an outgoing flux rope and an inward system of collapsing loops. Now, we combine LASCO white-light images of the outer corona with spectrally resolved EIT images of the inner corona to describe a similar event for which the separation occurs closer to the Sun. The evolution of this 2006 July 1-2 event had four phases: (1) an expansion phase in which magnetic loops rise slowly upward and increase the amount of open flux in the adjacent polar coronal hole and in the low-latitude hole of opposite polarity; (2) a stretching phase in which the legs of the rising loops pinch together to form a current sheet; (3) a transition phase in which field line reconnection produces an outgoing flux rope and a hot cusp of new loops; and (4) an end phase in which the reconnected loops become visible at lower temperatures, and the outgoing flux rope plows through the slow material ahead of it to form a traveling bow wave. During this time, the photospheric field was relatively weak and unchanging, as if the eruption had a nonmagnetic origin. We suppose that coronal heating gradually overpowers magnetic tension and causes the streamer to separate into a system of collapsing loops and a flux rope that is carried outward in the solar wind.

The solar magnetic field is the primary agent that drives solar activity and couples the Sun to the heliosphere. Although the details of this coupling depend on the quantitative properties of the field, many important aspects of the corona-solar wind connection can be understood by considering only the general topological properties of those regions on the Sun where the field extends from the photosphere out to interplanetary space, the so-called open field regions that are usually observed as coronal holes. From the simple assumptions that underlie the standard quasi-steady corona-wind theoretical models, and that are likely to hold for the Sun as well, we derive two conjectures as to the possible structure and dynamics of coronal holes: (1) coronal holes are unique in that every unipolar region on the photosphere can contain at most one coronal hole, and (2) coronal holes of nested polarity regions must themselves be nested. Magnetic reconnection plays the central role in enforcing these constraints on the field topology. From these conjectures we derive additional properties for the topology of open field regions, and propose several observational predictions for both the slowly varying and transient corona/solar wind.

Using instruments on the ACE and Wind spacecraft, we investigate the temporal evolution, spectra, and ionization states of solar energetic particle (SEP) Fe in the impulsive event of 2000 May 1. Proton and electron intensities and anisotropies were used to help constrain the characteristics of the interplanetary propagation, taking into account focusing, pitch-angle scattering, adiabatic deceleration, and convection. We find that the event was nearly scatter-free, with an interplanetary scattering mean free path larger than 1 AU. The Fe spectrum spectral form is consistent with stochastic acceleration, but the observed increase of the ionization state of Fe between 200-600 keV nucleon^-1 is larger than can be explained using a single temperature source even after including the effect of adiabatic deceleration in the solar wind. A two-temperature source region is required to fit the observed range of Fe charge states, with the bulk (>80%) of the particles coming from a T ~ 10⁶ K region, and the remainder from a region with T ~ 1.6 × 10⁷ K.

Virtually all X-class flares produce a coronal mass ejection (CME), and each CME carries magnetic helicity into the heliosphere. Using magnetograms from the Michelson Doppler Imager on the Solar and Heliospheric Observatory, we surveyed magnetic helicity injection into 48 X-flare-producing active regions recorded by the MDI between 1996 July and 2005 July. Magnetic helicity flux was calculated according to the method of Chae for the 48 X-flaring regions and for 345 non-X-flaring regions. Our survey revealed that a necessary condition for the occurrence of an X-flare is that the peak helicity flux has a magnitude >6 × 10³⁶ Mx² s^-1. X-flaring regions also consistently had a higher net helicity change during the ~6 day measurement intervals than nonflaring regions. We find that the weak hemispherical preference of helicity injection, positive in the south and negative in the north, is caused by the solar differential rotation, but it tends to be obscured by the intrinsic helicity injection, which is more disorganized and tends to be of opposite sign. An empirical fit to the data shows that the injected helicity over the range 10³⁹-10 ⁴³ Mx² s^-1 is proportional to magnetic flux squared. Similarly, over a range of 0.3-3000 days, the time required to generate the helicity in a CME is inversely proportional to the magnetic flux squared. Most of the X-flare regions generated the helicity needed for a CME in a few days to a few hours.

Solar microwave observations of the X3.4 Flare/CME event observed in Chinese solar broadband radiospectrometer (SBRS/Huairou) on 2006 December 13 show a series of very short period pulsations (VSP) with the period of <1.0 s in the frequency range of 2.60-3.80 GHz. Many pulsating events have the period of only several tens of milliseconds. These pulsations are quasi-periodic, broad bandwidth, and ubiquitous during all the phases of the flare/CME event. Based on theoretical analysis of the temporal behavior of the resistive tearing mode in the electric current-carrying flare loops, we propose that microwave pulsations are a result of the modulation of the tearing-mode oscillations in the current-carrying flare loops. Our calculation of the period of the tearing-mode oscillations are in good agreement with the observations.

We report a detailed analysis of successive flaring during the X1.5 event in the NOAA AR 0808 on 2005 September 13. We identify a filament lying at the southeast boundary of the active region as the physical linkage between the two flares in close succession. It is noticeable that the filament erupted ~13 minutes after the initial flare onset at ~19 : 22 UT near the central magnetic polarity inversion line (PIL). During this time period, the filament only showed a slow rising; meanwhile, a spatially associated large magnetic loop with one leg connecting to the initial flaring site began to brighten in the TRACE 195 Å channel. After ~19 : 35 UT, the filament abruptly erupted together with the bright TRACE loop. Besides the moving ribbons at the first flaring site, the filament eruption caused a secondary flare identified with another set of moving ribbons. This event thus provides a clear evidence for the successive flaring where the initial flare destabilizes the nearby flux loop system, leading to the filament eruption with the second flare. We also identify the initial flare core by finding rapid, irreversible enhancements of the photospheric transverse magnetic fields at a section of the PIL.

We investigate the temporal and spatial variation of the relative abundance of He to H in a sample of solar filaments by comparing cotemporal observations of Hα and He I λ10830 obtained at MLSO. Motivated by indications that cross-field diffusion of neutral filament material is an important mechanism in mass loss, the present study offers results that provide a convincing test of the mechanisms proposed in Gilbert and coworkers. Specifically, when observed across an entire disk passage, we find a majority of stable, quiescent filaments show a relative helium deficit in the upper portions of their structure coupled with a relative helium surplus in the lower regions, a consequence of the large loss timescale for neutral helium compared to neutral hydrogen. Moreover, we find that the variation of the relative He/H ratio is uniform across filament barbs and footpoints on both short and long timescales.

It has been frequently observed in longitudinal magnetograms that magnetic elements disappear when a magnetic polarity element approaches and collides with another polarity element. We examine 12 collision events observed with the Advanced Stokes Polarimeter. We find formation of new magnetic connection between the colliding opposite polarity elements both in the photosphere and in the corona. In some cases, the opposite polarity elements to be collided appear at different times and at widely separated positions. Magnetic fields horizontal to the solar surface are spontaneously formed on the polarity inversion line (PIL) between such colliding elements, and transient bright X-ray loops connecting the opposite polarity elements appear. We suggest that formation of the coronal loops and the photospheric horizontal fields are due to magnetic reconnection between the colliding field lines, possibly at multiple locations with different heights. We also find that a global change in the direction of the photospheric horizontal fields between the colliding elements occurs in association with formation and disappearance of Hα dark filaments. Initial horizontal fields perpendicular to the PIL become parallel to the PIL, when dark filaments are observed along the PIL. They return to being perpendicular to the PIL at around the time of the disappearance of the dark filament.

We seek to clarify the nature of running penumbral (RP) waves: are they chromospheric trans-sunspot waves or a visual pattern of upward-propagating waves? Full Stokes spectropolarimetric time series of the photospheric Si I λ10827 line and the chromospheric He I λ10830 multiplet were inverted using a Milne-Eddington atmosphere. Spatial pixels were paired between the outer umbral/inner penumbral photosphere and the penumbral chromosphere using inclinations retrieved by the inversion and the dual-height pairings of line-of-sight velocity time series were studied for signatures of wave propagation using a Fourier phase difference analysis. The dispersion relation for radiatively cooling acoustic waves, modified to incorporate an inclined propagation direction, fits well the observed phase differences between the pairs of photospheric and chromospheric pixels. We have thus demonstrated that RP waves are in effect low-β slow-mode waves propagating along the magnetic field.

We study the evolution of the flows and horizontal proper motions in and around a decaying follower sunspot based on time sequences of two-dimensional spectroscopic observations in the visible and white-light imaging data obtained over 6 days from 2005 June 7 to 12. During this time period the sunspot decayed gradually to a pore. The spectroscopic observations were obtained with the Fabry-Pérot-based Visible-Light Imaging Magnetograph (VIM) in conjunction with the high-order adaptive optics (AO) system operated at the 65 cm vacuum reflector of the Big Bear Solar Observatory (BBSO). We apply local correlation tracking (LCT) to the speckle-reconstructed time sequences of white-light images around 600 nm to infer horizontal proper motions, while the Doppler shifts of the scanned Fe I line at 630.15 nm are used to calculate line-of-sight (LOS) velocities with subarcsecond resolution. We find that the dividing line between radial inward and outward proper motions in the inner and outer penumbra, respectively, survives the decay phase. In particular the moat flow is still detectable after the penumbra disappeared. Based on our observations, three major processes removed flux from the sunspot: (1) fragmentation of the umbra, (2) flux cancelation of moving magnetic features (MMFs; of the same polarity as the sunspot) that encounter the leading opposite polarity network and plages areas, and (3) flux transport by MMFs (of the same polarity as the sunspot) to the surrounding network and plage regions that have the same polarity as the sunspot.

Magnetic helicity is now regarded as an important physical quantity in understanding solar magnetic activities. We have studied the injection of magnetic helicity through the photosphere of four active regions during the long periods of their lifetimes. The rate of helicity injection was determined as a function of time in each active region by applying the local correlation tracking (LCT) method to full-disk, 96 minute cadence magnetograms taken by SOHO MDI. As a result, we have found that most helicity was intensively injected while active regions emerged and grew, suggesting that magnetic flux emergence may be the major process of helicity injection. In all the active regions studied, the rate of helicity injection during flux emergence was initially low, increased and stayed high for a while, and then became low again, while magnetic flux steadily increased at a more or less constant rate all the time. Flux cancellation, as a minor process, resulted in some loss of coronal magnetic helicity, or inverse helicity injection. The contribution of differential rotation to helicity injection was found to be insignificant in these active regions. It was also found that the magnetic helicity budget of an active region strongly depends on its average magnetic flux.

We self-consistently derive the magnetic energy and relative magnetic helicity budgets of a three-dimensional linear force-free magnetic structure rooted in a lower boundary plane. For the potential magnetic energy we derive a general expression that gives results practically equivalent to those of the magnetic virial theorem. All magnetic energy and helicity budgets are formulated in terms of surface integrals applied to the lower boundary, thus avoiding computationally intensive three-dimensional magnetic field extrapolations. We analytically and numerically connect our derivations with classical expressions for the magnetic energy and helicity, thus presenting a unified treatment of the energy/helicity budgets in the constant-alpha approximation that is lacking so far. Applying our derivations to photospheric vector magnetograms of an eruptive and a noneruptive solar active region, we find that the most profound quantitative difference between these regions lies in the estimated free magnetic energy and relative magnetic helicity budgets. If this result is verified with a large number of active regions, it will advance our understanding of solar eruptive phenomena. We also find that the constant-alpha approximation gives rise to large uncertainties in the calculation of the free magnetic energy and the relative magnetic helicity. Therefore, care must be exercised when this approximation is applied to photospheric magnetic field observations. Despite its shortcomings, the constant-alpha approximation is adopted here because this study will form the basis of a comprehensive nonlinear force-free description of the energetics and helicity in the active region solar corona, which is our ultimate objective.

Time-distance helioseismology has shown that f-mode travel times contain information about horizontal flows in the Sun. The purpose of this study is to provide a simple interpretation of these travel times. We study the interaction of surface gravity waves with horizontal flows in an incompressible, plane-parallel solar atmosphere. We show that for uniform flows less than roughly 250 m s^-1, the travel-time shifts are linear in the flow amplitude. For stronger flows, perturbation theory up to third order is needed to model waveforms. The case of small-amplitude spatially varying flows is treated using the first-order Born approximation. We derive two-dimensional Fréchet kernels that give the sensitivity of travel-time shifts to local flows. We show that the effect of flows on travel times depends on wave damping and on the direction from which the observations are made. The main physical effect is the advection of the waves by the flow rather than the advection of wave sources or the effect of flows on wave damping. We compare the two-dimensional sensitivity kernels with simplified three-dimensional kernels that only account for wave advection and assume a vertical line of sight. We find that the three-dimensional f-mode kernels approximately separate in the horizontal and vertical coordinates, with the horizontal variations given by the simplified two-dimensional kernels. This consistency between quite different models gives us confidence in the usefulness of these kernels for interpreting quiet-Sun observations.

We present sensitive 5.5-7.6 μm spectra of comet C/2003 K4 (LINEAR) obtained on 2004 July 16 (r_h = 1.760 AU, Δ_Spitzer = 1.409 AU, phase angle = 35.4°) with the Spitzer Space Telescope. The ν₂ vibrational band of water is detected with a high signal-to-noise ratio (≳50). Model fitting to the best spectrum yields a water ortho-to-para ratio of 2.47 ± 0.27, which corresponds to a spin temperature of 28.5 K. Spectra acquired at different offset positions show that the rotational temperature decreases with increasing distance from the nucleus, which is consistent with evolution from thermal to fluorescence equilibrium. The inferred water production rate is (2.43 ± 0.25) × 10²⁹ molecules s^-1. The spectra do not show any evidence for emission from PAHs and carbonate minerals, in contrast to results reported for comets 9P/Tempel 1 and C/1995 O1 (Hale-Bopp). However, residual emission is observed near 7.3 μm, the origin of which remains unidentified.

A rare coincidence of scales in standard particle physics is needed to explain why Λ or the negative pressure of cosmological dark energy (DE) coincides with the positive pressure P₀ of random motion of dark matter (DM) in bright galaxies. Recently Zlosnik and coworkers proposed to modify the Einstein curvature by adding nonlinear pressure from a medium flowing with a four-velocity vector field U^μ. We propose to check whether a smooth extension of general relativity with a simple kinetic Lagrangian of U^μ can be constructed, and whether the pressure can bend spacetime sufficiently to replace the roles of DE, cold DM, and heavy neutrinos in explaining anomalous accelerations at all scales. As a specific proof of concept we find a vector-for-Λ model (VΛ model) and its variants. With essentially no free parameters, these appear broadly consistent with the solar system, gravitational potentials in dwarf spiral galaxies and the Bullet Cluster of galaxies, the early universe with inflation, structure formation, and big bang nucleosynthesis, and late acceleration with a 1 : 3 ratio of DM : DE.

We present Submillimeter Array observations of the z = 3.91 gravitationally lensed broad absorption line quasar APM 08279+5255 which spatially resolve the 1.0 mm (200 μm rest frame) dust continuum emission. At 0.4'' resolution, the emission is separated into two components, a stronger, extended one to the northeast (46 ± 5 mJy) and a weaker, compact one to the southwest (15 ± 2 mJy). We have carried out simulations of the gravitational lensing effect responsible for the two submillimeter components in order to constrain the intrinsic size of the submillimeter continuum emission. Using an elliptical lens potential, the best-fit lensing model yields an intrinsic (projected) diameter of ~80 pc, which is not as compact as the optical/near-infrared (NIR) emission and agrees with previous size estimates of the gas and dust emission in APM 08279+5255. Based on our estimate, we favor a scenario in which the 200 μm (rest frame) emission originates from a warm dust component (T_d = 150-220 K) that is mainly heated by the AGN rather than by a starburst. The flux is boosted by a factor of ~90 in our model, consistent with recent estimates for APM 08279+5255.

We report the discovery of an almost complete (~300°) Einstein ring of diameter 10'' in Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5). Spectroscopic data from the 6 m telescope of the Special Astrophysical Observatory reveal that the deflecting galaxy has a line-of-sight velocity dispersion in excess of 400 km s^-1 and a redshift of 0.444, while the source is a star-forming galaxy with a redshift of 2.379. From its color, luminosity, and velocity dispersion, we argue that this is the most massive galaxy lens hitherto discovered.

We report a sensitive search for the HCN(J = 2 → 1) emission line toward SDSS J114816.64+525150.3 (hereafter J1148+5251) at z = 6.42 with the Very Large Array (VLA). HCN emission is a star formation indicator, tracing dense molecular hydrogen gas [n(H₂) ≥ 10⁴ cm^-3] within star-forming molecular clouds. No emission was detected in the deep interferometer maps of J1148+5251. We derive a limit for the HCN line luminosity of L < 3.3 × 10⁹ K km s^-1 pc², corresponding to a HCN/CO luminosity ratio of L/L < 0.13. This limit is consistent with a fraction of dense molecular gas in J1148+5251 within the range of nearby ultraluminous infrared galaxies (ULIRGs; median value: L/L = 0.17) and HCN-detected z > 2 galaxies (0.17). The relationship between L and L_FIR is considered to be a measure of the efficiency at which stars form out of dense gas. In the nearby universe, these quantities show a linear correlation, and thus, a practically constant average ratio. In J1148+5251, we find L_FIR/L > 6600. This is significantly higher than the average ratios for normal nearby spiral galaxies (L_FIR/L = 580) and ULIRGs (740), but consistent with a rising trend as indicated by other z > 2 galaxies (predominantly quasars; 1525). It is unlikely that this rising trend can be accounted for by a contribution of active galactic nucleus (AGN) heating to L_FIR alone, and may hint at a higher median gas density and/or elevated star formation efficiency toward the more luminous high-redshift systems. There is marginal evidence that the L_FIR/L ratio in J1148+5251 may even exceed the rising trend set by other z > 2 galaxies; however, only future facilities with very large collecting areas such as the Square Kilometer Array (SKA) will offer the sensitivity required to further investigate this question.

We explore a simple model for the high luminosity of SN 2006gy involving photon diffusion of shock-deposited thermal energy. The distinguishing property of the model is that the large "stellar" radius of ~160 AU required to prevent adiabatic losses is not the true stellar radius, but rather, it is the radius of an opaque, unbound circumstellar envelope, created when ~10 M_☉ was ejected in the decade before the supernova in an eruption analogous to that of η Carinae. The supernova light is produced primarily by diffusion of thermal energy following the passage of the blast wave through this shell. This model differs from traditional models of supernova debris interacting with an external circumstellar medium (CSM) in that here the shell is optically thick and the escape of radiation is delayed. We show that any model attempting to account for SN 2006gy's huge luminosity with radiation emitted by ongoing CSM interaction fails for the following basic reason: the CSM density required to achieve the observed luminosity makes the same circumstellar envelope opaque (τ ≳ 300), forcing a thermal diffusion solution. In our model, the weaker CSM interaction giving rise to SN 2006gy's characteristic Type IIn spectrum and soft X-rays is not linked to the power source of the visual continuum; instead, it arises after the blast wave breaks free from the opaque shell into the surrounding wind. While a simple diffusion model can explain the gross properties of the early light curve of SN 2006gy, it predicts that the light curve must plummet rapidly at late times, unless an additional power source is present.

We report a radio detection of supernova SN 2004ip in the circumnuclear region of the luminous infrared galaxy IRAS 18293-3413 , using Very Large Array (VLA) observations at 8.4 GHz on 2007 June 11. SN 2004ip had been previously discovered at near-infrared wavelengths using adaptive optics observations, but its nature (core collapse or thermonuclear) could not be definitely established. Our radio detection, about 3 years after the explosion of the supernova, indicates a prominent interaction of the ejecta of SN 2004ip with the circumstellar medium, confirming that the supernova was a core collapse event (probably a Type II) and strongly suggesting that its progenitor was a massive star with a significant mass loss prior to its explosion. SN 2004ip has a 8.4 GHz luminosity of 3.5 × 10²⁷ ergs s^-1 Hz^-1, about 3 times as bright as SN 2000ft in NGC 7469 at a similar age; given its projected distance to the nucleus (~500 pc), it is one of the closest of all known radio SNe to a galaxy nucleus and one of the brightest radio SNe ever.

Patat et al. recently inferred the existence of circumstellar material around a normal Type Ia supernova (SN Ia) for the first time, finding time-variable Na I D absorption lines in the spectrum of SN 2006X. We present high-resolution spectroscopy of the bright SN Ia 2007af at three epochs and search for variability in any of the Na D absorption components. Over the time range from 4 days before to 24 days after maximum light, we find that the host-galaxy Na D lines appear to be of interstellar rather than circumstellar origin and do not vary down to the level of 18 mÅ (column density of 2 × 10¹¹ cm^-2). We limit any circumstellar absorption lines to be weaker than ~10 mÅ (6 × 10¹⁰ cm^-2). For the case of material distributed in spherically symmetric shells of radius ~10¹⁶ cm surrounding the progenitor system, we place an upper limit on the shell mass of ~(3 × 10^-8)/XM_☉, where X is the Na ionization fraction. We also show that SN 2007af is a photometrically and spectroscopically normal SN Ia. Assuming that the variable Na D lines in SN 2006X came from circumstellar matter, we therefore conclude that either there is a preferred geometry for the detection of variable absorption components in SNe Ia, or SN 2007af and SN 2006X had different types of progenitor systems.

The rapid variability of the VHE emission reported for some TeV blazars implies Doppler factors well in excess of those inferred from superluminal motions and unification schemes. We propose that those extreme flares may result from radiative deceleration of blobs on scales where local dissipation occurs. The minimum jet power estimated from the resolved synchrotron emission on VLBI scales appears to be consistent with this model. It is shown that if the energy distribution of nonthermal electrons accelerated locally in the blob is reasonably flat, then a background radiation field having a luminosity in the range 10⁴¹-10⁴² ergs s^-1 can give rise to a substantial deceleration of the blob, but still be transparent enough to allow the TeV γ-rays thereby produced to escape the system.

We compare the stellar structure of the isolated, Local Group dwarf galaxy Pegasus (DDO 216) with low-resolution H I maps from L. M. Young et al. Our comparison reveals that Pegasus displays the characteristic morphology of ram pressure stripping; in particular, the H I has a "cometary" appearance that is not reflected in the regular, elliptical distribution of the stars. This is the first time this phenomenon has been observed in an isolated Local Group galaxy. The density of the medium required to ram pressure strip Pegasus is at least 10^-5 to 10^-6 cm^-3. We conclude that this is strong evidence for an intergalactic medium associated with the Local Group.

The first map of interstellar acetylene (C₂H₂) has been obtained with the Infrared Spectrograph on board the Spitzer Space Telescope. A spectral line map of the ν₅ vibration-rotation band at 13.7 μm, carried out toward the star-forming region Cepheus A East, shows that the C₂H₂ emission peaks in a few localized clumps where gas-phase CO₂ emission was previously detected with Spitzer. The distribution of excitation temperatures, derived from fits to the C₂H₂ line profiles, ranges from 50 to 200 K, which is consistent with that derived for gaseous CO₂—suggesting that both molecules probe the same warm gas component. The C₂H₂ molecules are excited via radiative pumping by 13.7 μm continuum photons emanating from the HW2 protostellar region. We derive column densities ranging from a few × 10¹³ to ~ 7 × 10¹⁴ cm^-2, corresponding to C₂H₂ abundances of 1 × 10^-9 to 4 × 10^-8 with respect to H₂. The spatial distribution of the C₂H₂ emission and a roughly constant N(C₂H₂)/N(CO₂) strongly suggest an association with shock activity, most likely the result of the sputtering of acetylene in icy grain mantles.

Knowing the Kerr parameters, we can make quantitative calculations of the rotational energy of black holes. We show that Nova Sco, Il Lupi, XTE J1550-564, and GS 2023+338 are relics of gamma-ray bursts (GRBs) and hypernova explosions. They had more than enough rotational energy to power themselves. In fact, they had so much energy that they would have disrupted the accretion disk of the black hole that powered them by the communicated rotational energy, so that the energy delivery would have been self-limiting. The most important feature in producing high rotational energy in the binary is low donor (secondary star) mass. We suggest that V4641 Sgr and GRS 1915+105 underwent less energetic explosions, because of their large donor masses. Cyg X-1 had an even less energetic explosion, because of an even larger donor mass. We find that in the evolution of the soft X-ray transient sources the donor is tidally locked with the helium star, which evolved from the giant, as the hydrogen envelope is stripped off in common-envelope evolution. Depending on the mass of the donor, the black hole can be spun up to the angular momentum necessary to power the GRB and hypernova explosion. The donor decouples, acting as a passive witness to the explosion which, for the given angular momentum, then proceeds as in the Woosley collapsar model. High-mass donors which tend to follow from low metallicity give long GRBs because their lower energy can be accepted by the central engine.

The brightest supernova remnant in the Magellanic Clouds, N132D, belongs to the rare class of oxygen-rich remnants, about a dozen objects that show optical emission from pure heavy-element ejecta. They originate in explosions of massive stars that produce large amounts of O, although only a tiny fraction of that O is found to emit at optical wavelengths. We report the detection of substantial amounts of O at X-ray wavelengths in a recent 100 ks Chandra ACIS observation of N132D. A comparison between subarcsecond-resolution Chandra and Hubble images reveals a good match between clumpy X-ray and optically emitting ejecta on large (but not small) scales. Ejecta spectra are dominated by strong lines of He- and H-like O; they exhibit substantial spatial variations partially caused by patchy absorption within the LMC. Because optical ejecta are concentrated in a 5 pc radius elliptical expanding shell, the detected ejecta X-ray emission also originates in this shell.

We present the results of a high-accuracy (σ ≈ 0.005%) polarization monitoring of the Be star Achernar that was carried out between 2006 July 7 and November 5. Our results indicate that after a near-quiescent phase from 1998 to 2002, Achernar is currently in an active phase and has built a circumstellar disk. We detect variations both in the polarization level and position angle on timescales as short as 1 hr and as long as several weeks. Detailed modeling of the observed polarization strongly suggests that the short-term variations originate from discrete mass ejection events which produce transient inhomogeneities in the inner disk. Long-term variations, on the other hand, can be explained by the formation of an inner ring following one or several mass ejection events.

We present the results of Spitzer spectroscopic observations of two highly obscured massive X-ray binaries: IGR J16318-4848 and GX 301-2. Our observations reveal for the first time the extremely rich mid-infrared environments of this type of source, including multiple continuum emission components (a hot component with T > 700 K and a warm component with T ~ 180 K) with apparent silicate absorption features, numerous H I recombination lines, many forbidden lines of low ionization potential, and pure rotational H₂ lines. This indicates that both sources have hot and warm circumstellar dust, ionized stellar winds, extended low-density ionized regions, and photodissociated regions. It appears difficult to attribute the total optical extinction of both sources to the hot and warm dust, which suggests that there could be an otherwise observable colder dust component responsible for most of the optical extinction and silicate absorption features. The observed mid-infrared spectra are similar to those from luminous blue variables, indicating that the highly obscured massive X-ray binaries may represent a previously unknown evolutionary phase of X-ray binaries with early-type optical companions. Our results highlight the importance and utility of mid-infrared spectroscopy for investigating highly obscured X-ray binaries.

HD 209458b is a Jovian planet orbiting around a star with characteristics similar to the Sun, which has repeated transits across the stellar disk. The orbital period is of 3.5 days, with a mean distance to the star of 0.045 AU. The peculiar characteristic of this system is that the planet shows a larger apparent size when observed in Lyα (1214-1217 Å) than at optical wavelengths. This discrepancy is thought to be due to the existence of an extended upper atmosphere and/or a cometary wake. In this work we present 3D numerical simulations of this system, and study the absorption produced by the extended atmosphere. This "exosphere" is modeled as a wind outflowing from the planet at the escape velocity (60 km s^-1), which interacts with the stellar wind. Four runs where carried out with planetary mass loss rates in the 1.5 × 10^-17 to 1.5 × 10^-14M_☉ yr^-1 range. By comparing the Lyα absorption predicted from the models with the observations, we determine an _p = (1.8 ± 0.4) × 10^-16M_☉ yr^-1 value for the mass loss rate from HD 209458b (a value which is of course dependent on the model assumptions, the error reflecting only the propagation of the observational uncertainties). This mass loss rate is consistent with the lower limit for _p (=10¹⁰ g s^-1) determined by Vidal-Madjar and coworkers.

Many extrasolar planets orbit closely to their parent star. Their existence raises the fundamental problem of loss and gain in their mass. For exoplanet HD 209458b, reports on an unusually extended hydrogen corona and a hot layer in the lower atmosphere seem to support the scenario of atmospheric inflation by the strong stellar irradiation. However, difficulties in reconciling evaporation models with observations call for a reassessment of the problem. Here we use HST archive data to report a new absorption rate of ~8.9% ± 2.1% by atomic hydrogen during the HD 209458b transit and show that no sign of evaporation could be detected for the exoplanet. We also report evidence of time variability in the HD 209458 Lyα flux, a variability that was not accounted for in previous studies, which corrupted their diagnostics. Mass-loss rates thus far proposed in the literature in the range 5 × (10¹⁰–10¹¹) g s^-1 must induce a spectral signature in the Lyα line profile of HD 209458 that cannot be found in the present analysis. Either an unknown compensation effect is hiding the expected spectral feature or else the mass-loss rate of neutrals from HD 209458 is modest.

Knowledge of the stellar parameters for the parent stars of transiting exoplanets is a prerequisite for establishing the planet properties themselves, and often relies on stellar evolution models. GJ 436, which is orbited by a transiting Neptune-mass object, presents a difficult case because it is an M dwarf. Stellar models in this mass regime are not as reliable as for higher mass stars, and tend to underestimate the radius. Here we use constraints from published transit light curve solutions for GJ 436 along with other spectroscopic quantities to show how the models can still be used to infer the mass and radius accurately, and at the same time allow the radius discrepancy to be estimated. Similar systems should be found during the upcoming Kepler mission, and could provide in this way valuable constraints to stellar evolution models in the lower main sequence. The stellar mass and radius of GJ 436 are M_⋆ = 0.452M_☉ and R_⋆ = 0.464R_☉, and the radius is 10% larger than predicted by the standard models, in agreement with previous results from well-studied double-lined eclipsing binaries. We obtain an improved planet mass and radius of M_p = 23.17 ± 0.79 M_⊕ and R_p = 4.22R_⊕, a density of ρ_p = 1.69 g cm^-3, and an orbital semimajor axis of a = 0.02872 ± 0.00027 AU.

We calculate upper limits on the amount of short-lived radionuclei that can be produced by nonthermal nucleosynthesis in the early solar system. Using energetic constraints obtained from X-ray observations of young stellar objects, we show that irradiation of bare solids can produce ¹⁰Be and ⁴¹Ca at levels compatible with a homogeneous distribution over the entire protoplanetary disk up to the comet-forming region. ⁵³Mn and ³⁶Cl cannot be produced at canonical levels together with ¹⁰Be and ⁴¹Ca, unless we posit a heterogeneous spatial distribution. The high level of ⁷Be suggested recently is barely compatible with a coproduction of ¹⁰Be up to the cometary reservoir and may indicate the irradiation of a gas phase. Finally, we show that the maximum amount of irradiation-induced ²⁶Al can only account for a homogeneous distribution of this radionuclide over a rocky reservoir of 2-3 M_⊕ and that the well-defined canonical ²⁶Al/²⁷Al ratio observed in Ca-Al-rich inclusions is probably not compatible with an in situ production in the embedded phase of the Sun. If extinct ²⁶Al is detected in the cometary material from the Stardust mission, the nucleosynthetic process that produced this preeminent high-resolution chronometer should be searched for in stellar events contemporary with the birth of the Sun.

The high-speed solar wind that originates in coronal holes is permeated by evolved, Alfvénic-type fluctuations associated with MHD turbulence. We have used high temporal resolution (3 s) plasma and magnetic field measurements by the Wind spacecraft at 1 AU to identify and study Petschek-like reconnection exhausts in this turbulent flow for the first time. Despite the fact that the turbulent cascade produces numerous thin current sheets at 1 AU, such exhausts are relatively rare; we have identified only 34 reconnection exhausts in 1358 hr of high-speed solar wind data. With three exceptions, each exhaust was embedded within a relatively sharp, outward-propagating Alfvénic fluctuation. Local field shear angles across these reconnection exhausts ranged from 24° to 160°, with average and median values being ~90°. The vast majority (88%) of these exhausts were narrower than 4 × 10⁴ km and were convected past the spacecraft in <66 s. Multispacecraft observations indicate that current sheets, and thus also reconnection X-lines, in the turbulent, high-speed wind are considerably more localized than in the low-speed wind or in interplanetary coronal mass ejections. The Wind observations demonstrate that reconnection is one way in which solar wind turbulence is dissipated and the high-speed wind heated far from the Sun, although it is not yet clear how effective reconnection is overall in this regard.

The initiation and evolution of coronal mass ejections (CMEs) is studied by means of the breakout model embedded in a 2.5D axisymmetric solar wind in the framework of numerical magnetohydrodynamics (MHD). The initial, steady equilibrium contains a pre-eruptive region consisting of three arcades with alternating magnetic flux polarity and with correspondingly three neutral lines on the photosphere. The magnetic tension of the overlying closed magnetic field of the helmet streamer keeps this structure in place. The most crucial part of the initial breakout topology is the existence of an X-point on the leading edge of the central arcade. By shearing part of this arcade, the reconnection with the overlying streamer field is turned on. The initial phase of the erupting arcade then closely follows the original breakout scenario. The breakout reconnection opens the overlying field in an energetically efficient way leading to an ever faster eruption. However, from a certain moment two new reconnections set in on the sides of the erupting central arcade and the breakout reconnection stops. The consequence of this change in reconnection location is twofold: (1) the lack of breakout reconnection so that the breakout plasmoid fails to become a fast CME; and (2) an eventual disconnection of the large helmet top resulting in a slow CME.

We study the structure of 107 bright sigmoids using full-resolution (2.5'' pixels) images from the Yohkoh Soft X-Ray Telescope (SXT) obtained between 1991 December and 2001 December. We find that none of these sigmoids are made of single loops of S or inverse-S shape; all comprise a pattern of multiple loops. We also find that all S-shaped sigmoids are made of right-bearing loops and all inverse-S-shaped sigmoids of left-bearing loops, without exception. We co-align the SXT images with Kitt Peak magnetograms to determine the magnetic field directions in each sigmoid. We use a potential-field source surface model to determine the direction of the overlying magnetic field. We find that sigmoids for which the relative orientation of these two fields has a parallel component outnumber antiparallel ones by more than an order of magnitude. We find that the number of sigmoids per active region varies with the solar cycle in a manner that is consistent with this finding. Finally, those few sigmoids that are antiparallel erupt roughly twice as often as those that are parallel. We briefly discuss the implications of these results in terms of formation and eruption mechanisms of flux tubes and sigmoids.

We present novel evidence for fine structure observed in the net circular polarization (NCP) of a sunspot penumbra based on spectropolarimetric measurements utilizing the Zeeman-sensitive Fe I 630.2 nm line. For the first time we detect filamentary organized fine structure of the NCP on spatial scales that are similar to the inhomogeneities found in the penumbral flow field. We also observe an additional property of the visible NCP, a zero-crossing of the NCP in the outer parts of the center-side penumbra, which has not been recognized before. In order to interpret the observations we solve the radiative transfer equations for polarized light in a model penumbra with embedded magnetic flux tubes. We demonstrate that the observed zero-crossing of the NCP can be explained by an increased magnetic field strength inside magnetic flux tubes in the outer penumbra combined with a decreased magnetic field strength in the background field. Our results strongly support the concept of the uncombed penumbra.