Table of contents

Recent astronomical observations indicate that our universe is undergoing a period of accelerated expansion. While there are many cosmological models that explain this phenomenon, the main question remains which is the best one in light of available data. We consider 10 cosmological models of the accelerating universe and select the best one using the Bayesian model comparison method. We demonstrate that the ΛCDM model is most favored by the Bayesian statistical analysis of the SNIa, CMB, baryon acoustic oscillation, and H(z) data.

Using the largest cosmological reionization simulation to date (~24 billion particles), we use the genus curve to quantify the topology of the neutral hydrogen distribution on scales ≥1 Mpc h⁻¹ as it evolves during cosmological reionization. We find that the reionization process proceeds primarily in an inside-out fashion, in which higher density regions become ionized earlier than lower density regions. There are four distinct topological phases: (1) pre-reionization at z≳ 15, when the genus curve is consistent with a Gaussian density distribution; (2) preoverlap at 10≲ z≲ 15, during which the number of H II bubbles increases gradually with time, until percolation of H II bubbles starts to take effect (this phase is characterized by a very flat genus curve at high volume fractions); (3) overlap at 8≲ z≲ 10, when large H II bubbles rapidly merge, manifested by a precipitous drop in the amplitude of the genus curve; and (4) postoverlap at 6≲ z≲ 8, when H II bubbles have mostly overlapped, and the genus curve is consistent with a diminishing number of isolated neutral islands. After the end of reionization (z≲ 6), the genus of neutral hydrogen is consistent with Gaussian random phase, in agreement with observations.

We measure the three-dimensional topology of large-scale structure in the Sloan Digital Sky Survey (SDSS). This allows the genus statistic to be measured with unprecedented statistical accuracy. The sample size is now sufficiently large to allow the topology to be an important tool for testing galaxy formation models. For comparison, we make mock SDSS samples using several state-of-the-art N-body simulations: the Millennium run of Springel et al. (10 billion particles), the Kim & Park CDM models (1.1 billion particles), and the Cen & Ostriker hydrodynamic code models (8.6 billion cell hydro mesh). Each of these simulations uses a different method for modeling galaxy formation. The SDSS data show a genus curve that is broadly characteristic of that produced by Gaussian random-phase initial conditions. Thus, the data strongly support the standard model of inflation where Gaussian random-phase initial conditions are produced by random quantum fluctuations in the early universe. But on top of this general shape there are measurable differences produced by nonlinear gravitational effects and biasing connected with galaxy formation. The N-body simulations have been tuned to reproduce the power spectrum and multiplicity function but not topology, so topology is an acid test for these models. The data show a "meatball" shift (only partly due to the Sloan Great Wall of galaxies) that differs at the 2.5 σ level from the results of the Millenium run and the Kim & Park dark halo models, even including the effects of cosmic variance.

We investigate the clustering of dark energy within matter overdensities and voids. In particular, we derive an analytical expression for the dark energy density perturbations, which is valid in the linear, quasi-linear, and fully nonlinear regimes of structure formation. We also investigate the possibility of detecting such dark energy clustering through the integrated Sachs-Wolfe effect. In the case of uncoupled quintessence models, if the mass of the field is of order the Hubble scale today or smaller, dark energy fluctuations are always small compared to the matter density contrast. Even when the matter perturbations enter the nonlinear regime, the dark energy perturbations remain linear. We find that virialized clusters and voids correspond to local overdensities in dark energy, with δ_ϕ/(1 + w) ∼ O(10⁻⁵) for voids, δ_ϕ/(1 + w) ∼ O(10⁻⁴) for supervoids, and δ_ϕ/(1 + w) ∼ O(10⁻⁵) for a typical virialized cluster. If voids with radii of 100-300 Mpc exist within the visible universe, then δ_ϕ may be as large as 10⁻³(1 + w) . Linear overdensities of matter and superclusters generally correspond to local voids in dark energy; for a typical supercluster, δ_ϕ/(1 + w) ∼ O(− 10⁻⁵) . The approach taken in this work could be straightforwardly extended to study the clustering of more general dark energy models.

We use a simple optical/infrared (IR) photometric selection of high-redshift QSOs that identifies a Lyman break in the optical photometry and requires a red IR color to distinguish QSOs from common interlopers. The search yields 100 z ∼ 3 (U-dropout) QSO candidates with 19 < r' < 22 over 11.7 deg² in the ELAIS-N1 (EN1) and ELAIS-N2 (EN2) fields of the Spitzer Wide-area Infrared Extragalactic (SWIRE) Legacy Survey. The z ∼ 3 selection is reliable, with spectroscopic follow-up of 10 candidates confirming that they are all QSOs at 2.83 < z < 3.44. We find that our z ∼ 4 (g'-dropout) sample suffers from both unreliability and incompleteness but present seven previously unidentified QSOs at 3.50 < z < 3.89. Detailed simulations show our z ∼ 3 completeness to be ~80%-90% from 3.0 < z < 3.5, significantly better than the ~30%-80% completeness of the SDSS at these redshifts. The resulting luminosity function extends 2 mag fainter than SDSS and has a faint-end slope of β = − 1.42 ± 0.15, consistent with values measured at lower redshift. Therefore, we see no evidence for evolution of the faint-end slope of the QSO luminosity function. Including the SDSS QSO sample, we have now directly measured the space density of QSOs responsible for ~70% of the QSO UV luminosity density at z ∼ 3. We derive a maximum rate of H I photoionization from QSOs at z ∼ 3.2, Γ = 4.8 × 10⁻¹³ s⁻¹, about half of the total rate inferred through studies of the Lyα forest. Therefore, star-forming galaxies and QSOs must contribute comparably to the photoionization of H I in the intergalactic medium at z ∼ 3.

Recent multiwavelength observations of 3C 454.3, in particular during its giant outburst in 2005, put severe constraints on the location of the "blazar zone," its dissipative nature, and high-energy radiation mechanisms. As the optical, X-ray, and millimeter light curves indicate, a significant fraction of the jet energy must be released in the vicinity of the millimeter photosphere; i.e., at distances where, due to lateral expansion, the jet becomes transparent at millimeter wavelengths. We conclude that this region is located at ~10 pc, which is also the distance that coincides with the location of the hot dust region. This location is consistent with the high-amplitude variations observed on a ~10 day timescale, provided that the Lorentz factor of a jet is Γ_j ∼ 20. We argue that dissipation is driven by the reconfinement shock and demonstrate that X-rays and γ-rays are likely to be produced via inverse Compton scattering of near- and mid-IR photons emitted by the hot dust. We also infer that the largest gamma-to-synchrotron luminosity ratio ever recorded in this object, which took place during its lowest luminosity states, could be simply due to weaker magnetic fields carried by a less powerful jet.

The existence of a gradient in the Faraday rotation measure (RM) of the quasar 3C 273 jet is confirmed by follow-up observations. A gradient transverse to the jet axis is seen for more than 20 mas in projected distance. Taking account of the viewing angle, we estimate it to be more than 100 pc. Comparing to the distribution of the RM in 1995, we detect a time variation of it at the same distance from the core over 7 yr. We discuss the origin of the Faraday rotation based on this rapid time variation. We rule out foreground media such as a narrow-line region, and suggest a helical magnetic field in the sheath region as the origin of this gradient of the RM.

We use the narrow-lined broad-line region (BLR) of the Seyfert 1 galaxy, I Zw 1, as a laboratory for modeling the ultraviolet (UV) Fe II 2100-3050 Å emission complex. We calculate a grid of Fe II emission spectra representative of BLR clouds and compare them with the observed I Zw 1 spectrum. Our predicted spectrum for log [n_H/(cm ⁻³) ] = 11.0, log [Φ_H/(cm ⁻² s⁻¹) ] = 20.5, and ξ/(1 km s⁻¹) = 20, using Cloudy and an 830 level model atom for Fe II with energies up to 14.06 eV, gives a better fit to the UV Fe II emission than models with fewer levels. Our analysis indicates (1) the observed UV Fe II emission must be corrected for an underlying Fe II pseudocontinuum; (2) Fe II emission peaks can be misidentified as that of other ions in active galactic nuclei (AGNs) with narrow-lined BLRs possibly affecting deduced physical parameters; (3) the shape of 4200-4700 Å Fe II emission in I Zw 1 and other AGNs is a relative indicator of narrow-line region (NLR) and BLR Fe II emission; (4) predicted ratios of Lyα, C III], and Fe II emission relative to Mg II λ2800 agree with extinction corrected observed I Zw 1 fluxes, except for C IV λ1549; (5) the sensitivity of Fe II emission strength to microturbulence ξ casts doubt on existing relative Fe/Mg abundances derived from Fe II (UV)/Mg II flux ratios. Our calculated Fe II emission spectra, suitable for BLRs in AGNs, are available at http://iacs.cua.edu/people/verner/FeII.

We present the results of infrared L-band (3-4 μm) and M-band (4-5 μm) Very Large Telescope (VLT) ISAAC spectroscopy of five bright ultraluminous infrared galaxies (ULIRGs) hosting an AGN. From our analysis we distinguish two types of sources: ULIRGs in which the AGN is unobscured (with a flat continuum and no absorption features at 3.4 and 4.6 μm), and those with highly obscured AGNs (with a steep, reddened continuum and absorption features due to hydrocarbons and CO). Starburst activity is also present in all of the sources, as inferred from the 3.3 μm PAH emission line. A strong correlation is found between continuum slope and CO optical depth, which suggests that deep carbon monoxide absorption is a common feature of highly obscured ULIRG AGNs. Finally, we show that the AGN dominates the 3-4 μm emission, even if its contribution to the bolometric luminosity is small.

We present Sunyaev-Zel'dovich Effect (SZE) scaling relations for 38 massive galaxy clusters at redshifts 0.14 ⩽ z⩽ 0.89, observed with both the Chandra X-ray Observatory and the centimeter-wave SZE imaging system at the BIMA and OVRO interferometric arrays. An isothermal β-model with the central 100 kpc excluded from the X-ray data is used to model the intracluster medium and to measure global cluster properties. For each cluster, we measure the X-ray spectroscopic temperature, SZE gas mass, total mass, and integrated Compton y-parameters within r₂₅₀₀. Our measurements are in agreement with the expectations based on a simple self-similar model of cluster formation and evolution. We compare the cluster properties derived from our SZE observations with and without Chandra spatial and spectral information and find them to be in good agreement. We compare our results with cosmological numerical simulations and find that simulations that include radiative cooling, star formation, and feedback match well both the slope and normalization of our SZE scaling relations.

We examine the accuracy of strong gravitational lensing determinations of the mass of galaxy clusters by comparing the conventional approach with the numerical integration of the fully relativistic null geodesic equations in the case of weak gravitational perturbations on Robertson-Walker metrics. In particular, we study spherically symmetric, three-dimensional singular isothermal sphere models and the three-dimensional matter distribution of Navarro and coworkers which are both commonly used in gravitational lensing studies. In both cases we study two different methods for mass-density truncation along the line of sight: hard truncation and conventional (no truncation). We find that the relative error introduced in the total mass by the thin-lens approximation alone is less than 0.3% in the singular isothermal sphere model and less than 2% in the model of Navarro and coworkers. The removal of hard truncation introduces an additional error of the same order of magnitude in the best case and up to an order of magnitude larger in the worst case studied. Our results ensure that the future generation of precision cosmology experiments based on lensing studies will not require the removal of the thin-lens assumption, but they may require a careful handling of truncation.

X-ray observations of galaxy cluster merger shocks can be used to constrain nonthermal processes in the intracluster medium (ICM). The presence of nonthermal pressure components in the ICM, as well as the shock acceleration of particles and their escape, all affect shock jump conditions in distinct ways. Therefore, these processes can be constrained using X-ray surface brightness and temperature maps of merger shock fronts. Here we use these observations to place constraints on particle acceleration efficiency in intermediate Mach number () shocks and explore the potential to constrain the contribution of nonthermal components (e.g., cosmic rays, magnetic field, and turbulence) to ICM pressure in cluster outskirts. We model the hydrodynamic jump conditions in merger shocks discovered in the galaxy clusters A520 () and 1E 0657–56 () using a multifluid model comprising a thermal plasma, a nonthermal plasma, and a magnetic field. Based on the published X-ray spectroscopic data alone, we find that the fractional contribution of cosmic rays accelerated in these shocks is ≲10% of the shock downstream pressure. Current observations do not constrain the fractional contribution of nonthermal components to the pressure of the undisturbed shock upstream. Future X-ray observations, however, have the potential to either detect particle acceleration in these shocks through its effect on the shock dynamics, or place a lower limit on the nonthermal pressure contributions in the undisturbed ICM. We briefly discuss implications for models of particle acceleration in collisionless shocks and the estimates of galaxy cluster masses derived from X-ray and Sunyaev-Zel'dovich effect observations.

Using a set of deep imaging data obtained by the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST) shortly after its deployment, Yan, Windhorst, & Cohen found a large number of F775W-band dropouts (i-dropouts), which are consistent with being galaxies at z ≈ 6. The surface density of i-dropouts thus derived, however, is an order of magnitude higher than those subsequent studies found in other deep ACS fields, including the Hubble Ultra Deep Field (HUDF). Here we revisit this problem, using both existing and new data. We confirm that the large overdensity of i-dropouts does exist in this field, and that their optical-to-IR colors are similar to those in the HUDF. However, we have discovered that the i-dropout overdensity is accompanied by an even larger excess of faint field objects in this region and its vicinity. This large excess of field objects is most likely caused by the fact that we have resolved the faint diffuse light extending from an interacting galaxy pair in the Virgo Cluster, M60/NGC 4647, which lies several arcminutes away from the region where the excess is found. The i-dropouts in this field are within the magnitude range where this excess of field objects occurs, and their spatial distribution seems to follow the same gradient as the entire excess field population. This excess population is also red in color, and the red wing of its color distribution continuously extends to the regime where the i-dropouts reside. While we still cannot completely rule out the possibility that the overdensity of i-dropouts might be a genuine large-scale structure of galaxies at z ≈ 6, we prefer the interpretation that most of them are part of the excess stellar population related to M60/NGC 4647.

Based on a cosmological N-body simulation, we analyze spatial and kinematic alignments of satellite halos within 6 times the virial radius of group-sized host halos (r_vir). We measure three different types of spatial alignment: halo alignment between the orientation of the group central substructure (GCS) and the distribution of its satellites, radial alignment between the orientation of a satellite and the direction toward its GCS, and direct alignment between the orientation of the GCS and that of its satellites. Analogously, we use the directions of satellite velocities and probe three further types of alignment: the radial velocity alignment between the satellite velocity and the connecting line between the satellite and GCS, the halo velocity alignment between the orientation of the GCS and satellite velocities, and the autovelocity alignment between the satellite orientations and their velocities. We find that satellites are preferentially located along the major axis of the GCS within at least 6r_vir (the range probed here). Furthermore, satellites preferentially point toward the GCS. The most pronounced signal is detected on small scales, but a detectable signal extends out to ~6r_vir. The direct alignment signal is weaker; however, a systematic trend is visible at distances ≲2r_vir. All velocity alignments are highly significant on small scales. The halo velocity alignment is constant within 2r_vir and declines rapidly beyond. The halo and the autovelocity alignments are maximal at small scales and disappear beyond 1r_vir and 1.5r_vir, respectively. Our results suggest that the halo alignment reflects the filamentary large-scale structure that extends far beyond the virial radii of the groups. In contrast, the main contribution to the radial alignment arises from the adjustment of the satellite orientations in the group tidal field. The projected data reveal good agreement with recent results derived from large galaxy surveys.

We examine whether nonthermal protons energized during a cluster merger are simultaneously responsible for the Coma cluster's diffuse radio flux (via secondary decay) and the departure of its intracluster medium (ICM) from a thermal profile via Coulomb collisions between the quasi-thermal electrons and the hadrons. Rather than approximating the influence of nonthermal proton/thermal electron collisions as extremely rare events which cause an injection of nonthermal, power-law electrons (the knock-on approximation), we self-consistently solve (to our knowledge, for the first time) the covariant kinetic equations for the two populations. The electron population resulting from these collisions is out of equilibrium, yet not a power law, and importantly displays a higher bremsstrahlung radiative efficiency than a pure power law. Observations with GLAST will test this model directly.

This paper revisits the classical Kennicutt method for inferring the stellar IMF from the integrated light properties of galaxies. The large-size, uniform high-quality data set from the SDSS DR4 is combined with more in-depth modeling and quantitative statistical analysis to search for systematic IMF variations as a function of galaxy luminosity. Galaxy Hα equivalent widths are compared to a broadband color index to constrain the IMF. This parameter space is useful for breaking degeneracies that are traditionally problematic. Age and dust corrections are largely orthogonal to IMF variations. In addition, the effects of metallicity and smooth SFH e-folding times are small compared to IMF variations. We find that for the sample as a whole the best-fitting IMF slope above 0.5 M_☉ is Γ = 1.4535, with a negligible random error of ±0.0004 and a systematic error of ±0.1. Galaxies brighter than around M_r,0.1 = − 20 (including galaxies like the Milky Way, which has M_r,0.1 ∼ − 21) are well fitted by a universal Γ ∼ 1.4 IMF, similar to Salpeter, and smooth, exponential SFHs. Fainter galaxies prefer steeper IMFs, and the quality of the fits reveals that for these galaxies a universal IMF with smooth SFHs is actually a poor assumption. Several sources of sample bias are ruled out as the cause of these luminosity-dependent IMF variations. Analysis of bursting SFH models shows that an implausible coordination of burst times is required to fit a universal IMF to the M_r,0.1 = − 17 galaxies. This leads to the conclusions that the IMF in low-luminosity galaxies has fewer massive stars, by either steeper slope or lower upper mass cutoff, and is not universal.

"Galactic shocks" (Fujimoto; Roberts) are investigated using full three-dimensional hydrodynamic simulations that take into account the self-gravity of the ISM, radiative cooling, and star formation followed by energy feedback from supernovae. This is an essential improvement over previous numerical models, in which two-dimensional isothermal, non-self-gravitating gas is assumed. We find that the classic galactic shocks are unstable and transient, and they shift to a globally quasi-steady, inhomogeneous pattern due to the nonlinear development of instabilities in the disk. The spiral patterns consist of many giant molecular cloud-like dense condensations, but those local structures are not steady, and they evolve into irregular spurs in the interarm regions. Energy feedback from supernovae does not destroy the quasi-steady spiral arms; rather, it mainly contributes to the vertical motion and the structures of the ISM. The results and methods presented here are a starting point for a more consistent treatment of the ISM in spiral galaxies, in which the effects of magnetic fields, radiative transfer, chemistry, and dynamical evolution of a stellar disk are taken into account.

We present Hubble Space Telescope (HST) and Spitzer Space Telescope images and photometry of the extremely metal-poor (Z≅ 0.03 Z_☉) blue dwarf galaxy CGCG 269–049. The HST images reveal a large population of red giant and asymptotic giant branch stars, ruling out the possibility that the galaxy has recently formed. From the magnitude of the tip of the red giant branch, we measure a distance to CGCG 269–049 of only 4.9 ± 0.4 Mpc. The spectral energy distribution of the galaxy between ~3.6-70 μm is also best fitted by emission from predominantly ~10 Gyr old stars, with a component of thermal dust emission having a temperature of 52 ± 10 K. The HST and Spitzer photometry indicate that more than 60% of the CGCG 269–049 stellar mass consists of stars ~1-10 Gyr old, similar to other local blue dwarf galaxies. Our HST Hα image shows no evidence of a supernova-driven outflow that could be removing metals from the galaxy, nor do we find evidence that such outflows occurred in the past. Taken together with the CGCG 269–049 large ratio of neutral hydrogen mass to stellar mass (~10), these results are consistent with recent simulations in which the metal deficiency of local dwarf galaxies results mainly from inefficient star formation, rather than youth or the escape of supernova ejecta.

We present new kinematic results for 387 stars near the Milky Way satellite dwarf spheroidal galaxy Leo I. Spectra were obtained with the Hectochelle multiobject echelle spectrograph on the MMT, centered in the optical near 5200 Å. From 297 repeat measurements of 108 stars, we estimate the mean velocity error (1 σ) of our sample to be 2.4 km s⁻¹, with a systematic precision of ≤1 km s⁻¹. The final sample of 328 Leo I members gives a mean heliocentric velocity of 282.9 ± 0.5 km s⁻¹ and a dispersion of 9.2 ± 0.4 km s⁻¹. The dispersion profile of Leo I is flat to beyond its classical "tidal" radius. We fit the profile to various equilibrium dynamical models. We strongly rule out all models where mass follows light. Anisotropic Sérsic+NFW models fit the dispersion profile well, but isotropic models are ruled out at a 95% confidence level. Inside a projected radius of ~1040 pc, the mass and V-band mass-to-light ratio of Leo I from equilibrium models are in the ranges (5–7) × 10⁷M_☉ and 9-14 (solar units), respectively. Leo I members outside a "break radius" of R_b ∼ 400'' (500 pc) exhibit significant velocity anisotropy, whereas stars interior to this radius are consistent with an isotropic velocity distribution. We interpret the break radius as the tidal radius of Leo I at perigalacticon some 1-2 Gyr ago. This interpretation accounts for the complex star formation history of Leo I, population segregation within the galaxy, and Leo I's large outward galactocentric velocity. The lack of evident tidal arms in Leo I suggests that the galaxy may have been injected into its present highly elliptical orbit by a third body a few Gyr before its last perigalacticon. This scenario is plausible within current hierarchical structure formation models.

Using a sample of ~28,000 sources selected at 3.6-4.5 μm with Spitzer observations of the Hubble Deep Field North, the Chandra Deep Field South, and the Lockman Hole (surveyed area ~664 arcmin²), we study the evolution of the stellar mass content of the universe at 0 < z < 4. We calculate stellar masses and photometric redshifts, based on ~2000 templates built with stellar population and dust emission models fitting the ultraviolet to mid-infrared spectral energy distributions of galaxies with spectroscopic redshifts. We estimate stellar mass functions for different redshift intervals. We find that 50% of the local stellar mass density was assembled at 0 < z < 1 (average star formation rate [SFR] 0.048 M_☉ yr⁻¹ Mpc⁻³), and at least another 40% at 1 < z < 4 (average SFR 0.074 M_☉ yr⁻¹ Mpc⁻³). Our results confirm and quantify the "downsizing" scenario of galaxy formation. The most massive galaxies (M > 10^12.0M_☉) assembled the bulk of their stellar content rapidly (in 1-2 Gyr) beyond z ∼ 3 in very intense star formation events (producing high specific SFRs). Galaxies with 10^11.5 < M < 10^12.0M_☉ assembled half of their stellar mass before z ∼ 1.5, and more than 90% of their mass was already in place at z ∼ 0.6. Galaxies with M < 10^11.5M_☉ evolved more slowly (presenting smaller specific SFRs), assembling half of their stellar mass below z ∼ 1. About 40% of the local stellar mass density of 10^9.0 < M < 10^11.0M_☉ galaxies was assembled below z ∼ 0.4, most probably through accretion of small satellites producing little star formation. The cosmic stellar mass density at z > 2.5 is dominated by optically faint (R≳ 25) red galaxies (distant red galaxies or BzK sources), which account for ~30% of the global population of galaxies, but contribute at least 60% of the cosmic stellar mass density. Bluer galaxies (e.g., Lyman break galaxies) are more numerous but less massive, contributing less than 50% of the global stellar mass density at high redshift.

We present Spitzer IRS mid-infrared spectra for 15 gravitationally lensed, 24 μm-selected galaxies, and combine the results with four additional very faint galaxies with IRS spectra in the literature. The median intrinsic 24 μm flux density of the sample is 130 μJy, enabling a systematic survey of the spectral properties of the very faint 24 μm sources that dominate the number counts of Spitzer cosmological surveys. Six of the 19 galaxy spectra (32%) show the strong mid-IR continuua expected of AGNs; X-ray detections confirm the presence of AGNs in three of these cases, and reveal AGNs in two other galaxies. These results suggest that nuclear accretion may contribute more flux to faint 24 μm-selected samples than previously assumed. Almost all the spectra show some aromatic (PAH) emission features; the measured aromatic flux ratios do not show evolution from z = 0. In particular, the high signal-to-noise mid-IR spectrum of SMM J163554.2+661225 agrees remarkably well with low-redshift, lower luminosity templates. We compare the rest-frame 8 μm and total infrared luminosities of star-forming galaxies, and find that the behavior of this ratio with total IR luminosity has evolved modestly from z = 2 to z = 0. Since the high aromatic-to-continuum flux ratios in these galaxies rule out a dominant contribution by AGNs, this finding implies systematic evolution in the structure and/or metallicity of infrared sources with redshift. It also has implications for the estimates of star-forming rates inferred from 24 μm measurements, in the sense that at z ∼ 2, a given observed frame 24 μm luminosity corresponds to a lower bolometric luminosity than would be inferred from low-redshift templates of similar luminosity at the corresponding rest wavelength.

We present ~3'' resolution maps of CO, its isotopologues, and HCN from in the center of Maffei 2. The J = 1–0 rotational lines of ¹²CO,¹³CO, C¹⁸O and HCN, and the J = 2–1 lines of ¹³CO and C¹⁸O were observed with the Owens Valley Radio Observatory (OVRO) and Berkeley-Illinois-Maryland Association (BIMA) arrays. The lower opacity CO isotopologues give more reliable constraints on H₂ column densities and physical conditions than optically thick ¹²CO. The J = 2–1/1–0 line ratios of the isotopologues constrain the bulk of the molecular gas to originate in low-excitation, subthermal gas. From large velocity gradient (LVG) modeling, we infer that the central giant molecular clouds (GMCs) have n_H2 ∼ 10^2.75 cm⁻³ and T_k ∼ 30 K. Continuum emission at 3.4, 2.7, and 1.4 mm was mapped to determine the distribution and amount of H II regions and dust. Column densities derived from C¹⁸O and 1.4 mm dust continuum fluxes indicate the standard Galactic conversion factor overestimates the amount of molecular gas in the center of Maffei 2 by factors of ~2-4. Gas morphology and the clear "parallelogram" in the position-velocity diagram shows that molecular gas orbits within the potential of a nuclear (~220 pc) bar. The nuclear bar is distinct from the bar that governs the large-scale morphology of Maffei 2. Giant molecular clouds in the nucleus are nonspherical and have large line widths, due to tidal effects. Dense gas and star formation are concentrated at the sites of the x₁-x₂-orbit intersections of the nuclear bar, suggesting that the starburst is dynamically triggered.

The ISO LWS far-infrared spectrum of the ultraluminous galaxy Mrk 231 shows OH and H₂O lines in absorption from energy levels up to 300 K above the ground state, and emission in the [O I] 63 μm and [C II] 158 μm lines. Our analysis shows that OH and H₂O are radiatively pumped by the far-infrared continuum emission of the galaxy. The absorptions in the high-excitation lines require high far-infrared radiation densities, allowing us to constrain the properties of the underlying continuum source. The bulk of the far-infrared continuum arises from a warm (T_dust = 70–100 K), optically thick (τ_{100μ m} = 1–2) medium of effective diameter 200-400 pc. In our best-fit model of total luminosity L_IR, the observed OH and H₂O high-lying lines arise from a luminous (L/L_IR ∼ 0.56) region with radius ~100 pc. The high surface brightness of this component suggests that its infrared emission is dominated by the AGN. The derived column densities N(OH) ≳ 10¹⁷ cm⁻² and N(H₂O) ≳ 6 × 10¹⁶ cm⁻² may indicate X-ray dominated region (XDR) chemistry, although significant starburst chemistry cannot be ruled out. The lower-lying OH, [C II] 158 μm, and [O I] 63 μm lines arise from a more extended (~350 pc) starburst region. We show that the [C II] deficit in Mrk 231 is compatible with a high average abundance of C⁺ because of an extreme overall luminosity to gas mass ratio. Therefore, a [C II] deficit may indicate a significant contribution to the luminosity by an AGN, and/or by extremely efficient star formation.

We have mapped the warm molecular gas traced by the H₂S(0)-H₂S(5) pure rotational mid-infrared emission lines over a radial strip across the nucleus and disk of M51 (NGC 5194) using the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. The six H₂ lines have markedly different emission distributions. We obtained the H₂ temperature and surface density distributions by assuming a two-temperature model: a warm (T = 100–300 K) phase traced by the low J [S(0)-S(2)] lines and a hot phase (T = 400–1000 K) traced by the high J [S(2)-S(5)] lines. The lowest molecular gas temperatures are found within the spiral arms (T ∼ 155 K), while the highest temperatures are found in the inter-arm regions (T > 700 K). The warm gas surface density reaches a maximum of 11 M_☉ pc⁻² in the northwest spiral arm, whereas the hot gas surface density peaks at 0.24 M_☉ pc⁻² at the nucleus. The spatial offset between the peaks in the different phases suggests that the warm phase is more efficiently heated by star formation activity and the hot phase is more efficiently heated by nuclear activity. The warm H₂ is found in the dust lanes of M51 and is generally spatially coincident with the cold molecular gas traced by CO emission, consistent with excitation of the warm phase in dense photodissociation regions. The hot H₂ is most prominent in the nuclear region. Here, the hot H₂ coincides with [O IV] (25.89 μm) and X-ray emission indicating that shocks and/or X-rays are responsible for exciting this phase.

We present BIMA observations of a 2^' field in the northeastern spiral arm of M31. In this region we find six giant molecular clouds (GMCs) that have a mean diameter of 57 ± 13 pc, a mean velocity width of 6.5 ± 1.2 km s⁻¹, and a mean molecular mass of (3.0 ± 1.6) × 10⁵M_☉. The peak brightness temperature of these clouds ranges from 1.6 to 4.2 K. We compare these clouds to clouds in M33 observed by Wilson & Scoville using the OVRO millimeter array and some cloud complexes in the Milky Way observed by Dame and coworkers using the CfA 1.2 m telescope. In order to properly compare the single-dish data to the spatially filtered interferometric data, we project several well-known Milky Way complexes to the distance of Andromeda and simulate their observation with the BIMA interferometer. We compare the simulated Milky Way clouds with the M31 and M33 data using the same cloud identification and analysis technique and find no significant differences in the cloud properties in all three galaxies. Thus, we conclude that previous claims of differences in the molecular cloud properties between these galaxies may have been due to differences in the choice of cloud identification techniques. With the upcoming CARMA telescope, individual molecular clouds may be studied in a variety of nearby galaxies. With ALMA, comprehensive GMC studies will be feasible at least as far as the Virgo cluster. With these data, comparative studies of molecular clouds across galactic disks of all types and between different galaxy disks will be possible. Our results emphasize that interferometric observations combined with the use of a consistent cloud identification and analysis technique will be essential for such forthcoming studies that will compare GMCs in the Local Group galaxies to galaxies in the Virgo cluster.

We examine the possibility of observing gravitational lensing in the weak deflection regime by the supermassive black hole in the center of the galaxy M31. This black hole is significantly more massive than the black hole in the center of our Galaxy, qualifying itself as a more effective lens. However, it is also more distant, and the candidate stellar sources appear consequently fainter. We separately consider as potential sources stars belonging to the bulge, to the disk, and to the triple nucleus formed by P1 + P2 and by the recently discovered inner cluster P3. We calculate the number of simultaneously lensed stars at a given time as a function of the threshold magnitude required for the secondary image. For observations in the K band we find 1.4 expected stars having secondary images brighter than K = 24 and 182 brighter than K = 30. For observations in the V band we expect 1.3 secondary images brighter than V = 27 and 271 brighter than V = 33. The bulge stars have the highest chance of being lensed by the supermassive black hole, whereas the disk and the composite nucleus stars contribute 10% each. The typical angular separation of the secondary images from the black hole range from 1 mas to 0.1''. For each population we also show the distribution of the lensed sources as a function of their distance and absolute magnitude, the expected angular positions and velocities of the generated secondary images, and the rate and the typical duration of the lensing events.

We perform a linear magnetohydrodynamic perturbation analysis for a stratified magnetized envelope where the diffusion of heat is mediated by charged particles that are confined to flow along magnetic field lines. We identify an instability, the "Coulomb bubble instability," which may be thought of as standard magnetosonic fast and slow waves, driven by the rapid diffusion of heat along the direction of the magnetic field. We calculate the growth rate and stability criteria for the Coulomb bubble instability for various choices of equilibrium conditions. The Coulomb bubble instability is intimately related to the photon bubble instability. The bulk thermodynamic properties of both instability mechanisms are quite similar in that they require the timescale for heat to diffuse across a wavelength to be shorter than the corresponding wave crossing time. Furthermore, overstability occurs only as long as the driving resulting from the presence of the background heat flux can overcome diffusive Silk damping. However, the geometric and therefore mechanical properties of the Coulomb bubble instability are the complete mirror opposite of the photon bubble instability. The Coulomb bubble instability is most strongly driven for weakly magnetized atmospheres that are strongly convectively stable. We briefly discuss a possible application of astrophysical interest: diffusion of interstellar cosmic rays in the hot T ∼ 10⁶ K Galactic corona. We show that for commonly accepted values of the cosmic-ray and gas pressure as well as its overall characteristic dimensions, the Galactic corona is in a marginal state of stability with respect to a cosmic-ray Coulomb bubble instability. The implication being that a cosmic-ray Coulomb bubble instability plays a role in regulating both the pressure and transport properties of interstellar cosmic rays, while serving as a source of acoustic power above the Galactic disk.

For purposes of designing targeted cataclysmic variable (CV) detection surveys and interpreting results of other projects with many CV detections such as the ChaMPlane Survey, we have created a model of the CV distribution in the Galaxy. It is modeled as a warped, flared exponential disk with a Gaussian vertical distribution. Extinction is based on a detailed Galactic dust and gas model. A luminosity function for CVs is also incorporated, based on a smoothed version of published data. We calculate predicted field detection rates as a function of the limiting magnitude expected for the detecting system (i.e., WIYN/Hydra or NOAO 4 m/Mosaic). Monte Carlo techniques are used to assess statistical fluctuations in these rates. We have created maps of the expected CV distribution for the full nonbulge Galactic plane (20° < l < 340°,| b| < 15°) for use in both the ChaMPlane Survey and future CV surveys. Assuming a CV distribution with a scale height of 160 pc, the ChaMPlane observational result of 5 CVs in 13 northern fields is best fit by a CV local space density of 0.9^{+ 1.5}_−0.5 × 10⁻⁵ pc ⁻³, with the range representing the 95% confidence interval.

We show that the inclination to the line of sight of bipolar planetary nebulae strongly affects some of their observed properties. We model these objects as having a dusty equatorial density enhancement that produces extinction that varies with the viewing angle. Our sample of 29 nebulae taken from the literature shows a clear correlation between the inclination angle and the near-infrared and optical photometric properties as well as the detected luminosity of the objects. As the inclination angle increases (the viewing angle is closer to the equatorial plane) the objects become redder, their apparent luminosity decreases, and their projected expansion velocity becomes smaller. We compute two-dimensional models of stars embedded in dusty disklike structures of various shapes and compositions and show that the observed data can be reproduced by disk-star combinations with reasonable parameters. To compare with the observational data, we generate sets of model data by randomly varying the star and disklike structure parameters within a physically meaningful range. We conclude that a only a smooth pole to equator density gradient agrees with the observed phenomena.

We present the results of long-slit spectroscopy, in several positions, of the Orion Nebula. Our goal is to study the spatial distributions of a large number of nebular quantities, including line fluxes, physical conditions, and ionic abundances, at a spatial resolution of about 1''. In particular, we have compared the O⁺⁺ abundance determined from collisionally excited and recombination lines in 671 individual one-dimensional spectra covering different morphological zones of the nebula. We find that protoplanetary disks (proplyds) show prominent spikes of , which is probably produced by collisional deexcitation due to the high electron densities found in these objects. Herbig-Haro objects show also relatively high values of , but these are probably produced by local heating due to shocks. We also find that the spatial distribution of the pure recombination O II and [O III] lines is fairly similar. The abundance discrepancy factor (ADF) of O⁺⁺ remains rather constant along the slit positions, except in some particular small areas of the nebula, such as at the locations of the most conspicuous Herbig-Haro objects. There is also an apparent slight increase of the ADF in the inner 40'' around θ¹ Ori C. We find a negative radial gradient of and in the nebula, based on the projected distance from θ¹ Ori C. In addition, the ADF of O⁺⁺ seems to increase very slightly with the electron temperature. Finally, we estimate the value of the mean-square electron temperature fluctuation, the so-called t² parameter. Our results indicate that the hypothetical thermal inhomogeneities, if they exist, should be smaller than our spatial resolution element.

The spatial distribution of the cosmic-ray flux is important in understanding the interstellar medium (ISM) of the Galaxy. This distribution can be analyzed by studying different molecular species along different sight lines whose abundances are sensitive to the cosmic-ray ionization rate. Recently several groups have reported an enhanced cosmic-ray ionization rate (ζ = χ_CRζ_standard) in diffuse clouds compared to the standard value, ζ_standard (=2.5 × 10⁻¹⁷ s⁻¹), measured toward dense molecular clouds. In an earlier work we reported an enhancement χ_CR = 20 toward HD 185418. McCall et al. have reported χ_CR = 48 toward ζ Persei based on the observed abundance of H⁺₃, while Le Petit et al. found χ_CR ≈ 10 to be consistent with their models for this same sight line. Here we revisit ζ Persei and perform a detailed calculation using a self-consistent treatment of the hydrogen chemistry, grain physics, energy and ionization balance, and excitation physics. We show that the value of χ_CR deduced from the H⁺₃ column density, N(H⁺₃) , in the diffuse region of the sight line depends strongly on the properties of the grains because they remove free electrons and change the hydrogen chemistry. The observations are largely consistent with χ_CR ≈ 40, with several diagnostics indicating higher values. This underscores the importance of a full treatment of grain physics in studies of interstellar chemistry.

We study three quasar radio sources (B1257–326, B1519–273, and J1819+385) that show large-amplitude intraday and annual scintillation variability produced by the Earth's motion relative to turbulent-scattering screens located within a few parsecs of the Sun. We find that the lines of sight to these sources pass through the edges of partially ionized warm interstellar clouds where two or more clouds may interact. From the gas flow vectors of these clouds, we find that the relative radial and transverse velocities of these clouds are large and could generate the turbulence that is responsible for the observed scintillation. For all three sight lines the flow velocities of nearby warm local interstellar clouds are consistent with the fits to the transverse flows of the radio scintillation signals.

The G31.41+0.31 region hosts one of the most prominent hot molecular cores known. Coincident with the hot molecular core is an outflow whose orientation has been controversial. We report VLA-C observations of thermal methanol (7₀–6₁A⁺, 44 GHz) toward the position of the G31.41+0.31 hot molecular core. Our goals are to clarify the orientation of the outflow and to study the properties of a molecular outflow from a very young region of massive star formation. We confirm that the outflow is indeed associated with the hot molecular core. Our observations strongly suggest that the outflow is oriented in the northeast-southwest direction. The outflow is massive (≳15 M_☉), with a dynamical time of the order of ~4 × 10³ yr, and has a wide-angle bipolar morphology.

We present Spitzer IRAC images that indicate the presence of cavities cut into the dense outer envelope surrounding very young pre-main-sequence stars. These young stellar objects (YSOs) characterized by an outflow represent the earliest stages of star formation. Mid-infrared photons thermally created by the central protostar/disk are scattered by dust particles within the outflow cavity itself into the line of sight. We observed this scattered light from 27 nearby, cavity-resolved YSOs and quantified the shape of the outflow cavities. Using the grid models of Robitaille et al., we matched model spectral energy distributions (SEDs) to the observed SEDs of the 27 cataloged YSOs using photometry from IRAC, MIPS, and IRAS. This allows for the estimation of geometric and physical properties such as inclination angle, cavity density, and accretion rate. By using the relative parameter estimates determined by the models, we are able to deduce an evolutionary picture for outflows. Our work supports the concept that cavities widen with time, beginning as a thin jetlike outflow that widens to reveal the central protostar and disk until the protostellar envelope is completely dispersed by outflow and accretion.

We obtained deep near-infrared (NIR) images of two embedded clusters at the northern and southern CO peaks of Cloud 2, which is one of the most distant star-forming regions in the extreme outer Galaxy, at the Galactic radius (R_g) of ~19 kpc. We detected cluster members with a mass detection limit of <0.1 M_☉, which is well into the substellar regime (K ∼ 21 mag, 5 σ). These high-quality data enable a comparison of EOG values to those in the solar neighborhood on the same basis for the first time. We first constructed the NIR color-color diagram in the Mauna Kea Observatory (MKO) filter system and also for the low-metallicity environment, since the metallicity in the EOG is much lower than it is in the solar neighborhood. The estimated stellar density suggests that "isolated-type" star formation is ongoing in Cloud 2-N, while "cluster-type" star formation is ongoing in Cloud 2-S. Despite this different star formation mode, other characteristics of the two clusters are almost identical: (1) the K-band luminosity functions (KLFs) of the two clusters are quite similar, as are the estimated IMFs and ages (~0.5-1 Myr) from the KLF fitting; (2) the estimated star formation efficiencies (SFEs) for both clusters are typical compared to those of embedded clusters in the solar neighborhood. The similarity of two independent clusters with a large separation (~25 pc) strongly suggests that their star formation activities were triggered by the same mechanism, probably a supernova remnant (GSH 138-01-94), as suggested in our earlier works.

We have mapped the protobinary source IRAS 16293–2422 in CO 2-1,¹³CO 2-1, and CO 3-2 with the Submillimeter Array (SMA). The maps with resolution of 1.5''-5'' reveal a single small-scale (~3000 AU) bipolar molecular outflow along the east-west direction. We found that the blueshifted emission of this small-scale outflow mainly extends to the east and the redshifted emission to the west from the position of IRAS 16293A. A comparison with the morphology of the large-scale outflows previously observed by single-dish telescopes at millimeter wavelengths suggests that the small-scale outflow may be the inner part of the large-scale (~15,000 AU) east-west outflow. On the other hand, there is no clear counterpart of the large-scale northeast-southwest outflow in our SMA maps. Comparing analytical models to the data suggests that the morphology and kinematics of the small-scale outflow can be explained by a wide-angle wind with an inclination angle of ~30°-40° with respect to the plane of the sky. The high-resolution CO maps show that there are two compact, bright spots in the blueshifted velocity range. An LVG analysis shows that the one located 1'' to the east of source A is extremely dense, n(H₂) ~10⁷ cm⁻³, and warm, T_kin > 55 K. The other one located 1^'' southeast of source B has a higher temperature of T_kin > 65 K but slightly lower density of n(H₂) ~10⁶ cm⁻³. It is likely that these bright spots are associated with the hot core-like emission observed toward IRAS 16293. Since both bright spots are blueshifted from the systemic velocity and are offset from the protostellar positions, they are likely formed by shocks.

We present the first high spatial resolution X-ray study of NGC 2244, the 2 Myr old stellar cluster in the Rosette Nebula, using Chandra. Over 900 X-ray sources are detected; 77% have optical or FLAMINGOS NIR stellar counterparts and are mostly previously uncataloged young cluster members. The X-ray-selected population is estimated to be nearly complete between 0.5 and 3 M_☉. A number of further results emerge from our analysis: (1) The X-ray LF and the associated K-band LF indicate a normal Salpeter IMF for NGC 2244. This is inconsistent with the top-heavy IMF reported from earlier optical studies that lacked a good census of <4 M_☉ stars. By comparing the NGC 2244 and Orion Nebula Cluster XLFs, we estimate a total population of ~2000 stars in NGC 2244. (2) The spatial distribution of X-ray stars is strongly concentrated around the central O5 star, HD 46150. The other early O star, HD 46223, has few companions. The cluster's stellar radial density profile shows two distinctive structures: a power-law cusp around HD 46150 that extends to ~0.7 pc, surrounded by an isothermal sphere extending out to 4 pc with core radius 1.2 pc. This double structure, combined with the absence of mass segregation, indicates that this 2 Myr old cluster is not in dynamical equilibrium. (3) The fraction of X-ray-selected cluster members with K-band excesses caused by inner protoplanetary disks is 6%, slightly lower than the 10% disk fraction estimated from the FLAMINGOS study based on the NIR-selected sample. (4) X-ray luminosities for 24 stars earlier than B4 confirm the long-standing log (L_X/L_bol) ∼ − 7 relation. The Rosette OB X-ray spectra are soft and consistent with the standard model of small-scale shocks in the inner wind of a single massive star.

We present Spitzer images of the relatively sparse, low-luminosity young cluster L988e, as well as complementary near-infrared (NIR) and submillimeter images of the region. The cluster is asymmetric, with the western region of the cluster embedded within the molecular cloud, and the slightly less dense eastern region to the east of, and on the edge of, the molecular cloud. With these data, as well as with extant Hα data of stars primarily found in the eastern region of the cluster, and a molecular ¹³CO gas emission map of the entire region, we investigate the distribution of forming young stars with respect to the cloud material, concentrating particularly on the differences and similarities between the exposed and embedded regions of the cluster. We also compare star formation in this region to that in denser, more luminous and more massive clusters already investigated in our comprehensive multiwavelength study of young clusters within 1 kpc of the Sun.

We investigate the effect of X-ray echo emission in gamma-ray bursts (GRBs). We find that the echo emission can provide an alternative way of understanding X-ray shallow decays and jet breaks. In particular, a shallow decay followed by a "normal" decay and a further rapid decay of X-ray afterglows can together be explained as being due to the echo from prompt X-ray emission scattered by dust grains in a massive wind bubble around a GRB progenitor. We also introduce an extra temporal break in the X-ray echo emission. By fitting the afterglow light curves, we can measure the locations of the massive wind bubbles, which will bring us closer to finding the mass-loss rate, wind velocity, and age of the progenitors prior to the GRB explosions.

The collapsar model is the most promising scenario to explain the huge release of energy associated with long-duration gamma-ray bursts (GRBs). Within this scenario GRBs are believed to be powered by accretion through a rotationally supported torus or by fast rotation of a compact object. In both cases rotation of the progenitor star is a key property, because it must be high enough for the torus to form, the compact object to rotate very fast, or both. Here, we check what rotational properties a progenitor star must have in order to sustain torus accretion over relatively long activity periods such as observed in most GRBs. We show that simple, often cited, estimates of the total mass available for torus formation and consequently the duration of a GRB are only upper limits. We revise these estimates by taking into account the long-term effect that as the compact object accretes, the minimum specific angular momentum needed for torus formation increases. This in turn leads to a smaller fraction of the stellar envelope that can form a torus. We demonstrate that this effect can lead to a significant (an order of magnitude) reduction of the total energy and overall duration of a GRB event. This of course can be mitigated by assuming that the progenitor star rotates faster then we assumed. However, our assumed rotation is already high compared to observational and theoretical constraints. We estimate the GRB duration times, first by assuming a constant accretion rate, an also by explicitly calculating the free-fall time of the gas during the collapse. We discuss the implications of our results.

The Swift XRT data for 179 GRBs (050124 to 070129) and the optical afterglow data for 57 pre- and post-Swift GRBs are analyzed to explore whether the observed breaks in the afterglow light curves can be interpreted as jet breaks, as well as their implications for jet energetics. We find that no burst is included in our "Platinum" sample, in which the data fully satisfy the jet break criteria. By relaxing one or more of the requirements for a jet break, candidates to various degrees are identified. In the X-ray band, 42 of 103 well-sampled X-ray light curves have a decay slope ≳1.5 in the postbreak segment (the "Bronze" sample), and 27 of these also satisfy the closure relations of the forward-shock models ("Silver" sample). The numbers of "Bronze" and "Silver" candidates in the optical light curves are 27 and 23, respectively. The X-ray break time is earlier than that in the optical bands. Among 13 bursts having both optical and X-ray light curves, only seven have an achromatic break, and even in these cases, only in one band do the data satisfy the closure relations ("Gold" sample). These results raise concerns about interpreting the breaks as jet breaks and further inferring GRB energetics. Assuming that the "Silver" and "Gold" breaks are jet breaks, we derive jet opening angles (θ_j) and kinetic energies (E_K) or lower limits on them and find that the E_K distribution is much more scattered than the pre-Swift sample, but a tentative anticorrelation between θ_j and E_K,iso is found, indicating that the E_K could still be quasi-universal.

GRB 021206 is one of the brightest GRBs ever observed. Its prompt emission, as measured by RHESSI, shows an unexpected spectral feature. The spectrum has a peak energy of about 700 keV and can be described by a Band function up to 4.5 MeV. Above 4.5 MeV, the spectrum hardens again, so the Band function fails to fit the whole RHESSI energy range up to 17 MeV. Nor does the sum of a blackbody function plus a power law, even though such a function can describe a spectral hardening. The cannonball model, on the other hand, predicts such a hardening, and we found that it fits the spectrum of GRB 021206 perfectly. We also analyzed other strong GRBs observed by RHESSI, namely, GRBs 020715, 021008, 030329, 030406, 030519B, 031027, and 031111. We found that all their spectra can be fit by the cannonball model, as well as by a Band function.

We combine a large database of population synthesis calculations, models for the star formation history of the universe, and a simple selection model for bursts to predict short GRB detection rates, redshift distributions, and host galaxy distributions. We compare our space of possible models with observations of short GRBs (rates and redshifts) and, when assuming short GRBs are produced from NS-NS binaries, the current estimates for NS-NS merger rates from close binary pulsars in the Milky Way. Whether short GRBs are assumed to arise from BH-NS or NS-NS mergers, we conclude that a fraction of models are in agreement with available short GRB and binary pulsar observations. We do not need to introduce artificial models with long delay times. Most commonly, models produce mergers preferentially in spiral galaxies if short GRBs arise from NS-NS mergers alone. On the other hand, typically BH-NS mergers can also occur in elliptical galaxies, in agreement with existing observations. We expect that a higher proportion of short GRBs should occur at moderate to high redshift (e.g., z > 1) than has presently been observed, in agreement with recent observations which suggest a strong selection bias toward successful follow-up of low-redshift short GRBs. Finally, if we add plausible additional assumptions about what BH-NS mergers could produce short GRBs based on the work of Belczynski and coworkers, then we expect only a small fraction of BH-NS models could be consistent with all current available data.

Relativistically colliding plasma is modeled by particle-in-cell simulations in one and two spatial dimensions, with an ion-to-electron mass ratio of 400 and a temperature of 100 keV. The energy of an initial quasi-parallel magnetic field is 1% of the plasma kinetic energy. Energy dissipation by a growing wave pulse of mixed polarity, probably an oblique whistler wave, and different densities of the colliding plasma slabs result in the formation of an energetic electromagnetic structure within milliseconds. The structure, which develops for an initial collision speed of 0.9c, accelerates electrons to Lorentz factors of several hundred. A downstream region forms, separating the forward and reverse shocks. In this region, the plasma approaches an energy equipartition between electrons, ions, and the magnetic field. The electron energy spectrum N(E) resembles a power law at high energies, with an exponent close to –2.7, or N(E) ∝ E^−2.7. The magnetic field reflects upstream ions, which form a beam and drag the electrons along to preserve the plasma quasineutrality. The forward and reverse shocks are asymmetric due to the unequal slab densities. The forward shock may be representative for the internal shocks of gamma-ray bursts.

Hypercritical accretion flows onto stellar mass black holes (BHs) are commonly believed to be as a promising model of central engines of gamma-ray bursts (GRBs). In this model a certain fraction of the gravitational binding energy of accreting matter is deposited to the energy of relativistic jets via neutrino annihilation and/or magnetic fields. However, some recent studies have indicated that the energy deposition rate by neutrino annihilation is somewhat smaller than that needed to power a GRB. To overcome this difficulty, Ramirez-Ruiz and Socrates proposed that high-energy neutrinos from the hot corona above the accretion disk might enhance the efficiency of the energy deposition. We elucidate the disk corona model in the context of hypercritical accretion flows. From the energy balance in the disk and the corona, we can calculate the disk and coronal temperature, T_d and T_c, and neutrino spectra, taking into account the neutrino cooling processes by neutrino-electron scatterings and neutrino pair productions. The calculated neutrino spectra consist of two peaks: one by the neutrino emission from the disk and the other by that from the corona. We find that the disk corona can enhance the efficiency of energy release but only by a factor of 1.5 or so, unless the height of the corona is very small, H≪ r. This is because the neutrino emission is very sensitive to the temperature of the emitting region, and then the ratio T_c/T_d cannot be very large.

Extreme mass ratio bursts (EMRBs) have been proposed as a possible source for future space-borne gravitational wave detectors, such as the Laser Interferometer Space Antenna (LISA). These events are characterized by long-period, nearly radial orbits of compact objects around a central massive black hole. The gravitational radiation emitted during such events consists of a short burst, corresponding to periapse passage, followed by a longer, silent interval. In this paper we investigate the impact of including relativistic corrections to the description of the compact object's trajectory via a geodesic treatment, as well as including higher order multipole corrections in the waveform calculation. The degree to which the relativistic corrections are important depends on the EMRB's orbital parameters. We find that relativistic EMRBs (v_max/c > 0.25) are not rare and actually account for approximately half of the events in our astrophysical model. The relativistic corrections tend to significantly change the waveform amplitude and phase relative to a Newtonian description, although some of this dephasing could be mimicked by parameter errors. The dephasing over several bursts could be of particular importance not only to gravitational wave detection, but also to parameter estimation, since it is highly correlated to the spin of the massive black hole. Consequently, we postulate that if a relativistic EMRB is detected, such dephasing might be used to probe the relativistic character of the massive black hole and obtain information about its spin.

We study the late evolution of solar metallicity stars in the transition region between white dwarf formation and core collapse. This includes the super-asymptotic giant branch (super-AGB, SAGB) stars, which ignite carbon burning and form an oxygen-neon (ONe) core. SAGB star cores may grow to the Chandrasekhar mass because of continued H- and He-shell burning, ending as core-collapse supernovae. From stellar evolution models we find that the initial mass range for SAGB evolution is 7.5–9.25 M_☉. We perform calculations with three different stellar evolution codes to judge the robustness of our results. The mass range significantly depends on the treatment of semiconvective mixing and convective overshooting. To consider the effect of a large number of thermal pulses, as expected in SAGB stars, we construct synthetic SAGB models that are calibrated through stellar evolution simulations. The synthetic model enables us to compute the evolution of the main properties of SAGB stars from the onset of thermal pulses until the core reaches the Chandrasekhar mass or is uncovered by the stellar wind. Thereby, we differentiate the stellar initial mass ranges that produce ONe WDs from that leading to electron-capture SNe. The latter is found to be 9.0–9.25 M_☉ for our fiducial model, implying that electron-capture SNe would constitute about 4% of all SNe in the local universe. The error in this determination due to uncertainties in the third dredge-up efficiency and AGB mass-loss rate could lead to about a doubling of the number of electron-capture SNe, which provides a firm upper limit to their contribution to all supernovae of ~20%.

We present extensive optical (UBVRI), near-infrared (JK) light curves and optical spectroscopy of the Type Ia supernova SN 2006X in the nearby galaxy NGC 4321 (M100). Our observations suggest that either SN 2006X has an intrinsically peculiar color evolution or it is highly reddened [E(B − V)_host = 1.42 ± 0.04 mag ] with R_V = 1.48 ± 0.06, much lower than the canonical value of 3.1 for the average Galactic dust. SN 2006X also has one of the highest expansion velocities ever published for an SN Ia. Compared with the other SNe Ia we analyzed, SN 2006X has a broader light curve in the U band, a more prominent bump/shoulder feature in the V and R bands, a more pronounced secondary maximum in the I and NIR bands, and a remarkably smaller late-time decline rate in the B band. The B − V color evolution shows an obvious deviation from the Lira-Phillips relation at 1-3 months after maximum brightness. At early times, optical spectra of SN 2006X displayed strong, high-velocity features of both intermediate-mass elements (Si, Ca, and S) and iron peak elements, while at late times they showed a relatively blue continuum, consistent with the blue U − B and B − V colors at similar epochs. A light echo and/or the interaction of the SN ejecta and its circumstellar material may provide a plausible explanation for its late-time photometric and spectroscopic behavior. Using the Cepheid distance of M100, we derive a Hubble constant of 72.8 ± 8.2 km s⁻¹ Mpc⁻¹ (statistical) from the normalized dereddened luminosity of SN 2006X. We briefly discuss whether abnormal dust is a universal signature for all SNe Ia and whether the most rapidly expanding objects form a subclass with distinct photometric and spectroscopic properties.

We analyze the Type II plateau supernovae (SNe IIP) SN 2005cs and SN 2006bp with the non-LTE model atmosphere code CMFGEN. We fit 13 spectra in the first month for SN 2005cs and 18 for SN 2006bp. Swift ultraviolet photometry and ground-based optical photometry calibrate each spectrum. Our analysis shows that both objects were discovered less than 3 days after they exploded, making these the earliest SN IIP spectra ever studied. They reveal broad and very weak lines from highly ionized fast ejecta with an extremely steep density profile. We identify He II λ4686 emission in the SN 2006bp ejecta. Days later, the spectra resemble the prototypical Type IIP SN 1999em, which had a supergiant-like photospheric composition. Despite the association of SN 2005cs with possible X-ray emission, the emergent UV and optical light comes from the photosphere, not from circumstellar emission. We surmise that the very steep density falloff we infer at early times may be a fossil of the combined actions of the shock wave passage and radiation driving at shock breakout. Based on tailored CMFGEN models, the direct fitting technique and the expanding photosphere method both yield distances and explosion times that agree within a few percent. We derive a distance to NGC 5194, the host of SN 2005cs, of 8.9 ± 0.5 Mpc and 17.5 ± 0.8 Mpc for SN 2006bp in NGC 3953. The luminosity of SN 2006bp is 1.5 times that of SN 1999em and 6 times that of SN 2005cs. Reliable distances to SNe IIP that do not depend on a small range in luminosity provide an independent route to the Hubble constant and improved constraints on other cosmological parameters.

Motivated by the recent discovery of 30 new millisecond pulsars in Terzan 5, made using the Green Bank Telescope's S-band receiver and the Pulsar Spigot spectrometer, we have set out to use the same observing system in a systematic search for pulsars in other globular clusters. Here we report on the discovery of five new pulsars in NGC 6440 and three in NGC 6441; each cluster previously had one known pulsar. Using the most recent distance estimates to these clusters, we conclude that there are as many potentially observable pulsars in NGC 6440 and NGC 6441 as in Terzan 5. We present timing solutions for all of the pulsars in these globular clusters. Four of the new discoveries are in binary systems; one of them, PSR J1748–2021B (NGC 6440B), has a wide (P_b = 20.5 days) and eccentric (e = 0.57) orbit. This allowed a measurement of its rate of advance of periastron, = 0.00391(18)° yr⁻¹. If this is a purely relativistic effect, the total mass of this binary system is 2.92 ± 0.20 M_☉ (1 σ) implying a median pulsar mass of 2.74 ± 0.21 M_☉. There is a 1% probability that the inclination is low enough that pulsar mass is below 2 M_☉, and 0.10% probability that it is between 1.20 and 1.44 M_☉. If confirmed, this anomalously large mass would strongly constrain the equation of state for dense matter. The other highly eccentric binary, PSR J1750–37A, has e = 0.71, and = 0.0055(3)° yr⁻¹, implying a total system mass of 1.97 ± 0.15 M_☉ and, along with the mass function, maximum and median pulsar masses of 1.65 and 1.26 M_☉, respectively.

An observation of the Vela-like pulsar B1823–13 and its synchrotron nebula with the Chandra X-Ray Observatory allowed us to resolve the pulsar and the fine structure of the nebula. The pulsar's spectrum fits a power-law model with a photon index Γ_PSR ≈ 2.4 for the plausible hydrogen column density n_H = 1 × 10²² cm⁻², corresponding to the luminosity L_PSR ≈ 8 × 10³¹ ergs s⁻¹ in the 0.5-8 keV band, at a distance of 4 kpc. The pulsar radiation likely includes magnetospheric and thermal components, but they cannot be reliably separated, because of the small number of counts detected and strong interstellar absorption. The pulsar is surrounded by a compact, ~25'' × 10'', pulsar wind nebula (PWN) elongated in the east-west direction, which includes a brighter inner component ≈7'' × 3'', elongated in the northeast-southwest direction. The slope of the compact PWN spectrum is Γ_comp ≈ 1.3, and the 0.5-8 keV luminosity is L_comp ∼ 3 × 10³² ergs s⁻¹. The compact PWN is surrounded by asymmetric diffuse emission (extended PWN) seen up to at least 2.4' south of the pulsar, with a softer spectrum (Γ_ext ≈ 1.9 for n_H = 1 × 10²² cm⁻²), and the 0.5-8 keV luminosity L_ext ∼ 10³³-10³⁴ ergs s⁻¹. We also measured the pulsar's proper motion using archival VLA data, μ_α = 23.0 ± 2.5 mas yr⁻¹ and μ_δ = − 3.9 ± 3.1 mas yr⁻¹ (in LSR), which corresponds to the transverse velocity v_⊥ ≈ 440 km s⁻¹. The direction of the proper motion is approximately parallel to the elongation of the compact PWN, but it is nearly perpendicular to that of the extended PWN and to the direction toward the center of the bright VHE γ-ray source HESS J1825–137, which is believed to be powered by PSR B1823–13.

We report on our Spitzer observations of the anomalous X-ray pulsar 4U 0142+61, made following a large X-ray burst that occurred on 2007 February 7. To search for mid-infrared flux variations, four imaging observations were carried out at 4.5 and 8.0 μm with the Infrared Array Camera from February 14 to 21. No significant flux variations were detected, and the average fluxes were 32.1 ± 2.0 μJy at 4.5 μm and 59.8 ± 8.5 μJy at 8.0 μm, consistent with those obtained in 2005. The nondetection of variability is interesting in light of reported rapid variability from this source in the near-infrared but consistent with the fact that the source already went back to its quiescent state before our observations began, as indicated by contemporaneous X-ray observations. In order to understand the origin of the near-infrared variability, frequent, simultaneous multiwavelength observations are needed.

Results from the most extensive study of the time-evolving dust structure around the prototype "pinwheel" nebula WR 104 are presented. Encompassing 11 epochs in three near-infrared filter bandpasses, a homogeneous imaging data set spanning more than 6 yr (or 10 orbits) is presented. Data were obtained from the highly successful Keck Aperture Masking Experiment, which can recover high-fidelity images at extremely high angular resolutions, revealing the geometry of the plume with unprecedented precision. Inferred properties for the (unresolved) underlying binary and wind system are orbital period 241.5 ± 0.5 days and angular outflow velocity 0.28 ± 0.02 mas day⁻¹. An optically thin cavity of angular size 13.3 ± 1.4 mas was found to lie between the central binary and the onset of the spiral dust plume. Rotational motion of the wind system induced by the binary orbit is found to have important ramifications: entanglement of the winds results in strong shock activity far downstream from the nose of the bow shock. The far greater fraction of the winds participating in the collision may play a key role in gas compression and the nucleation of dust at large radii from the central binary and shock stagnation point. Investigation of the effects of radiative braking points toward significant modifications of the simple hydrostatic colliding wind geometry, extending the relevance of this phenomenon to wider binary systems than previously considered. Limits placed on the maximum allowed orbital eccentricity of e≲ 0.06 argue strongly for a prehistory of tidal circularization in this system. Finally, we discuss the implications of Earth's polar (i≲ 16°) vantage point onto a system likely to host supernova explosions at future epochs.

We present highlights and an overview of 20 FUSE and HST STIS observations of the bright symbiotic binary EG And. The main motivation behind this work is to obtain spatially resolved information on an evolved giant star in order to understand the mass-loss processes at work in these objects. The system consists of a low-luminosity white dwarf and a mass-losing, nondusty M2 giant. The ultraviolet observations follow the white dwarf continuum through periodic gradual occultations by the wind and chromosphere of the giant, providing a unique diagnosis of the circumstellar gas in absorption. Unocculted spectra display high-ionization features, such as the O VI resonance doublet, which is present as a variable (hourly timescales), broad wind profile, which diagnose the hot gas close to the dwarf component. Spectra observed at stages of partial occultation display a host of low-ionization, narrow absorption lines, with transitions observed from lower energy levels up to ~5 eV above ground. This absorption is due to chromospheric/wind material, with most lines due to transitions of Si II, P II, N I, Fe II, and Ni II, as well as heavily damped H I Lyman series features. No molecular features are observed in the wind acceleration region despite the sensitivity of FUSE to H₂. From analysis of the ultraviolet data set, as well as optical data, we find that the dwarf radiation does not dominate the wind acceleration region of the giant and that observed thermal and dynamic wind properties are most likely representative of isolated red giants.

We have derived isotopic fractions of europium, samarium, and neodymium in two metal-poor giants with differing neutron-capture nucleosynthetic histories. These isotopic fractions were measured from new high-resolution (R ∼ 120,000), high signal-to-noise ratio (S/N ~ 160-1000) spectra obtained with the 2d-coudé spectrograph of McDonald Observatory's 2.7 m Smith telescope. Synthetic spectra were generated using recent high-precision laboratory measurements of hyperfine and isotopic subcomponents of several transitions of these elements and matched quantitatively to the observed spectra. We interpret our isotopic fractions by the nucleosynthesis predictions of the stellar model, which reproduces s-process nucleosynthesis from the physical conditions expected in low-mass, thermally pulsing stars on the AGB, and the classical method, which approximates s-process nucleosynthesis by a steady neutron flux impinging on Fe-peak seed nuclei. Our Eu isotopic fraction in HD 175305 is consistent with an r-process origin by the classical method and is consistent with either an r- or an s-process origin by the stellar model. Our Sm isotopic fraction in HD 175305 suggests a predominantly r-process origin, and our Sm isotopic fraction in HD 196944 is consistent with an s-process origin. The Nd isotopic fractions, while consistent with either r-process or s-process origins, have very little ability to distinguish between any physical values for the isotopic fraction in either star. This study for the first time extends the n-capture origin of multiple rare earths in metal-poor stars from elemental abundances to the isotopic level, strengthening the r-process interpretation for HD 175305 and the s-process interpretation for HD 196944.

Stellar models have been computed for stars having [ Fe/H ] = 0.0 (assuming both the Grevesse & Sauval and Asplund et al. heavy-element mixtures) and –2.0 to determine the effects on the predicted T_eff scale of using boundary conditions derived from the latest MARCS model atmospheres. The latter were fitted in a fully consistent way to the interior models at the photosphere and at τ = 100: the resultant evolutionary sequences on the H-R diagram were found to be nearly independent of the chosen fitting point. Tracks were also computed in which the pressure at T = T_eff was obtained by integrating the hydrostatic equation together with either the classical gray T(τ , T_eff) relation or that derived by Krishna Swamy from an empirical solar atmosphere. Due to the effects of differences in the solar-calibrated values of the mixing-length parameter, α_MLT, very similar tracks were obtained for the different treatments of the atmosphere, except at solar abundances, where the models based on the Krishna Swamy T(τ , T_eff) relationship predicted ~150 K hotter giant branches than the others, in good agreement with the inferred temperatures of giants in the open cluster M67 from recent (V − K) -T_eff relations. Tracks that used new ``scaled solar, differentially corrected'' MARCS atmospheres were found to agree well with those that employed the Krishna Swamy T(τ , T_eff) relationship, independently of the assumed metal abundance. (Gray atmospheres are quite different from MARCS models.) Fits of isochrones for [ Fe/H ] = − 2.0 to the CMD of the globular cluster M68, as well as the possibility that α_MLT varies with stellar parameters, are also discussed.

For the investigation of collisions among protoplanetesimal dust aggregates, we performed microgravity experiments in which the impacts of high-porosity millimeter-sized dust aggregates into 2.5 cm high-porosity dust aggregates can be studied. The dust aggregates consisted either of monodisperse spherical, quasi-monodisperse irregular, or polydisperse irregular micrometer-sized dust grains and were produced by random ballistic deposition with porosities between 85% and 93%. Impact velocities ranged from ~0.1 to ~3 m s⁻¹, and impact angles were almost randomly distributed. In addition to the smooth surfaces of the target aggregates formed in our experiments, we "molded" target aggregates such that the radii of the local surface curvatures corresponded to the projectile radii, decreasing the targets' porosities to 80%-85%. The experiments showed that impacts into the highest porosity targets almost always led to sticking, whereas for the less porous dust aggregates, consisting of monodisperse spherical dust grains, the collisions with intermediate velocities and high impact angles resulted in the bouncing of the projectile with a mass transfer from the target to the projectile aggregate. Sticking probabilities for the impacts into the "molded" target aggregates were considerably decreased. For the impacts into smooth targets, we measured the depth of intrusion and the crater volume, and were able to derive some interesting dynamical properties which can help to derive a collision model for protoplanetesimal dust aggregates. Future models of the aggregate growth in protoplanetary disks should take into account noncentral impacts, impact compression, the influence of the local radius of curvature on the collisional outcome, and the possible mass transfer between the target and projectile agglomerates in nonsticking collisions.

Only a few solar-type main-sequence stars are known to be orbited by warm dust particles; the most extreme is the G0 field star BD +20 307 that emits ~4% of its energy at mid-infrared wavelengths. We report the identification of a similarly dusty star HD 23514, an F6-type member of the Pleiades. A strong mid-IR silicate emission feature indicates the presence of small warm dust particles, but with the primary flux density peak at the nonstandard wavelength of ~9 μm. The existence of so much dust within an AU or so of these stars is not easily accounted for given the very brief lifetime in orbit of small particles. The apparent absence of very hot (≳1000 K) dust at both stars suggests the possible presence of a planet closer to the stars than the dust. The observed frequency of the BD +20 307/HD 23514 phenomenon indicates that the mass equivalent of Earth's Moon must be converted, via collisions of massive bodies, to tiny dust particles that find their way to the terrestrial planet zone during the first few hundred million years of the life of many (most?) Sun-like stars. Identification of these two dusty systems among youthful nearby solar-type stars suggests that terrestrial planet formation is common.

We report precise Doppler measurements of two stars, obtained at Lick Observatory as part of our search for planets orbiting intermediate-mass subgiants. Periodic variations in the radial velocities of both stars reveal the presence of substellar orbital companions. These two stars are notably massive with stellar masses of 1.80 and 1.64 M_☉, respectively, indicating that they are former A-type dwarfs that have evolved off of the main sequence and are now K-type subgiants. The planet orbiting κ CrB has a minimum mass M_Psin i = 1.8 M_Jup, eccentricity e = 0.146 and a 1208 day period, corresponding to a semimajor axis a = 2.7 AU. The planet around HD 167042 has a minimum mass M_Psin i = 1.7 M_Jup and a 412.6 day orbit, corresponding to a semimajor axis a = 1.3 AU. The eccentricity of HD 167042b is consistent with circular (e = 0.027 ± 0.04), adding to the rare class of known exoplanets in long-period, circular orbits similar to the solar system gas giants. Like all of the planets previously discovered around evolved A stars, κ CrBb and HD 167042b orbit beyond 0.8 AU.

We report 18 years of Doppler shift measurements of a nearby star, 55 Cancri, that exhibits strong evidence for five orbiting planets. The four previously reported planets are strongly confirmed here. A fifth planet is presented, with an apparent orbital period of 260 days, placing it 0.78 AU from the star in the large empty zone between two other planets. The velocity wobble amplitude of 4.9 m s⁻¹ implies a minimum planet mass Msin i = 45.7 M_⊕. The orbital eccentricity is consistent with a circular orbit, but modest eccentricity solutions give similar χ²_ν fits. All five planets reside in low-eccentricity orbits, four having eccentricities under 0.1. The outermost planet orbits 5.8 AU from the star and has a minimum mass Msin i = 3.8 M_Jup, making it more massive than the inner four planets combined. Its orbital distance is the largest for an exoplanet with a well-defined orbit. The innermost planet has a semimajor axis of only 0.038 AU and has a minimum mass, Msin i, of only 10.8 M_⊕, making it one of the lowest mass exoplanets known. The five known planets within 6 AU define a minimum-mass protoplanetary nebula to compare with the classical minimum-mass solar nebula. Numerical N-body simulations show this system of five planets to be dynamically stable and show that the planets with periods of 14.65 and 44.3 days are not in a mean motion resonance. Millimagnitude photometry during 11 years reveals no brightness variations at any of the radial velocity periods, providing support for their interpretation as planetary.

This study addresses the long-term evolution of possible two-planet extrasolar systems that are initially trapped in 3:1 mean motion resonance. A planar general three-body problem model is used, and its resonant dynamics are examined by computing periodic orbits in a rotating frame, Poincaré maps, and maps of dynamical stability. We computed the families of symmetric resonant periodic orbits that obey bifurcations, giving rise to families of asymmetric periodic orbits. The linear stability of such orbits has also been computed, and their relation to the long-term stability of their nearby phase-space domain has been studied. The maps of dynamical stability reveal a complicated structure in the phase space, where chaos and order coexist and alternate as the initial eccentricities or the phases of the planets change. The regular orbits are classified into various types according to the librating or rotating evolution of the resonant angles. Apsidal symmetric librations are common in the domain of resonant motion, but asymmetric ones are associated exclusively with the existence of asymmetric periodic orbits. Such a stable asymmetric configuration seems to correspond to the companions b and c of the 55 Cnc extrasolar system, which are trapped in the 3:1 mean motion resonance according to the study of McArthur et al. However, a recent study by Fischer et al. shows the existence of a new planet (the companion f) in the system, and that the planets b and c are not in mean motion resonance.

We present results from a set of over 300 pseudospectral simulations of atmospheric circulation on extrasolar giant planets with circular orbits. The simulations are of high enough resolution (up to 341 total and sectoral modes) to resolve small-scale eddies and waves, required for reasonable physical accuracy. In this work, we focus on the global circulation pattern that emerges in a shallow, "equivalent barotropic," turbulent atmosphere on both tidally synchronized and unsynchronized planets. A full exploration of the large physical and numerical parameter space is performed to identify robust features of the circulation. For some validation, the model is first applied to solar system giant planets. For extrasolar giant planets with physical parameters similar to HD 209458b—a presumably synchronized extrasolar giant planet, representative in many dynamical respects—the circulation is characterized by the following features: (1) a coherent polar vortex that revolves around the pole in each hemisphere; (2) a low number (typically two or three) of slowly varying, broad zonal (east-west) jets that form when the maximum jet speed is comparable to, or somewhat stronger than, those observed on the planets in the solar system; and (3) a motion-associated temperature field, whose detectability and variability depend on the strength of the net heating rate and the global rms wind speed in the atmosphere. In many ways, the global circulation is Earth-like, rather than Jupiter-like. However, if extrasolar giant planets rotate faster and are not close-in (therefore not synchronized), their circulations become more Jupiter-like, for Jupiter-like rotation rates.

The properties of solar energetic particles (SEPs) in solar flares are studied through remote imaging in the radio, hard X-ray, and γ-ray energy ranges. However, the heliospheric SEP populations are observed only in situ by satellite measurements, which drastically limits our understanding of their spatial and temporal variations. Can those SEP populations be remotely imaged, as are the solar SEPs? We consider two possibilities for detecting faint γ-ray emission from SEP interactions with solar wind (SW) ions. First, the 6.13 and 4.44 MeV γ-ray lines of ¹⁶O and ¹²C, respectively, produced by the interactions of the SEPs from a large low-energy (E < 30 MeV) gradual event are calculated and found to be far below a detectable level. Then the expected π⁰-decay γ-ray emission is calculated for the intense ground-level event (GLE) of 2005 January 20 and compared with (1) the observed Galactic and extragalactic background and (2) the expected near-solar emission from inverse-Compton scattering of solar photons by cosmic-ray electrons and from Galactic cosmic-ray collisions with the solar atmosphere. It appears feasible to detect the π⁰-decay emission from that event with a detector of the size of the Large Area Telescope on GLAST. Earlier 1982 and 1991 flare observations of long-duration (hours) π⁰-decays were attributed to E > 300 MeV protons captured in strong coronal loops, but we suggest that the observed emission was due to SEP-SW collisions following shock acceleration on open field lines.

During relatively quiet solar conditions throughout the spring and summer of 2007, the SECCHI HI2 white-light telescope on the STEREO B solar-orbiting spacecraft observed a succession of wave fronts sweeping past Earth. We have compared these heliospheric images with in situ plasma and magnetic field measurements obtained by near-Earth spacecraft, and we have found a near perfect association between the occurrence of these waves and the arrival of density enhancements at the leading edges of high-speed solar wind streams. Virtually all of the strong corotating interaction regions are accompanied by large-scale waves, and the low-density regions between them lack such waves. Because the Sun was dominated by long-lived coronal holes and recurrent solar wind streams during this interval, there is little doubt that we have been observing the compression regions that are formed at low latitude as solar rotation causes the high-speed wind from coronal holes to run into lower speed wind ahead of it.

Intensities of abundance diagnostic lines of Ca XV, Ca XVI, Ni XVII, Ar XIII, and Ar XV have been derived for a classic flare loop system observed during the Skylab mission. These have been used to test for photospheric or coronal origin of the flare loop material. The resulting FIP-bias factors are between 1.7 and 4.6 with a majority of the values around 4.5 indicating a source with material modified by the FIP effect. The loop system bias factors are similar to those observed in a sample of Skylab prominences, suggesting that the disrupted mass of the preflare embedded filament provided the loop system material.

The X-Ray Telescope on Hinode has observed individual loops of plasma moving downward in a manner that is consistent with field line shrinkage in the aftermath of reconnection at higher altitudes. An on-disk B3.8 flare observed on 2007 May 2 has loops that clearly change in shape from cusp-shaped to more rounded. In addition, bright loops are observed that decrease in altitude with a speed of approximately 5 km s⁻¹, and fainter, higher loop structures shrink with a velocity of 48 km s⁻¹. A C2.1 flare observed on 2006 December 17 also has loops that change shape. Many bright features are seen to be moving downward in this event, and we estimate their speed to be around 2-4 km s⁻¹. We measure the shrinkage in both of these events, and find that it is 17%-27%, which is consistent with theoretical predictions.

The solar corona is a complex magnetic environment where several kinds of waves can propagate. In this work, the properties of fast, Alfvén, and slow magnetohydrodynamic waves in a simple curved structure are investigated. We consider the linear regime, i.e., small-amplitude waves. We study the time evolution of impulsively generated waves in a coronal arcade by solving the ideal magnetohydrodynamic equations. We use a numerical code specially designed to solve these equations in the low-β regime. The results of the simulations are compared with the eigenmodes of the arcade model. Fast modes propagate nearly isotropically through the whole arcade and are reflected at the photosphere, where line-tying conditions are imposed. On the other hand, Alfvén and slow perturbations are very anisotropic and propagate along the magnetic field lines. Because of the different physical properties in different field lines, there is a continuous spectrum of Alfvén and slow modes. Curvature can have a significant effect on the properties of the waves. Among other effects, it considerably changes the frequency of oscillation of the slow modes and enhances the possible dissipation of the Alfvén modes due to phase mixing.

A simple one-dimensional model of the pulsed dissipation of a spontaneous current sheet is presented. Parker has demonstrated that spontaneous current sheets, or magnetic tangential discontinuities, must inevitably form and dissipate in the electrically near-perfectly conducting solar corona. Under the simplifying assumption that the effects of magnetic diffusion and near-ideal MHD relaxation to equilibrium may be idealized to occur separately in time, exact analytical one-dimensional isothermal solutions for successive alternate equilibrium and diffused states are obtained showing explicitly the gradual removal of the magnetic discontinuity. For discontinuities between both parallel and antiparallel fields, this removal results in the liberation of energy stored in the field. Energy losses in the antiparallel cases are found to be larger than for the parallel cases, and this difference increases as the plasma β decreases. Diffusions of parallel fields are found to result in magnetic flux transfer across the current sheet during the evolution while discontinuities between parallel fields cause flux annihilation, the creation of magnetic neutral points, inflows, and localized plasma accumulations. Numerical solutions obtained using the Versatile Advection Code reproduce these basic properties under isothermal, nonisothermal adiabatic, and nonadiabatic conditions and with magnetic diffusion and MHD relaxation occurring simultaneously. The relevance of these models to more complicated systems is discussed.

An analytical and numerical treatment is given of a constrained version of the tectonics model developed by Priest et al.. We begin with a uniform magnetic field B = B₀ that is line-tied at the surfaces z = 0 and z = L. This initial configuration is twisted by photospheric footpoint motion that is assumed to depend on only one coordinate (x) transverse to the initial magnetic field. The geometric constraints imposed by our assumption preclude the occurrence of reconnection and secondary instabilities but enable us to follow for long times the dissipation of energy due to the effects of resistivity and viscosity. In this limit, we demonstrate that when the coherence time of random photospheric footpoint motion is much smaller by several orders of magnitude compared with the resistive diffusion time, the heating due to ohmic and viscous dissipation becomes independent of the resistivity of the plasma. Furthermore, we obtain scaling relations that suggest that even if reconnection and/or secondary instabilities were to limit the buildup of magnetic energy in such a model, the overall heating rate will still be independent of the resistivity.

The Zeeman pattern of Mn I lines is sensitive to hyperfine structure (HFS), and because of this, they respond to hectogauss magnetic field strengths differently than the lines commonly used in solar magnetometry. This peculiarity has been employed to measure magnetic field strengths in quiet-Sun regions, assuming the magnetic field to be constant over a resolution element. This assumption is clearly insufficient, biasing the measurements. The diagnostic potential of Mn I lines can be fully exploited only after one understands the sense and magnitude of such bias. We present the first syntheses of Mn I lines in realistic quiet-Sun model atmospheres. The Mn I lines weaken with increasing field strength. In particular, kilogauss magnetic concentrations produce Mn I λ5538 circular polarization signals (Stokes V) that can be up to 2 orders of magnitude smaller than what the weak magnetic field approximation predicts. The polarization emerging from an atmosphere having weak and strong fields is biased toward the weak fields, and HFS features characteristic of weak fields show up even when the magnetic flux and energy are dominated by kilogauss fields. For the HFS feature of Mn I λ5538 to disappear, the filling factor of kilogauss fields has to be larger than the filling factor of subkilogauss fields. Since the Mn I lines are usually weak, Stokes V depends on magnetic field inclination according to the simple cosine law. Atmospheres with unresolved velocities produce very asymmetric line profiles, which cannot be reproduced by simple one-component model atmospheres. Using the HFS constants available in the literature, we reproduce the observed line profiles of nine lines with varied HFS patterns.

We evaluate the skill of three solar cycle predictors, namely, polar magnetic flux, flux crossing the equator, and tachocline toroidal flux, using both observations and a calibrated dynamo model. Polar flux measurements are available only for the past three sunspot cycles, implying poor statistics. However, the correlation between observed north and south polar flux peaks, and peaks of the next sunspot cycle is r = 0.785. We find that the correlation between the observed cross-equatorial flux and the observed peak of the next solar cycle is also high, close to that of Cameron & Schüssler, and the statistics are more reliable. Thus, the cross-equatorial flux is a better predictor of the next cycle than is the polar flux. From the dynamo model, the correlations with observed cycle peaks for polar flux, cross-equatorial flux, and toroidal flux are 0.48, 0.76, and 0.96, respectively. All these correlations decline when the northern and southern hemispheres are simulated separately, as well as with shortening of the averaging length in input data. A very high correlation between the model polar flux at the end of a cycle and the observed peak of that cycle implies that, within a calibrated flux transport dynamo, the polar flux follows the sunspot cycle, rather than being a precursor to it. With short-term averaging of input data, the polar and cross-equatorial fluxes retain much more short-term variability than does the toroidal flux. This is because the long traversal time of the input poloidal fields to the bottom of the convection zone in a mean-field model smooths out the short-term variability in the toroidal flux. The observed slowdown in meridional flow during 1996-2004 leads to a weaker polar flux, but a stronger cross-equatorial flux compared to the case with steady meridional flow. We infer that it is unlikely that both the cross-equatorial and the polar fluxes can be good predictors of solar cycle peaks.

We present a sensitive 3 σ upper limit of 1.1% for the HNC/HCN abundance ratio in comet 73P/Schwassmann-Wachmann (fragment B), obtained on 2006 May 10-11, using Caltech Submillimeter Observatory (CSO). This limit is a factor of ~7 lower than the values measured previously in moderately active comets at 1 AU from the Sun. Comet 73P/Schwassmann-Wachmann was depleted in most volatile species, except of HCN. The low HNC/HCN ratio thus argues against HNC production from polymers produced from HCN. However, thermal degradation of macromolecules, or polymers, produced from ammonia and carbon compounds, such as acetylene, methane, or ethane appears a plausible explanation for the observed variations of the HNC/HCN ratio in moderately active comets, including the very low ratio in comet 73P/Schwassmann-Wachmann reported here. Similar polymers have been invoked previously to explain anomalous ¹⁴N/¹⁵N ratios measured in cometary CN.

Since Type Ia supernovae (SNe) explode in galaxies, they can, in principle, be used as the same tracer of the large-scale structure as their hosts to measure baryon acoustic oscillations (BAOs). To realize this, one must obtain a dense integrated sampling of SNe over a large fraction of the sky, which may only be achievable photometrically with future projects such as the Large Synoptic Survey Telescope. The advantage of SN BAOs is that SNe have more uniform luminosities and more accurate photometric redshifts than galaxies, but the disadvantage is that they are transitory and hard to obtain in large number at high redshift. We find that a half-sky photometric SN survey to redshift z = 0.8 is able to measure the baryon signature in the SN spatial power spectrum. Although dark energy constraints from SN BAOs are weak, they can significantly improve the results from SN luminosity distances of the same data, and the combination of the two is no longer sensitive to cosmic microwave background priors.

We report on time-dependent axisymmetric simulations of an X-ray-excited flow from a parsec-scale, rotating, cold torus around an active galactic nucleus. Our simulations account for radiative heating and cooling and radiation pressure force. The simulations follow the development of a broad biconical outflow induced mainly by X-ray heating. We compute synthetic spectra predicted by our simulations. The wind characteristics and the spectra support the hypothesis that a rotationally supported torus can serve as the source of a wind which is responsible for the warm absorber gas observed in the X-ray spectra of many Seyfert galaxies.

We show an interesting correlation between the surface brightness and temperature structure of the relaxed clusters RX J1720.1+2638 and MS 1455.0+2232, hosting a pair of cold fronts, and their central core-halo radio source. We discuss the possibility that the origin of this diffuse radio emission may be strictly connected with the gas sloshing mechanism suggested to explain the formation of cold fronts in non-major-merging clusters. We show that the radiative lifetime of the relativistic electrons is much shorter than the timescale on which they can be transported from the central galaxy up to the radius of the outermost cold front. This strongly indicates that the observed diffuse radio emission is likely produced by electrons reaccelerated via some kind of turbulence generated within the cluster volume limited by the cold fronts during the gas sloshing.

From the Sloan Digital Sky Survey (SDSS) Data Release 5 (DR5), we extract a sample of 4594 galaxies at redshifts 0.02 < z < 0.03, complete down to a stellar mass of M = 10¹⁰M_☉. We quantify their structure (Sérsic index), morphology (Sérsic index + "Bumpiness"), and local environment. We show that morphology and structure are intrinsically different galaxy properties, and we demonstrate that this is a physically relevant distinction by showing that these properties depend differently on galaxy mass and environment. Structure mainly depends on galaxy mass whereas morphology mainly depends on environment. This is driven by variations in star formation activity, as traced by color, which only weakly affects the structure of a galaxy but strongly affects its morphological appearance. The implication of our results is that the existence of the morphology-density relation is intrinsic and not just due to a combination of more fundamental, underlying relations. Our findings have consequences for high-redshift studies, which often use some measure of structure as a proxy for morphology. A direct comparison with local samples selected through visually classified morphologies may lead to biases in the inferred evolution of the morphological mix of the galaxy population, and misinterpretations in terms of how galaxy evolution depends on mass and environment.

We present observations of the nearby barred starburst galaxy M83 (NGC 5236), with the new Fabry-Perot interferometer GHαFaS mounted on the 4.2 m William Herschel Telescope on La Palma. The unprecedented high-resolution observations, of 16 pc FWHM⁻¹, of the Hα-emitting gas cover the central 2 kpc of the galaxy. The velocity field displays the dominant disk rotation with signatures of gas inflow from kpc scales down to the nuclear regions. At the inner inner Lindblad resonance radius of the main bar and centered at the dynamical center of the main galaxy disk, a nuclear (5.5 ± 0.9) × 10⁸M_☉ rapidly rotating disk with scale length of 60 ± 20 pc has formed. The nuclear starburst is found in the vicinity as well as inside this nuclear disk, and our observations confirm that gas spirals in from the outer parts to feed the nuclear starburst, giving rise to several star formation events at different epochs, within the central 100 pc radius of M83.

By assuming diffusive shock acceleration of the nonthermal particles in the shell, we model a time-dependent nonthermal particle and photon spectra for the radio-bright shell-type supernova remnant (SNR) IC 443 with radio, optical, and X-ray emission concentrated toward the rim, whereas the γ-rays detected by EGRET are located at the center of the SNR, and the very high energy (VHE) γ-rays detected by MAGIC are displaced to the south, in direct correlation with a molecular cloud (MC). In this model, the nonthermal photon emission consists of two components, one comes from the shell evolving in the interstellar medium (ISM) and the other from the shell interacting with an MC. Our results indicate that (1) the emission from radio to soft X-ray bands is dominated by synchrotron radiation in the shell evolving in the ISM and (2) the detected high-energy emission (>10 MeV) from the SNR is from the shell evolving in the MC; i.e., the high-energy photons with energies from 10 MeV to 0.1 TeV are dominated both by bremsstrahlung and by p-p interaction, and the VHE γ-rays are produced mainly via p-p interaction.

We observed the first known very high energy (VHE) γ-ray-emitting unidentified source, TeV J2032+4130, for 94 hr with the MAGIC telescope. The source was detected with a significance of 5.6 σ. The flux, position, and angular extension are compatible with the previous ones measured by the HEGRA telescope system 5 years ago. The integral flux amounts to (4.5 ± 0.3_stat ± 0.35_sys) × 10⁻¹³ photons cm⁻² s⁻¹ above 1 TeV. The source energy spectrum, obtained with the lowest energy threshold to date, is compatible with a single power law with a hard photon index of Γ = –2.0 ± 0.3_stat ± 0.2_sys.

The nonthermal 3.6 cm radio continuum emission from the young stars S1 and DoAr 21 in the core of Ophiuchus has been observed with the Very Long Baseline Array (VLBA) at 6 and 7 epochs, respectively, between June 2005 and August 2006. The typical separation between successive observations was 2-3 months. Thanks to the remarkably accurate astrometry delivered by the VLBA, the trajectory described by both stars on the plane of the sky could be traced very precisely, and modeled as the superposition of their trigonometric parallax and a uniform proper motion. The best fits yield distances to S1 and DoAr 21 of 116.9_−6.4^+7.2 and 121.9_−5.3^+5.8 pc, respectively. Combining these results, we estimate the mean distance to the Ophiuchus core to be 120.0_−4.2^+4.5 pc, a value consistent with several recent indirect determinations, but with a significantly improved accuracy of 4%. Both S1 and DoAr 21 happen to be members of tight binary systems, but our observations are not frequent enough to properly derive the corresponding orbital parameters. This could be done with additional data, however, and would result in a significantly improved accuracy on the distance determination.

APEX single-dish observations at submillimeter wavelengths toward a sample of massive star-forming regions reveal that C₂H is almost omnipresent toward all covered evolutionary stages from infrared dark clouds via high-mass protostellar objects to ultracompact H II regions. High-resolution data from the Submillimeter Array toward one hot-core-like high-mass protostellar object show a shell-like distribution of C₂H with a radius of ~9000 AU around the central submillimeter continuum peak position. These observed features are well reproduced by a 1D cloud model with power-law density and temperature distributions and a gas-grain chemical network. The reactive C₂H radical (ethynyl) is abundant from the onset of massive star formation, but later it is rapidly transformed to other molecules in the core center. In the outer cloud regions the abundance of C₂H remains high due to constant replenishment of elemental carbon from CO being dissociated by the interstellar UV photons. We suggest that C₂H may be a molecule well suited to study the initial conditions of massive star formation.

We present a strong case for a transiting hot Jupiter planet identified during a single-field transit survey toward the Lupus Galactic plane. The object, Lupus-TR-3b, transits a V = 17.4 K1 V host star every 3.91405 days. Spectroscopy and stellar colors indicate a host star with effective temperature 5000 ± 150 K, with a stellar mass and radius of 0.87 ± 0.04 M_☉ and 0.82 ± 0.05 R_☉, respectively. Limb-darkened transit fitting yields a companion radius of 0.89 ± 0.07 R_J and an orbital inclination of 88.3_−0.8^+1.3 deg. Magellan 6.5 m MIKE radial velocity measurements reveal a 2.4 σ K = 114 ± 25 m s⁻¹ sinusoidal variation in phase with the transit ephemeris. The resulting mass is 0.81 ± 0.18 M_J and density 1.4 ± 0.4 g cm⁻³. Y-band PANIC image deconvolution reveals a V ⩾ 21 red neighbor 0.4'' away which, although highly unlikely, we cannot conclusively rule out as a blended binary with current data. However, blend simulations show that only the most unusual binary system can reproduce our observations. This object is very likely a planet, detected from a highly efficient observational strategy. Lupus-TR-3b constitutes the faintest ground-based detection to date, and one of the lowest mass hot Jupiters known.

New solar wind data from the Voyager 1 and Voyager 2 spacecraft, together with the SOHO SWAN measurements of the direction in which neutral hydrogen enters into the inner heliosheath and neutral helium measurements provided by multiple observations, are expected to provide more reliable constraints on the ionization ratio of the local interstellar medium (LISM) and the direction and magnitude of the interstellar magnetic field (ISMF). In this Letter we use the currently most sophisticated numerical model of the heliospheric interface, which is based on an MHD treatment of the ion flow and kinetic modeling of neutral particles, to analyze an ISMF-induced asymmetry of the heliosphere in the presence of the interplanetary magnetic field and neutral particles. It is shown that secondary hydrogen atoms modify the LISM properties leading to its shock-free deceleration at the heliopause. We determine the deflection of hydrogen atoms from their original trajectory in the unperturbed LISM and show that it occurs not only in the plane defined by the ISMF and LISM velocity vectors, but also, to a lesser extent, perpendicular to this plane. We also consider the possibility of using 2-3 kHz radio emission data to further constrain the ISMF direction.

We analyze suprathermal ions and plasma wave spectra upstream of interplanetary shocks driven by coronal mass ejection events. In particular, we analyze the competition between two processes: (1) the upstream wave generation by suprathermal protons accelerated at the shock, and (2) the cascading of wave energy in the inertial range of solar wind turbulence. We derive the cascading timescale from the comparison of particle and turbulent wave spectra with theory and conclude that amplified solar wind turbulence upstream of interplanetary traveling shocks is better described by Iroshnikov-Kraichnan-type rather than Kolmogorov-type wave diffusion.

There is observational evidence that the elongation of an Earth-directed coronal mass ejection (CME) may indicate the orientation of the underlying erupting flux rope. In this study, we compare orientations of CMEs, magnetic clouds (MCs), EIT (EUV Imaging Telescope) posteruption arcades, and the coronal neutral line (CNL). We report on good correlations between (1) the directions of the axial field in the EIT arcades and the elongations of halo CMEs, and (2) the tilt of the CNL and MC axis orientations. We found that majority of the eruptions that had EIT arcades, CMEs, and MCs similarly oriented also had the CNL co-aligned with them. On the other hand, those events that showed no agreement between orientations of the EIT arcades, CMEs, and MCs had their MCs aligned with the CNL. We speculate that the axis of the ejecta may be rotated in such a way that it is locally aligns itself with the heliospheric current sheet.

Two methods are used to observationally determine the solar radius: One is the observation of the intensity profile at the limb; the other one uses f-mode frequencies to derive a "seismic" solar radius which is then corrected to optical depth unity. The two methods are inconsistent and lead to a difference in the solar radius of ~0.3 Mm. Because of the geometrical extension of the solar photosphere and the increased path lengths of tangential rays the Sun appears to be larger to an observer who measures the extent of the solar disk. Based on radiative transfer calculations we show that this discrepancy can be explained by the difference between the height at disk center where τ₅₀₀₀ = 1 (τ_Ross = 2/3) and the inflection point of the intensity profile on the limb. We calculate the intensity profile of the limb for the MDI continuum and the continuum at 5000 Å for two atmosphere structures and compare the position of the inflection points with the radius at τ₅₀₀₀ = 1 (τ_Ross = 2/3). The calculated difference between the seismic radius and the inflection point is 0.347 ± 0.006 Mm with respect to τ₅₀₀₀ = 1, and 0.333 ± 0.008 Mm with respect to τ_Ross = 2/3. We conclude that the standard solar radius in evolutionary models has to be lowered by 0.333 ± 0.008 Mm and is 695.66 Mm. Furthermore, this correction reconciles inflection point measurements and the seismic radii within the uncertainties.