Table of contents

The fragmentation process of primordial-gas cores during prestellar collapse is studied using three-dimensional nested-grid hydrodynamics. Starting from the initial central number density of n_c ∼ 10³ cm ⁻³, we follow the evolution of rotating spherical cores up to the stellar density n_c≃ 10²² cm ⁻³. An initial condition of the cores is specified by three parameters: the ratios of the rotation and thermal energies to the gravitational energy (β₀ and α₀, respectively), and the amplitude of the bar-mode density perturbation (A_ϕ). Cores with rotation β₀ > 10⁻⁶ are found to fragment during the collapse. The fragmentation condition hardly depends on either the initial thermal energy α₀ or amplitude of bar-mode perturbation A_ϕ. Since the critical rotation parameter for fragmentation is lower than that expected in first-star formation, binaries or multiples are also common for the first stars.

We study the nature of faint red-selected galaxies at z ∼ 2-3 using the Hubble Ultra Deep Field (HUDF) and Spitzer Infrared Array Camera (IRAC) photometry. Given the magnitude limit of the HST data, we detect candidate galaxies to H_AB < 26 mag, probing lower luminosity (lower mass) galaxies at these redshifts. We identify 32 galaxies satisfying the (J₁₁₀ − H₁₆₀)_AB > 1.0 mag color selection, 16 of which have unblended [3.6 μm] and [4.5 μm] photometry from Spitzer. Using this multiwavelength data set, we derive photometric redshifts, masses, and stellar population parameters for these objects. We find that the selected objects span a diverse range of properties over a large range of redshifts, 1≲ z≲ 3.5. A substantial fraction (11/32) of the (J₁₁₀ − H₁₆₀)_AB > 1.0 mag population appear to be lower redshift (z≲ 2.5), heavily obscured dusty galaxies or edge-on spiral galaxies, while others (12/32) appear to be galaxies at 2≲ z≲ 3.5 whose light at rest-frame optical wavelengths is dominated by evolved stellar populations. We argue that longer wavelength data (≳1 μm, rest frame) are essential for interpreting the properties of the stellar populations in red-selected galaxies at these redshifts. Interestingly, by including Spitzer data, many candidates for galaxies dominated by evolved stellar populations are rejected, and for only a subset of the sample (6/16) do the data favor this interpretation. These objects have a surface density of ~1 arcmin⁻². We place an upper limit on the space density of candidate massive evolved galaxies with 2.5 < z < 3.5 and H_{AB 160} ⩽ 26 mag of n = 6.6^{+ 2.0}_−3.0 × 10⁻⁴ Mpc⁻³, with a corresponding upper limit on the stellar mass density of ρ^* = 5.6^{+ 4.4}_−2.8 × 10⁷M_☉ Mpc⁻³. The z > 2.5 objects that are dominated by evolved stellar populations have a space density at most one-third that of z ∼ 0 red early-type galaxies. Therefore, at least two-thirds of present-day early-type galaxies assemble or evolve into their current configuration at redshifts below 2.5. We find a dearth of candidates for low-mass (≲2 × 10¹⁰M_☉) galaxies at 1.5 < z≲ 3 that are dominated by passively evolving stellar populations even though the data should be sensitive to them; thus, at these redshifts, galaxies whose light is dominated by evolved stellar populations are restricted to only those galaxies that have assembled high stellar mass.

We present results from a pilot HST ACS deep imaging study in broadband V of five low-redshift QSO host galaxies classified in the literature as ellipticals. The aim of our study is to determine whether these early-type hosts formed at high redshift and have since evolved passively, or whether they have undergone relatively recent mergers that may be related to the triggering of the nuclear activity. We perform two-dimensional modeling of the light distributions to analyze the host galaxies' morphology. We find that, while each host galaxy is reasonably well fitted by a de Vaucouleurs profile, the majority of them (4/5) reveal significant fine structure such as shells and tidal tails. These structures contribute between ~5% and 10% to the total V-band luminosity of each host galaxy within a region of r ∼ 3r_eff and are indicative of merger events that occurred between a few hundred Myr and a Gyr ago. These timescales are comparable to starburst ages in the QSO hosts previously inferred from Keck spectroscopy. Our results thus support a consistent scenario in which most of the QSO host galaxies suffered mergers with accompanying starbursts that likely also triggered the QSO activity in some way, but we are also left with considerable uncertainty on physical mechanisms that might have delayed this triggering for several hundred Myr after the merger.

We select a sample of ~4200 traditionally defined broad absorption line quasars (BALQs) from the Fifth Data Release quasar catalog of the Sloan Digital Sky Survey. For a statistically homogeneous quasar sample with 1.7 ⩽ z⩽ 4.2, the BAL quasar fraction is ~14% and is almost constant with redshift. We measure the autocorrelation of non-BAL quasars (non-BALQs) and the cross-correlation of BALQs with non-BALQs using this statistically homogeneous sample, both in redshift space and using the projected correlation function. We find no significant difference between the clustering strengths of BALQs and non-BALQs. Assuming a power-law model for the real space correlation function ξ (r) = (r/r₀)^−1.8, the correlation length for non-BALQs is r₀ = 7.6 ± 0.8 h⁻¹ Mpc ; for BALQs the cross-correlation length is r₀ = 7.4 ± 1.1 h⁻¹ Mpc . Our clustering results suggest that BALQs live in similar large-scale environments to non-BALQs.

We have used archival Chandra and XMM-Newton observations of quasars hosting intrinsic narrow UV absorption lines (intrinsic NALs) to carry out an exploratory survey of their X-ray properties. Our sample consists of three intrinsic NAL quasars and one "mini-BAL" quasar, plus four quasars without intrinsic absorption lines for comparison. These were drawn in a systematic manner from an optical/UV-selected sample. The X-ray properties of intrinsic NAL quasars are indistinguishable from those of "normal" quasars. We do not find any excess absorption in quasars with intrinsic NALs, with upper limits of N_H≲ few × 10²² cm ⁻². We compare the X-ray and UV properties of our sample quasars by plotting the equivalent width and blueshift velocity of the intrinsic NALs and the X-ray spectral index against the "optical-to-X-ray" slope, α_ox. When BAL quasars and other AGNs with intrinsic NALs are included, the plots suggest that intrinsic NAL quasars form an extension of the BAL sequences and tend to bridge the gap between BAL and normal quasars. Observations of larger samples of intrinsic NAL quasars are needed to verify these conclusions. We also test two competing scenarios for the location of the NAL gas in an accretion disk wind. Our results strongly support a location of the NAL gas at high latitudes above the disk, closer to the disk axis than the dense BAL wind. We detect excess X-ray absorption only in Q0014+8118, which does not host intrinsic NALs. The absorbing medium very likely corresponds to an intervening system at z = 1.1, which also produces strong absorption lines in the rest-frame UV spectrum of this quasar. In an appendix we discuss the connection between UV and X-ray attenuation and its effect on α_ox.

We present the results of a 3 year monitoring campaign of the Seyfert 1 galaxy Markarian 509, using X-ray data from the Rossi X-Ray Timing Explorer and optical data taken by the SMARTS consortium. Both light curves show significant variations and are strongly correlated with the optical flux leading the X-ray flux by 15 days. The X-ray power spectrum shows a steep high-frequency slope of –2.0, breaking to a slope of –1.0 at a timescale of 34 days. The lag from optical to X-ray emission is most likely caused by variations in the accretion disk propagating inward.

In this paper, standard accretion disk models of active galactic nuclei (AGNs) are tested using light curves of 26 objects that have been well observed using reverberation mapping. Timescales of variations are estimated by the most common definition of the variability timescale and the zero-crossing time of the autocorrelation function of the optical light curves for each source. The timescales of variations measured by the two methods are consistent with each other. If the typical value of the viscosity parameter α ∼ 0.1 is adopted, the measured optical variability timescales are closest to the thermal timescales of the standard disks. If α is allowed to range from ~0.03 to ~0.2, the measured timescales are consistent with the thermal timescales of the standard disks. There is a linear relation between the measured variability timescales and black hole masses; this linear relation is qualitatively consistent with expectation of the standard accretion disk models. The time lags measured by the z-transformed discrete correlation function (ZDCF) between different bands are on the order of days. The measured time lags of NGC 4151 and NGC 7469 are marginally consistent with the time lags estimated in the case of continuum thermal reprocessing for the standard accretion disk models. However, the measured time lags of NGC 5548 and Fairall 9 are unlikely to be the case of continuum thermal reprocessing. Our results are unlikely to be inconsistent with, or are likely to be conditionally in favor of, the standard accretion disk models of AGNs.

We investigated the correlation between nuclear/circumnuclear starbursts around active galactic nuclei (AGNs) and the AGN activities for 43 Seyfert galaxies in the CfA and 12 μm samples. We found that the circumnuclear starburst luminosity, as well as the nuclear starburst luminosity, is positively correlated with the AGN luminosity. Moreover, the nuclear starburst luminosity is more strongly correlated with the AGN luminosity normalized with the AGN Eddington luminosity than is the circumnuclear starburst luminosity. This implies that starbursts nearer the AGN could have a greater effect on AGN mass accretion. We also discuss these results from the viewpoint of the radiation effects from starbursts and sequential starbursts.

We present a detailed analysis of week-long simultaneous observations of the blazar Mrk 421 at 2-60 keV X-rays (RXTE) and TeV γ-rays (Whipple and HEGRA) in 2001. Accompanying optical monitoring was performed with the Mt. Hopkins 48 inch telescope. The unprecedented quality of this data set enables us to establish the existence of the correlation between the TeV and X-ray luminosities, and also to start unveiling some of its characteristics, in particular its energy dependence and time variability. The source shows strong variations in both X-ray and γ-ray bands, which are highly correlated. No evidence of an X-ray/γ-ray interband lag τ is found on the full week data set, with τ ≲ 3 ks. A detailed analysis of the March 19 flare, however, reveals that data are not consistent with the peak of the outburst in the 2-4 keV X-ray and TeV band being simultaneous. We estimate a 2.1 ± 0.7 ks TeV lag. The amplitudes of the X-ray and γ-ray variations are also highly correlated, and the TeV luminosity increases more than linearly with respect to the X-ray one. The high degree of correlation lends further support to the standard model in which a unique electron population produces the X-rays by synchrotron radiation and the γ-ray component by inverse Compton scattering. However, the finding that for the individual best observed flares the γ-ray flux scales approximately quadratically with respect to the X-ray flux poses a serious challenge to emission models for TeV blazars, as it requires rather special conditions and/or fine tuning of the temporal evolution of the physical parameters of the emission region. We briefly discuss the astrophysical consequences of these new findings in the context of the competing models for the jet emission in blazars.

We report the discovery using Spitzer's high-resolution spectrograph of seven active galactic nuclei (AGNs) in a sample of 32 late-type galaxies that show no definitive signatures of AGNs in their optical spectra. These observations suggest that the AGN detection rate in late-type galaxies is possibly 4 times larger than what optical spectroscopic observations alone suggest. We demonstrate using photoionization models with an input AGN and an extreme UV-bright starburst ionizing radiation field that the observed mid-infrared line spectrum, which includes the high-ionization [Ne V] 14 μm and/or 24 μm lines, cannot be replicated unless an AGN contribution, in some cases as little as 10% of the total galaxy luminosity, is included. These models show that when the fraction of the total luminosity due to the AGN is low, optical diagnostics are insensitive to the presence of the AGN. In this regime of parameter space, the mid-infrared diagnostics offer a powerful tool for uncovering AGNs missed by optical spectroscopy. The AGN bolometric luminosities in our sample range from ~3 × 10⁴¹ to ~2 × 10⁴³ ergs s⁻¹, which, based on the Eddington limit, corresponds to a lower mass limit for the black hole that ranges from ~3 × 10³ to as high as ~1.5 × 10⁵M_☉. These lower mass limits, however, do not put a strain on the well-known relationship between the black hole mass and the host galaxy's stellar velocity dispersion established in predominantly early-type galaxies. Our findings add to the growing evidence that black holes do form and grow in low-bulge environments and that they are significantly more common than optical studies indicate.

The Spitzer Space Telescope has revealed a significant population of high-redshift (z ∼ 2) dust-obscured galaxies with large mid-infrared to ultraviolet luminosity ratios. Due to their optical faintness, these galaxies have been previously missed in traditional optical studies of the distant universe. We present a simple method for selecting this high-redshift population based solely on the ratio of the observed mid-infrared 24 μm to optical R-band flux density. We apply this method to observations of the ≈8.6 deg² NOAO Deep Wide-Field Survey Boötes field, and uncover ≈2600 dust-obscured galaxy candidates [i.e., 0.089 arcmin⁻²) with 24 μm flux densities F_{24 μ m} ⩾ 0.3 mJy and (R − [ 24]) ⩾ 14 (i.e., F_ν(24 μ m)/F_ν(R) ≳ 1000]. These galaxies have no counterparts in the local universe. They represent 7% ± 0.6% of the 24 μm source population at F_{24 μ m} ⩾ 1 mJy but increase to ≈13% ± 1% of the population at ≈0.3 mJy. These galaxies exhibit evidence of both star formation and AGN activity, with the brighter 24 μm sources being more AGN-dominated. We have measured spectroscopic redshifts for 86 of these galaxies, and find a broad redshift distribution centered at . The space density of this population is Σ_DOG(F_{24μ m} ⩾ 0.3 mJy) = (2.82 ± 0.05) × 10⁻⁵h³₇₀ Mpc ⁻³, similar to that of bright submillimeter-selected galaxies at comparable redshifts. These redshifts imply large luminosities, with median ν L_ν(8 μ m) ≈ 4 × 10¹¹L_☉. The infrared luminosity density contributed by this relatively rare dust-obscured galaxy population is log (IRLD) ≈ 8.23^{+ 0.18}_−0.30. This is ≈60^{+ 40}₋₁₅% of that contributed by z ∼ 2 ultraluminous infrared galaxies (ULIRGs, with L_IR > 10¹²L_☉); our simple selection thus identifies a significant fraction of z ∼ 2 ULIRGs. This IRLD is ≈26% ± 14% of the total contributed by all z ∼ 2 galaxies. We suggest that these dust-obscured galaxies are the progenitors of luminous (~4L*) present-day galaxies, seen undergoing an extremely luminous, short-lived phase of both bulge and black hole growth. They may represent a brief evolutionary phase between submillimeter-selected galaxies and less obscured quasars or galaxies.

We present mid-infrared spectra of 32 high-redshift ultraluminous infrared galaxies, selected via the stellar photospheric feature at rest-frame 1.6 μm, and an observed-frame 24 μm flux of >500 μJy. Nearly all the sample reside in a redshift range of ⟨ z⟩ = 1.71 ± 0.15 and have rest-frame 1-1000 μm luminosities of 10^12.9-10^13.8L_☉. Most of the spectra exhibit prominent polycyclic aromatic hydrocarbon emission features and weak silicate absorption, consistent with a starburst origin for the IR emission. Our selection method appears to be a straightforward and efficient way of finding distant, IR-luminous, star-forming galaxies in narrow redshift ranges. There is, however, evidence that the mid-IR spectra of our sample differ systematically from those of local ULIRGs; our sample have comparable PAH equivalent widths but weaker apparent silicate absorption, and (possibly) enhanced PAH 6.2 μm/7.7 μm and 6.2 μm/11.2 μm flux ratios. Furthermore, the composite mid-IR spectrum of our sample is almost identical to that of local starbursts with IR luminosities of 10¹⁰-10¹¹L_☉, rather than that of local ULIRGs. These differences are consistent with a reduced dust column, which can plausibly be obtained via some combination of (1) star formation that is extended over spatial scales of 1-4 kpc and (2) star formation in unusually gas-rich regions.

We use the photometric information contained in individual pixels of 44,964 (0.019 < z < 0.125 and –23.5 < M_r < − 20.5) galaxies in the Fourth Data Release (DR4) of the Sloan Digital Sky Survey to investigate the effects of environment on galaxy star formation (SF). We use the pixel-z technique, which combines stellar population synthesis models with photometric redshift template fitting on the scale of individual pixels in galaxy images. Spectral energy distributions are constructed, sampling a wide range of properties such as age, star formation rate (SFR), dust obscuration, and metallicity. By summing the SFRs in the pixels, we demonstrate that the distribution of total galaxy SFR shifts to lower values as the local density of surrounding galaxies increases, as found in other studies. The effect is most prominent in the galaxies with the highest SF, and we see the break in the SFR-density relation at a local galaxy density of ≈0.05 (Mpc h⁻¹)⁻³. Since our method allows us to spatially resolve the SF distribution within galaxies, we can calculate the mean SFR of each galaxy as a function of radius. We find that on average the mean SFR is dominated by SF in the central regions of galaxies, and that the trend for suppression of SFR in high-density environments is driven by a reduction in this nuclear SF. We also find that the mean SFR in the outskirts is largely independent of environmental effects. This trend in the mean SFR is shared by galaxies which are highly star forming, while those which are weakly star forming show no statistically significant correlation between their environment and the mean SFR at any radius.

OH megamasers (OHMs) emit primarily in the main lines at 1667 and 1665 MHz and differ from their Galactic counterparts due to their immense luminosities, large line widths, and 1667/1665 MHz flux ratios, which are always greater than 1. We find that these maser properties result from strong 53 μm radiative pumping combined with line overlap effects caused by turbulent line widths ~20 km s⁻¹; pumping calculations that do not include line overlap are unreliable. A minimum dust temperature of ~45 K is needed for inversion, and maximum maser efficiency occurs for dust temperatures ~80-140 K. We find that warmer dust can support inversion at lower IR luminosities, in agreement with observations. Our results are in good agreement with a clumpy model of OHMs, with clouds sizes ≲1 pc and OH column densities ~5 × 10¹⁶ cm⁻², that is able to explain both the diffuse and compact emission observed for OHMs. We suggest that all OH main-line masers may be pumped by far-IR radiation, with the major differences between OHMs and Galactic OH masers caused by differences in line width produced by line overlap. Small Galactic maser line widths tend to produce stronger 1665 MHz emission. The large OHM line widths lead to inverted ground-state transitions having approximately the same excitation temperature, producing 1667/1665 MHz flux ratios greater than 1 and weak satellite line emission. Finally, the small observed ratio of pumping radiation to dense molecular gas, as traced by HCN and HCO⁺, is a possible reason for the lack of OH megamaser emission in NGC 6240.

High-resolution X-ray observations have revealed cavities and "cold fronts" with sharp edges in temperature and density within galaxy clusters. Their presence poses a puzzle, since these features are not expected to be hydrodynamically stable or to remain sharp in the presence of diffusion. However, a moving core or bubble in even a very weakly magnetized plasma necessarily sweeps up enough magnetic field to build up a dynamically important sheath; the layer's strength is set by a competition between "plowing up" and slipping around of field lines, and depends primarily on the ram pressure seen by the moving object. In this inherently three-dimensional problem, our analytic arguments and numerical experiments show that this layer modifies the dynamics of a plunging core, greatly modifying the hydrodynamic instabilities and mixing, changing the geometry of stripped material, and slowing the core through magnetic tension. We derive an expression for the maximum magnetic field strength and thickness of the layer, as well as for the opening angle of the magnetic wake. The morphology of the magnetic draping layer implies the suppression of thermal conduction across the layer, thus conserving strong temperature gradients. The intermittent amplification of the magnetic field as well as the injection of magnetohydrodynamic turbulence in the wake of the core is identified to be due to vorticity generation within the magnetic draping layer. These results have important consequences for understanding the complex gas-dynamical processes of the intracluster medium and apply quite generally to motions through other magnetized environments, e.g., the interstellar medium.

We determine the age of 1104 early-type galaxies in eight rich clusters (z = 0.0046 to 0.175) using a new continuum color technique. We find that galaxies in clusters divide into two populations, an old population with a mean age similar to the age of the universe (12 Gyr) and a younger population with a mean age of 9 Gyr. The older population follows the expected relations for mass and metallicity that imply a classic monolithic collapse origin. Although total galaxy metallicity is correlated with galaxy mass, it is uncorrelated with age. It is impossible, with the current data, to distinguish between a later epoch of star formation, longer duration of star formation, or late bursts of star formation to explain the difference between the old and young populations. However, the global properties of this younger population are correlated with cluster environmental factors, which implies secondary processes, postformation epoch, operate on the internal stellar population of a significant fraction of cluster galaxies. In addition, the mean age of the oldest galaxies in a cluster are correlated with cluster velocity dispersion, implying that galaxy formation in massive clusters begins at earlier epochs than less massive clusters.

The Bekenstein-Milgrom gravity theory with a modified Poisson equation is tested here for the existence of triaxial equilibrium solutions. Using the nonnegative least-square method, we show that self-consistent triaxial galaxies exist for baryonic models with a mild density cusp, ρ ∼ Σ/r. Self-consistency is achieved for a wide range of central concentrations, Σ ∼ 10-1000 M_☉ pc⁻², representing low to high surface brightness galaxies. Our results demonstrate for the first time that the orbit superposition technique is fruitful for constructing galaxy models beyond Newtonian gravity, and triaxial cuspy galaxies might exist without the help of cold dark matter.

From the main galaxy sample of the Sloan Digital Sky Survey Data Release 5, we construct samples of paired and isolated galaxies to investigate the effects of interactions on galaxy properties. To account for the radial selection effect in the main galaxy sample, we divide the whole redshift region into 18 bins of width Δz = 0.01 and perform a comparative study of statistical properties between paired and isolated galaxies in each redshift bin. We find that the early-type fraction of paired galaxies is far higher than that of isolated galaxies, which supports the hypothesis that interactions are the major morphological driver. We also explore the dependence of paired galaxies' properties on the three-dimensional separation between pair members and find an obvious trend in which the early-type fraction of paired galaxies depends on the separation: closer pairs have a higher early-type fraction. This could result from the higher proportion of interactions and mergers in close pairs than in pairs with greater three-dimensional separation.

We report the discovery of two concentric Einstein rings around the gravitational lens SDSS J0946+1006. The main lens is at redshift z_l = 0.222, while the inner ring (1) is at redshift z_s1 = 0.609 (R_{Ein 1} = 1.43'' ± 0.01''). The wider image separation (R_{Ein 2} = 2.07''± 0.02'') of the outer ring (2) implies a higher redshift than that of ring 1; the detection of ring 2 in the F814W ACS filter implies an upper limit of z_s2≲ 6.9. The main lens can be described by a power-law total mass density profile ρ_tot ∝ r^−γ' with γ' = 2.00 ± 0.03 and velocity dispersion σ_SIE = 287 ± 5 km s⁻¹ (the stellar velocity dispersion is σ_v,* = 284 ± 24 km s⁻¹). The strong lensing configuration is inconsistent with light traces mass. Adopting a prior on the stellar mass-to-light ratio from previous SLACS work, we infer a 73% ± 9% dark matter fraction within the cylinder of radius equal to the effective radius of the lens. We find that, for the case of SDSS J0946+1006, the geometry of the two rings does not place interesting constraints on cosmography because of the suboptimal redshifts of lens and sources. We then consider the perturbing effect of the mass associated with ring 1 building a compound lens model. This introduces minor changes to the mass of the main lens and provides an estimate of z_s2 = 3.1^{+ 2.0}_−1.0 and of the mass of the source responsible for ring 1 (σ_{SIE ,s1} = 94^{+ 27}₋₄₇ km s⁻¹). We conclude by examining the prospects of doing cosmography with a sample of 50 double rings, expected from future space-based surveys. Accounting for uncertainties in the mass profile of the lens and the effects of the perturber, we find that such a sample would constrain Ω_m and w within 10%, assuming flatness.

We report the discovery of a light echo (LE) from the Type Ia supernova (SN) 2006X in the nearby galaxy M100. The presence of the LE is supported by analysis of both the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) images and the Keck optical spectrum that we obtained at ~300 days after maximum brightness. In the image procedure, both the radial-profile analysis and the point-spread function (PSF) subtraction method resolve significant excess emission at 2-5 ACS pixels (~0.05''-0.13'') from the center. In particular, the PSF-subtracted ACS images distinctly appear to have an extended, ringlike echo. Due to limitations of the image resolution, we cannot confirm any structure or flux within 2 ACS pixels from the SN. The late-time spectrum of SN 2006X can be reasonably fit with two components: a nebular spectrum of a normal SN Ia and a synthetic LE spectrum. Both image and spectral analysis show a rather blue color for the emission of the LE, suggestive of a small average grain size for the scattering dust. Using the Cepheid distance to M100 of 15.2 Mpc, we find that the dust illuminated by the resolved LE is ~27-170 pc from the SN. The echo inferred from the nebular spectrum appears to be more luminous than that resolved in the images (at the ~2 σ level), perhaps suggesting the presence of an inner echo at <2 ACS pixels (~0.05''). It is not clear, however, whether this possible local echo was produced by a distinct dust component (i.e., the local circumstellar dust) or by a continuous, larger distribution of dust as with the outer component. Nevertheless, our detection of a significant echo in SN 2006X confirms that this supernova was produced in a dusty environment having small dust particles.

We have used multiband high-resolution HST WFPC2 and ACS observations combined with wide-field ground-based observations to study the blue straggler star (BSS) population in the Galactic globular cluster NGC 6388. As in several other clusters we have studied, the BSS distribution is found to be bimodal: highly peaked in the cluster center, rapidly decreasing at intermediate radii, and rising again at larger radii. In other clusters the sparsely populated intermediate-radius region (or "zone of avoidance") corresponds well to that part of the cluster where dynamical friction would have caused the more massive BSSs or their binary progenitors to settle to the cluster center. Instead, in NGC 6388, BSSs still populate a region that should have been cleaned out by dynamical friction effects, thus suggesting that dynamical friction is somehow less efficient than expected. As a by-product of these observations, the peculiar morphology of the horizontal branch (HB) is also confirmed. In particular, within the (very extended) blue portion of the HB we are able to clearly characterize three subpopulations: ordinary blue HB stars, extreme HB stars, and blue hook stars. Each of these populations has a radial distribution which is indistinguishable from normal cluster stars.

Despite the efforts of the past decade, the origin of the bimodal horizontal branch (HB) found in some globular clusters (GCs) remains a conundrum. Inspired by the discovery of multiple stellar populations in the most massive Galactic GC, ω Centauri, we investigate the possibility that two distinct populations may coexist and are responsible for the bimodal HBs in the third and fifth brightest GCs, NGC 6388 and NGC 6441. Using the population synthesis technique, we examine two different chemical "self-enrichment" hypotheses in which a primordial GC was sufficiently massive to contain two or more distinct populations as suggested by the populations found in ω Cen: (1) the age-metallicity relation scenario in which two populations with different metallicity and age coexist, following an internal age-metallicity relation, and (2) the super-helium-rich scenario in which GCs contain a certain fraction of helium-enhanced stars, for instance, the second-generation stars formed from the helium-enriched ejecta of the first. The comparative study indicates that the detailed color-magnitude diagram morphologies and the properties of the RR Lyrae variables in NGC 6388 and NGC 6441 support the latter scenario: i.e., the model which assumes a minor fraction (~15%) of helium excess (Y≃ 0.3) stars. The results suggest that helium content is the main driver behind the HB bimodality found most often in massive GCs. If confirmed, the GC-to-GC variation of helium abundance should be considered a local effect, further supporting the argument that age is the global second parameter of HB morphology.

On the basis of optical, IR, and X-ray studies of Cas A, we propose a geometry for the remnant based on a "jet-induced" scenario with significant systematic departures from axial symmetry. In this model, the main jet axis is oriented in the direction of strong blueshifted motion at an angle of 110°-120° east of north and about 40°-50° to the east of the line of sight. Normal to this axis would be an expanding torus as predicted by jet-induced models. In the proposed geometry, iron-peak elements in the main jetlike flow could appear "beyond" the portions of the remnant rich in silicon by projection effects, not the effect of mixing. In the context of the proposed geometry, the displacement of the compact object from the kinematic center of the remnant at a position angle of ~169° can be accommodated if the motion of the compact object is near to, but slightly off from, the direction of the main "jet" axis by of order 30°. In this model, the classical NE "jet," the SW "counterjet," and other protrusions, particularly the "hole" in the north, are nonaxisymmetric flows approximately in the equatorial plane, e.g., out through the perimeter of the expanding torus, rather than being associated with the main jet. We explore the spokelike flow in the equatorial plane in terms of Rayleigh-Taylor, Richtmyer-Meshkov, and Kelvin-Helmholz instabilities and illustrate these instabilities with a jet-induced simulation.

Emission-line abundances have been uncertain for more than a decade due to unexplained discrepancies in the relative intensities of the forbidden lines and weak permitted recombination lines in PNe and H II regions. The observed intensities of forbidden and recombination lines originating from the same parent ion differ from their theoretical values by factors of more than an order of magnitude in some of these nebulae. In this study we observe UV resonance line absorption in the central stars of PNe produced by the nebular gas and from the same ions that emit optical forbidden lines. We then compare the derived absorption column densities with the emission measures determined from ground-based observations of the nebular forbidden lines. We find for our sample of PNe that the collisionally excited forbidden lines yield column densities that are in basic agreement with the column densities derived for the same ions from the UV absorption lines. A similar comparison involving recombination line column densities produces poorer agreement, although near the limits of the formal uncertainties of the analyses. An additional sample of objects with larger abundance discrepancy factors will need to be studied before a stronger statement can be made that recombination line abundances are not correct.

We carried out high spatial resolution N-band imaging and spectroscopic observations of the planetary nebula BD +30 3639 with the Cooled Mid-Infrared Camera and Spectrometer (COMICS) mounted on the Subaru telescope. Mid-infrared images reveal a rectangle shell structure of the nebula extending over 4'', which is supposed to be formed by the superwind from the central star. We have detected [Ne II] 12.8 μm and the unidentified infrared (UIR) bands at 8.6 and 11.2 μm, together with the broad plateau emission in 11-13 μm. In addition, the spectra clearly indicate the presence of silicate absorption around 10 μm for the first time, which increases sharply at the outside of the shell. This can be attributed to silicate grains that were formed in the O-rich phase of the central star, which is now C-rich. The present data indicate that these silicate grains are located in a very thin shell, suggesting that BD +30 3639 underwent a short period (~100 yr) of a high mass-loss rate (~3 × 10⁻³M_☉ yr⁻¹) at the last epoch of the O-rich phase. N-band spectra show that the distribution of [Ne II] is not clearly different from that of the UIR band and suggest that the UIR band carriers coexist with the ionized gas to some extent. This may be attributed to a slow destruction of the band carriers in the ionized region of the high electron density. The 10 μm continuum and the 11-13 μm plateau emission are dominant in the shell region, while the UIR 11.2 μm band becomes stronger in the outside of the shell, suggesting a change in the dust composition or the dust size distribution between the shell and the outside of the shell region. On the contrary, the peak wavelength of the UIR 11.2 μm band agrees with the class B spectrum, which is often seen in planetary nebulae, and does not change between the shell and the outside of the shell region. These facts suggest that the UIR band carriers are originally formed in the carbon-rich stellar wind and partially destroyed in the shell by shocks. The change of the UIR band spectrum from class B to A, which is generally seen in H II regions, should take place outside of the nebula.

The negative molecular ion C₃N⁻ has been detected at millimeter wavelengths in a low-pressure laboratory discharge, and then with frequencies derived from the laboratory data in the molecular envelope of IRC+10216. Spectroscopic constants derived from laboratory measurements of 12 transitions between 97 and 378 GHz allow the rotational spectrum to be calculated well into the submillimeter-wave band to 0.03 km s⁻¹ or better in equivalent radial velocity. Four transitions of C₃N⁻ were detected in IRC+10216 with the IRAM 30 m telescope at precisely the frequencies calculated from the laboratory measurements. The column density of C₃N⁻ is 0.5% that of C₃N, or approximately 20 times greater than that of C₄H⁻ relative to C₄H. The C₃N⁻ abundance in IRC+10216 is compared with a chemical model calculation by Petrie & Herbst. An upper limit in TMC-1 for C₃N⁻ relative to C₃N (<0.8%) and a limit for C₄H⁻ relative to C₄H (<0.004%) that is 5 times lower than that found in IRC+10216, were obtained from observations with the NRAO 100 m Green Bank Telescope (GBT). The fairly high concentration of C₃N⁻ achieved in the laboratory implies that other molecular anions containing the CN group may be within reach.

We used the Submillimeter Array to map the angular distribution of the H30α recombination line (231.9 GHz) in the circumstellar region of the peculiar star MWC 349A. The resolution was 1.2'', but because of high signal-to-noise ratio we measured the positions of all maser components to accuracies better than 0.01'', at a velocity resolution of 1 km s⁻¹. The two strongest maser components (called high-velocity components) at velocities near –14 and 32 km s⁻¹ are separated by 0.048'' ± 0.001'' (60 AU) along a position angle of 102° ± 1°. The distribution of maser emission at velocities between and beyond these two strongest components were also provided. The continuum emission lies at the center of the maser distribution to within 10 mas. The masers appear to trace a nearly edge-on rotating disk structure, reminiscent of the water masers in Keplerian rotation in the nuclear accretion disk of the galaxy NGC 4258. However, the maser components in MWC 349A do not follow a simple Keplerian kinematic prescription with v ∼ r^−1/2, but have a larger power-law index. We explore the possibility that the high-velocity masers trace spiral density or shock waves. We also emphasize caution in the interpretation of relative centroid maser positions where the maser is not clearly resolved in position or velocity, and we present simulations that illustrate the range of applicability of the centroiding method.

We study the turbulent velocity dispersion spectra of the coexistent HCN and HCO⁺ molecular species as a function of length scale in the M17 star-forming molecular cloud. We show that the observed downward shift of the ion's spectrum relative to that of the neutral is readily explained by the existence of an ambipolar diffusion range within which ion and neutral turbulent energies dissipate differently. We use these observations to evaluate this decoupling scale and show how to estimate the strength of the plane-of-the-sky component of the embedded magnetic field in a completely novel way.

Studies of gamma-ray burst (GRB) host galaxies are crucial to understanding GRBs. However, since they are identified by the superposition in the plane of the sky of a GRB afterglow and a galaxy, there is always a possibility that an association represents a chance alignment, rather than a physical connection. We examine a uniform sample of 72 GRB fields to explore the probability of chance superpositions. There is typically a ~1% chance that an optical afterglow will coincide with a galaxy by chance. While spurious host galaxy detections will, therefore, be rare, the possibility must be considered when examining individual GRB/host galaxy examples. It is also tempting to use the large and uniform collection of X-ray afterglow positions to search for GRB-associated galaxies. However, we find that approximately half of the 14 superpositions in our sample are likely to occur by chance, so in the case of GRBs localized only by an X-ray afterglow, even statistical studies are suspect.

We present the spectral analysis of duration-integrated broadband spectra (in ~30 keV–200 MeV) of 15 bright BATSE gamma-ray bursts (GRBs). Some GRB spectra are very hard, with their spectral peak energies being above the BATSE LAD passband limit of ~2 MeV. In such cases, their high-energy spectral parameters (peak energy and high-energy power-law indices) cannot be adequately constrained by BATSE LAD data alone. A few dozen bright BATSE GRBs were also observed with EGRET's calorimeter, TASC, in multi-MeV energy band, with a large effective area and fine energy resolution. Combining the BATSE and TASC data, therefore, affords spectra that span four decades of energy (30-200 MeV), allowing for a broadband spectral analysis with good statistics. Studying such broadband high-energy spectra of GRB prompt emission is crucial, as they provide key clues to understanding its gamma-ray emission mechanism. Among the 15 GRB spectra, we found two cases with a significant high-energy excess, and another case with a extremely high peak energy (E_peak≳ 170 MeV). There have been very limited number of GRBs observed at MeV energies and above, and only a few instruments have been capable of observing GRBs in this energy band with such high sensitivity. Thus, our analysis results presented here should also help predict GRB observations with current and future high-energy instruments such as AGILE and GLAST, as well as with ground-based very-high-energy telescopes.

Massive black hole binary coalescences are prime targets for space-based gravitational wave (GW) observatories such as LISA. GW measurements can localize the position of a coalescing binary on the sky to an ellipse with a major axis of a few tens of arcminutes to a few degrees, depending on source redshift, and a minor axis which is 2-4 times smaller. Neglecting weak gravitational lensing, the GWs would also determine the source's luminosity distance to better than percent accuracy for close sources, degrading to several percent for more distant sources. Weak lensing cannot, in fact, be neglected and is expected to limit the accuracy with which distances can be fixed to errors no less than a few percent. Assuming a well-measured cosmology, the source's redshift could be inferred with similar accuracy. GWs alone can thus pinpoint a binary to a three-dimensional "pixel" which can help guide searches for the hosts of these events. We examine the time evolution of this pixel, studying it at merger and at several intervals before merger. One day before merger, the major axis of the error ellipse is typically larger than its final value by a factor of ~1.5-6. The minor axis is larger by a factor of ~2-9, and, neglecting lensing, the error in the luminosity distance is larger by a factor of ~1.5-7. This large change over a short period of time is due to spin-induced precession, which is strongest in the final days before merger. The evolution is slower as we go back further in time. For z = 1, we find that GWs will localize a coalescing binary to within ~10 deg² as early as a month prior to merger and determine distance (and hence redshift) to several percent.

We present a detailed analysis of archival Hubble Space Telescope data that we use to measure the proper motion of the Crab pulsar, with the primary goal of comparing the direction of its proper motion with the projected axis of its pulsar wind nebula (the projected spin axis of the pulsar). We demonstrate that our measurement, using 47 observations spanning >10 yr, is robust and has an uncertainty of only ±0.4 mas yr⁻¹ on each component of the proper motion. However, we then consider the various uncertainties that arise from the need to correct the proper motion that we measure to the local standard of rest at the position of the pulsar and find μ_α = − 11.8 ± 0.4 ± 0.5 mas yr ⁻¹ and μ_δ = + 4.4 ± 0.4 ± 0.5 mas yr ⁻¹ relative to the pulsar's standard of rest, where the two uncertainties are from the measurement and the reference frame, respectively. Comparing this proper motion to the symmetry axis of the pulsar wind nebula, we must consider the unknown velocity of the pulsar's progenitor (assumed to be ~10 km s⁻¹), and hence add an additional uncertainty of ±2 mas yr⁻¹ to each component of the proper motion. This implies a projected misalignment with the nebular axis of 14°± 2°± 9°, consistent with a broad range of values including perfect alignment. We use our proper motion to derive an independent estimate for the site of the supernova explosion with an accuracy that is 2-3 times better than previous estimates. We conclude that the precision of individual measurements which compare the direction of motion of a neutron star to a fixed axis will often be limited by fundamental uncertainties regarding reference frames and progenitor properties.

For a variety of fully relativistic polytropic neutron star models we calculate the star's tidal Love number k₂. Most realistic equations of state for neutron stars can be approximated as a polytrope with an effective index n ≈ 0.5–1.0. The equilibrium stellar model is obtained by numerical integration of the Tolman-Oppenheimer-Volkhov equations. We calculate the linear l = 2 static perturbations to the Schwarzschild spacetime following the method of Thorne and Campolattaro. Combining the perturbed Einstein equations into a single second-order differential equation for the perturbation to the metric coefficient g_tt and matching the exterior solution to the asymptotic expansion of the metric in the star's local asymptotic rest frame gives the Love number. Our results agree well with the Newtonian results in the weak field limit. The fully relativistic values differ from the Newtonian values by up to ~24%. The Love number is potentially measurable in gravitational wave signals from inspiralling binary neutron stars.

We show that a large-scale, weak magnetic field threading a turbulent accretion disk tends to be advected inward, contrary to previous suggestions that it will be stopped by outward diffusion. The efficient inward transport is a consequence of the diffuse, magnetically dominated surface layers of the disk, where the turbulence is suppressed and the conductivity is very high. This structure arises naturally in three-dimensional simulations of magnetorotationally unstable disks, and we demonstrate here that it can easily support inward advection and compression of a weak field. The advected field is anchored in the surface layer but penetrates the main body of the disk, where it can generate strong turbulence and produce values of α (i.e., the turbulent stress) that are large enough to match observational constraints; typical values of the vertical magnetic field merely need to reach a few percent of equipartition for this to occur. Overall, these results have important implications for models of jet formation that require strong, large-scale magnetic fields to exist over a region of the inner accretion disk.

We use four Chandra gratings spectra of the neutron star low-mass X-ray binary 4U 1820–30 to better understand the nature of certain X-ray absorption lines in X-ray binaries, including the Ne II, Ne III, Ne IX, O VII, and O VIII lines. The equivalent widths of the lines are generally consistent between the observations, as expected if these lines originate in the hot interstellar medium. No evidence was found that the lines were blueshifted, again supporting the interstellar medium origin, although this may be due to poor statistics. There is apparent variability in the O VIII Lyα line equivalent width providing some evidence that at least some of the O VIII absorption arises within the system. However, the significance is marginal (2.4 σ), and the lack of variation in the other lines casts some doubt on the reality of the variability. From calculating the equivalent hydrogen column densities for a range of Doppler parameters, we find they are consistent with the interstellar origin of the lines. In addition, we fit the spectra with photoionization models for locally absorbing material, and find that they can reproduce the spectrum well, but only when there is an extremely low filling factor. We conclude that both the ISM and local absorption remain possible for the origin of the lines, but that more sensitive observations are needed to search for low-level variability.

Chang and coworkers reported millisecond duration dips in the X-ray intensity of Sco X-1 and attributed them to occultations of the source by small trans-Neptunian objects (TNOs). We have found multiple lines of evidence that these dips are not astronomical in origin, but rather the result of high-energy charged particle events in the RXTE PCA detectors. Our analysis of the RXTE data indicates that at most 10% of the observed dips in Sco X-1 could be due to occultations by TNOs, and, furthermore, we find no positive or supporting evidence for any of them being due to TNOs. We therefore believe that it is a mistake to conclude that any TNOs have been detected via occultation of Sco X-1.

Motivated by the recently discovered class of faint (10³⁴-10³⁵ ergs s⁻¹) X-ray transients in the Galactic center region, we investigate the 2-10 keV properties of classical and recurrent novae. Existing data are consistent with the idea that all classical novae are transient X-ray sources with durations of months to years and peak luminosities in the 10³⁴-10³⁵ ergs s⁻¹ range. This makes classical novae a viable candidate class for the faint Galactic center transients. We estimate the rate of classical novae within a 15^' radius region centered on the Galactic center (roughly the field of view of XMM-Newton observations centered on Sgr A*) to be ~0.1 yr⁻¹. Therefore, it is plausible that some of the Galactic center transients that have been announced to date are unrecognized classical novae. The continuing monitoring of the Galactic center region carried out by Chandra and XMM-Newton may therefore provide a new method to detect classical novae in this crowded and obscured region, where optical surveys are not, and can never hope to be, effective. Therefore, X-ray monitoring may provide the best means of testing the completeness of the current understanding of the nova populations.

We report observations of the nova RS Ophiuchi (RS Oph) using the Keck Interferometer Nuller (KIN), approximately 3.8 days following the most recent outburst that occurred on 2006 February 12. These observations represent the first scientific results from the KIN, which operates in N band from 8 to 12.5 μm in a nulling mode. The nulling technique is the sparse aperture equivalent of the conventional coronagraphic technique used in filled aperture telescopes. In this mode the stellar light itself is suppressed by a destructive fringe, effectively enhancing the contrast of the circumstellar material located near the star. By fitting the unique KIN data, we have obtained an angular size of the mid-infrared continuum emitting material of 6.2, 4.0, or 5.4 mas for a disk profile, Gaussian profile (FWHM), and shell profile, respectively. The data show evidence of enhanced neutral atomic hydrogen emission and atomic metals including silicon located in the inner spatial regime near the white dwarf (WD) relative to the outer regime. There are also nebular emission lines and evidence of hot silicate dust in the outer spatial region, centered at ~17 AU from the WD, that are not found in the inner regime. Our evidence suggests that these features have been excited by the nova flash in the outer spatial regime before the blast wave reached these regions. These identifications support a model in which the dust appears to be present between outbursts and is not created during the outburst event. We further discuss the present results in terms of a unifying model of the system that includes an increase in density in the plane of the orbit of the two stars created by a spiral shock wave caused by the motion of the stars through the cool wind of the red giant star.

We report the serendipitous detection of a very bright, very nearby microlensing event. In late 2006 October, an otherwise unremarkable A0 star at a distance of ~1 kpc (GSC 3656–1328) brightened achromatically by a factor of nearly 40 over the span of several days and then decayed in an apparently symmetrical way. We present a light curve of the event based on optical photometry from the Center for Backyard Astrophysics and the All Sky Automated Survey, as well as near-infrared photometry from the Peters Automated Infrared Imaging Telescope. This light curve is well fit by a generic microlensing model. We also report optical spectra and Swift X-ray and UV observations that are consistent with the microlensing interpretation. We discuss and reject alternative explanations for this variability. The lens star is probably a low-mass star or brown dwarf, with a relatively high proper motion of ≳20 mas yr⁻¹, and may be visible using precise optical/infrared imaging taken several years from now. A modest, all-sky survey telescope could detect ~10 such events per year, which would enable searches for very low mass planetary companions to relatively nearby stars.

When calculating stellar initial mass functions (IMFs) for young clusters, one has to take into account that (1) most massive stars are born in multiple systems, (2) most IMFs are derived from data that cannot resolve such systems, and (3) multiple chance superpositions between members are expected to happen if the cluster is too distant. In this article I use numerical experiments to model the consequences of those phenomena on the observed color-magnitude diagrams and the IMFs derived from them. Real multiple systems affect the observed or apparent massive-star MF slope little but can create a significant population of apparently ultramassive stars. Chance superpositions produce only small biases when the number of superimposed stars is low but, once a certain number threshold is reached, they can affect both the observed slope and the apparent stellar upper mass limit. I apply these experiments to two well known massive young clusters in the Local Group, NGC 3603 and R136. In both cases I show that the observed population of stars with masses above 120 M_☉ can be explained by the effects of unresolved objects, mostly real multiple systems for NGC 3603 and a combination of real and chance-alignment multiple systems for R136. Therefore, the case for the reality of a stellar upper mass limit at solar or near-solar metallicities is strengthened, with a possible value even lower than 150 M_☉. An IMF slope somewhat flatter than Salpeter or Kroupa with γ between –1.6 and –2.0 is derived for the central region of NGC 3603, with a significant contribution to the uncertainty arising from the imprecise knowledge of the distance to the cluster. The IMF at the very center of R136 cannot be measured with the currently available data but the situation could change with new HST observations.

We study collisions between dust aggregates to construct a model of their structural evolution in protoplanetary disks. We carry out three-dimensional simulations of aggregate collisions and examine their compression and disruption processes following our previous two-dimensional simulations. We take clusters of ballistic cluster-cluster aggregation (BCCA) formed by a hit-and-stick process as initial structures and study their head-on collisions with the use of realistic binding forces. Our numerical results indicate that the energy criteria for compression and disruption of BCCA clusters are consistent with previous two-dimensional simulations. For aggregate compression at a collision, we succeed in obtaining a scaling law in which the gyration radius of the resultant aggregate is proportional to E^−0.10_imp, where E_imp is the impact energy. Furthermore, we derive an "equation of state" of aggregates which reproduces the scaling law for compression. The equation of state is useful for describing the density evolution of dust aggregates during their growth.

We study dust accumulation by photophoresis in optically thin gas disks. Using formulae for the photophoretic force that are applicable for the free molecular regime and for the slip-flow regime, we calculate dust accumulation distances as a function of particle size. It is found that photophoresis pushes particles (smaller than 10 cm) outward. For a Sun-like star, these particles are transported to 0.1-100 AU, depending on their size, and form an inner disk. Radiation pressure pushes small particles (≲1 mm) out further to form an extended outer disk. Consequently, an inner hole opens inside ~0.1 AU. The radius of the inner hole is determined by the condition that the mean free path of the gas molecules equal the maximum size of the particles that photophoresis effectively works on (100 μm-10 cm, depending on the dust properties). The dust disk structure formed by photophoresis can be distinguished from the structure of model gas-free dust disks, because the particle sizes in the outer disk are larger, and the inner hole radius depends on the gas density.

We present refined values for the physical parameters of transiting exoplanets, based on a self-consistent and uniform analysis of transit light curves and the observable properties of the host stars. Previously it has been difficult to interpret the ensemble properties of transiting exoplanets because of the widely different methodologies that have been applied in individual cases. Furthermore, previous studies often ignored an important constraint on the mean stellar density that can be derived directly from the light curve. The main contributions of this work are (1) a critical compilation and error assessment of all reported values for the effective temperature and metallicity of the host stars, (2) the application of a consistent methodology and treatment of errors in modeling the transit light curves, and (3) more accurate estimates of the stellar mass and radius based on stellar evolution models, incorporating the photometric constraint on the stellar density. We use our results to revisit some previously proposed patterns and correlations within the ensemble. We confirm the mass-period correlation and find evidence for a new pattern within the scatter about this correlation: planets around metal-poor stars are more massive than those around metal-rich stars at a given orbital period. Likewise, we confirm the proposed dichotomy of planets according to their Safronov number, and we find evidence that the systems with small Safronov numbers are more metal-rich on average. Finally, we confirm the trend that led to the suggestion that higher metallicity stars harbor planets with a greater heavy-element content.

The hot Jupiter HD 189733b was observed during its primary transit using the Infrared Array Camera on the Spitzer Space Telescope. The transit depths were measured simultaneously at 3.6 and 5.8 μm. Our analysis yields values of 2.356% ± 0.019% and 2.436% ± 0.020% at 3.6 and 5.8 μm, respectively, for a uniform source. We estimated the contribution of the limb-darkening and starspot effects on the final results. We concluded that although the limb darkening increases by ~0.02%-0.03% the transit depths, the differential effects between the two IRAC bands is even smaller, 0.01%. Furthermore, the host star is known to be an active spotted K star with observed photometric modulation. If we adopt an extreme model of 20% coverage with spots 1000 K cooler of the star surface, it will make the observed transits shallower by 0.19% and 0.18%. The difference between the two bands will be only of 0.01%, in the opposite direction to the limb-darkening correction. If the transit depth is affected by limb darkening and spots, the differential effects between the 3.6 and 5.8 μm bands are very small. The differential transit depths at 3.6 and 5.8 μm and the recent one published by Knutson and coworkers) at 8 μm are in agreement with the presence of water vapor in the upper atmosphere of the planet. This is the companion paper to Tinetti et al., where the detailed atmosphere models are presented.

The Parker or field line tangling model of coronal heating is studied comprehensively via long-time high-resolution simulations of the dynamics of a coronal loop in Cartesian geometry within the framework of reduced magnetohydrodynamics. Slow photospheric motions induce a Poynting flux which saturates by driving an anisotropic turbulent cascade dominated by magnetic energy. In physical space this corresponds to a magnetic topology where magnetic field lines are barely entangled; nevertheless, current sheets (corresponding to the original tangential discontinuities hypothesized by Parker) are continuously formed and dissipated. Current sheets are the result of the nonlinear cascade that transfers energy from the scale of convective motions (~1000 km) down to the dissipative scales, where it is finally converted to heat and/or particle acceleration. Current sheets constitute the dissipative structure of the system, and the associated magnetic reconnection gives rise to impulsive "bursty" heating events at the small scales. This picture is consistent with the slender loops observed by state-of-the-art (E)UV and X-ray imagers which, although apparently quiescent, shine brightly in these wavelengths with little evidence of entangled features. The different regimes of weak and strong magnetohydrodynamic turbulence that develop and their influence on coronal heating scalings are shown to depend on the loop parameters, and this dependence is quantitatively characterized: weak turbulence regimes and steeper spectra occur in stronger loop fields and lead to larger heating rates than in weak field regions.

The theoretical and experimental study of fast electron beams attracts much attention in astrophysics and the laboratory. In the case of solar flares, the problem of reliable beam detection and diagnostics is of exceptional importance. This paper explores the fact that electron beams moving obliquely to the magnetic field or along the field with some angular scatter around the beam's propagation direction can generate microwave continuum bursts through the gyrosynchrotron mechanism. The characteristics of the microwave bursts produced by beams differ from those in the case of isotropic or loss-cone distributions, which suggests a new quantitative diagnostic for beams in the solar corona. To demonstrate the potential of this tool, we analyze a radio burst that occurred during an impulsive class 1B/M6.7 flare on 2001 March 10 (NOAA AR 9368; N27°, W42°). Based on detailed analysis of the spectral, temporal, and spatial relationships, we obtain firm evidence that the microwave continuum burst was produced by electron beams. We develop and apply a new forward-fitting algorithm based on the exact gyrosynchrotron formulae and employing both total-power and polarization measurements to solve the inverse problem of the beam diagnostics. The burst is found to have been generated by an oblique beam in a region of reasonably strong magnetic field (~200-300 G) and observed at a quasi-transverse viewing angle. We find that the lifetime of the emitting electrons in the radio source was relatively short, τ_l ≈ 0.5 s, consistent with a single reflection of the electrons from a magnetic mirror at the footpoint with the stronger magnetic field. We discuss the implications of these findings for electron acceleration in flares and beam diagnostics.

Frontside halo coronal mass ejections (CMEs) are generally considered as potential candidates for producing geomagnetic storms, but there was no definite way to predict whether they will hit the Earth or not. Recently Moon et al. suggested that the degree of CME asymmetries, as defined by the ratio of the shortest to the longest distances of the CME front measured from the solar center, be used as a parameter for predicting their geoeffectiveness. They called this quantity a direction parameter, D, as it suggests how much CME propagation is directed to Earth, and examined its forecasting capability using 12 fast halo CMEs. In this paper, we extend this test by using a much larger database (486 frontside halo CMEs from 1997 to 2003) and more robust statistical tools (contingency table and statistical parameters). We compared the forecast capability of this direction parameter to those of other CME parameters, such as location and speed. We found the following results: (1) The CMEs with large direction parameters (D ⩾ 0.4) are highly associated with geomagnetic storms. (2) If the direction parameter increases from 0.4 to 1.0, the geoeffective probability rises from 52% to 84%. (3) All CMEs associated with strong geomagnetic storms (Dst ⩽ − 200 nT) are found to have large direction parameters (D ⩾ 0.6). (4) CMEs causing strong geomagnetic storms (Dst ⩽ − 100 nT), in spite of their northward magnetic field, have large direction parameters (D ⩾ 0.6). (5) Forecasting capability improves when statistical parameters (e.g., "probability of detection—yes" and "critical success index") are employed, in comparison with the forecast solely based on the location and speed of CMEs. These results indicate that the CME direction parameter can be an important indicator for forecasting CME geoeffectiveness.

We present the first in-depth statistical survey of all X-ray microflares observed by RHESSI between 2002 March and 2007 March, a total of 25,705 events, an order of magnitude larger then previous studies. These microflares were found using a new flare-finding algorithm designed to search the 6-12 keV count rate when RHESSI's full sensitivity was available in order to find the smallest events. The peak and total count rate are automatically obtained along with count spectra at the peak and the microflare centroid position. Our microflare magnitudes are below GOES C class, on average GOES A class (background subtracted). They are found to occur only in active regions, not in the "quiet" Sun, and are similar to large flares. The monthly average microflaring rate is found to vary with the solar cycle and ranges from 90 to 5 flares a day during active and quiet times, respectively. Most flares are found to be impulsive (74%), with rise times shorter than decay times. The mean flare duration is ~6 minutes with a 1 minute minimum set by the flare-finding algorithm. The frequency distributions of the peak count rate in the energy bands, 3-6, 6-12, and 12-25 keV, can be represented by power-law distributions with a negative power-law index of 1.50 ± 0.03, 1.51 ± 0.03, and 1.58 ± 0.02, respectively. We find that these power-law indices are constant as a function of time. The X-ray photon spectra for individual events can be approximated with a power-law spectrum [dJ/d(hν) ∼ (hν)^−γ]. Using the ratio of photon fluxes between 10-15 and 15-20 keV, we find 4 < γ < 12, with an average of 7.4. Based on these values, the nonthermal power is calculated. The microflare occurrence frequency varies with the rate of energy release consistent with a power law with an exponent of -1.7 ± 0.1. We estimate the total energy flux deposited in active regions by microflare-associated accelerated electrons (>10 keV) over the five years of observations to be, on average, below 10²⁶ erg s⁻¹.

Observations of transition region emission in solar active regions represent a powerful tool for determining the properties of hot coronal loops. We present the analysis of new observations of active region moss taken with the Extreme Ultraviolet Imaging Spectrometer (EIS) on the Hinode satellite. EIS observations of a density sensitive Fe XII line ratio suggest moss densities of approximately 10¹⁰ cm⁻³ and pressures of 3 × 10¹⁶ cm⁻³ K. We find that the moss intensities predicted by steady, uniformly heated loop models are too intense relative to the observations, consistent with previous work. To bring the steady heating model into agreement with the observations a filling factor is required. Our analysis indicates that the filling factor in the moss is nonuniform and varies inversely with the loop pressure. The intensities predicted by steady uniform heating are generally consistent with the EIS moss observations. There are, however, significant discrepancies for the coolest emission line available in the data we analyze.

We continue our studies of alignment of atoms by radiation in diffuse media and their realignment by the ambient magnetic field. We understand atomic alignment as the alignment of atoms or ions in their ground or metastable states that have more than two fine or hyperfine sublevels. In particular, we consider the alignment in interstellar and circumstellar media, with the goal of developing new diagnostics of magnetic fields in these environments. We provide predictions of the polarization that arises from astrophysically important aligned atoms (ions) with fine structure of the ground level, namely, O I, S II, and Ti II. Unlike our earlier papers which dealt with weak fields only, a part of our current paper is devoted to the studies of atomic alignment when magnetic fields get strong enough to affect the emission from the excited level. This is a regime of the Hanle effect, but modified by the atomic alignment of the atomic ground state. We also discuss the ground Hanle effect where the magnetic splitting is comparable to the pumping rate. Using an example of emission and absorption lines of the S II ion we demonstrate how polarimetric studies can probe magnetic fields in circumstellar regions and accretion disks. In addition, we show that atomic alignment induced by anisotropic radiation can induce substantial variations of magnetic dipole transitions within the ground state, thus affecting abundance studies based on this emission. Moreover, we show that the radio emission arising this way is polarized, which provides a new way to study magnetic fields, e.g., at the epoch of universe reionization.

We report numerical simulations and laboratory demonstrations of a four-quadrant phase mask (FQPM) coronagraph equipped with a Jacquinot pupil as a Lyot stop. We demonstrate that the Jacquinot-Lyot stop can effectively suppress the residual stellar intensity due to tip-tilt errors for unresolved stars. We also show that the achievable contrast with the Jacquinot-Lyot stop depends on the direction of the tip-tilt errors, which suggests that the contrast can be improved if the direction of these errors is monitored simultaneously with the data acquisition. Furthermore, we conduct laboratory experiments with a polarization differential imager in which the Jacquinot-Lyot stop is introduced into a four-quadrant polarization mask (FQPoM) coronagraph. In these experiments we also demonstrate two image processing techniques, a cross-correlation technique and a polarization-degree analysis, to extract planetary signal from residual stellar speckle noise.

It has long been known how to analytically relate the clustering properties of the collapsed structures (halos) to those of the underlying dark matter distribution for Gaussian initial conditions. Here we apply the same approach to physically motivated non-Gaussian models. The techniques we use were developed in the 1980s to deal with the clustering of peaks of non-Gaussian density fields. The description of the clustering of halos for non-Gaussian initial conditions has recently received renewed interest, motivated by the forthcoming large galaxy and cluster surveys. For inflationary-motivated non-Gaussianities, we find an analytic expression for the halo bias as a function of scale, mass, and redshift, employing only the approximations of high peaks and large separations.

We derive a new peak lag versus peak luminosity relation in gamma-ray burst (GRB) pulses. We demonstrate conclusively that GRB spectral lags are pulse rather than burst properties and show how the lag versus luminosity relation determined from CCF measurements of burst properties is essentially just a rough measure of this newly derived relation for individual pulses. We further show that most GRB pulses have correlated properties: short-lag pulses have shorter durations, are more luminous, and are harder within a burst than long-lag pulses. We also uncover a new pulse duration versus pulse peak luminosity relation, and indicate that long-lag pulses often precede short-lag pulses. Although most pulse behaviors are supportive of internal shocks (including long-lag pulses), we identify some pulse shapes that could result from external shocks.

Two long γ-ray bursts, GRB 060505 and GRB 060614, occurred in nearby galaxies at redshifts of 0.089 and 0.125, respectively. Due to their proximity and durations, deep follow-up campaigns to search for supernovae (SNe) were initiated. However, none were found in either case, to limits more than 2 orders of magnitude fainter than the prototypical GRB-associated SN, 1998bw. It was suggested that the bursts, in spite of their durations (~4 and 102 s), belonged to the population of short GRBs which has been shown to be unrelated to SNe. In the case of GRB 060614 this argument was based on a number of indicators, including the negligible spectral lag, which is consistent with that of short bursts. GRB 060505 has a shorter duration, but no spectral lag was measured. We present the spectral lag measurements of GRB 060505 using Suzaku's Wide Area Monitor and the Swift Burst Alert Telescope. We find that the lag is 0.36 ± 0.05 s, inconsistent with the lags of short bursts and consistent with the properties of long bursts and SN GRBs. These results support the association of GRB 060505 with other low-luminosity GRBs also found in star-forming galaxies and indicate that at least some massive stars may die without bright SNe.

We report the discovery of a compact supercluster structure at z = 0.9. The structure comprises three optically selected clusters, all of which are detected in X-rays and spectroscopically confirmed to lie at the same redshift. The Chandra X-ray temperatures imply individual masses of ~5 × 10¹⁴M_☉. The X-ray masses are consistent with those inferred from optical-X-ray scaling relations established at lower redshift. A strongly lensed z ∼ 4 Lyman break galaxy behind one of the clusters allows a strong-lensing mass to be estimated for this cluster, which is in good agreement with the X-ray measurement. Optical spectroscopy of this cluster gives a dynamical mass in good agreement with the other independent mass estimates. The three components of the RCS 2319+00 supercluster are separated from their nearest neighbor by a mere <3 Mpc in the plane of the sky and likely <10 Mpc along the line of sight, and we interpret this structure as the high-redshift antecedent of massive (~10¹⁵M_☉) z ∼ 0.5 clusters such as MS 0451.5–0305.

In this Letter, we present a new self-consistent theory for the production of the relativistic outflows observed from radio-loud black hole candidates and active galaxies as a result of particle acceleration in hot, viscous accretion disks containing standing, centrifugally supported isothermal shocks. This is the first work to obtain the structure of such disks for a relatively large value of the Shakura-Sunyaev viscosity parameter (α = 0.1), and to consider the implications of the shock for the acceleration of relativistic particles in viscous disks. In our approach, the hydrodynamics and the particle acceleration are coupled and the solutions are obtained self-consistently based on a rigorous mathematical method. We find that particle acceleration in the vicinity of the shock can provide enough energy to power the observed relativistic jet in M87.

We present preliminary results from a deep (600 ks) Chandra observation of the hot interstellar medium of the nearby early-type galaxy Centaurus A. We find a surface brightness discontinuity in the gas ~3.5 kpc from the nucleus spanning a 120° arc. The temperature of the gas is 0.60 ± 0.05 keV (0.68 ± 0.10 keV) interior (exterior) to the discontinuity. The elemental abundance is poorly constrained by the spectral fits, but if the abundance is constant across the discontinuity, there is a factor of 2.3 ± 0.4 pressure jump across the discontinuity. This would imply that the gas is moving at 470 ± 100 km s⁻¹, or Mach 1.0 ± 0.2 (1.2 ± 0.2) relative to the sound speed of the gas external (internal) to the discontinuity. Alternatively, pressure balance could be maintained if there is a large (factor of ~7) discontinuity in the elemental abundance. We suggest that the observed discontinuity is the result of nonhydrostatic motion of the gas core (i.e., sloshing) due to the recent merger. In this situation, both gas motions and abundance gradients are important in the visibility of the discontinuity. Cen A is in the late stages of merging with a small late-type galaxy, and a large discontinuity in density and abundance across a short distance demonstrates that the gas of the two galaxies remains poorly mixed, even several hundred million years after the merger. The pressure discontinuity may have had a profound influence on the temporal evolution of the kiloparsec-scale jet. The jet could have decollimated, crossing the discontinuity and thereby forming the northeast radio lobe.

The CCP radical (X²Π_r) has been detected in the circumstellar gas of IRC +10216, the fifth phosphorus-bearing molecule identified in interstellar space. This identification was made on the basis of new laboratory millimeter/submillimeter direct absorption measurements, conducted in the range 120-413 GHz. Four rotational transitions of this species were observed using the Arizona Radio Observatory (ARO) 12 m telescope on Kitt Peak at 2 and 3 mm in wavelength. Each transition consists of lambda-doublets, which are well-separated in frequency in IRC +10216; five of these eight possible lines of CCP were clearly detected, while the remaining three were contaminated by stronger emission from other species. The column density derived for CCP was N_tot= 1.2 × 10¹² cm⁻² and T_rot = 21 K. Modeling of the line profiles suggests that CCP arises from an extended shell with a maximum radius of ~40''. The abundance of this radical, relative to H₂, is f ~ 1 × 10⁻⁹—roughly comparable to that of PN and CP in this source. CCP may be produced from radical-radical reactions of CP, or ion-molecule chemistry involving P⁺ and HCCH. The identification of CCP is additional evidence that phosphorus chemistry is active in carbon-rich circumstellar gas.

Recent discovery of the year-scale variability in the synchrotron X-ray emission of the supernova remnant (SNR) RX J1713.7–3946 has initiated our study of multiepoch X-ray images and spectra of the young SNR Cassiopeia A based on the Chandra archive data taken in 2000, 2002, and 2004. We have found year-scale time variations in the X-ray intensity for a number of X-ray filaments or knots associated with the reverse-shocked regions. The X-ray spectra of the variable filaments are characterized by a featureless continuum, and described by a power law with a photon index within 1.9-2.3. The upper limits on the iron K-line equivalent width are 110 eV, which favors a synchrotron origin of the X-ray emission. The characteristic variability timescale of 4 yr can be explained by the effects of fast synchrotron cooling and diffusive shock acceleration with a plausible magnetic field of 1 mG. The X-ray variability provides a new effective way of studying particle acceleration at supernova shocks.

The WD+MS channel of the single-degenerate scenario is currently favorable for progenitors of Type Ia supernovae (SNe Ia). Incorporating the results of detailed binary evolution calculations for this channel into the latest version of a binary population synthesis code, I obtained the distributions of many properties of the companion stars at the moment of SN explosion. The properties can be verified by future observations.

We have discovered a partially eclipsing white dwarf, low-mass M dwarf binary (3.015114 hr orbital period), SDSS J143547.87+373338.5, from 2007 May observations at the WIYN telescope. Here we present blue-band photometry of three eclipses. Eclipse fitting gives main-sequence solutions to the M dwarf companion of M_S = 0.15–0.35 M_☉ and R_S = 0.17–0.32 R_☉. Analysis of the SDSS spectrum constrains the M dwarf further to be of type M4-M6 with M_S = 0.11–0.20 M_☉. Once full radial velocity curves are measured, high-precision determinations of the masses and radii of both components will be easily obtained without any knowledge of stellar structure or evolution. ZZ Ceti pulsations from the white dwarf were not found at our 4 mmag detection limit.

The atmospheres of close-in extrasolar planets absorb most of the incident stellar radiation, advect this energy, then reradiate photons in preferential directions. Those photons carry away momentum, applying a force on the planet. Here we evaluate the resulting secular changes to the orbit, known as the Yarkovsky effect. For known transiting planets, typical fractional changes in semimajor axis are about 1% over their lifetime, but could be up to ~5% for close-in planets such as OGLE-TR-56b or inflated planets such as TrES-4. We discuss the origin of the correlation between semimajor axis and surface gravity of transiting planets in terms of various physical processes, finding that radiative thrusters are too weak by about a factor of 10 to establish the lower boundary that causes the correlation.

Eleven years after its perihelion, comet C/1995 O1 (Hale-Bopp) is still active. Between 2007 October 20 and 22, we detected a diffuse coma of 180 × 10³ km in diameter with a slight elongation toward the north-south direction. The integrated brightness was 20.04 mag in R_C, implying Afρ = 300 m and albedo × dust surface a_RC = 4300 km². The coma was relatively red at V − R = 0.66 mag, which is consistent with that of the dust in other comets. The observed properties and the overall fading in brightness between 10 and 26 AU follow the predicted behavior of CO-driven activity. This is the most distant cometary activity ever observed.

The electron-cyclotron maser (ECM) instability is an important mechanism that directly amplifies electromagnetic radiation by nonthermal energetic electrons trapped in magnetic fields. Major astrophysical observations, on the other hand, imply that the energetic electrons frequently exhibit an energy distribution with a negative power-law spectrum. In this Letter, the lower energy cutoff behavior of the power-law electrons and its effects on the ECM instability are discussed. We introduce steepness index δ and cutoff energy E_c to describe the cutoff behavior of the power-law electrons and demonstrate that the power-law electrons with the steepness cutoff of δ > α can efficiently excite the ECM instability, where α is the spectrum of the power-law electrons, and that the growth rates of radiative waves in the O and X modes all increase with both the steepness index δ and cutoff energy E_c, but the growth rate of the X mode is considerably larger than that of the O mode. For the saturation cutoff case of δ ⩽ α , on the other hand, the power-law electrons cannot drive the ECM instability.

Type II radio bursts are produced near the local plasma frequency f_p and near 2f_p by shocks moving through the corona and solar wind. In the present Letter eight well-defined coronal type II radio bursts (30-300 MHz) are analyzed. Three results are presented. First, it is found that the dependence of the central frequency on time can be fitted to a power-law model, f ∝ (t − t₀)^−α, with 0.6 ⩽ α ⩽ 1.3. Assuming a constant shock velocity, these results provide evidence that the density profile n_e(r) in the type II source regions closely resembles the solar wind, with n_e(r) ∝ r⁻². One possible interpretation is that the solar wind starts within a few solar radii of the photosphere, most probably within 1 solar radius. Another relies on a gasdynamic Whitham analysis and demonstrates a possibility for blast shocks to accelerate, thereby reducing apparent power-law indices to solar wind-like values. Second, for the events considered it is found that radio burst emission in the form of 1/f versus t dynamic spectra closely follows straight lines. In future this will allow much more objective identification of type II bursts in solar radio data and plausibly real-time correlation with coronagraph and other solar radar. Third, it is demonstrated that 1/f versus t dynamic spectra can provide direct evidence for acceleration of the shock deep in the corona, thus complementing coronagraph studies.

Coronal mass ejections (CMEs) are often assumed to be magnetic flux ropes, but direct proof has been lacking. A key feature, resulting from the translational symmetry of a flux rope, is that the total transverse pressure as well as the axial magnetic field has the same functional form over the vector potential along any crossing of the flux rope. We test this feature (and hence the flux-rope structure) by reconstructing the 2007 May 22 magnetic cloud (MC) observed at STEREO B, Wind/ACE, and possibly STEREO A with the Grad-Shafranov (GS) method. The model output from reconstruction at STEREO B agrees fairly well with the magnetic field and thermal pressure observed at ACE/Wind; the separation between STEREO B and ACE/Wind is about 0.06 AU, almost half of the MC radial width. For the first time, we reproduce observations at one spacecraft with data from another well-separated spacecraft, which provides compelling evidence for the flux-rope geometry and is of importance for understanding CME initiation and propagation. We also discuss the global configuration of the MC at different spacecraft on the basis of the reconstruction results.

The occurrence of f⁻¹ noise in interplanetary magnetic fields (in the 1 × 10⁻⁵ to 1 × 10⁻⁴ Hz band) and other plasma parameters has now been known for about 20 years and has been recently identified also in the photospheric magnetic fields. However, the relationship between interplanetary and solar fluctuation spectra and the identification of their sources at the Sun are problems that still need to be addressed. Moreover, interplanetary density and magnetic field power spectra show a f⁻² interval at frequencies smaller that ~6 × 10⁻⁴ Hz whose source on the Sun is at present not fully understood. In this work we report on the first study of low-frequency density fluctuations in the solar corona at 2.1 R_☉. In 2006 June the Ultraviolet Coronagraph Spectrometer (SOHO UVCS) observed over a period of about 9.2 days H Lyα intensity fluctuations at 2.1 R_☉ over a polar coronal hole. The Lyα intensity power spectra S(f) (related mainly to density fluctuations) showed a S(f) ∝ f⁻² frequency interval between 2.6 × 10⁻⁶ and 3.0 × 10⁻⁵ Hz and a S(f) ∝ f⁻¹ frequency interval between 3.0 × 10⁻⁵ and 1.3 × 10⁻⁴ Hz. The detection of a f⁻² interval, in agreement with interplanetary density and magnetic field power spectra, has been also predicted in solar wind models as a consequence of phase-mixing mechanisms of waves propagating in coronal holes. High-latitude power spectra show a f⁻¹ band approximately in the same frequency interval where f⁻¹ noise has been detected in interplanetary densities, and interplanetary and photospheric magnetic fields, providing a connection between photospheric, coronal, and interplanetary f⁻¹ noises.

The measurements of magnetic twist of EUV coronal loops for 14 loops are presented here. EUV coronal loops are thin, EUV-emitting structures of hot plasma tracing magnetic field lines in the corona. The constriction of plasma into a loop without dispersion may be explained if the magnetic field of the loop is twisted. On the basis of this idea, Chae and Moon developed a method of determining magnetic twist of coronal loops by analyzing coronal images and photospheric magnetograms together. By applying this method to as many coronal loops observed by TRACE 171 Å as possible, we attempt to determine a statistically meaningful value of magnetic twist of coronal loops for the first time. We have selected a number of conspicuous loops which are bright enough and well separated from other adjacent loops on TRACE EUV images. We have constructed and examined coronal magnetic fields of selected active regions containing the loops with consideration of the projection effect, and we can identify 14 coronal loops whose magnetic field lines are well represented by a linear force-free field. We have found that these loops have absolute twist values from 0.22π to 1.73π, which suggests that the absolute winding number of EUV coronal loop may be mostly less than one turn. Our results support the idea that EUV coronal loops may be a product of magnetic reconnection of magnetic flux tubes.

Convective instability has been a mechanism used to explain the formation of solar photospheric flux tubes with kG field strength. However, the turbulence of the Earth's atmosphere has prevented ground-based observers from examining the hypothesis with precise polarimetric measurement on the subarcsecond scale flux tubes. Here we discuss observational evidence of this scenario based on observations with the Solar Optical Telescope (SOT) aboard Hinode. The cooling of an equipartition field strength flux tube precedes a transient downflow reaching 6 km s⁻¹ and the intensification of the field strength to 2 kG. These observations agree very well with the theoretical predictions.

We discuss a numerical 3D radiation-MHD simulation of penumbral fine structure in a small sunspot. This simulation shows the development of short filamentary structures with horizontal flows, similar to observed Evershed flows, and an inward propagation of these structures at a speed compatible with observations. Although the lengths of these filaments are much shorter than observed, we conjecture that this simulation qualitatively reproduces the mechanisms responsible for filament formation and Evershed flows in penumbrae. We conclude that the Evershed flow represents the horizontal-flow component of overturning convection in gaps with strongly reduced field strength. The top of the flow is always directed outward—away from the umbra—because of the broken symmetry due to the inclined magnetic field. Upflows occur in the inner parts of the gaps and most of the gas turns over radially (outward and sideways), and descends back down again. The ascending, cooling, and overturning flow tends to bend magnetic field lines down, forcing a weakening of the field that makes it easier for gas located in an adjacent layer—farther in—to initiate a similar sequence of motion, aided by lateral heating, thus causing the inward propagation of the filament.

Laboratory spectra of carbon nanoparticles (CNPs) near 3.29 μm show a band whose profile is an excellent match to that of the aromatic CH feature observed in type A emission sources. This band is associated with the CH stretching vibration in polycyclic aromatic hydrocarbon (PAH) molecules having peripheral aliphatic hydrocarbon substituents. Comparison between experimental and observational spectra suggests that the PAH species responsible for the appearance of a CH absorption feature at 3.275 μm is likely the same as that producing emission at 3.29 μm. We also detect a feature at 3.255 μm, attributed to small PAHs with aliphatic side chains, that corresponds to the aromatic CH band detected in molecular clouds. Our results indicate that the common form for PAHs in the interstellar medium is as rings having one or more aliphatic side chains. The proportion of side chains increases as excitation decreases, so that the overall composition of substituted PAHs in the DISM approaches that of hydrogenated amorphous carbon.