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We report observations of two nearby Type Ia supernovae (SNe Ia) for which observations of Cepheid variables in the host galaxies have been obtained with the Hubble Space Telescope: SN 1994ae in NGC 3370 and SN 1998aq in NGC 3982. For NCG 3370, we used the Advanced Camera for Surveys to observe 64 Cepheids that yield a distance of 29 Mpc, the farthest direct measurement of Cepheids. We have measured emission lines from H II regions in both host galaxies that provide metallicity-dependent corrections to their period-luminosity relations. These two SNe Ia double the sample of "ideal" luminosity calibrators: objects with well-observed and well-calibrated light curves of typical shape and with low reddening. By comparing them to all similarly well-measured SNe Ia in the Hubble flow, we find that H₀ = 73 ± 4 (statistical) ± 5 (systematic) km s^-1 Mpc^-1. A detailed analysis demonstrates that most of the past disagreement over the value of H₀ as determined from SNe Ia is abated by the replacement of past, problematic data by more accurate and precise, modern data.

Recent observations have revealed a population of red massive galaxies at high redshift that are challenging to explain in terms of hierarchical galaxy formation models. We analyze this "massive galaxy problem" with two different types of hydrodynamic simulations—Eulerian total variation diminishing (TVD) and smoothed particle hydrodynamics (SPH)—of a concordance Λ cold dark matter (ΛCDM) universe. We consider two separate but connected aspects of the problem posed by these extremely red objects (EROs): (1) the mass scale of these galaxies and (2) their red colors. We perform spectrophotometric analyses of simulated galaxies in B, z, R, I, J_s, K_s, and K filters and compare their near-infrared (near-IR) properties with observations at redshift z = 1-3. We find that the simulated galaxies brighter than the magnitude limit of K_Vega = 20 mag have stellar masses M_⋆ ≳ 10¹¹h^-1M_☉ and a number density of a few times 10^-4h³ Mpc^-3 at z ~ 2, in good agreement with the observed number density in the K20 survey. Therefore, our hydrodynamic simulations do not exhibit the mass-scale problem. The answer to the "redness problem" is less clear because of our poor knowledge of the amount of dust extinction in EROs and the uncertain fraction of star-forming EROs. However, our simulations can account for the observed comoving number density of ~1 × 10^-4 Mpc^-3 at z = 1-2 if we assume a uniform extinction of E(B - V) = 0.4 for the entire population of simulated galaxies. Upcoming observations of the thermal emission of dust in 24 μm by the Spitzer Space Telescope will help to better estimate the dust content of EROs at z = 1-3 and thus to further constrain the star formation history of the universe and theoretical models of galaxy formation.

We use a serendipitously discovered overdensity of extremely red objects (EROs) to study the morphologies and cumulative surface number density of EROs in a dense environment. Our extremely deep imaging allows us to select very faint EROs, reaching K_S = 21, or ~2 mag fainter than the L* of passively evolving ellipticals at z = 1.5. We find that the shape of the ERO cumulative surface number density in our overdense field mimics that of the field ERO population over all magnitudes down to K_S = 21 but with a factor of 3-4 higher normalization. The excellent seeing in our images (04 in K_S and 06 in R) allows for morphological classification of the brighter (K_S < 19) EROs, and we find a mix of morphologies including interacting systems and disks; the fraction of pure bulges (at most 38%), galaxies with disks (at least 46%), and interacting systems (at least 21%) is consistent with morphological fractions in field ERO studies. The similarity in the shape of the cumulative surface density and morphological mix between our overdense field and the field ERO population suggests that ERO galaxies in overdense regions at z ~ 1-2 may not have had an appreciably different history from those in the field.

Color-color diagrams for the clump and interclump emission in 10 clump-cluster galaxies of the Hubble Ultra Deep Field (UDF) are made from B,V, i, and z images and compared with models to determine redshifts, star formation histories, and galaxy masses. These galaxies are members of a class dominated by 5-10 giant clumps, with no exponential disk or bulge. The redshifts are found to be in the range from 1.6 to 3. The clump emission is typically 40% of the total galaxy emission, and the luminous clump mass is 19% of the total galaxy mass. The clump colors suggest declining star formation over the last ~0.3 Gyr, while the interclump emission is redder than the clumps, corresponding to a greater age. The clump luminous masses are typically 6 × 10⁸M_☉, and their diameters average 1.8 kpc, making their average density ~0.2 M_☉ pc^-3. Including the interclump populations, assumed to begin forming at z = 6, the total galaxy luminous masses average 6.5 × 10¹⁰M_☉ and their diameters average 19 kpc to the 2 σ noise level. The expected galaxy rotation speeds average ~150 km s^-1 if they are uniformly rotating disks. The ages of the clumps are longer than their internal dynamical times by a factor of ~8, so they are stable star clusters, but the clump densities are only ~10 times the limiting tidal densities, so they could be deformed by tidal forces. This is consistent with the observation that some clumps have tails. The clumps could form by gravitational instabilities in accreting disk gas and then disperse on a ~1 Gyr timescale, building up the interclump disk emission, or they could be captured as gas-rich dwarf galaxies, flaring up with star formation at first and then dispersing. Support for this second possibility comes from the high abundance of nearly identical clumps in the UDF, smaller than 6 pixels, whose distributions on color-magnitude and color-color plots are the same as the galaxy clumps studied here. The distribution of axial ratios for the combined population of chain and clump-cluster galaxies in the UDF is compared with models and shown to be consistent with a thick-disk geometry. If these galaxies evolve into today's disk galaxies, then we are observing a stage in which accretion and star formation are extremely clumpy and the resulting high velocity dispersions form thick disks. Several clump-clusters have disk densities that are much larger than in local disks, however, suggesting an alternate model in which they do not survive until today, but get converted into ellipticals by collisions.

We investigate how the shapes and angular momenta of galaxy and group mass dark matter halos in a ΛCDM N-body simulation are correlated internally and how they are aligned with respect to the location and properties of surrounding halos. We explore these relationships down to halos of much lower mass (10¹¹h^-1M_☉) than previous studies. The halos are triaxial, with c/a ratios of 0.6 ± 0.1 and a mean two-dimensional projected ellipticity of = 0.24. More massive halos are more flattened. The axis ratios rise out to 0.6r_vir, beyond which they drop. The principal axes, in particular the minor axes, are very well aligned within 0.6 r_vir. High-mass halos show particularly strong internal alignment. The angular momentum vectors are also reasonably well aligned except between the very outermost and very innermost regions of the halo. The angular momentum vectors tend to align with the minor axes, with a mean misalignment of ~25°, and lie perpendicular to the major and intermediate axes. The properties of a halo at 0.4 r_vir are quite characteristic of the properties at most other radii within the halo. There is a very strong tendency for the minor axes of halos to lie perpendicular to large-scale filaments, and a much weaker tendency for the major axes to lie along the filaments. This alignment extends to much larger separations for group and cluster mass halos than for galaxy mass halos. As a consequence, the intrinsic alignments of galaxies are likely weaker than previous predictions, which were based on the shapes of cluster mass halos. The angular momenta of the highest concentration halos tend to point toward other halos. The angular momenta of galaxy mass halos point parallel to filaments, while those of group and cluster mass halos show a very strong tendency to point perpendicular to the filaments. This suggests that group and cluster mass halos acquire most of their angular momentum from major mergers along filaments, while the accretion history of mass and angular momentum onto galaxy mass halos has been smoother.

Very recent observations of the ⁶Li isotope in halo stars reveal a ⁶Li plateau about 1000 times above the predicted big bang nucleosynthesis abundance. We calculate the evolution of ⁶Li versus redshift generated from an initial burst of cosmological cosmic rays (CCRs) up to the formation of the Galaxy. We show that the pre-Galactic production of the ⁶Li isotope can account for the ⁶Li plateau observed in metal-poor halo stars without additional overproduction of ⁷Li. The derived relation between the amplitude of the CCR energy spectra and the redshift of the initial CCR production puts constraints on the physics and history of the objects, such as Population III stars, responsible for these early cosmic rays. Consequently, we consider the evolution of ⁶Li in the Galaxy. Since ⁶Li is also produced in Galactic cosmic-ray nucleosynthesis, we argue that halo stars with metallicities between [Fe/H] = -2 and -1 must be somewhat depleted in ⁶Li.

We have used the VLA to study radio variability among a sample of 18 low-luminosity active galactic nuclei (LLAGNs) on timescales of a few hours to 10 days. The goal was to measure or limit the sizes of the LLAGN radio-emitting regions in order to use the size measurements as input to models of the radio emission mechanisms in LLAGNs. We detect variability on typical timescales of a few days at a confidence level of 99% in half the target galaxies. Either variability that is intrinsic to the radio-emitting regions or that is caused by scintillation in the Galactic interstellar medium is consistent with the data. For either interpretation, the brightness temperature of the emission is below the inverse Compton limit for all our LLAGNs and has a mean value of about 10¹⁰ K. The variability measurements plus VLBI upper limits imply that the typical angular size of the LLAGN radio cores at 8.5 GHz is 0.2 mas, plus or minus a factor of 2. The ~10¹⁰ K brightness temperature strongly suggests that a population of high-energy nonthermal electrons must be present, in addition to a hypothesized thermal population in an accretion flow, in order to produce the observed radio emission.

A study of the photometric accuracy of deconvolved astronomical images was undertaken by processing two simulated images with several well-known algorithms. The simulations consisted of an image of a simple star field and an image of an active galactic nucleus (AGN). Each image was mathematically convolved with a known PSF and then deconvolved to recover the original image. Measurements and comparisons were made before and after the deconvolution to check for photometric accuracy. The best result for the star field was obtained with the multiscale Richardson-Lucy algorithm. When measuring the differential photometry of the nucleus of the AGN between different frames, leaving the image in its convolved state gave a closer variability result to the original than any deconvolution method.

The XMM-Newton observation of the optically type 1 active galactic nucleus (AGN) AX J0447-0627 (z = 0.214) unambiguously reveals a complex, bright, and prominent set of lines in the 4-8 keV rest-frame energy range. Although from a phenomenological point of view, the observed properties can be described by a simple power-law model plus five narrow Gaussian lines (at rest-frame energies of ~4.49, ~5.55, ~6.39, ~7.02, and ~7.85 keV), we find that a model comprising a power law (Γ ~ 2.2), a reflected relativistic continuum, a narrow Fe I Kα line from neutral material, as well as a broad Fe Kα relativistic line from a ionized accretion disk, represents a good physical description of the data. The "double-horned" profile of the relativistic line implies an inclination of the accretion disk of ~45° and an origin in a narrow region of the disk, from R_in ~ 19GM/c² to R_out ~ 30GM/c². The narrow Fe I Kα line from neutral material is probably produced far from the central black hole, most likely in the putative molecular torus. Although some of these properties have already been found in other type 1 AGNs and discussed in the literature, at odds with the objects reported so far we measure high equivalent widths of the observed lines: ~1.4 keV for the double-horned relativistic line and ~0.4 keV for the narrow line.

We present results from a Chandra ACIS-S observation of the type 2 low-ionization nuclear emission-line region (LINER) radio galaxy NGC 4261 (3C 270). The X-ray data show that this galaxy hosts a heavily obscured (N_H ~ 8 × 10²² cm^-2) active galactic nucleus (AGN) with an intrinsic luminosity of 2.4 × 10⁴¹ ergs s^-1, which dominates the energy output of the nucleus but is much less than the Eddington luminosity of its central massive black hole. The multiwavelength properties of this component are consistent with either radiatively inefficient accretion flows or emission from the inner part of a jet within 20 mas (3.4 pc) from the nucleus. A softer, unobscured power-law component, which we identify as synchrotron X-ray emission related to the unobscured part of the inner jet, produces ~10% of the 0.1-10.0 keV emission from the nucleus and is likely to be associated with the arcsecond-scale optical/UV emission measured with the Hubble Space Telescope (HST). These results indicate that the energy source of some type 2 LINERs could be a heavily obscured AGN, when obscuration of the ionizing continuum might be responsible for the low ionization state of the line-emitting gas.

To investigate the relationship between black holes and their host galaxies, many groups have used the width of the [O III] λ5007 line as a substitute for the stellar velocity dispersion (σ_*) of galaxy bulges. We directly test this assumption with a large and homogeneous sample of narrow-line active galactic nuclei from the Sloan Digital Sky Survey. We consider multiple transitions ([O II] λ3727, [O III] λ5007, and [S II] λλ6716, 6731) and various techniques for quantifying the line width in order to obtain a calibration between the gas velocity dispersion, σ_g, and σ_*. We find that σ_g of the low-ionization lines traces σ_*, as does σ_g for the core of [O III] after its asymmetric blue wing is properly removed, although in all cases the correlation between σ_g and σ_* has considerable scatter. While the gas kinematics of the narrow-line region of active galaxies are primarily governed by the gravitational potential of the stars, the accretion rate, as traced by the Eddington luminosity ratio, seems to play an important secondary role. Departures from virial motions correlate systematically with accretion rate. We discuss the implications of these results for previous studies that use [O III] line widths to infer stellar velocity dispersions in quasars and narrow-line Seyfert 1 galaxies.

Chandra observations of the merging galaxy cluster A520 reveal a prominent bow shock with M = 2.1. This is only the second clear example of a substantially supersonic merger shock front in clusters. Comparison of the X-ray image with that of the previously known radio halo reveals a coincidence of the leading edge of the halo with the bow shock, offering an interesting experimental setup for determining the role of shocks in the radio halo generation. The halo in A520 apparently consists of two spatially distinct parts, the main turbulence-driven component and a cap-like forward structure related to the shock, where the latter may provide preenergized electrons for subsequent turbulent reacceleration. The radio edge may be caused by electron acceleration by the shock. If so, the synchrotron spectrum should have a slope of α ≃ 1.2 right behind the edge, with quick steepening farther away from the edge. Alternatively, if shocks are inefficient accelerators, the radio edge may be explained by an increase in the magnetic field and density of preexisting relativistic electrons due to gas compression. In the latter model, there should be radio emission in front of the shock with the same spectrum as that behind it, but 10-20 times fainter. If future sensitive radio measurements do not find such preshock emission, then the electrons are indeed accelerated (or reaccelerated) by the shock, and one will be able to determine its acceleration efficiency. We also propose a method to estimate the magnetic field strength behind the shock, based on measuring the dependence of the radio spectral slope upon the distance from the shock. In addition, the radio edge provides a way to constrain the diffusion speed of the relativistic electrons.

We present V and I photometry of the resolved stars in the cometary dwarf irregular galaxy NGC 2366, using Wide Field Planetary Camera 2 images obtained with the Hubble Space Telescope. The resulting color-magnitude diagram reaches down to I ~ 26.0 mag. It reveals not only a young population of blue main-sequence stars (age ≲30 Myr) but also an intermediate-age population of blue and red supergiants (20 Myr ≲ age ≲100 Myr) and older evolved populations of asymptotic giant branch (AGB) stars (age ≳100 Myr) and red giant branch (RGB) stars (age ≳1 Gyr). The measured magnitude I = 23.65 ± 0.10 mag of the RGB tip results in a distance modulus m - M = 27.67 ± 0.10, which corresponds to a distance of 3.42 ± 0.15 Mpc, in agreement with previous distance determinations. The youngest stars are associated with the bright complex of H II regions NGC 2363 (=Mrk 71) in the southwest extremity of the galaxy. As a consequence of the diffusion and relaxation processes of stellar ensembles, the older the stellar population is, the smoother and more extended is its spatial distribution. An underlying population of older stars is found throughout the body of NGC 2366. The most notable feature of this older population is the presence of numerous relatively bright AGB stars. The number ratio of AGB to RGB stars and the average absolute brightness of AGB stars in NGC 2366 are appreciably higher than in the BCD VII Zw 403, indicating a younger age of the AGB stars in NGC 2366. In addition to the present burst of age ≲100 Myr, there has been strong star formation activity in the past of NGC 2366, from ~100 Myr to ≲3 Gyr ago.

Using synthetic spectra, we construct a simple model of an elliptical galaxy with a velocity dispersion σ = 200 km s^-1. Absorption feature indices are defined that are sensitive to the abundances of C, N, O, Na, Mg, Al, Si, S, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Ba, and Eu as a first step in determining the abundances of these elements in stellar populations such as elliptical galaxies for which integrated spectra are available. Using these indices and assuming a photon statistical error such that S/N = 100 around 5000 Å, the feasibility of measuring individual elements in real galaxies is assessed. Of the elements studied, only S, K, Cu, Zn, and Eu appear to be difficult to determine. The rest appear to be measurable with high-quality data and models. The effects of abundance changes on Lick IDS indices, Rose indices, and BRVI colors are also tabulated.

This paper presents deep spectra of 14 planetary nebulae (PNs) at a variety of angular distances from the center of the flattened elliptical galaxy NGC 4697, which is located at ~11 Mpc from us. Both near the center and among the most outlying PNs, we have found several examples of very strong [O III] λ5007, about 20 times brighter than Hβ. This, together with strong [Ne III] lines, implies a lower limit for O and Ne abundances near solar at the center and also at more than 2 effective radii from the center of NGC 4697. Thus we have found, for the first time from individual stars, direct evidence of the existence of a metal-rich population in elliptical galaxies, confirming metallicities derived from absorption-line indices and extending the information to angular distances where the stellar surface brightness is too low for absorption-line studies. A comparison with abundances obtained closer to the center of NGC 4697 from integrated absorption-line spectra indicates that the metallicity gradient, which is quite steep within 1 effective radius, becomes much less pronounced beyond 1 effective radius. The only alternative to this interpretation would be the presence of a very metal-poor PN population with [Z/H] below -1, which we cannot rule out, but it would require an extremely bimodal metallicity distribution with almost no intermediate metallicities.

In spite of a large number of global three-dimensional MHD simulations of accretion flows and jets being made recently, their astrophysical relevance for realistic situations is not well known. In order to examine to what extent the simulated MHD flows can account for the observed SED of Sgr A*, for the first time we calculate the emergent spectra from three-dimensional MHD flows in a wide range of wavelengths (from radio to X-ray) by solving the three-dimensional radiative transfer equations. We use the simulation data by Kato and coworkers and perform Monte Carlo radiative transfer simulations, in which synchrotron emission/absorption, free-free emission/absorption, and Compton/inverse Compton scattering are taken into account. We assume two-temperature plasmas and calculate electron temperatures by solving the electron energy equation. Only thermal electrons are considered. It is found that the three-dimensional MHD flow generally overproduces X-rays by means of bremsstrahlung radiation from the regions at large radii. A flatter density profile, ρ ∝ r^-a with a < 1, than that of the ADAF, ρ ∝ r^-3/2, is the main reason for this. If we restrict the size of the emission region to be as small as ~10r_S, where r_S is the Schwarzschild radius, the MHD model can reproduce the basic features of the observed SED of Sgr A* during its flaring state. Yet, the spectrum in the quiescent state remains to be understood. We discuss how to resolve this issue in the context of MHD flow models. Possibilities include modifications of the MHD flow structure by the inclusion of radiative cooling and/or significant contributions by nonthermal electrons. It is also possible that the present spectral results may be influenced by particular initial conditions. We also calculate the time-dependent spectral changes, finding that the fluxes fluctuate in a wide range of the frequency and the flux at each wavelength does not always vary coherently.

In the neighborhood of Sgr A*, several stars (S2, S12, S14, S1, S8, and S13) enjoy an accurate determination of their orbital parameters. General relativity predicts that the central black hole acts as a gravitational lens on these stars, generating a secondary image and two infinite series of relativistic images. For each of these six stars, we calculate the light curves for the secondary and the first two relativistic images, in the Schwarzschild black hole hypothesis, throughout their orbital periods. The curves are peaked around the periapse epoch, but two subpeaks may arise in nearly edge-on orbits, when the source is behind or in front of Sgr A*. We show that for most of these stars the secondary image should be observable during its brightness peak. In particular, S14 is the best candidate, since its secondary image reaches K = 23.3 with an angular separation of 0.125 mas from the apparent horizon of the central black hole. The detection of such images by future instruments could represent the first observation of gravitational lensing beyond the weak-field approximation.

Full-polarization MERLIN and VLBA observations of the 1720 MHz maser emission from the OH molecule in the supernova remnant W44 are presented. Zeeman splitting has been directly resolved between right- and left-circularly polarized spectra indicating magnetic field strengths of || ≈ 0.50 mG. The position angle of the linear polarization is in good agreement with the orientation of the shocked gas at the supernova remnant/molecular cloud interface. Based on MERLIN data having a resolution of 200 mas and VLBA data with a resolution of 15 mas, the masers are measured to have deconvolved angular sizes of 50-350 mas (150-1000 AU) with compact cores 20 mas (60 AU) in size, consistent with theoretical expectation and previous observations.

We present hydrodynamical simulations of the photoevaporation of a cloud with large-scale density gradients, giving rise to an ionized, photoevaporation flow. The flow is found to be approximately steady during the large part of its evolution, during which it can resemble a "champagne flow" or a "globule flow" depending on the curvature of the ionization front. The distance from source to ionization front and the front curvature uniquely determine the structure of the flow, with the curvature depending on the steepness of the lateral density gradient in the neutral cloud. We compare these simulations with both new and existing observations of the Orion Nebula and find that a model with a mildly convex ionization front can reproduce the profiles of emission measure, electron density, and mean line velocity for a variety of emitting ions on scales of 10¹⁷-10¹⁸ cm. The principal failure of our model is that we cannot explain the large observed widths of the [O I] λ6300 line that form at the ionization front.

Molecular line observations of NH₃ (J,K) = (1,1) and (2, 2) and CH₃OH at 24.93 GHz taken with the Australian Telescope Compact Array (ATCA) toward the massive twin cores NGC 6334I and NGC 6334I(N) reveal significant variations in the line emission between the two massive cores. The UC H II region/hot core NGC 6334I exhibits strong thermal NH₃ and CH₃OH emission adjacent to the UC H II region and coincident with two millimeter continuum peaks observed by T. R. Hunter et al. In contrast, we find neither compact NH₃ nor thermal CH₃OH line emission toward NGC 6334I(N). There the NH₃ emission is distributed over a broad region (>1') without a clear peak, and we find Class I CH₃OH maser emission with peak brightness temperatures up to 7000 K. The maser emission peaks appear to be spatially associated with the interfaces between the molecular outflows and the ambient dense gas. Peak NH₃ (1, 1) line brightness temperatures ≥70 K in both regions indicate gas temperatures on the same order. NH₃ emission is also detected toward the outflow in NGC 6334I, resulting in an estimated rotational temperature of T_rot ~ 19 K. Furthermore, we observe CH₃OH and NH₃ absorption toward the UC H II region; the velocity structure is consistent with expanding molecular gas around the UC H II region. Thermal and kinematic effects possibly imposed from the UC H II region on the molecular core are also discussed.

We report on new observations of a purported parsec-scale outflow in the hostile environment at the boundary of the Rosette Nebula, a well-known H II region driven by several O stars in the open cluster NGC 2244, and the Rosette Molecular Cloud (RMC). Several emission features were first reported by Ybarra & Phelps in 2004 , but with only optical [S II] imaging, their nature could not then be established. Our new near-infrared (NIR) H₂ imaging observations reveal bulleted NIR emission between a Two Micron All Sky Survey (2MASS) NIR excess point source and the optical [S II] emission for RMC C of Ybarra & Phelps. Analysis of the 2MASS colors indicates that the point source is likely to be a Class I young stellar object (YSO). Together these results imply that RMC C is in fact part of a true YSO outflow, now designated HH 871. The presence of additional [S II] emission, apparently connected to the HH 871 outflow but separated by ~1.2 pc, assuming a distance of 1600 pc for the RMC, also lends support to the Ybarra & Phelps hypothesis that a parsec-scale outflow is present in this harsh ionizing environment.

The results of experiments on the formation of molecular hydrogen on low-density and high-density amorphous ice surfaces are analyzed using a rate equation model. The activation energy barriers for the relevant diffusion and desorption processes are obtained. The more porous morphology of the low-density ice gives rise to a broader spectrum of energy barriers compared to the high-density ice. Inserting these parameters into the rate equation model under steady-state conditions, we evaluate the production rate of molecular hydrogen on ice-coated interstellar dust grains.

Afterglow observations are commonly used to determine the parameters of GRB explosions, the energy E, surrounding density n, postshock magnetic field equipartition fraction _B, and electron equipartition fraction _e, under the frequently made assumption that the efficiency of electron "injection" into relativistic shock acceleration is high, i.e., that the fraction f of electrons that undergo acceleration is f ≈ 1. We show that the value of f cannot be determined by current observations, since currently testable model predictions for a parameter choice {E' = E/f, n' = n/f, = f_B, = f_e} are independent of the value of f for m_e/m_p ≤ f ≤ 1. Current observations imply that the efficiency f is similar for highly relativistic and subrelativistic shocks and plausibly suggest that f ~ 1, quite unlike the situation in the Crab Nebula. However, values m_e/m_p ≤ f ≪ 1 cannot be ruled out, implying a factor m_e/m_p uncertainty in determination of model parameters. We show that early, ≤10 hr, radio afterglow observations, which will be far more accessible in the Swift era, may provide constraints on f. Such observations will therefore provide a powerful diagnostic of GRB explosions and of the physics of particle acceleration in collisionless shocks.

This paper is devoted to the analysis of particle acceleration in gamma-ray bursts (GRBs) and its radiative consequences. Therefore, we get on one hand constraints on the physics and on the other hand possible signatures of particle acceleration that could be recorded by new gamma-ray instruments. In a recent paper we have shown that UHECRs can be generated in GRBs even with conservative assumptions about the magnetic field and the scattering capability of its perturbations, provided that a suitable relativistic Fermi process is at work during the so-called internal shock phase. In this paper we extend the analysis of the consequences of these assumptions to the whole prompt emission of both electrons and protons. Indeed, assuming that the magnetic field decays as 1/r² and that the scattering time of particles is longer than the Bohm assumption, in particular, with a rule derived from Kolmogorov scaling, we show that the four following events naturally happen with no other parameter adaptation than the intensity of the magnetic field, which turns out to be subequipartition: (1) UHECRs can be generated with a sufficient flux (≃1 km^-2 yr^-1) within the GZK sphere to account for the CR spectrum at the ankle. (In the previous paper, we showed that the associated pγ-neutrino emission is tiny.) (2) A thermal component below the so-called E_peak is often unavoidable and even amplified when the shocks start before the photosphere. (3) The CRs could radiate gamma rays around 67 MeV (in the comoving frame, which implies ≃20 GeV for the observer) due to π⁰ decay and a low-energy neutrino emission (around 0.2 GeV) associated with neutron decay and also neutrinos of energy between 5 and 150 GeV due to muon decay (as predicted in the previous paper). (4) The UHECRs radiate high-energy gamma rays between a few hundred MeV and 10 GeV (taking the pair creation process into account) due to their synchrotron emission, with a sufficient flux to be observable.

We present the results from a Hubble Space Telescope ACS search for supernovae associated with X-ray flashes 020903, 040701, 040812, and 040916. We find strong evidence that XRF 020903 (z = 0.25) was associated with a SN 1998bw-like supernova and confirm this using optical spectroscopy at t ~ 25 days. We find no evidence, however, for SN 1998bw-like supernovae associated with the other three events. In the case of XRF 040701 (z = 0.21), we rule out even a faint supernova similar to SN 2002ap, using template light curves for several local Type Ic supernovae. For the two cases in which the redshift is not known, XRFs 040812 and 040916, we derive robust redshift limits, assuming that they were accompanied by supernovae similar to SN 1998bw, and compare these limits with photometric redshift constraints provided by their host galaxies. We supplement this analysis with results for three additional events (XRFs 011030, 020427, and 030723) and discuss the observed diversity of supernovae associated with X-ray flashes and gamma-ray bursts. We conclude that XRF-SNe exist but can be significantly fainter than SN 1998bw, possibly consistent with the observed spread in local Type Ibc supernovae.

We summarize the analysis of a uniform set of both previously known and newly discovered scattered-light echoes, detected within 30'' of SN 1987A in 10 years of optical imaging, and with which we have constructed the most complete three-dimensional model of the progenitor's circumstellar environment. Surrounding the SN is a richly structured bipolar nebula. An outer, double-lobed "peanut," which we believe is the contact discontinuity between the red supergiant and main-sequence winds, is a prolate shell extending 28 lt-yr along the poles and 11 lt-yr near the equator. Napoleon's Hat, previously believed to be an independent structure, is the waist of this peanut, which is pinched to a radius of 6 lt-yr. Interior, the innermost circumstellar material lies along a cylindrical hourglass, 1 lt-yr in radius and 4 lt-yr long, which connects to the peanut by a thick equatorial disk. The nebulae are inclined 41° south and 8° east of the line of sight, slightly elliptical in cross section, and marginally offset west of the SN. The three-dimensional geometry of the three circumstellar rings is studied, suggesting the northern and southern rings are located 1.3 and 1.0 lt-yr from the SN, while the equatorial ring is elliptical (b/a ≲ 0.98) and spatially offset in the same direction as the hourglass. Dust-scattering models of the observed echo fluxes suggest that between the hourglass and bipolar lobes: the gas density drops from 1 to 3 cm^-3 to ≳0.03 cm^-3; the maximum dust-grain size increases from ~0.2 to 2 μm; and the silicate : carbonaceous dust ratio decreases. The nebulae have a total mass of ~1.7 M_☉, yielding a red-supergiant mass loss around 5 × 10^-6M_☉ yr^-1. We compare these results to current formation models and find that no model has successfully reproduced this system. However, our results suggest a heuristic evolutionary sequence in which the progenitor evolves through two "blue loops," perhaps accompanied by a close binary companion.

We report on Chandra ACIS and XMM-Newton MOS/pn imaging observations of two pulsar wind nebulae (K3/PSR J1420-6048 and G313.3+0.1, the "Rabbit") associated with the Galactic unidentified γ-ray source GeV J1417-6100. With excellent ACIS imaging the very energetic pulsar PSR J1420-6048 is separated from its surrounding nebula. This nebula has surprisingly little compact structure, although a faint arc is seen near the pulsar. Similarly, two point sources are resolved in the Rabbit Nebula. The large XMM-Newton collecting area provides useful spectral constraints on the Rabbit and the associated point sources. Based on spectra and X-ray morphology, we identify one point source as a plausible pulsar counterpart. Large backgrounds and low source counts limited pulse search sensitivities, but we report pulse upper limits and a candidate 108 ms period for the Rabbit Pulsar based on the XMM-Newton data and an ACIS CC observation. Comparison of the X-ray images with high-resolution ATCA radio maps shows that the nonthermal X-ray emission corresponds well with the radio structure.

We report the precise optical and X-ray localization of the 3.2 ms accretion-powered X-ray pulsar XTE J1814-338 with data from the Chandra X-Ray Observatory as well as optical observations conducted during the 2003 June discovery outburst. Optical imaging of the field during the outburst of this soft X-ray transient reveals an R = 18 star at the X-ray position. This star is absent (R > 20) from an archival 1989 image of the field and brightened during the 2003 outburst, and we therefore identify it as the optical counterpart of XTE J1814-338. The best source position derived from optical astrometry is R.A. = 18^h13^m3904, decl. = -33^°46'223 (J2000). The featureless X-ray spectrum of the pulsar in outburst is best fit by an absorbed power law (with photon index γ = 1.41 ± 0.06) plus blackbody (with kT = 0.95 ± 0.13 keV) model, where the blackbody component contributes approximately 10% of the source flux. The optical broadband spectrum shows evidence for an excess of infrared emission with respect to an X-ray heated accretion disk model, suggesting a significant contribution from the secondary or from a synchrotron-emitting region. A follow-up observation performed when XTE J1814-338 was in quiescence reveals no counterpart to a limiting magnitude of R = 23.3. This suggests that the secondary is an M3 V or later-type star and therefore very unlikely to be responsible for the soft excess, making synchrotron emission a more reasonable candidate.

We present here the high-resolution X-ray spectrum of the accreting binary X-ray pulsar GX 1+4, obtained with the High Energy Transmission Grating (HETG) instrument of the Chandra X-Ray Observatory. This was supplemented by a simultaneous observation with the Proportional Counter Array (PCA) of the Rossi X-Ray Timing Explorer (RXTE). During this observation, the source was in a somewhat low intensity state, and the pulse profile with both Chandra and RXTE shows a narrow dip, characteristic of GX 1+4 in medium- and low-intensity states. The continuum X-ray spectrum obtained with HETG and PCA can be fitted well with a high-energy cutoff power-law model with line-of-sight absorption. Interestingly, we find that this low state is accompanied by a relatively small absorption column density. A 6.4 keV narrow emission line with an equivalent width of 70 eV is clearly detected in the HETG spectrum. The fluorescence iron line, or at least part of it, is produced in the neutral or lowly ionized iron in the circumstellar material that also causes most of the line-of-sight absorption. In the HETG spectrum, we have found evidence for a weak (equivalent width ~30 eV) emission line at 6.95 keV. This line is identified as Lyα emission line from hydrogen-like iron, and the spectrum does not show emission lines from helium-like iron. We discuss various emission regions for the hydrogen-like iron emission line, such as gas diffused into the Alfvén sphere or an accretion curtain flowing from the inner accretion disk to the magnetic poles.

RX J0806.3+1527 is a candidate double-degenerate binary with possibly the shortest known orbital period. The source shows an ≈100% X-ray intensity modulation at the putative orbital frequency of 3.11 mHz (321.5 s). If the system is a detached, ultracompact binary, gravitational radiation should drive spin-up with a magnitude of ~ 10^-16 Hz s^-1. Efforts to constrain the X-ray frequency evolution to date have met with mixed success, principally due to the sparseness of earlier observations. Here we describe the results of the first phase-coherent X-ray monitoring campaign on RX J0806.3+1527 with Chandra. We obtained a total of 70 ks of exposure in six epochs logarithmically spaced over 320 days. With these data we conclusively show that the X-ray frequency is increasing at a rate of (3.77 ± 0.8) × 10^-16 Hz s^-1. Using the ephemeris derived from the new data, we are able to phase up all the earlier Chandra and ROSAT data and show that they are consistent with a constant = (3.63 ± 0.06) × 10^-16 Hz s^-1 over the past decade. This value appears consistent with that recently derived by Israel et al., largely from monitoring of the optical modulation, and is in rough agreement with the solutions reported initially by Hakala et al., based on ground-based optical observations. The large and stable over a decade is consistent with gravitational radiation losses driving the evolution. An intermediate polar (IP) scenario in which the observed X-ray period is the spin period of an accreting white dwarf appears less tenable because the observed requires an ≈ 2 × 10^-8M_☉ yr^-1, which is much larger than that inferred from the observed X-ray luminosity (although this depends on the uncertain distance and bolometric corrections), and it is difficult to drive such a high in a binary system with parameters consistent with all the multiwavelength data. If the ultracompact scenario is correct, then the X-ray flux cannot be powered by stable accretion, which would drive the components apart, suggesting that a new type of energy source (perhaps electromagnetic) may power the X-ray flux.

We present high-resolution spectroscopy of the neutron star/low-mass X-ray binaries (LMXBs) 4U 1850-087 and 4U 0513-40 as part of our continuing study of known and candidate ultracompact binaries. The LMXB 4U 1850-087 is one of four systems in which we had previously inferred an unusual Ne/O ratio in the absorption along the line of sight, most likely from material local to the binaries. However, our recent Chandra X-Ray Observatory LETGS spectrum of 4U 1850-087 finds a Ne/O ratio by number of 0.22 ± 0.05, smaller than previously measured and consistent with the expected interstellar value. We propose that variations in the Ne/O ratio due to source variability, as previously observed in these sources, can explain the difference between the low- and high-resolution spectral results for 4U 1850-087. Our XMM-Newton RGS observation of 4U 0513-40 also shows no unusual abundance ratios in the absorption along the line of sight. We also present spectral results from a third candidate ultracompact binary, 4U 1822-000, whose spectrum is well fit by an absorbed power-law + blackbody model with absorption consistent with the expected interstellar value. Finally, we present the nondetection of a fourth candidate ultracompact binary, 4U 1905+000, with an upper limit on the source luminosity of <1 × 10³² ergs s^-1. Using archival data, we show that the source has entered an extended quiescent state.

I present data on the shell of classical Nova Persei (1901) obtained by the Advanced CCD Imaging Spectrometer S3 detector on board the Chandra X-Ray Observatory. The X-ray nebula is affected mostly by the complex interstellar medium around the nova and has not developed a regular shell. The X-ray nebula is lumpy and asymmetric, with the bulk of the emission coming from the southwestern quadrant. The brightest X-ray emission is detected as an arc that covers the region from the west to the south of the central source. Part of this feature, which is cospatial with the brightest nonthermal radio emission region, is found to be a source of nonthermal (synchrotron) X-ray emission with a power-law photon index of 2.3 and α = 0.68 at about a flux of 1.7 × 10^-13 ergs cm^-2 s^-1. This confirms that the shell is a site of particle acceleration, mainly in the reverse shock zone. There are strong indications for nonlinear diffusive shock acceleration occurring in the forward shock/transition zone with an upper limit on the nonthermal X-ray flux of 1.0 × 10^-14 ergs cm^-2 s^-1. The total X-ray spectrum of the nebula consists of two prominent components of emission (other than the resolved synchrotron X-ray emission). The component dominant below 2 keV is most likely a nonequilibrium ionization thermal plasma of kT_s = 0.1-0.3 keV with an X-ray flux of 1.6 × 10^-11 ergs cm^-2 s^-1. There is also a higher temperature, kT_s = 0.5-2.6 keV, embedded, N_H = (4.0-22.0) × 10²² cm^-2 emission component prominent above 2 keV. The unabsorbed X-ray flux from this component is 1.5 × 10^-10 ergs cm^-2 s^-1. The X-ray-emitting plasma is of solar composition except for enhancement in the elemental abundances (mean abundances over the remnant) of Ne/Ne_☉ and N/N_☉ in the ranges 13-21 and 1-5, respectively. A distinct emission line of neon, He-like Ne IX, is detected, which reveals a distribution of several emission knots/blobs and shows a conelike structure with wings extending toward the northwest and southeast at expansion velocities ~2600 km s^-1 in the X-ray wavelengths. The emission measures yield an average electron density in the range 0.6-11.2 cm^-3 for both of the components (filling factor = 1). The electron density increases to higher values ~300 cm^-3 if the filling factor is decreased substantially. The mass in the X-ray-emitting nebula is (2.1-38.5) × 10^-4M_☉. The X-ray luminosity of the forward shock ~4.3 × 10³² ergs s^-1 indicates that it is adiabatic. The shocked mass, the X-ray luminosity, and comparisons with other wavelengths suggest that the remnant has started cooling and most likely is in a Sedov phase.

In a previous paper we have described a technique to derive constraints on the differential emission measure (DEM) distribution, a measure of the temperature distribution, of collisionally ionized hot plasmas from their X-ray emission line spectra. We apply this technique to the Chandra HETGS spectra of all of the nine hot stars available to us at the time that this project was initiated. We find that DEM distributions of six of the seven O stars in our sample are very similar, but that θ¹ Ori C has an X-ray spectrum characterized by higher temperatures. The DEM distributions of both of the B stars in our sample have lower magnitudes than those of the O stars, and one, τ Sco, is characterized by higher temperatures than the other, β Cru. These results confirm previous work in which high temperatures have been found for θ¹ Ori C and τ Sco and taken as evidence for channeling of the wind in magnetic fields, the existence of which is related to the stars' youth. Our results demonstrate the utility of our method for deriving temperature information for large samples of X-ray emission-line spectra.

We present the first simultaneous, multiwavelength observations of an L dwarf, the L3.5 candidate brown dwarf 2MASS J00361617+1821104, conducted with the Very Large Array, the Chandra X-Ray Observatory, and the Kitt Peak 4 m telescope. We detect strongly variable and periodic radio emission (P = 3 hr) with a fraction of about 60% circular polarization. No X-ray emission is detected to a limit of L_X/L_bol ≲ 2 × 10^-5, several hundred times below the saturation level observed in early M dwarfs. Similarly, we do not detect Hα emission to a limit of L_Hα/L_bol ≲ 2 × 10^-7, the deepest for any L dwarf observed to date. The ratio of radio to X-ray luminosity is at least 4 orders of magnitude in excess of that observed in a wide range of active stars (including M dwarfs), providing the first direct confirmation that late-M and L dwarfs violate the radio/X-ray correlation. The radio emission is due to gyrosynchrotron radiation in a large-scale magnetic field of about 175 G, which is maintained on timescales longer than 3 yr. The detected 3 hr period may be due to (1) the orbital motion of a companion at a separation of about 5 stellar radii, similar to the configuration of RS CVn systems, (2) an equatorial rotation velocity of about 37 km s^-1 and an anchored, long-lived magnetic field, or (3) periodic release of magnetic stresses in the form of weak flares. In the case of orbital motion, the magnetic activity may be induced by the companion, possibly explaining the unusual pattern of activity and the long-lived signal. We conclude that fully convective stars can maintain a large-scale and stable magnetic field, but the lack of X-ray and Hα emission indicates that the atmospheric conditions are markedly different than in early-type stars and even M dwarfs. Similar observations are therefore invaluable for probing both the internal and external structure of low-mass stars and substellar objects, and for providing constraints on dynamo models.

We present some new ideas about the possibility of life developing around subgiant and red giant stars. Our study concerns the temporal evolution of the habitable zone. The distance between the star and the habitable zone, as well as its width, increases with time as a consequence of stellar evolution. The habitable zone moves outward after the star leaves the main sequence, sweeping a wider range of distances from the star until the star reaches the tip of the asymptotic giant branch. Currently there is no clear evidence as to when life actually formed on the Earth, but recent isotopic data suggest life existed at least as early as 7 × 10⁸ yr after the Earth was formed. Thus, if life could form and evolve over time intervals from 5 × 10⁸ to 10⁹ yr, then there could be habitable planets with life around red giant stars. For a 1 M_☉ star at the first stages of its post-main-sequence evolution, the temporal transit of the habitable zone is estimated to be several times 10⁹ yr at 2 AU and around 10⁸ yr at 9 AU. Under these circumstances life could develop at distances in the range 2-9 AU in the environment of subgiant or giant stars, and in the far distant future in the environment of our own solar system. After a star completes its first ascent along the red giant branch and the He flash takes place, there is an additional stable period of quiescent He core burning during which there is another opportunity for life to develop. For a 1 M_☉ star there is an additional 10⁹ yr with a stable habitable zone in the region from 7 to 22 AU. Space astronomy missions, such as proposed for the Terrestrial Planet Finder (TPF) and Darwin, that focus on searches for signatures of life on extrasolar planets, should also consider the environments of subgiants and red giant stars as potentially interesting sites for understanding the development of life. We performed a preliminary evaluation of the difficulty of interferometric observations of planets around red giant stars compared to a main-sequence star environment. We show that pathfinder missions for TPF and Darwin, such as Eclipse and FKSI, have sufficient angular resolution and sensitivity to search for habitable planets around some of the closest evolved stars of the subgiant and red giant class.

We study the dynamical origin of the structures observed in the scattered-light images of the resolved debris disk around HD 141569A. The disk has two conspicuous spiral rings and two large-scale spiral arms. We explore the roles of radiation pressure from the central star, gas drag from the gas disk, and the tidal forces from two nearby stars in creating and maintaining these structures. The disk's color, scattering function, and infrared emission suggest that submicron-sized grains dominate the dust population observed in scattered light. CO observations indicate the presence of up to 60 M_⊕ of gas. The dust grains are subject to the competing effects of expulsive radiation pressure (β > 1, where β is the ratio of the radiation and gravitational forces) and retentive gas drag. We use a simple one-dimensional axisymmetric model to show that the presence of the gas helps confine the dust and that a broad ring of dust is produced if a central hole exists in the disk. This model also suggests that the disk is in a transient, excited dynamical state, as the observed dust creation rate applied over the age of the star is inconsistent with submillimeter mass measurements. We model in two dimensions the effects of a flyby encounter between the disk and a binary star in a prograde, parabolic, coplanar orbit. We track the spatial distribution of the disk's gas, planetesimals, and dust. We conclude that the surface density distribution reflects the planetesimal distribution for a wide range of parameters. Our most viable model features a disk with initial radius 400 AU, a gas mass of 50 M_⊕, and β = 4 and suggests that the system is being observed within 4000 yr of the flyby periastron. The model reproduces some features of HD 141569A's disk, such as a broad single ring and large spiral arms, but it does not reproduce the observed multiple spiral rings or disk asymmetries nor the observed clearing in the inner disk. For the latter, we consider the effect of a 5M_J planet in an eccentric orbit on the planetesimal distribution of HD 141569A.

One of the most remarkable properties of extrasolar planets revealed by the ongoing radial velocity surveys is their high orbital eccentricities, which are difficult to explain with our current theoretical paradigm for planet formation. Observations have shown that at least ~20% of these planets, including some with particularly high eccentricities, are orbiting a component of a wide binary star system. The presence of a distant binary companion can cause significant secular perturbations to the orbit of a planet. In particular, at high relative inclinations, a planet can undergo a large-amplitude eccentricity oscillation. This so-called Kozai mechanism is effective at a very long range, and its amplitude is purely dependent on the relative orbital inclination. In this paper, we address the following simple question: assuming that every host star with a detected giant planet also has a (possibly unseen, e.g., substellar) distant companion, with reasonable distributions of orbital parameters and masses, how well could secular perturbations reproduce the observed eccentricity distribution of planets? Our calculations show that the Kozai mechanism consistently produces an excess of planets with very high (e ≳ 0.6) and very low (e ≲ 0.1) eccentricities. Assuming an isotropic distribution of relative orbital inclination, we would expect that 23% of planets do not have sufficiently high inclination angles to experience the eccentricity oscillation. By a remarkable coincidence, only 23% of currently known extrasolar planets have eccentricities e < 0.1. However, this paucity of near-circular orbits in the observed sample cannot be explained solely by secular perturbations. This is because, even with high enough inclinations, the Kozai mechanism often fails to produce significant eccentricity perturbations when there are other competing sources of orbital perturbations on secular timescales, such as general relativity. Our results show that, with any reasonable set of mass and initial orbital parameters, the Kozai mechanism always leaves more than 50% of planets on near-circular orbits. On the other hand, the Kozai mechanism can produce many highly eccentric orbits. Indeed, the overproduction of high eccentricities observed in our models could be combined with plausible circularizing mechanisms (e.g., friction from residual gas) to create more intermediate eccentricities (e ≃ 0.1-0.6).

One of the obstacles in the search for exoplanets via transits is the large number of candidates that must be followed up, few of which ultimately prove to be exoplanets. Any method that could make this process more efficient by somehow identifying the best candidates and eliminating the worst would therefore be very useful. Seager & Mallén-Ornelas demonstrated that it was possible to discern between blends and exoplanets using only the photometric characteristics of the transits. However, these techniques are critically dependent on the shape of the transit, characterization of which requires very high precision photometry of a sort that is atypical for candidates identified from transit searches. We present a method relying only on transit duration, depth, and period, which require much less precise photometry to determine accurately. The numerical tool we derive, the exoplanet diagnostic η, is intended to identify the subset of candidates from a transit search that is most likely to contain exoplanets and is thus most worthy of subsequent follow-up studies. The effectiveness of the diagnostic is demonstrated with its success in separating modeled exoplanetary transits and interlopers and by applying it to actual OGLE transit candidates.

We present a discussion of the time evolution of the mass and energy of a model coronal mass ejection (CME), analyzing both synthetic coronograph images and three-dimensional data of the numerical ideal magnetohydrodynamics (MHD) simulation. Our global steady state coronal model possesses high-latitude coronal holes and a helmet streamer structure with a current sheet near the equator, reminiscent of near solar minimum conditions. Within this model system, we drive a CME to erupt by the introduction of a Gibson-Low magnetic flux rope that is embedded in the helmet streamer in an initial state of force imbalance. The flux rope rapidly expands and is ejected from the corona with maximum speeds in excess of 1000 km s^-1 driving a fast-mode shock that propagates from the inner corona to a distance of 1 AU. We study the mass and energetics of the CME inferred from the three-dimensional results of the simulation, as well as calculated from synthetic coronograph images produced at different times. The two-dimensional plane-of-sky density structure of a CME is discussed for wide-angle coronographs, such as the Heliospheric Imager (HI-2) on board STEREO (Solar Terrestrial Relations Observatory), and compared with the three-dimensional density structure. We found that the CME mass derived from the synthetic coronographic images is an underestimate by about 50% of the total mass of the CME according to the three-dimensional data. Two main reasons can be invoked: the poor assumption that all the mass of the CME is in the plane of the sky and the wrapping of the front shock around the density-depleted cavity, which leads to an apparent decrease in brightness of the front shock.

The Bastille Day flare on 2000 July 14 was well observed by several space- and ground-based observatories and studied extensively by many researchers. Recently, we discovered that a large fraction of X-class flares are associated with a very interesting evolutionary pattern in δ sunspots: part of the outer δ spot structure decays rapidly after major flares; in the meantime, central umbral and/or penumbral structure becomes darker. These changes take place in about 1 hour and are permanent. We find that the active region NOAA AR 9077 has sunspot structure change similar to that associated with the 2000 July 14 X5.7 flare. We provide additional evidence supporting that we detected the real change in the sunspot structure after the flare. The new evidence presented in this paper include the following: (1) the Evershed velocity of decayed penumbral segments was weakened significantly following the flare, indicating actual weakening of penumbral structure; (2) based on available vector magnetograms before and after the flare, the transverse field strength decreased at the areas of penumbral decay and increased significantly near the flaring neutral line; (3) a new electric current system is found near the flare neutral line after the flare; and (4) the center-of-mass positions of opposite magnetic polarities converged toward magnetic neutral line immediately following the onset of the flare, and magnetic flux of the active region decreased steadily following the flare. There is no flare model capable of interpreting all the aspects of observations. A simple quadrupolar magnetic reconnection model may explain most of our observations: two magnetic dipoles join at the δ configuration before the flare; magnetic reconnection creates two new sets of loops: a compact flare loop and a large-scale expanding loop that might be the source of the CME. The outer penumbral fields become more vertical due to this reconnection, corresponding to the penumbral decay. Following initiation of magnetic reconnection associated with the flare, reconnected fields near the magnetic neutral line are first enhanced, then gradually weakened as it submerges. However, this model is questionable from one aspect of the observations: we failed to identify two far-end footpoints of this quadrupolar magnetic reconnection. We discuss other existing flare models in the context of our observations as well.

Travel times of acoustic waves are calculated from Dopplergrams of solar oscillations obtained using the Global Oscillation Network Group (GONG) ground-based network and the Michelson Doppler Imager (MDI) instrument on board the SOHO satellite. These travel times are inverted using a standard ray approximation to ascertain the sound-speed anomalies below two active regions. Some simple methods for ignoring the possibly corrupted measurements from within a sunspot are considered, as are diagnostics for optimizing the inversion. Results are then presented for two different spot regions, and the results of the instruments are compared: both regions behave in similar ways, and the agreement between the two instruments is good. First-skip and second-skip data are found to produce similar results for deeper layers of the model, but the significance of the shallower results from second-skip data is questionable. We conclude that GONG data are appropriate for time-distance analysis.

Recent solar photospheric abundance analyses (by Asplund et al. and Lodders) revise the C, N, O, Ne, and Ar abundances downward by 0.15-0.2 dex compared to previous determinations by Grevesse & Sauval. The abundances of Fe and other elements are reduced by smaller amounts, 0.05-0.1 dex. With these revisions, the photospheric Z/X decreases to 0.0165 (or 0.0177, according to Lodders), and Z decreases to ~0.0122 (or 0.0133, according to Lodders). A number of papers (by, e.g., Basu & Antia, Montalban et al., Bahcall & Pinsonneault, Turck-Chièze et al., and Antia & Basu) report that solar models evolved with standard opacities and diffusion treatment using these new abundances give poor agreement with helioseismic inferences for sound-speed and density profile, convection-zone helium abundance, and convection-zone depth. These authors also considered a limited set of models with increased opacities, enhanced diffusion, or abundance variations to improve agreement, finding no entirely satisfactory solution. Here we explore evolved solar models with varying diffusion treatments, including enhanced diffusion with separate multipliers for helium and other elements, to reduce the photospheric abundances, while keeping the interior abundances about the same as earlier standard models. While enhanced diffusion improves agreement with some helioseismic constraints compared to a solar model evolved with the new abundances using nominal input physics, the required increases in thermal diffusion rates are unphysically large, and none of the variations tried completely restores the good agreement attained using the earlier abundances. A combination of modest opacity increases, diffusion enhancements, and abundance increases near the level of the uncertainties, while somewhat contrived, remains the most physically plausible means to restore agreement with helioseismology. The case for enhanced diffusion would be improved if the inferred convection-zone helium abundance could be reduced; we recommend reconsidering this derivation in light of new equations of state with modified abundances and other improvements. We also recommend considering, as a last resort, diluting the convection zone, which contains only 2.5% of the Sun's mass, by accretion of material depleted in the more volatile elements C, N, O, Ne, and Ar after the Sun arrived on the main sequence.

We present low-resolution K-band spectra taken at the Gemini 8 m telescope of (90377) Sedna and (90482) Orcus (provisional designations 2003 VB₁₂ and 2004 DW, respectively), currently the two minor planets with the greatest absolute magnitudes (i.e., the two most reflective minor planets). We place crude limits on the surface composition of these two bodies using a Hapke model for a wide variety of assumed albedos. The unusual minor planet Sedna was discovered on UT 2003 November 14 at roughly 90 AU, with 1.6 times the heliocentric and perihelion distances of any other bound minor planet. It is the first solar system object discovered between the Kuiper Belt and the Oort Cloud and may represent a transition population between the two. The reflectance spectrum of Sedna appears largely featureless at the current signal-to-noise ratio, suggesting a surface likely to be highly processed by cosmic rays. For large-grain models (100 μm to 1 cm) we find that Sedna cannot have more than 70% surface coverage of water ice and cannot have more than 60% surface coverage of methane ice, to 3 σ confidence. Minor planet Orcus shows strong water ice absorption corresponding to less than a 50% surface fraction for grain models 25 μm and larger. Orcus cannot have more than 30% of its surface covered by large (100 mm to 1 cm) methane grains, to 3 σ confidence.

We present high-resolution laboratory spectra of K-shell X-ray lines from inner-shell excited and ionized ions of oxygen, obtained with a reflection grating spectrometer on the electron beam ion trap (EBIT-I) at the Lawrence Livermore National Laboratory. Only with a multi-ion model including all major atomic collisional and radiative processes are we able to identify the observed K-shell transitions of oxygen ions from O III to O VI. The wavelengths and associated errors for some of the strongest transitions are given, taking into account both the experimental and modeling uncertainties. The present data should be useful in identifying the absorption features present in astrophysical sources, such as active galactic nuclei and X-ray binaries. They are also useful in providing benchmarks for the testing of theoretical atomic structure calculations.

We reanalyze the galaxy-mass correlation function measured by the Sloan Digital Sky Survey to obtain host dark matter halo masses at galaxy and galaxy-group scales. We extend the data to galaxy clusters in the 2MASS catalog and study the relation between central galaxy luminosity and halo mass. While the central galaxy luminosity scales as ~M^0.7 to M^0.8 at low masses, the relation flattens to shallower than ~M^0.3 above ~4 × 10¹³M_☉. The total luminosity of galaxies in the halo, however, continues to grow as a power law ~M^0.8-M^0.9. Starting from the hypothesis that the central galaxies grow by hierarchical merging ("galactic cannibalism"), we develop a simple model for the evolution of their luminosities as a consequence of the accretion of satellite galaxies, tracking the merging of dark matter halos. The luminosity-mass relation flattens when the timescale on which dynamical friction induces orbital decay in the satellite galaxies exceeds the age of the dark matter halo. Then the growth of the central galaxy is suppressed, as it can cannibalize only the rare, massive satellite galaxies. The model takes the dependence of the total luminosity of galaxies in a halo on its mass and the global galaxy luminosity function as input and reproduces the observed central galaxy luminosity-mass relation over 3 decades in halo mass, 10¹²-10¹⁵M_☉. The success of the model suggests that gas cooling and subsequent star formation did not play an important role in the final assembly of central galaxies from sub-L_⋆ precursors.

Using the empirical relations between the central galaxy luminosity and the halo mass, and between the total galaxy luminosity in a halo and the halo mass, we construct the galaxy luminosity function (LF). To the luminosity of the central galaxy in a halo of a given mass, we assign lognormal scatter with a mean calibrated against the observations. In halos where the total galaxy luminosity exceeds that of the central galaxy, satellite galaxies are distributed as a power law in luminosity. Combined with the halo mass function, this description reproduces the observed characteristics of the galaxy LF, including a shape consistent with the Schechter function. The L_⋆ in the LF is the luminosity above which the central galaxy luminosity-halo mass relation begins to flatten in halos above ~10¹³M_☉. In surveys in which central galaxies in massive clusters are neglected, either by design or because of the cosmic variance, L_⋆ is simply the mean luminosity of central galaxies in halos at the upper end of the selected mass range. The smooth, exponential decay of the Schechter function toward high luminosities reflects the intrinsic scatter in the central galaxy luminosity-halo mass relation. In addition to the LF, the model successfully reproduces the observed dependence of galaxy clustering bias on luminosity.

The supermassive black hole at the center of a distant galaxy can be weighed, in rare but realistic cases, when the galaxy acts as a strong gravitational lens. The central image that should be produced by the lens is either destroyed or accompanied by a second central image, depending on the mass of the black hole. We demonstrate that when a central image pair is detected, the mass of the black hole can be determined with an accuracy of ≲0.1 dex, if the form of the smooth mass distribution near the galaxy core is known. Uncertainty in the central mass distribution introduces a systematic error in the black hole mass measurement. However, even with nearly complete ignorance of the inner mass distribution, the black hole mass can still be determined to within a factor of 10. Central image pairs should be readily observable with future radio interferometers, allowing this technique to be used for a census of supermassive black holes in inactive galaxies at significant redshift (0.2 ≲ z ≲ 1.0).

We investigate morphological structure parameters and local environments of distant moderate-luminosity active galactic nucleus (AGN) host galaxies in the overlap between the HST/ACS observations of the Great Observatories Origins Deep Survey (GOODS) and the two Chandra Deep Fields. We compute near-neighbor counts and BViz asymmetry (A) and concentration (C) indices for ≈35,500 GOODS/ACS galaxies complete to z₈₅₀ ≈ 26.6, including the resolved hosts of 322 X-ray-selected AGNs. Distributions of (1) z₈₅₀ asymmetry for 130 z₈₅₀ < 23 AGN hosts and (2) near-neighbor counts for 173 z₈₅₀ < 24 AGN hosts are both consistent with non-AGN control samples. This implies no close connection between recent galaxy mergers and moderate-luminosity AGN activity out to appreciable look-back times (z ≲ 1.3), approaching the epoch of peak AGN activity in the universe. The distribution of z₈₅₀C for the AGN hosts is offset by ΔC ≈ +0.5 compared to the non-AGN, a 6.4 σ discrepancy much larger than can be explained by the possible influence of unresolved emission from the AGN or a circumnuclear starburst. The local universe association between AGN and bulge-dominated galaxies thus persists to substantial look-back time. We discuss implications in the context of the low-redshift supermassive central black hole mass correlation with host galaxy properties, including concentration.

Two recent studies based on composite reddened quasar spectra have indicated the presence of "gray" dust in quasar environments. This gray dust has a relatively flat extinction law in the UV, consistent with the theoretical expectation of a lack of small dust grains close to a quasar. In contrast, individual reddened quasars in the Sloan Digital Sky Survey tend to have steep extinction laws in the UV, similar to that in the SMC. We analyze the method used in determining extinction laws from composite quasar spectra in order to resolve this discrepancy. We show that quasars reddened by SMC-type dust that are present in quasar samples have a negative correlation between E_B-V and redshift, due to selection effects. The fact that the highest redshift quasars (which contribute to the UV part of a composite spectrum) are less extincted leads to shallower extinction in the UV. We construct a composite quasar spectrum from a simulated sample of quasars reddened by SMC-type dust and show that the extinction curve derived from the composite does not recover the intrinsic extinction law. We conclude there is no evidence of gray dust in quasar environments.

We present VLA H I 21 cm observations of HIJASS J1021+6842, which has been discovered in the direction of the M81 Group. Our synthesis imaging reveals that the H I is distributed over a larger angular extent and velocity range than the single-dish discovery observations. Assuming that HIJASS J1021+6842 is at the distance of the M81 Group, we detect 1.5 × 10⁸M_☉ of H I distributed over as much as 30 kpc, i.e., substantially larger than the biggest dwarf galaxies in the same group. At the depth of our imaging, the H I appears to be confined to at least seven clouds. Peak H I column densities are ~1.8 × 10²⁰ atoms cm^-2, which is well below the canonical star formation threshold of ~10²¹ atoms cm^-2 and therefore consistent with the fact that no optical counterpart has as yet been identified. A gradient in velocity is observed across the extent of the detected H I; assuming that the object is gravitationally bound we derive a dynamical mass of 7 × 10⁹M_☉ and a dark-to-luminous mass ratio of >10. Alternatively, a tidal origin may also result in the observed velocity gradient, which would lead to a considerably lower dynamical mass. Given the above properties and the absence of evidence of a stellar population, HIJASS J1021+6842 is unique amongst the other systems in the M81 Group.

A double-peak spectral energy density of prompt γ-rays, similar to that observed in blazars, is expected in the cannonball (CB) model of gamma-ray bursts (GRBs) produced in supernova (SN) explosions. The first sub-MeV ordinary peak is formed by inverse Compton scattering (ICS) of the ambient SN light by the CBs' electrons, while the second peak at GeV-TeV energies is formed by ICS of interstellar medium electrons accelerated by the jetted CBs from the SN explosion. Usually the second peak is in the GeV-TeV range. However, in X-ray flashes with a low peak energy, which in the CB model are normal GRBs viewed far off-axis, the second peak energy moves to the MeV range. In far off-axis GRBs, such as 980425 and 031203, the second peak may have been confused with the normal GRB peak. In most GRBs that have been observed so far, the γ-ray detectors ran out of statistics far below the second peak. However, in bright GRBs, the two peaks may be resolved by simultaneous measurements with Swift, INTEGRAL, and GLAST.

Dust emission from the Type II supernova SN 2002hh in NGC 6946 has been detected at mid-infrared wavelengths by the Spitzer Space Telescope from 590 to 758 days after outburst and confirmed by higher angular resolution Gemini North mid-IR observations. The day 600 5.8-24 μm emission can be fit by a 290 K blackbody having a luminosity of 1.6 × 10⁷L_☉. The minimum emitting radius of 1.1 × 10¹⁷ cm is too large for the emitting dust to have been formed in the supernova ejecta. Using radiative transfer models and realistic dust grain parameters, fits to the observed flux distribution could be obtained with an optically thick dust shell having a mass of 0.10-0.15 M_☉, corresponding to a total dust+gas mass in excess of 10 M_☉, suggesting a massive M supergiant or luminous blue variable precursor to this self-obscured object.

We report the detection of a new interstellar molecule: CH₃CCD, a deuterated isotopomer of methyl acetylene. Analysis of lines detected at seven positions along the ridge of the dark cloud TMC-1 indicates that the CH₃CCD column density and fractionation exhibit strong spatial gradients, with D/H ratios ranging from a minimum of 0.04 at the cyanopolyyne peak (CP) position to as high as 0.18. We find that the column density in CH₂DCCH is almost constant along the ridge, whereas that of CH₃CCD increases and that of CH₃CCH decreases in moving away from the CP. Chemical mechanisms that may explain the observed fractionation trends in CH₃CCD and CH₂DCCH are briefly discussed.

The recent detection of shock precursors toward the very young L1448-mm outflow offers us the possibility to study the grain chemistry during the first stages of the shock evolution, constraining the molecules ejected from grains and the species formed in gas phase. Observations of key molecules in the grain chemistry such as SiO, CH₃OH, SO, CS, H₂S, OCS, and SO₂ toward this outflow are presented. The line profiles and the derived abundances show three distinct velocity regimes that trace the shock evolution: the preshock, the shock precursor, and the postshock gas. The SiO, CH₃OH, SO, and CS abundances are enhanced with respect to the quiescent gas by 1 order of magnitude in the shock-precursor component, and by 3 orders of magnitude in the postshock gas. The derived SiO and CH₃OH abundances are consistent with the recent ejection of these molecules from grains. Since H₂S is only enhanced in the shock-precursor component and OCS and SO₂ are undetected, SO and CS are the most abundant sulfur-bearing species in the grain mantles of L1448-mm. The ejection of mainly SO and CS rather than H₂S or OCS from grains suggests that the sulfur chemistry will depend on the chemical "history" of the grain mantles in outflows and hot cores.

The possible existence of intermediate-mass binary black holes (IMBBHs) in globular clusters (GCs) offers us a unique geometry in which to detect spacetime oscillations. For certain pulsar-IMBBH configurations possible within a GC, the usual far-field plane wave approximation for the IMBBH metric perturbation severely underestimates the induced pulse time-of-arrival (TOA) fluctuations. In this Letter, the expected TOA fluctuations induced by an IMBBH lying close to the line of sight between a pulsar and the Earth are calculated for the first time. For an IMBBH consisting of 10 and 10³M_☉ components, a 10 yr orbital period, and located 0.1 lt-yr from the Earth-pulsar line of sight, the induced TOA fluctuations will be of order 5-500 ns.

Soft γ-ray repeaters (SGRs) are neutron stars that emit short (≲1 s) and energetic (≲10⁴² ergs s^-1) bursts of soft γ-rays. Only four of them are currently known. Occasionally, SGRs have been observed to emit much more energetic "giant flares" (~10⁴⁴-10⁴⁵ ergs s^-1). These are exceptional and rare events. We report here on serendipitous observations of the intense γ-ray flare from SGR 1806-20 that occurred on 2004 December 27. Unique data from the Cluster and Double Star TC-2 satellites, designed to study the Earth's magnetosphere, provide the first observational evidence of three separate timescales within the early (first 100 ms) phases of this class of events. These observations reveal that in addition to the initial very steep (<0.25 ms) X-ray onset, there is first a 4.9 ms exponential rise timescale followed by a continued exponential rise in intensity on a timescale of 70 ms. These three timescales are a prominent feature of current theoretical models, including the timescale (several milliseconds) for fracture propagation in the crust of the neutron star.

We report on the results of an ~30 ks Chandra pointing of the soft gamma-ray repeater SGR 1806-20, the first X-ray observation with high spectral resolution performed after the 2004 December 27 giant flare. The source was found in a bursting active phase and with a significantly softer spectrum than that of the latest observations before the giant flare. The observed flux in the 2-10 keV range was ~2.2 × 10^-11 ergs cm^-2 s^-1, about 20% lower than that measured 3 months before the event. This indicates that although its giant flare was ≈100 times more intense than those previously observed in two other soft gamma-ray repeaters, the postflare X-ray flux decay of SGR 1806-20 has been much faster. The pulsed fraction was ~3%, a smaller value than that observed before the flare. We discuss the different properties of the postflare evolution of SGR 1806-20 in comparison to those of SGR 1900+14 and interpret the results as strong evidence that a magnetospheric untwisting occurred (or is occurring) after the giant flare.

We present new X-ray timing and spectral results on the 8.0 s X-ray pulsar CXOU J010043.1-721134 from a series of observations using the Chandra X-Ray Observatory. We found a spin period in 2004 January of 8.020392 ± 0.000009 s. A comparison of this to the 2001 Chandra observations implies a period derivative = (1.88 ± 0.08) × 10^-11 s s^-1, leading to an inferred dipole surface magnetic field of 3.9 × 10¹⁴ G. The spectrum is well fit to an absorbed blackbody of temperature kT = 0.38 ± 0.02 keV with a power-law tail of photon index Γ = 2.0 ± 0.6. We find that the source has an unabsorbed X-ray flux (0.5-10 keV) of 4 × 10^-13 ergs cm^-2 s^-1 and a corresponding X-ray luminosity of ~2 × 10³⁵ ergs s^-1 for a distance of 60 kpc. These properties support the classification of CXOU J010043.1-721134 as the seventh confirmed anomalous X-ray pulsar, the eleventh confirmed magnetar, and the first magnetar to be identified in the Small Magellanic Cloud.

V838 Monocerotis had an intriguing, nova-like outburst in 2002 January that has subsequently led to several studies of the object. It is now recognized that the outburst of V838 Mon and its evolution are different from those of a classical nova or other classes of well-known eruptive variables. V838 Mon, along with two other objects that have analogous properties, appears to make up a new class of eruptive variables. There are limited infrared studies of V838 Mon. Here we present near-infrared H-band (1.5-1.75 μm) spectra of V838 Mon from late 2002 to the end of 2004. The principal new result from our work is the detection of several rotation-vibration lines of water in the H-band spectra. The observed water lines have been modeled to first establish that they are indeed due to water. Subsequently the temperature and column densities of the absorbing material, from where the water absorption features originate, are derived. From our analysis, we find that the water features arise from a cool, ~750-900 K, region around V838 Mon that appears to be gradually cooling with time.

Employing spectra obtained with the new Keck I HIRES near-UV-sensitive detector, we have performed a comprehensive chemical composition analysis of the binary blue metal-poor star CS 29497-030. Abundances for 29 elements and upper limits for an additional seven have been derived, concentrating on elements largely produced by means of neutron-capture nucleosynthesis. Included in our analysis are the two elements that define the termination point of the slow neutron-capture process, lead and bismuth. We determine an extremely high value of [Pb/Fe] = +3.65 ± 0.07 (σ = 0.13) from three features, supporting the single-feature result obtained in previous studies. We detect Bi for the first time in a metal-poor star. Our derived Bi/Pb ratio is in accord with those predicted from the most recent FRANEC calculations of the slow neutron-capture process in low-mass asymptotic giant branch (AGB) stars. We find that the neutron-capture elemental abundances of CS 29497-030 are best explained by an AGB model that also includes very significant amounts of pre-enrichment of rapid neutron-capture process material in the protostellar cloud out of which the CS 29497-030 binary system formed. Mass transfer is consistent with the observed [Nb/Zr] ~ 0. Thus, CS 29497-030 is both an r+s and "extrinsic AGB" star. Furthermore, we find that the mass of the AGB model can be further constrained by the abundance of the light odd-element Na.

Recent analysis of high-resolution Chandra X-ray spectra has shown that the Ne/O abundance ratio is remarkably constant in stellar coronae. Based on this result, we point out the utility of the Ne/O ratio as a discriminant for accretion-related X-rays from T Tauri stars and for probing the measure of grain depletion of the accreting material in the inner disk. We apply the Ne/O diagnostic to the classical T Tauri stars BP Tau and TW Hya—the two stars found to date whose X-ray emission appears to originate, at least in part, from accretion activity. We show that TW Hya appears to be accreting material that is significantly depleted in O relative to Ne. In contrast, BP Tau has an Ne/O abundance ratio consistent with that observed for post-T Tauri stars. We interpret this result in terms of the different ages and evolutionary states of the circumstellar disks of these stars. In the young BP Tau disk (age ~0.6 Myr), dust is still present near the disk corotation radius and can be ionized and accreted, rereleasing elements depleted onto grains. In the more evolved TW Hya disk (age ~10 Myr), evidence points to ongoing coagulation of grains into much larger bodies, and possibly planets, that can resist the drag of inward-migrating gas, and the accreting gas is consequently depleted of grain-forming elements.

The commonly used minimum-mass power-law representation of the early solar nebula is reanalyzed using a new cumulative mass model. This model is a first integral of the planetary data and predicts a smoother surface density approximation compared with methods based on direct computation of surface density. The density is quantified using two independent analytical formulations. First, a best-fit transcendental function is applied directly to the basic planetary data. Next, a solution to the time-dependent disk evolution equation is parametrically adapted to the solar nebula data. The latter model is shown to be a good approximation to the finite-size early solar nebula and, by extension, to extrasolar protoplanetary disks.

We present calculations of the oscillator strengths of Fe II λλ2507, 2509, which are the strongest observed lines in the emission spectrum of the B and D Weigelt blobs of η Carinae. Unlike previous determinations of these oscillator strengths, our results are compatible with the modeling undertaken by Verner et al.