Table of contents

We study the baryonic gas clouds (the intergalactic medium) in the universe before reionization with the lognormal (LN) model, which has been shown to be dynamically legitimate in describing the fluctuation evolution in quasilinear as well as nonlinear regimes in recent years. The probability distribution function of the mass field in the LN model is long-tailed and so plays an important role in rare events, such as the formation of the first generation of baryonic objects. Since in this model the nonlinear field is directly mapped from the corresponding linear field, we can calculate the density and velocity distributions of the intergalactic medium at very high spatial resolutions. We simulate the distributions at a resolution of 0.15 kpc from z = 7 to 15 in the low-density cold dark matter cosmological model. We analyze statistics on the hydrogen clouds at high redshifts, including column densities, clumping factors, sizes, masses, and spatial number densities. One of our goals is to identify which hydrogen clouds are going to collapse. By inspecting the mass density profiles and the velocity profiles of clouds, we find that the velocity outflow significantly postpones the collapsing process in less massive clouds, even when their masses are larger than the Jeans mass. This indicates that the formation of collapsed clouds with small mass at high redshift is substantially suppressed. Consequently, only massive (>10⁵M_☉) clouds can form objects at higher redshifts, and less massive (10⁴-10⁵M_☉) collapsed objects are formed later. Although the mass fraction in clouds with sizes larger than the Jeans length is already larger than 1% at z = 15, there is only a tiny fraction of mass (10^-8) in collapsed clouds. If all the ionizing photons and the ~10^-2 metallicity observed at low redshift are produced by the first 1% of the mass of collapsed baryonic clouds, the majority of that first generation of objects would be occurring not much earlier than z = 10.

We investigate formation of molecules and first objects in a model with a decaying cosmological term (DCT). In particular, we show how the formation of important molecules such as H₂ and HD is influenced during the epoch of the redshift z from 10⁴ to 1. We find that for a phenomenological DCT molecular formation is significantly shifted to the earlier epoch by Δz ~ 10³ compared to the case in the standard big bang cosmology, as a result of the rapid decrease in the matter temperature. The mass of first objects is estimated using the timescales relevant to their formation. We suggest that the formation scenarios of the objects studied so far could be revised if a DCT is taken into account.

We investigate the Butcher-Oemler effect using samples of galaxies brighter than observed-frame K* + 1.5 in 33 clusters at 0.1 ≲ z ≲ 0.9. We attempt to duplicate as closely as possible the methodology of Butcher & Oemler. Apart from selecting in the K band, the most important difference is that we use a brightness limit fixed at 1.5 mag below an observed-frame K* rather than the nominal limit of rest-frame M_V = -20 used by Butcher & Oemler. For an early-type galaxy at z = 0.1, our sample cutoff is 0.2 mag brighter than rest-frame M_V = -20, while at z = 0.9, our cutoff is 0.9 mag brighter. If the blue galaxies tend to be faint, then the difference in magnitude limits should result in our measuring lower blue fractions. A more minor difference from the Butcher & Oemler methodology is that the area covered by our galaxy samples has a radius of 0.5 or 0.7 Mpc at all redshifts, rather than R₃₀, the radius containing 30% of the cluster population. In practice our field sizes are generally similar to those used by Butcher & Oemler. We find that the fraction of blue galaxies in our K-selected samples is lower on average than that derived from several optically selected samples and that it shows little trend with redshift. However, at the redshifts z < 0.6, at which our sample overlaps with that of Butcher & Oemler, the difference in f_B as determined from our K-selected samples and those of Butcher & Oemler is much reduced. The large scatter in the measured f_B, even in small redshift ranges, in our study indicates that determining the f_B for a much larger sample of clusters from K-selected galaxy samples is important.

As a test of our methods, our data allow us to construct optically selected samples down to rest-frame M_V = -20, as used by Butcher & Oemler, for four clusters that are common between our sample and theirs. For these rest-frame V-selected samples, we find similar fractions of blue galaxies to Butcher & Oemler, while the K-selected samples for the same four clusters yield blue fractions that are typically half as large. This comparison indicates that selecting in the K band is the primary difference between our study and previous optically based studies of the Butcher & Oemler effect. Selecting in the observed K band is more nearly a process of selecting galaxies by their mass than is the case for optically selected samples. Our results suggest that the Butcher-Oemler effect is at least partly due to low-mass galaxies whose optical luminosities are boosted. These lower mass galaxies could evolve into the rich dwarf population observed in nearby clusters.

We study the structure and substructure of halos obtained in N-body simulations for a ΛCDM cosmology with non-Gaussian initial conditions. The initial statistics are lognormal in the gravitational potential field with positive (LN_p) and negative (LN_n) skewness; the sign of the skewness is conserved by the density field, and the power spectrum is the same for all the simulations. Our aim is not to test a given non-Gaussian statistics but to explore the generic effect of positive- and negative-skew statistics on halo properties. From our low-resolution simulations, we find that LN_p (LN_n) halos are systematically more (less) concentrated than their Gaussian counterparts. This result is confirmed by our Milky Way- and cluster-sized halos resimulated with high resolution. In addition, they show inner density profiles that depend on the statistics: the innermost slopes of LN_p (LN_n) halos are steeper (shallower) than those obtained from the corresponding Gaussian halos. A subhalo population embedded in LN_p halos is more susceptible to destruction than its counterpart inside Gaussian halos. On the other hand, subhalos in LN_n halos tend to survive longer than subhalos in Gaussian halos. The spin parameter probability distribution of LN_p (LN_n) halos is skewed to smaller (larger) values with respect to the Gaussian case. Our results show how the statistics of the primordial density field can influence some halo properties, opening the possibility of constraining, albeit indirectly, the primordial statistics at small scales.

We present a semianalytic model to investigate the merger history, destruction rate, and survival probability of substructure in hierarchically formed dark matter halos and use it to study the substructure content of halos as a function of input primordial power spectrum. For a standard cold dark matter "concordance" cosmology (ΛCDM; n = 1, σ₈ = 0.95) we successfully reproduce the subhalo velocity function and radial distribution profile seen in N-body simulations and determine that the rate of merging and disruption peaks ~10-12 Gyr in the past for Milky Way-like halos, while surviving substructures are typically accreted within the last ~0-8 Gyr. We explore power spectra with normalizations and spectral "tilts" spanning the ranges σ₈ ≃ 1-0.65 and n ≃ 1-0.8, and include a "running-index" model with dn/d ln k = -0.03 similar to the best-fit model discussed in the first-year Wilkinson Microwave Anisotropy Probe (WMAP) report. We investigate spectra with truncated small-scale power, including a broken-scale inflation model and three warm dark matter cases with m_W = 0.75-3.0 keV. We find that the mass fraction in substructure is relatively insensitive to the tilt and overall normalization of the primordial power spectrum. All of the CDM-type models yield projected substructure mass fractions that are consistent with, but on the low side, of published estimates from strong lens systems: f₉ = 0.4%-1.5% (64th percentile) for subhalos smaller than 10⁹M_☉ within projected cylinders of radius r < 10 kpc. Truncated models produce significantly smaller fractions, f₉ = 0.02%-0.2% for m_W ≃ 1 keV, and are disfavored by lensing estimates. This suggests that lensing and similar probes can provide a robust test of the CDM paradigm and a powerful constraint on broken-scale inflation/warm particle masses, including masses larger than the ~1 keV upper limits of previous studies. We compare our predicted subhalo velocity functions with the dwarf satellite population of the Milky Way. Assuming that dwarfs have isotropic velocity dispersions, we find that the standard n = 1 model overpredicts the number of Milky Way satellites at V_max ≲ 35 km s^-1, as expected. Models with less small-scale power do better because subhalos are less concentrated and the mapping between observed velocity dispersion and halo V_max is significantly altered. The running-index model, or a fixed tilt with σ₈ ~ 0.75, can account for the local dwarfs without the need for differential feedback (for V_max ≳ 20 km s^-1); however, these comparisons depend sensitively on the assumption of isotropic velocities in satellite galaxies.

We study high-redshift structure formation and reionization in a ΛCDM universe under the assumption that the spectral power index of primordial density fluctuations is a function of length scale. We adopt a particular formulation of the "running" spectral index (RSI) model as suggested by the combined analysis of the recent Wilkinson Microwave Anisotropy Probe (WMAP) data and two other large-scale structure observations. We carry out high-resolution cosmological simulations and use them to study the formation of primordial gas clouds where the first stars are likely to form. While early structure forms hierarchically in the RSI model, quite similar to the standard power-law ΛCDM model, the reduced power on small scales causes a considerable delay in the formation epoch of low-mass (~10⁶M_☉) "minihalos" compared with the ΛCDM model. The abundance of primordial star-forming gas clouds in such halos also differs by more than an order of magnitude at z > 15 between the two models. The extremely small number of gas clouds in the RSI model indicates that reionization is initiated later than z < 15, generally resulting in a smaller total Thomson optical depth than in the ΛCDM model. By carrying out radiative transfer calculations, we also study reionization by stellar populations formed in galaxies. We show that, in order to reionize the universe by z ~ 7, the escape fraction of ultraviolet photons from galaxies in the RSI model must be as high as 0.6 throughout the redshift range 5 < z < 18 for a stellar population similar to that of the local universe. Even with a top-heavy initial mass function representing an early population of massive stars and/or an extraordinarily high photon emission rate from galaxies, the total optical depth can only be as large as τ_e ~ 0.1 for reasonable models of early star formation. The RSI model is thus in conflict with the large Thomson optical depth inferred by the WMAP satellite.

All the analyses of cosmic microwave background (CMB) temperature maps up to date show that CMB anisotropies follow a Gaussian distribution. On the other hand, astrophysical foregrounds, which hamper the detection of the CMB angular power spectrum, are not Gaussian-distributed on the sky. Therefore, they should give a sizeable contribution to the CMB bispectrum. In fact, the first-year data of the Wilkinson Microwave Anisotropy Probe (WMAP) mission have allowed the first detection of the extragalactic source contribution to the CMB bispectrum at 41 GHz and, at the same time, much tighter limits than before to non-Gaussian primordial fluctuations. In view of the above, and for achieving higher precision in current and future CMB measurements of non-Gaussianity, in this paper we discuss a comprehensive assessment of the bispectrum due to either uncorrelated or clustered extragalactic point sources in the entire frequency interval around the CMB intensity peak. Our calculations, based on current cosmological evolution models for sources, show that the reduced angular bispectrum due to point sources b_ps should be detectable in all WMAP and Planck frequency channels. We also find agreement with the results for b_ps at 41 GHz coming from the analysis of the first-year WMAP data. Moreover, by comparing b_ps with the primordial reduced CMB bispectrum, we find that only the peak value of the primordial bispectrum (which appears at l ≃ 200) results in greater than b_ps in a frequency window around the intensity peak of the CMB. The amplitude of this window basically depends on the capability of the source detection algorithms (i.e., on the achievable flux detection limit S_lim for sources). Finally, our current results show that at low frequencies (i.e., ν ≤ 100 GHz) the angular bispectrum of a clustered distribution of sources does not seem substantially different from that of Poisson-distributed ones, by using realistic angular correlation functions suitable to apply to the relevant source populations. On the other hand, we also find that at higher frequencies (i.e., ν ≥ 300 GHz) the clustering term can greatly enhance the normalization of b_ps.

We present a measurement of the angular spectrum of the cosmic microwave background from l = 26 to 225 from the 30 and 40 GHz channels of the MAT/TOCO experiment based on two seasons of observations. At comparable frequencies, the data extend to a lower l than the recent Very Small Array and DASI results. After accounting for known foreground emission in a self-consistent analysis, a rise from the Sachs-Wolfe plateau to a peak of δT_l ≈ 80 μK near l ≈ 200 is observed.

We report measurements of Ω_M, Ω_Λ, and w from 11 supernovae (SNe) at z = 0.36-0.86 with high-quality light curves measured using WFPC2 on the Hubble Space Telescope (HST). This is an independent set of high-redshift SNe that confirms previous SN evidence for an accelerating universe. The high-quality light curves available from photometry on WFPC2 make it possible for these 11 SNe alone to provide measurements of the cosmological parameters comparable in statistical weight to the previous results. Combined with earlier Supernova Cosmology Project data, the new SNe yield a measurement of the mass density Ω_M = 0.25 (statistical) ± 0.04 (identified systematics), or equivalently, a cosmological constant of Ω_Λ = 0.75 (statistical) ± 0.04 (identified systematics), under the assumptions of a flat universe and that the dark energy equation-of-state parameter has a constant value w = -1. When the SN results are combined with independent flat-universe measurements of Ω_M from cosmic microwave background and galaxy redshift distortion data, they provide a measurement of w = -1.05 (statistical) ± 0.09 (identified systematic), if w is assumed to be constant in time. In addition to high-precision light-curve measurements, the new data offer greatly improved color measurements of the high-redshift SNe and hence improved host galaxy extinction estimates. These extinction measurements show no anomalous negative E(B-V) at high redshift. The precision of the measurements is such that it is possible to perform a host galaxy extinction correction directly for individual SNe without any assumptions or priors on the parent E(B-V) distribution. Our cosmological fits using full extinction corrections confirm that dark energy is required with P(Ω_Λ > 0) > 0.99, a result consistent with previous and current SN analyses that rely on the identification of a low-extinction subset or prior assumptions concerning the intrinsic extinction distribution.

The inability of standard models to explain the flux ratios in many four-image gravitational lens systems has been presented as evidence for significant small-scale structure in lens galaxies. That claim has generally relied on detailed lens modeling, so it is both model dependent and somewhat difficult to interpret. We present a more robust and generic method for identifying lenses with small-scale structure. For a close triplet of images created when the source lies near an ideal cusp catastrophe, the sum of the signed magnifications should exactly vanish, independent of any global properties of the lens potential. For realistic cusps, the magnification sum vanishes only approximately, but we show that it is possible to place strong upper bounds on the degree to which the magnification sum can deviate from zero. Lenses with flux ratio "anomalies," or fluxes that significantly violate the upper bounds, can be said with high confidence to have structure in the lens potential on scales of the image separation or smaller. Five observed lenses have such flux ratio anomalies: B2045+265 has a strong anomaly at both radio and optical/near-IR wavelengths; B0712+472 has a strong anomaly at optical/near-IR wavelengths and a marginal anomaly at radio wavelengths; 1RXS J1131-1231 has a strong anomaly at optical wavelengths; RX J0911+0551 appears to have an anomaly at optical/near-IR wavelengths, although the conclusion in this particular lens is subject to uncertainties in the typical strength of octopole density perturbations in early-type galaxies; and finally, SDSS J0924+0219 has a strong anomaly at optical wavelengths. Interestingly, analysis of the cusp relation does not reveal a significant anomaly in B1422+231, even though this lens is known to be anomalous from detailed modeling. Methods that are more sophisticated (and less generic) than the cusp relation may therefore be necessary to uncover flux ratio anomalies in some systems. Although these flux ratio anomalies might represent either millilensing or microlensing, we cannot identify the cause of the anomalies using only broadband flux ratios in individual lenses. Rather, the conclusion we can draw is that the lenses have significant structure in the lens potential on scales comparable to or smaller than the separation between the images. Additional arguments must be invoked to specify the nature of this small-scale structure.

In this paper, we exploit the gravitational potential of the rich cluster A2218 as a magnifying glass. We demonstrate that the magnification due to the cluster allows us to observe distant background galaxies at a comparable level of detail to galaxies at z ~ 0.1. Using the Gemini Multiobject Spectrograph (GMOS) integral field unit (IFU) on Gemini North, we observed the spatially resolved [O II] λ3727 emission line spectrum for a lensed disk galaxy at z = 1.034. Using a detailed model for the cluster mass distribution, we are able to correct for the lensing by the cluster and reconstruct the source morphology. We find that the overall magnification is a factor of 4.92 ± 0.15, and the rest-frame absolute I-band magnitude is M = -22.4 ± 0.2, where the error bars include conservative estimates of the uncertainty in the source-plane reconstruction. The inclination-corrected circular velocity is 206 ± 18 km s ^-1. The galaxy lies very close to the mean Tully-Fisher relation of present-day spirals. Although our results are based on a single object, they demonstrate that gravitational lensing can be viably used to make detailed studies of the evolution of the structure of distant field galaxies.

We describe a method for the determination of black hole masses based on information inferred from high-energy spectra. It is required that the spectral energy distribution consist of thermal and Comptonized components. One can then, in principle, infer the depth of the gravitational potential well for sources of known distance. The thermal component is inferred by the integration of a blackbody spectral form over the disk. We assume that the color temperature distribution in the disk has a specific shape given by the Shakura-Sunyaev disk model that goes to zero at the inner disk radius and at infinity and has a maximum at 4.2 R_S. In this formulation there is only one parameter, the so-called color correction factor, relating the apparent temperature to effective temperature, which characterizes the thermal emission component. We have made use of improved Galactic black hole binary dynamical mass determinations to derive, in effect, an empirical calibration of this factor. We then present our analysis of observational data for representative objects of several classes: Galactic black hole X-ray binaries, narrow-line Seyfert galaxies (NLS1s), and "ultraluminous" extragalactic X-ray sources (ULXs). We then apply our mass determination calculation and present our results. We argue that this approach can potentially fill a void in the current knowledge of NLS1 and ULX properties and discuss how a deeper understanding of both classes has relevance to the broader issues of how cosmic black holes, beyond the stellar-mass realm, are formed and what is their overall mass distribution.

Near-infrared spectra of 15 high-redshift radio galaxies (HzRGs) located at 2 < z < 2.6 were obtained by the OH Airglow Suppressor spectrograph mounted on the Subaru telescope. The UV-optical line ratio diagnostic diagrams indicate that half of the observed HzRGs have extended emission-line regions with low metal abundance, photoionized by a flat-continuum active galactic nucleus such as a quasar. We also found two probable correlations between radio and rest-optical parameters: (1) HzRGs with massive hosts tend to have a redder rest-optical continuum, and (2) HzRGs with smaller radio sizes also show a redder optical continuum. On the basis of the correlations, the nature of HzRGs at 2 < z < 2.6 is discussed.

We present new Chandra X-ray observations of the luminous and cosmologically significant X-ray cluster of galaxies, MS 0451.6-0305, at z = 0.5386. Spectral imaging data for the cluster are consistent with an isothermal cluster of (10.0-10.6) ± 1.6 keV, with an intracluster Fe abundance of (0.32-0.40) ± 0.13 solar. The systematic uncertainties, arising from calibration and model uncertainties, of the temperature determination are nearly the same size as the statistical uncertainties, since the time-dependent correction for absorption on the detector is uncertain for these data. We discuss the effects of this correction on the spectral fitting. The effects of statistics and fitting assumptions of two-dimensional models for the X-ray surface brightness are thoroughly explored. This cluster appears to be elongated, and so we quantify the effects of assuming an ellipsoidal gas distribution on the gas mass and the total gravitating mass estimates. These data are also jointly fitted with previous Sunyaev-Zeldovich observations to obtain an estimate of the cluster's distance (D_A = 1219 ± 387 Mpc, statistical followed by systematic uncertainties) assuming spherical symmetry. If, instead, we assume a Hubble constant, the X-ray and Sunyaev-Zeldovich data are used together to test the consistency of an ellipsoidal gas distribution and to weakly constrain the intrinsic axis ratio. The mass derived from the X-ray data is consistent with the weak-lensing mass and is only marginally less than the mass determined from the optical velocities. We confirm that this cluster is very hot and massive, further supporting the conclusion of previous analyses that the universe has a low matter density and that cluster properties have not evolved much since z ~ 0.5. Furthermore, the presence of iron in this high-redshift cluster at an abundance that is the same as that of low-redshift clusters implies that there has been very little evolution of the cluster iron abundance since z ~ 0.5. We discuss the possible detection of a faint, soft, extended component that may be the by-product of hierarchical structure formation.

Results of optical identification of the ASCA Lynx Deep Survey are presented. Six X-ray sources are detected in the 2-7 keV band using the Solid-State Imaging Spectrometer in a ~20' × 20' field of view with fluxes larger than ~4 × 10^-14 ergs s^-1 cm^-2 in the band. Follow-up optical spectroscopic observations were made, and five out of six sources are identified with active galactic nuclei/quasi-stellar objects (AGNs/QSOs) at redshifts of 0.5-1.3. We also identify two more additional X-ray sources detected in a soft X-ray band with AGNs/QSOs. We find that three QSOs identified are located at z ~ 1.3. Two rich clusters and several groups of galaxies are also placed at the same redshift in the surveyed field, and projected separations between the QSOs and the clusters are 3-8 Mpc at the redshift.

We present a comprehensive report on the high-energy properties of the γ-ray blazar PKS 0208-512 as observed with EGRET and the Imaging Compton Telescope (COMPTEL) of the Compton Gamma Ray Observatory (CGRO), spanning the entire nine-year mission (1991-2000). More high-significance detections were recorded with EGRET of PKS 0208-512 (nine at greater than 6 σ confidence) than for any other γ-ray blazar. On timescales of weeks to months, PKS 0208-512 is one of the five most variable of the γ-ray blazars. We find a statistically significant correlation between intensity and spectral index for PKS 0208-512, and this source constitutes the best example of spectral hardening with intensity within the EGRET database of blazar observations. Under the assumption of isotropic Eddington-limited emission, we infer a black hole mass of approximately 6 × 10⁷M_☉ at the nucleus of PKS 0208-512. Gamma-ray transparency arguments, however, clearly support the hypothesis of relativistic beaming in PKS 0208-512, with a minimum relativistic Doppler factor for the jet of δ ≳ 3.2 at GeV energies. We have also carried out a comprehensive reanalysis of the COMPTEL data associated with PKS 0208-512 at MeV energies and reexamined the earlier reports of excess 1-3 MeV emission from this source. For individual CGRO viewing periods, we find upper limits only for MeV emission from PKS 0208-512. Our reanalysis of the original COMPTEL data for the period 1993 May-June leads us to conclude that the significance of the original reported detection is marginal at best and that there is no compelling evidence at present for the existence of a distinct flaring state for PKS 0208-512 at MeV energies.

A long 280 ks observation of the Seyfert 1 galaxy NGC 3783 with XMM-Newton is reported. We focus on the oxygen line complex between 17 and 24 Å as measured with the Reflection Grating Spectrometer. Accurate absorption column densities and emission-line fluxes are obtained. We explore several options for the geometry and physical form of the emitting and absorbing gas. The lack of change in ionization in the absorber despite an increase in continuum flux during the observation restricts the high-ionization (O-K) and the low-ionization (Fe-M) gas to distances of at least 0.5 and 2.8 pc, respectively, away from the central source. Given the P Cygni type profiles in the resonance spectral lines and the similar velocity widths, column densities, and ionization structure inferred separately from the emission and absorption lines, it is tempting to relate the X-ray narrow-line emitting plasma with the X-ray-absorbing gas. Under this assumption, the scenario of dense clumped clouds can be ruled out. Conversely, extended ionization cones (r ≳ 10 pc) are consistent with the observation independent of this assumption. These findings are in stark contrast to the picture of numerous clumpy (n_e ≳ 10⁹ cm^-3) clouds drawn recently from UV spectra, but they are consistent with the extended X-ray emission cones observed directly in Seyfert 2 galaxies.

Several strong TeV γ-ray flares were detected from Mrk 421 in the years 2000 and 2001 by the Tibet III air shower array at a level of statistical significance of 5.1 σ. Mrk 421 was unprecedentedly active at X-ray and TeV γ-ray energies during this period, and a positive correlation was found between the change of the all-sky monitor Rossi X-Ray Timing Explorer X-ray flux and the Tibet TeV γ-ray flux. When a power-law energy spectrum for γ-rays from this source is assumed, the spectral index is calculated to be -3.24 ± 0.69 at the most active phase in 2001. The spectral index observed by the Tibet air shower array is consistent with those obtained via imaging air Cerenkov telescopes.

We present a detailed Chandra study of the galaxy group NGC 1550. For its temperature (1.37 ± 0.01 keV) and velocity dispersion (~300 km s^-1), the NGC 1550 group is one of the most luminous known galaxy groups (L_bol = 1.65 × 10⁴³ ergs s^-1 within 200 kpc, or 0.2r_vir). We find that within ~60 kpc, where the gas cooling time is less than a Hubble time, the gas temperature decreases continuously toward the center, implying the existence of a cooling core. The temperature also declines beyond ~100 kpc (or 0.1r_vir). The temperature profile of NGC 1550 is remarkably similar to those of two other 1 keV groups with accurate temperature determination. The temperature begins to decline at 0.07r_vir-0.1r_vir, while in hot clusters the decline begins at or beyond 0.2r_vir. Thus, there are at least some 1 keV groups that have temperature profiles significantly different from those of hot clusters, which may reflect the role of nongravitational processes in intracluster medium/intergalactic medium evolution. NGC 1550 has no isentropic core in its entropy profile, in contrast to the predictions of "entropy floor" simulations. We compare the scaled entropy profiles of three 1 keV groups (including NGC 1550) and three 2-3 keV groups. The scaled entropy profiles of 1 keV groups show much larger scatter than those of hotter systems, which implies varied preheating levels. We also discuss the mass content of the NGC 1550 group and the abundance profile of heavy elements.

Using the Sloan Digital Sky Survey (SDSS), we probe the halo mass distribution by studying the velocities of satellites orbiting isolated galaxies. In a subsample that covers 2500 deg² on the sky, we detect about 3000 satellites with absolute blue magnitudes going down to M_B = -14; most of the satellites have M_B = -16 to -18, comparable to the magnitudes of M32 and the Magellanic Clouds. After a careful, model-independent removal of interlopers, we find that the line-of-sight velocity dispersion of satellites declines with distance to the primary. For an L_* galaxy the rms line-of-sight velocity changes from ≈120 km s^-1 at 20 kpc to ≈60 km s^-1 at 350 kpc. This decline agrees remarkably well with theoretical expectations, as all modern cosmological models predict that the density of dark matter in the peripheral parts of galaxies declines as ρ_DM ∝ r^-3. Thus, for the first time we find direct observational evidence of the density decline predicted by cosmological models; we also note that this result contradicts alternative theories of gravity such as modified Newtonian dynamics (MOND). We also find that the velocity dispersion of satellites within 100 kpc scales with the absolute magnitude of the central galaxy as σ ∝ L^0.3; this is very close to the Tully-Fisher relation for normal spiral galaxies.

We report the results of the analysis of the extended soft emission discovered in the Chandra ACIS pointing at the merging system NGC 4038/4039 ("the Antennae"). We present a multicolor X-ray image that suggests both extensive absorption by the dust in this system, peaking in the contact region, as well as variations in the temperature of different emitting regions of the hot interstellar medium (ISM). Spectral fits to multicomponent thermal emission models confirm this picture and give a first evaluation of the parameters of the hot plasma. We compare the diffuse X-ray emission with radio continuum (6 cm), H I, CO, and Hα images to take a first look at the multiphase ISM of "the Antennae" galaxies. We find that the hot (X-ray) and cold (CO) gas have comparable thermal pressures in the two nuclear regions. We also conclude that the displacement between the peak of the diffuse X-ray emission in the north of the galaxy system, toward the inner regions of the northern spiral arm (as defined by Hα, radio continuum, and H I), could result from ram pressure of infalling H I clouds.

About 45% of the point sources detected in the 2 Ms Chandra exposure of the Hubble Deep Field-North (HDF-N) can be matched with moderately bright galaxies with z < 1.4 that have been studied by the Caltech Faint Galaxy Redshift Survey. Although the optical spectra of these galaxies appear normal, based on their X-ray properties ~20% of them appear to contain weak active galactic nuclei (AGNs). More than 90% of the X-ray photons detected by Chandra from galaxies within the redshift regime 0.4 < z < 1.1 are powered by accretion onto massive black holes. For the sample of galaxies in common, we use their emitted luminosity in the 3727 Å line of [O II] to estimate their star formation rates (SFRs). The X-ray-emitting galaxies are not those with the highest rest-frame equivalent width in this emission line, but rather are among those with the highest SFRs. With SFRs corrected for inclination effects, the distant galaxies show an L_X-SFR relationship that is comparable to that of local galaxies. The HDF sample has a significantly higher median SFR and median SFR/galaxy stellar mass than does a sample of local star-forming galaxies. We demonstrate that the observed SFR for most of the galaxies at z ~ 1 in the HDF sample, if maintained as constant over their ages, suffices to produce the stellar mass observed in these galaxies. A rise in SFR at still earlier times is not required. We provide further evidence to support the conclusion that, once AGNs are eliminated, X-ray emission in these distant star-forming galaxies is related to the SFR through the same physical mechanisms that prevail locally.

We investigate radiatively inefficient accretion flow models for Sgr A*, the supermassive black hole in our Galactic center, in light of new observational constraints. Confirmation of linear polarization in the submillimeter emission argues for accretion rates much less than the canonical Bondi rate. We consider models with low accretion rates and calculate the spectra produced by a hybrid electron population consisting of both thermal and nonthermal particles. The thermal electrons produce the submillimeter emission and can account for its linear polarization properties. As noted in previous work, the observed low-frequency radio spectrum can be explained if a small fraction (≈1.5%) of the electron thermal energy resides in a soft power-law tail. In the innermost region of the accretion flow, turbulence and/or magnetic reconnection events may occasionally accelerate a fraction of the electrons into a harder power-law tail. We show that the synchrotron emission from these electrons, or the Compton upscattering of synchrotron photons by the same electrons, may account for the X-ray flares observed by Chandra.

The merger of a metal-poor satellite galaxy with the Milky Way about 5-6 Gyr ago is postulated to resolve three great unexplained conflicts presented by mainstream presolar stardust SiC grains. The model allows all of the asymptotic giant branch (AGB) carbon stars that donated these grains to have been formed nearly simultaneously in a starburst generated by gaseous mixing, despite their great apparent age differences when evaluated in terms of Galactic chemical evolution (GCE). The model explains why a precisely measured linear correlation exists between the ratios ²⁹Si/²⁸Si and ³⁰Si/²⁸Si in the initial compositions of those AGB stars. It suggests why the slope of that normalized correlation line is m = 4/3 rather than unity, as predicted by GCE. It also suggests why the solar silicon isotopes lie near the bottom of that mainstream correlation line rather than near its top, as expected by current astrophysical ideas. By addressing many isotopic puzzles found within the solar composition, the model also yields a fresh view of the origin of the Sun and of its relationship to the Galaxy. The model is remarkable in reading dynamic events of the presolar history of the Milky Way from precise isotopic ratios measured in terrestrial laboratories within individual micron-sized presolar grains that have been extracted from meteorites that formed 4.56 Gyr ago but that fell only recently to Earth.

The unusual class of magnetized nonthermal radio filaments, threads, and streaks, with their unique physical characteristics, is found only within the inner couple of degrees of the Galactic center. Also, a number of young, mass-losing, and rare stellar clusters are recognized as lying in the Galactic center region. The latter characteristic of the Galactic center region is used to explain the origin of the nonthermal radio filaments. We consider a mechanism in which the collective winds of massive W-R and OB stars within a dense stellar environment produce shock waves that can accelerate particles to relativistic energies. This mechanism is an extension of a model originally proposed in 1996 by Rosner & Bodo, who suggested that energetic nonthermal particles are produced in a terminal shock of mass-losing stars. The large-scale distribution of the magnetic field in the context of this model is argued to be neither poloidal in geometry nor pervasive throughout the Galactic center region.

We report ~600 days of BATSE earth occultation observations of the total gamma-ray (30 keV to 1.7 MeV) emission from the Crab Nebula between 1991 May 24 (TJD 8400) and 1994 October 3 (TJD 9628). Light curves from 35-100, 100-200, 200-300, 300-400, 400-700, and 700-1000 keV show that positive fluxes were detected by BATSE in each of these six energy bands at significances of approximately 31, 20, 9.2, 4.5, 2.6, and 1.3 σ, respectively, per day. We also observed significant flux and spectral variations in the 35-300 keV energy region, with timescales of days to weeks. The spectra below 300 keV, averaged over typical Compton Gamma Ray Observatory viewing periods of 6-13 days, can be well described by a broken power law with average indices of ~2.1 and ~2.4, varying around a spectral break at ~100 keV. Above 300 keV, the long-term averaged spectra, averaged over three 400 day periods (TJD 8400-8800, 8800-9200, and 9200-9628, respectively), are well represented by the same power law with index of ~2.34 up to ~670 keV, plus a hard spectral component extending from ~670 keV to ~1.7 MeV, with a spectral index of ~1.75. The latter component could be related to a complex structure observed by COMPTEL in the 0.7-3 MeV range. Above 3 MeV, the extrapolation of the power-law continuum determined by the low-energy BATSE spectrum is consistent with fluxes measured by COMPTEL in the 3-25 MeV range and by EGRET from 30-50 MeV. We interpret these results as synchrotron emission produced by the interaction of particles ejected from the pulsar with the field in different dynamical regions of the nebula system, as observed recently by the Hubble Space Telescope, XMM-Newton, and Chandra.

We present the first linear multicolor polarization observations for a sample of 35 stars in the direction of the Galactic cluster NGC 6231. We have found a complex pattern in the angles of the polarimetric vectors. Near the core of this cluster the structure shows a semicircular pattern that we have interpreted as a reorientation of the dust particles showing the morphology of the magnetic field. We propose that a supernova event occurred some time ago and produced a shock on the local ISM. We discuss in this paper independent confirmations of this event, both from the studies on the diffuse interstellar absorptions and the results of the pre-main-sequence stars. We also show that a supernova is supported by the evolutionary status of the cluster.

Calculations are performed for the spectral line profiles and images of astrophysical maser radiation that emerges from isolated spheres and thin disks viewed edge-on. In contrast to previous investigations in which various approximations are made, the full equations are solved here for the frequency-dependent radiative transport that includes the thermal motion of the molecules. The spectral line profiles for spheres and disks are found to rebroaden to the full thermal Doppler breadth with increasing saturation in essentially the same way as is well known to occur for a linear maser. The variation with frequency in the apparent angular sizes of masing spheres and thin disks is found to be negligible at frequencies within the spectral line at which the flux is significant. Calculations are also performed for spherical and disk masers that are not isolated but for which the seed radiation for the masers is incident from one side, as would occur when a strong continuum source is on the far side of the masers. Again, the spectral line profiles are found to rebroaden to the full thermal breadths with increasing saturation, and there are no significant variations in the apparent angular sizes with frequency. However, the full rebroadening does occur at somewhat higher saturation, and the variation of the apparent angular sizes as a function of the degree of saturation is quite different from that of the isolated masers. Spheres and disks have served as idealized geometries with which to examine possible deviations from the linear approximation for astrophysical masers.

It is well known that the sight line toward HD 204827 in the cluster Trumpler 37 shows a UV extinction curve that does not follow the average Galactic extinction relation. However, when a dust component, foreground to the cluster, is removed, the residual extinction curve is identical to that found in the SMC within the uncertainties. The curve is very steep and has little or no 2175 Å bump. The position of HD 204827 in the sky is projected onto the edge of the Cepheus IRAS bubble. In addition, HD 204827 has an IRAS bow shock, indicating that it may be embedded in dust swept up by the supernova that created the IRAS bubble. Shocks due to the supernova may have led to substantial processing of this dust. The HD 204827 cloud is dense and rich in carbon molecules. The 3.4 μm feature indicating a C-H grain mantle is present in the dust toward HD 204827. The environment of the HD 204827 cloud dust may be similar to the dust associated with HD 62542, which lies on the edge of a stellar wind bubble and is also dense and rich in molecules. This sight line may be a Rosetta Stone if its environment can be related to those in the SMC having similar dust.

We present results of a sensitive 76 ks Chandra observation of the young stellar cluster in NGC 2024, lying at a distance of ~415 pc in the Orion B giant molecular cloud. Previous infrared observations have shown that this remarkable cluster contains several hundred embedded young stars, most of which are still surrounded by circumstellar disks. Thus, it presents a rare opportunity to study X-ray activity in a large sample of optically invisible protostars and classical T Tauri stars (CTTSs) undergoing accretion. Chandra detected 283 X-ray sources, of which 248 were identified with counterparts at other wavelengths, mostly in the near-infrared. Astrometric registration of Chandra images against the Two Micron All Sky Survey (2MASS) resulted in positional offsets of ≈025 near field center, yielding high confidence identifications of infrared counterparts. The Chandra detections are characterized by hard heavily absorbed spectra and spectacular variability. Spectral analysis of more than 100 of the brightest X-ray sources yields a mean extinction ⟨A_V⟩ ~ 10.5 mag and typical plasma energies ⟨kT⟩ ~ 3 keV. The range of variability includes rapid impulsive flares and persistent low-level fluctuations indicative of strong magnetic activity, as well as slow rises and falls in count rate whose origin is more obscure. Some slowly evolving outbursts reached sustained temperatures of kT ~ 6-10 keV. Chandra detected all but one of a subsample of 27 CTTSs identified from previous near- and mid-infrared photometry, and their X-ray and bolometric luminosities are correlated. We also report the X-ray detection of IRS 2b, which is thought to be a massive embedded late O or early B star that may be the ionizing source of NGC 2024. Seven millimeter-bright cores (FIR 1-7) in NGC 2024 that may be protostellar were not detected, with the possible exception of faint emission near the unusual core FIR 4.

We present the polarization detections in DR 21(OH) from both the thermal dust emission at 1.3 mm and the CO J = 2 → 1 line obtained with the Berkeley-Illinois-Maryland Association array. Our results are consistent with the prediction of the Goldreich-Kylafis effect that the CO polarization is either parallel or perpendicular to the magnetic field direction. The detection of the polarized CO emission is over a more extended region than the dust polarization, while the dust polarization provides an aid in resolving the ambiguity of the CO polarization. The combined results suggest that the magnetic field direction in DR 21(OH) is parallel to the CO polarization and therefore parallel to the major axis of DR 21(OH). The strong correlation between the CO and dust polarization suggests that magnetic fields are remarkably uniform throughout the envelope and the cores. The dispersion in polarization position angles implies a magnetic field strength in the plane of the sky of about 1 mG, compared to about 0.5 mG inferred for the line-of-sight field from previous CN Zeeman observations. Our CO data also show that both MM 1 and MM 2 power high-velocity outflows with v ≳ 25 km s^-1 relative to the systemic velocity.

The fluctuations observed in the light curves of some gamma-ray burst (GRB) afterglows (such as GRB 021004) provide a useful tool to probe the circumburst density profile and to probe the variations in the energy of the blast wave with time. We present a general formalism that reduces the calculation of the observed light curve from a Blandford-McKee blast wave to the evaluation of a one-dimensional integral. Using this formalism we obtain a simple approximation to the general light curve that arises in more complex situations where the afterglow's energy or the external density varies. The solution is valid for spherically symmetric profiles, and it takes full consideration of angular time delay effects. We present the light curves of several external density profiles and demonstrate the effects of density variations on the light curve. We also revisit the afterglow of GRB 021004 and find that the steep decay after the first bump (~4000 s) cannot result from a spherically symmetric density variation or from the passage of the synchrotron frequency through the optical band. This suggests that an angular structure is responsible for some of the observed features in the light curve. This may be the first evidence that an angular structure is important in the early stages of the afterglow.

We formulate and solve the diffusion problem of line photon propagation in a bulk outflow from a compact object (black hole or neutron star) using a generic assumption regarding the distribution of line photons within the outflow. Thomson scattering of the line photons within the expanding flow leads to a decrease of their energy which is of first order in v/c, where v is the outflow velocity and c is the speed of light. We demonstrate that the emergent line profile is closely related to the time distribution of photons diffusing through the flow (the light curve) and consists of a broad redshifted feature. We analyzed the line profiles for the general case of outflow density distribution. We emphasize that the redshifted lines are intrinsic properties of the powerful outflow that are supposed to be in many compact objects.

In a globular cluster, hierarchical triple black hole systems can be produced through binary-binary interaction. It has been proposed recently that the Kozai mechanism could drive the inner binary of the triple system to merge before it is interrupted by interactions with other field stars. We investigate qualitatively and numerically the evolution of the eccentricities in these binaries under gravitational radiation (GR) reaction. We predict that ~30% of the systems will possess eccentricities greater than 0.1 when their emitted gravitational waves pass through 10 Hz frequency. The implications for gravitational wave detection, especially the relevance to data analyses for broadband laser interferometer gravitational wave detectors, are discussed.

We examine a number of evolutionary scenarios for the recently discovered class of accretion-powered millisecond X-ray pulsars in ultracompact binaries, including XTE J0929-314 and XTE J1751-305. These systems have very short orbital periods of P_orb = 43.6 and 42.4 minutes, respectively, and extremely small mass functions. We focus on a particular scenario that can naturally explain the present-day properties of these systems. This model invokes a donor star that was either very close to the main-sequence turnoff at the onset of mass transfer or had sufficient time to evolve during the mass-transfer phase. We have run a systematic set of binary evolution calculations with a wide range of initial conditions. We find that these two ultracompact binaries can best be fitted by models wherein the donors start to lose mass at orbital periods of ~15 hr. The orbital periods then decrease to a minimum value of ≲40 minutes and finally evolve back up to about 43 minutes. We present the results of our binary evolution calculations for these systems, including interior profiles for the donor stars. We find that the initial properties of the donor star and the exact mode of orbital angular momentum losses do not have to be individually fine-tuned in order to reproduce the observed properties. We also carry out an analysis based on the measured mass functions of XTE J0929-314 and XTE J1751-305 to establish formal probability distributions for the current donor masses, chemical compositions, and thermal bloating factors of these systems. These distributions are evaluated in the context of our binary evolution models. We conclude that the donor masses are likely to be in the range ~0.012-0.025 M_☉ and have radii of ~0.042-0.055 R_☉. These radii are factors of ~1.1-1.3 times larger than the corresponding radii of zero-temperature stars of the same mass and chemical composition. According to the evolutionary scenario proposed in this paper, the interiors of the donors are largely composed of He, and the surface H abundances are almost certainly less than 10% (by mass). The orbital period derivative of these systems is very likely to be positive, i.e., _orb/P_orb > 0, with typical values in the range ~3 × 10^-10 to 2 × 10^-8 yr^-1. Long-term average values of the mass-transfer rate could be as high as ~10^-11 to 3 × 10^-10M_☉ yr^-1 but is more likely to be ~10^-11M_☉ yr^-1. Our study supports the hypothesis that X-ray irradiation has had a minimal effect on enhancing the radius of the low-mass donor. We also show how, in the context of this same basic evolutionary scenario, we can model the properties of the SAX 1808.4-3658 binary millisecond X-ray pulsar (P_orb = 2 hr). Finally, we point out that if the proposed scenario to explain the ultracompact systems is correct, then these binaries truly link (as evolutionary cousins) systems that (1) evolve to become wide binary millisecond pulsars containing low-mass He dwarfs and (2) those ordinary low-mass X-ray binaries in which the H-rich donors are slowly reduced to planetary masses.

We investigate the axisymmetric magnetosphere of an aligned rotating magnetic dipole surrounded by an ideal force-free plasma. We concentrate on the magnetic field structure around the point of intersection of the separatrix between the open- and closed-field line regions and the equatorial plane. We first study the case in which this intersection point is located at the light cylinder. We find that in this case the separatrix equilibrium condition implies that all the poloidal current must return to the pulsar in the open-field region, i.e., there should be no finite current carried by the separatrix/equator current sheet. We then perform an asymptotic analysis of the pulsar equation near the intersection point and find a unique, self-similar solution; however, a light surface inevitably emerges right outside the light cylinder. We then perform a similar analysis for the situation in which the intersection point lies somewhere inside the light cylinder, in which case a finite current flowing along the separatrix and the equator is allowed. We find a very simple behavior in this case, characterized by a 90° angle between the separatrix and the equator and by a finite vertical field in the closed-field region. Finally, we discuss the implications of our results for global numerical studies of pulsar magnetospheres.

The challenge in searching for non-radio-pulsing isolated neutron stars (INSs) is in excluding association with objects in the very large error boxes (~13'', 1 σ radius) typical of sources from the largest X-ray all-sky survey, the ROSAT All-Sky Survey Bright Source Catalogue (RASS BSC). We search for candidate INSs using statistical analysis of optical (USNO-A2), infrared (IRAS), and radio (NVSS) sources near the ROSAT X-ray localization and show that this selection would find 20% of the INSs in the RASS BSC. This selection finds 32 candidates at declinations δ > -39^°, among which are two previously known INSs, 17 sources that we show are not INSs, and 13 the classification of which are as yet undetermined. These results require a limit of less than 67 INSs (90% confidence, full sky, assuming isotropy) in the RASS BSC. This limit modestly constrains a naive and optimistic model for cooling neutron stars in the Galaxy.

We report Chandra and XMM-Newton observations of the transient neutron star low-mass X-ray binary GRS 1741.9-2853. Chandra detected the source in outburst on 2000 October 26 at an X-ray luminosity of ~10³⁶ ergs s^-1 (2-8 keV; 8 kpc), and in quiescence on 2001 July 18 at ~10³² ergs s^-1. The latter observation is the first detection of GRS 1741.9-2853 in quiescence. We obtain an accurate position for the source of 17^h45^m233, -28°54'497 (J2000), with an uncertainty of 07. GRS 1741.9-2853 was not detected significantly in three other Chandra observations, nor in three XMM-Newton observations, indicating that the luminosity of the source in quiescence varies by at least a factor of 5 from (<0.9-5.0) × 10³² ergs s^-1 (2-8 keV). A weak X-ray burst with a peak luminosity of 5 × 10³⁶ ergs s^-1 above the persistent level was observed with Chandra during the outburst on 2000 October 26. The energy of this burst, 10³⁸ ergs, is unexpectedly low and may suggest that the accreted material is confined to the polar caps of the neutron star. A search of the literature reveals that GRS 1741.9-2853 was observed in outburst with ASCA in the fall of 1996 as well, when the BeppoSAX WFC detected the three previous X-ray bursts from this source. The lack of X-ray bursts from GRS 1741.9-2853 at other epochs suggests that it produces bursts only during transient outbursts when the accretion rate onto the surface of the neutron star is about 10^-10M_☉ yr^-1. A similar situation may hold for other low-luminosity bursters recently identified from WFC data.

We report the discovery of millisecond oscillations in an X-ray burst from the X-ray transient SAX J1748.9-2021 in the globular cluster NGC 6440. Oscillations at a frequency of 409.7 ± 0.3 Hz were present in one of 15 X-ray bursts observed with the Proportional Counter Array on the Rossi X-Ray Timing Explorer during the outburst that occurred in 2001. The burst was relatively dim and had the second longest duration and decay time. The average peak luminosity of two bursts showing radius expansion is (3.6 ± 0.4) × 10³⁸ ergs s^-1, consistent with the Eddington luminosity for a 1.4 M_☉ and 10 km radius neutron star burning hydrogen-poor matter. We speculate that the dichotomy observed between sources with burst oscillations at once versus twice the frequency difference of kHz quasiperiodic oscillations in the persistent emission may be related to the magnetic field geometry of the neutron stars.

We present an analysis of the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of the ultramassive (M = 1.31 M_☉), magnetic (B_s = 2.3 MG) white dwarf PG 1658+441. The far-ultraviolet (FUV) spectrum exhibits very broad Lyman lines and quasi-molecular Lyβ satellites, but weak Lyγ satellites may also be present. PG 1658+441 is the hottest white dwarf known to show these satellite features. We fit the Lyman lines with stellar models and obtain atmospheric parameters consistent with a published analysis of the Balmer lines. By averaging results obtained for the different FUSE segments, we determine T_eff = 29,620 ± 500 K and log g = 9.31 ± 0.07. The models match the data over large portions of the spectrum, but discrepancies remain near the satellite features. Finally, no trace elements have been identified in the FUV spectrum, and we provide abundance upper limits for C, N, Si, and P.

We report on the first near-simultaneous X-ray and optical observations of RX J0806.3+1527. The source is believed to be a 321 s orbital period ultracompact binary system hosting an X-ray-emitting white dwarf. Data were obtained with Chandra and the ESO Very Large Telescope (VLT) in 2001 November. We found an optical/X-ray phase shift in the periodic modulation of about 0.5, strongly favoring the existence of two distinct emission regions in the two bands (for the pulsed fluxes). The Chandra data allow us to study, for the first time, the spectral continuum of RX J0806.3+1527 in soft X-rays. This was well fitted by a blackbody spectrum with kT ~ 65 eV and hydrogen column density of N_H ~ 5 × 10²⁰ cm^-2. The average (unabsorbed) source 0.1-2.5 keV luminosity during the modulation-on phase is L_X ~ 5 × 10³² ergs s^-1 (assuming a distance of 500 pc). Such a value is lower than the luminosity expected if stable mass transfer between two white dwarfs were driven by gravitational radiation. Evidence for absorption-like features are present in the phase-averaged spectrum at about 0.53, 0.64, and 1.26 keV, which may be attributed to heavy elements (C and N). We compare and discuss these findings with other binary systems hosting an accreting white dwarf.

Quiescent low-mass X-ray binaries (qLMXBs) containing neutron stars have been identified in several globular clusters using Chandra or XMM X-ray observations, via their distinctive soft thermal spectra. We report a complete census of the qLMXB population in these clusters, identifying three additional probable qLMXBs in NGC 6440. We conduct several analyses of the qLMXB population and compare it with the harder, primarily cataclysmic variable (CV), population of low-luminosity X-ray sources with 10³¹ ergs s^-1 < L_X < 10^32.5 ergs s^-1. The radial distribution of our qLMXB sample suggests an average system mass of 1.5M_☉, consistent with a neutron star and low-mass companion. Spectral analysis reveals that no globular cluster qLMXBs, other than the transient in NGC 6440, require an additional hard power-law component, as often observed in field qLMXBs. We identify an empirical lower luminosity limit of ~10³² ergs s^-1 among globular cluster qLMXBs. The bolometric luminosity range of qLMXBs implies (in the deep crustal heating model of Brown and collaborators) low time-averaged mass-transfer rates, below the disk stability criterion. The X-ray luminosity functions of the CV populations alone in NGC 6397 and 47 Tuc are shown to differ. The distribution of qLMXBs among globular clusters is consistent with their dynamical formation by either tidal capture or exchange encounters, allowing us to estimate that 7 times more qLMXBs than bright LMXBs reside in globular clusters. The distribution of harder sources (primarily CVs) has a weaker dependence on density than that of the qLMXBs. Finally, we discuss possible effects of core collapse and globular cluster destruction on X-ray source populations.

We report our analysis of a Chandra X-ray observation of the rich globular cluster M80, in which we detect some 19 sources to a limiting 0.5-2.5 keV X-ray luminosity of 7 × 10³⁰ ergs s^-1 within the half-mass radius. X-ray spectra indicate that two of these sources are quiescent low-mass X-ray binaries containing neutron stars. We identify five sources as probable cataclysmic variables (CVs), one of which seems to be heavily absorbed, implying high inclination. The brightest CV may be the X-ray counterpart of nova 1860 T Sco. The concentration of the X-ray sources within the cluster core implies an average mass of 1.2 ± 0.2 M_☉, consistent with the binary nature of these systems and very similar to the radial distribution of the blue stragglers in this cluster. The X-ray and blue straggler source populations in M80 are compared to those in the similar globular cluster 47 Tuc.

V Sagittae shows quasi-periodic optical high (soft X-ray-off) and low (soft X-ray-on) states with the total period of ~300 days. A binary model is presented to explain orbital light curves for both the high and low states, as well as the transition mechanism between them. The binary model consists of a white dwarf (WD), a disk around the WD, and a lobe-filling main-sequence companion. In the optical low state, the mass-transfer rate to the WD is small and the size of the disk is as small as its Roche lobe size. In the optical high state, the mass-transfer rate to the WD exceeds the critical rate of ~1 × 10^-6M_☉ yr^-1 and the WD blows an optically thick, massive wind. Surface layers of the disk are blown in the wind, and the disk surface extends to the companion or over. As a result, optical luminosity of the disk increases by a magnitude because of its large irradiation effect. The massive wind completely obscures soft X-rays. This corresponds to the optical high/soft X-ray off state. The transition between optical high and low states is driven by an attenuation of the mass transfer from the secondary. During the optical high state, the wind from the WD hits the companion and strips off its surface layer. The mass transfer from the companion gradually reduces and stops. As the mass supply stops, the WD wind weakens and eventually stops. The disk shrinks to a Roche lobe size, and the optical magnitude drops. This phase corresponds to the optical low/soft X-ray on state. Then a rapid mass transfer resumes. The mass of the WD envelope increases and blows a wind again. This cycle is repeated like a limit cycle. During this intermittent wind phase, the WD can grow in mass at the critical rate and eventually reach the Chandrasekhar mass limit. This process is called "accretion wind evolution," which is a key evolutionary process in a recently developed evolutionary scenario of Type Ia supernovae. This evolutionary process was first confirmed in the LMC supersoft X-ray source RX J0513.9-6951, although it commonly occurs in the supersoft X-ray sources when the mass-transfer rate exceeds the critical rate. Thus, V Sge is the second example of accretion wind evolution.

We report Chandra observations of the dwarf nova WX Hyi in quiescence. The X-ray spectrum displays strong and narrow emission lines of N, O, Mg, Ne, Si, S, and Fe. The various ionization states implied by the lines suggest that the emission is produced within a flow spanning a wide temperature range, from T ~ 10⁶ K to T ≳ 10⁸ K. Line diagnostics indicate that most of the radiation originates from a very dense region, with n ~ 10¹³-10¹⁴ cm^-3. The Chandra data allow the first tests of specific models proposed in the literature for the X-ray emission in quiescent dwarf novae. We have computed the spectra for a set of models ranging from hot boundary layers, to hot settling flow solutions, to X-ray-emitting coronae. WX Hyi differs from other dwarf novae observed at minimum in having much stronger low-temperature lines, which prove difficult to fit with existing models, and possibly a very strong, broad O VII line, perhaps produced in a wind moving at a few times 10³ km s^-1. The accretion rate inferred from the X-rays is lower than the value inferred from the UV. The presence of high-velocity mass ejection could account for this discrepancy while at the same time explaining the presence of the broad O VII line. If this interpretation is correct, it would provide the first detection of a wind from a dwarf nova in quiescence.

We present a spectrum of the symbiotic star V1016 Cyg observed with the 3.6 m Canada-France-Hawaii Telescope, in order to illustrate a method to measure the covering factor of the neutral scattering region around the giant component with respect to the hot emission region around the white dwarf component. In the spectrum, we find broad wings around Hα and a broad emission feature around 6545 Å that is blended with the [N II] λ6548 line. These two features are proposed to be formed by Raman scattering by atomic hydrogen, where the incident radiation is proposed to be UV continuum radiation around Lyβ in the former case and the He II λ1025 emission line arising from n = 6 → n = 2 transitions for the latter feature. We remove the Hα wings by a template Raman scattering wing profile and subtract the [N II] λ6548 line using the 3 times stronger [N II] λ6583 feature in order to isolate the He II Raman-scattered 6545 Å line. We obtain the flux ratio F₆₅₄₅/F₆₅₆₀ = 0.24 of the He II λ6560 emission line and the 6545 Å feature for V1016 Cyg. Under the assumption that the He II emission from this object is isotropic, this ratio is converted to the ratio Φ₆₅₄₅/Φ₁₀₂₅ = 0.17 of the number of the incident photons and that of the scattered photons. This implies that the scattering region with H I column density N ≥ 10²⁰ cm^-2 covers 17% of the emission region. By combining the presumed binary period of ~100 yr for this system we infer that a significant fraction of the slow stellar wind from the Mira component is ionized and that the scattering region around the Mira extends a few tens of AU, which is closely associated with the mass loss process of the Mira component. It is argued that the Raman-scattered He II λ6545 line is an important and useful tool to investigate the mass-loss process occurring in the late stage of stellar evolution.

We incorporate into an evolutionary code a unified treatment of turbulent transport due to convective and shear instabilities in a rotating star. The effects of composition gradients are included in a consistent way. We use the code to follow the evolution of rotating stars from the pre-main sequence to near the tip of the red giant branch. We find that at a certain stage in the red giant evolution of a 1 M_☉ star, when the convectively unstable region occupies an extensive outer envelope, shear instability sets in in a zone just below the H-burning shell. However, at a later time, shear instability also develops in a second (outer) region that lies between the H-burning shell and the convective envelope. The onset of the outer region of shear instability coincides closely in time with a well-defined feature of red giant evolution: the "bump" (so-called because of its effect on the luminosity function of clusters). This evolutionary bump has recently been identified as a significant empirical marker among red giants: cool massive winds have been found to set in abruptly after the star evolves through the bump. The results we report in the present paper suggest that the onset of mass loss in red giants may be correlated with the onset of an outer region of shear instability inside the star. We offer some speculations concerning the possible physics of such a correlation.

We present the results of a Monte Carlo radiative transfer code that calculates the polarization produced by multiple Thomson scattering and variable absorptive opacity in a circumstellar disk around one component of a close detached binary system. We consider in detail the polarization variations over the binary cycle that result from the disk's illumination both by the external star and by its own volume emission. We identify key features of these polarization phase curves and investigate their behavior as functions of optical depth, albedo, and inclination for geometrically thin and thick disks. The polarization due to disk self-illumination is sensitive to the internal optical characteristics of the disk, while the polarization arising from external illumination is sensitive mainly to the disk's geometrical thickness. With appropriate flux weighting, these results, combined with those for an internally illuminated disk, allow simulation of the polarization signature from an arbitrary binary-disk system.

In the first of a new series of papers on open cluster distances, we use updated stellar evolution models to construct an isochrone appropriate for the Hyades and compare it with the Hyades eclipsing binary system VB 22. We find that the absolute and relative luminosities of the two stars are in good agreement with the model but that the radii do not match the values inferred from eclipse data. We present evidence that there is a consistency problem with the flux ratios and the inferred radii and discuss possible theoretical effects that could be responsible for the mismatch in the radii. We derive a helium abundance for the Hyades of Y = 0.271 ± 0.006, which is equal within the errors to the Sun's initial helium abundance, even though the Hyades is considerably more metal-rich.

We have frequency-analyzed 6391 variables classified earlier as fundamental-mode RR Lyrae (RR0) stars in the MACHO database on the Large Magellanic Cloud (LMC). The overwhelming majority (i.e., 96%) of these variables have been proved to be indeed RR0 stars, whereas the remaining ones have fallen into one of the following categories: single- and double-mode Cepheids, binaries, first-overtone and double-mode RR Lyrae stars, and nonclassified variables. Special attention has been paid to the properties of the amplitude- and phase-modulated RR0 stars (the Blazhko stars). We found altogether 731 Blazhko variables showing either a doublet or an equidistant triplet pattern at the main pulsation component in their frequency spectra. This sample overwhelmingly exceeds the number of Blazhko stars known in all other systems combined. The incidence rate of the Blazhko variables among the RR0 stars in the LMC is 11.9%, which is 3 times higher than their rate among the first-overtone RR Lyrae stars. No difference is found in the average brightness between the single-mode and Blazhko variables. However, the latter ones show a somewhat lower degree of skewness in their average light curves and a concomitant lower total amplitude in their modulation-free light curves. From the frequency spectra we found that variables with larger modulation amplitudes at the higher frequency side of the main pulsation component are 3 times more common than the ones showing the opposite amplitude pattern. A search for a modulation component with the Blazhko period in the average brightness of the individual variables showed the existence of such a modulation with an overall amplitude of ≈0.006 mag. On the other hand, a similar search for quadruple modulation patterns around the main pulsation component has failed to clearly detect such components at the ≈0.004 mag level. This means that the amplitudes of the quadruple components (if they exist) should be, on average, at least 10 times smaller than those of the triplet components. This finding and the existence of Blazhko variables with highly asymmetric modulation amplitudes not only question the validity of the magnetic oblique rotator model but also put stringent constraints on models based on mode-coupling theories.

We have used the High Resolution Camera (HRC-I) of the Chandra X-Ray Observatory to search for coronal (T ~ 10⁶ K) emission from the archetype "noncoronal" red giants Arcturus (α Bootis=HD 124897, K1 III) and Aldebaran (α Tauri=HD 29139, K5 III). Our program follows up previous detections of ultraviolet coronal proxies such as C IV λ1548 (T ~ 1 × 10⁵ K) and O VI λ1031 (T ~ 3 × 10⁵ K). The deep (~19 ks) HRC-I pointings obtained a tentative 3 σ detection of Arcturus, with f_X(0.2-2 keV) = 1.0 × 10^-15 ergs cm^-2 s^-1 (95% confidence limits [CLs]), but failed to record Aldebaran, with an upper limit of ≲1.5 × 10^-15 ergs cm^-2 s^-1 (also at 95% CL). The corresponding L_X/L_bol ratios are a factor of ten thousand less than the Sun, a low-activity coronal dwarf. At the same time, Hubble Space Telescope Imaging Spectrograph far-ultraviolet spectra suggest the presence of a "cool absorber," probably near the base of the red giant chromosphere, imprinting discrete low-excitation absorptions on top of highly ionized features such as Si IV λ1393. The hot emission zones thus are at least partially buried under a large column of chromospheric material, which would severely attenuate any soft X-rays that might be emitted. The submerged hot structures presumably are magnetic because of their high temperatures and broad C IV profiles (FWHM ~ 130 km s^-1). Perhaps these structures are analogous to small-scale ephemeral bipolar regions seen ubiquitously on the Sun throughout the sunspot cycle and thought to be of direct convective origin. If small-scale magnetic fields indeed are present in the lower atmospheres of red giants such as Arcturus and Aldebaran, they might play a role in initiating the cool winds of such stars, perhaps through a mechanism similar to solar spicules.

In a separate paper, we have reexamined the observations of IR excess obtained with the Infrared Space Observatory satellite and discussed the ages of stars with excess. The amount of dust (measured by the luminosity fraction f_d = L_IR/L_*) seen around main-sequence stars of different ages shows several interesting trends. To discuss these results in the context of a physical model, we develop in this paper an analytical model for the dust production in Vega-type systems. Previously it has been claimed that a power-law slope of about -2 in the diagram plotting amount of dust versus time could be explained by a simple collisional cascade. We show that such a cascade in fact results in a power law f_d ∝ t^-1 if the dust removal processes are dominated by collisions. A power law f_d ∝ t^-2 only results when the dust removal processes become dominated by Pointing-Robertson drag. This might be the case in the Kuiper Belt of our own solar system, but it is certainly not the case in any of the observed disks. A steeper slope can, however, be created by including continuous stirring into the models. We show that the existence of both young and old Vega-like systems with large amounts of dust (f_d ≃ 10^-3) can be explained qualitatively by Kuiper Belt-like structures with delayed stirring. Finally, the absence of young stars with intermediate amounts of dust might be due to the fact that stirring due to planet formation may not be active in young low-mass disks. The considerations in this paper support the picture of simultaneous stirring and dust production proposed by Kenyon and Bromley.

We study the time dependency of Vega-like excesses using infrared studies obtained with the imaging photopolarimeter ISOPHOT on board the Infrared Space Observatory. We review the different studies published on this issue and critically check and revise ages and fractional luminosities in the different samples. The conclusions of our study differ significantly from those obtained by other authors (e.g., Holland and coworkers; Spangler and coworkers), who suggested that there is a global power law governing the amount of dust seen in debris disks as a function of time. Our investigations lead us to conclude that (1) for stars at most ages, a large spread in fractional luminosity occurs, but (2) there are few very young stars with intermediate or small excesses; (3) the maximum excess seen in stars of a given age is about f_d ≈ 10^-3, independent of time; and (4) Vega-like excess is more common in young stars than in old stars.

The final masses of Jovian planets are attained when the tidal torques that they exert on their surrounding protostellar disks are sufficient to open gaps in the face of disk viscosity, thereby shutting off any further accretion. In sufficiently well ionized disks, the predominant form of disk viscosity originates from the magnetorotational instability (MRI) that drives hydromagnetic disk turbulence. In the region of sufficiently low ionization rate, the so-called dead zone, turbulence is damped and we show that lower mass planets will be formed. We considered three ionization sources (X-rays, cosmic rays, and radioactive elements) and determined the size of a dead zone for the total ionization rate by using a radiative, hydrostatic equilibrium disk model developed by Chiang and coworkers. We studied a range of surface mass density (Σ₀ = 10³-10⁵ g cm^-2) and X-ray energy (kT_X = 1-10 keV). We also compared the ionization rate of such a disk by X-rays with cosmic rays and find that the latter dominate X-rays in ionizing protostellar disks unless the X-ray energy is very high (5-10 keV). Among our major conclusions are that for typical conditions, dead zones encompass a region extending out to several AU, the region in which terrestrial planets are found in our solar system. Our results suggest that the division between low- and high-mass planets in exosolar planetary systems is a consequence of the presence of a dead zone in their natal protoplanetary disks. We also find that the extent of a dead zone is mainly dependent on the disk's surface mass density. Our results provide further support for the idea that Jovian planets in exosolar systems must have migrated substantially inward from their points of origin.

Using spectral line observations to study minor ion motions in a solar coronal hole, a test was made for the presence of Alfvén waves. From this we conclude that the Alfvén wave flux into the upper portion of the hole is not significant. The SOHO/SUMER ultraviolet spectrograph was used to observe 22 coronal ions with masses ranging from 14m_H to 56m_H and charge states ranging from +4 to +11. The line widths of 33 vacuum ultraviolet (VUV) lines radiated by these ions were measured off-limb in a coronal hole at heights between 1.02 and 1.3 solar radii (R_☉), and the height dependencies of sufficiently intense lines were measured to test for the increase of width with height expected for undamped Alfvén wave propagation. Line widths from ions formed at coronal hole temperatures were found to increase with height to 1.1 R_☉ and were constant for greater heights. If this line broadening is a manifestation of Alfvén wave propagation, the waves are strongly damped above 1.1 R_☉. Consequently, another mechanism must transport energy to the solar wind acceleration region. In addition, the observed dependence of line-of-sight ion speed squared on the reciprocal of the mass indicates that there is not a common nonthermal velocity if there is a common ion temperature. This suggests that Alfvén waves do not cause the anomalously large effective temperatures measured.

The relative motions of myriads of magnetic fragments in the solar surface are likely to drive magnetic reconnection and therefore heating among the magnetic field lines that spread from these fragments into the solar corona. We suggest that the fundamental mechanism is one of "binary reconnection" due to the motion of a given magnetic source relative to its nearest neighbor. The heating is due to several effects: (1) the three-dimensional reconnection of field lines that start out joining the two sources and end up joining the largest source to other more distant sources (or vice versa), so that the field line footpoints are exchanged; (2) the viscous or resistive damping of the waves that are emitted by the sources as their relative orientation rotates; and (3) the relaxation of the nonlinear force-free fields that join the two sources and that are built up by the relative motion of the sources.

We report the results of a statistical study of the relationship between eruptive solar flares and an observed Hα preflare phenomenon we call moving blueshift events (MBSEs). The Hα data were gathered using the Mees Solar Observatory CCD imaging spectrograph (MCCD). The 16 events in our data set were observed by both the MCCD and the Yohkoh Soft X-Ray Telescope, typically for at least 3 hr prior to the flare and in some cases repeatedly for several days prior to the flare. The data set contains both eruptive and noneruptive flares, without bias. Focusing on 3 hr periods before and after the flares, we found that the average rate of MBSEs prior to the flares was ~5 times greater prior to the 11 eruptive flares than prior to the five noneruptive ones. Also, the average rate of MBSEs dropped by a factor of ~6 after the eruptive flares. Earlier studies inferred that MBSEs reflect motions that originate in the readjustment of magnetic fields after magnetic reconnection. From the high correlation between eruptive flares and preflare MBSEs in the several hours prior to such events, we conclude that reconnection in the chromosphere or low corona plays an important role in establishing the conditions that lead to solar flare eruptions.

We perform a multiwavelength study of a two-ribbon flare on 2002 September 29 and its associated filament eruption, observed simultaneously in the Hα line by a ground-based imaging spectrograph and in hard X-rays by RHESSI. The flare ribbons contain several Hα-bright kernels that show different evolutionary behaviors. In particular, we find two kernels that may be the footpoints of a loop. A single hard X-ray source appears to cover these two kernels and to move across the magnetic neutral line. We explain this as a result of the merging of two footpoint sources that show gradually asymmetric emission owing to an asymmetric magnetic topology of the newly reconnected loops. In one of the Hα kernels, we detect a continuum enhancement at the visible wavelength. By checking its spatial and temporal relationship with the hard X-ray emission, we ascribe it to electron-beam precipitation. In addition, we derive the line-of-sight velocity of the filament plasma based on the Doppler shift of the filament-caused absorption in the Hα blue wing. The filament shows rapid acceleration during the impulsive phase. These observational features are in principal consistent with the general scenario of the canonical two-ribbon flare model.

We studied the magnetic and velocity fields of four pores situated close to the disk center and its surrounding regions. We find the following results from our analysis: The velocity inside the pore is very close to zero, whereas there is a strong and narrow downflow around the pore. The vertical velocity gradient observed at the edge of the pore is stronger than the velocity gradient seen in intergranular lanes. Immediately surrounding these narrow downflows, normal granular convection is observed. This observation is consistent with the theoretical picture of an isolated flux tube embedded in a quiet region surrounded by a downflow driven by radiative energy losses. Needle-like structures were seen around the pore, with the head of the needle showing an upflow. The needle tail ends in the downflow surrounding the pore. Assuming the flow is horizontal in the body of the needle, the needle-like structures would represent a possible signature of circular flow system surrounding the pore. The radial extent of this observed flow system (which likely feeds the downflow around the pore) is about 10''. A pore with relatively large fill fraction shows a small upflow in the center surrounded by the downflow, whereas a pore with small fill fraction shows downflows throughout the pore. The asymmetries of the observed Stokes V profiles and their temporal variations are studied. We find temporal variations of V-profile asymmetries observed within pores on timescales of 5 minutes.

After the discovery of CO, CO₂, and more recently H₂O in Titan's atmosphere, the chemistry of oxygenated organic compounds in this environment has been limited to theoretical studies only. Both kinetic and thermochemical models predict the formation of methanol and formaldehyde as the main O-organic products. Here we show that oxirane (also named ethylene oxide) is the main product in experiments designed to simulate the upper atmosphere of Titan; methanol and formaldehyde are not produced in detectable amounts. Oxirane has been previously detected in the interstellar medium and hot molecular cloud cores. The first opportunity to detect it in Titan will occur in 2005 January during the entry of the Huygens probe into the satellite's atmosphere. Therefore, oxirane becomes a molecule of wide astrophysical interest.

We have determined the relativistic light deflection of the quasar J0842+1835 as Jupiter passed within 37 on 2002 September 8, by measuring the time delay using the Very Long Baseline Array (VLBA) and Effelsberg radio telescopes at 8.4 GHz. At closest approach, general relativity (GR) predicts a radial (static) deflection of 1190 μas and a tangential (retarded) deflection in the direction of Jupiter's motion of 51 μas. Our experiment achieved an rms position error of ≤10 μas and measured this retarded deflection to be 0.98 ± 0.19 (rms error) times that predicted by GR. The increased positional accuracy for this VLBI phase-referencing experiment was achieved by using two calibrator sources. Comments on the interpretation of this experiment are given.

The unprecedented sky coverage and photometric uniformity of the Two Micron All Sky Survey (2MASS) provides a rich resource for investigating the galaxies populating the local universe. A full characterization of the large-scale clustering distribution is important for theoretical studies of structure formation. 2MASS offers an all-sky view of the local galaxy population at 2.15 μm, unbiased by young stellar light and minimally affected by dust. We use 2MASS to map the local distribution of galaxies, identifying the largest structures in the nearby universe. The inhomogeneity of these structures causes an acceleration on the Local Group of galaxies, which can be seen in the dipole of the cosmic microwave background (CMB). We find that the direction of the 2MASS clustering dipole is 16° from the CMB dipole, confirming that the local galaxy distribution accelerates the Local Group. From the magnitude of the dipole, we find a value of the linear bias parameter b = 1.06 ± 0.17 in the K_s band. Thus, the linear bias parameter of K_s-selected galaxies is similar to the bias parameter found in other wave bands.

A scenario is put forth for the formation of supermassive black holes at the centers of galaxies. It depends uon the formation of a Mestel disk with a flat rotation curve, M_<r ∝ r and Σ ∝ 1/r. Such disks could form from the collapse of uniformly rotating, isolated, gaseous clouds, either protogalactic, galaxy-mass damped Lyα clouds or the gas that survives galaxy mergers. We propose that in either case the disk will be unstable to the Rossby vortex instability (RVI). This instability grows from any large, steep pressure gradient in an optically thick disk. Such pressure gradients either occur adjacent to compact objects or could be triggered by heating from individual supernovae in and around the disk. Upon excitation, the RVI transports angular momentum outward, accreting nearly all mass within the initiation radius. We have calculated that in very thin disks, the nonlinear vortices initiated by the RVI can transport angular momentum far more efficiently than turbulence. Compared to a viscosity-based Shakura-Sunyaev disk, the RVI transports angular momentum out to a much larger radius, so more mass is accreted into the central black hole. A typical galaxy rotational velocity is v_rot = 200 km s^-1, and the critical column density, necessary to initiate the RVI, is Σ_CCD ≃ 100 g cm^-2. For M_<r = 2πr²Σ, we have r_CCD = v/(2πΣ_CCDG), and the mass accreted becomes M_BH = v/(2πΣ_CCDG²) = 3 × 10⁷M_☉. Both the black hole mass M_BH and its v dependence are in good agreement with recent observations, because v_rot = σ_c, where σ_c is the velocity dispersion of the bulge at the radius of mutual contact.

The gamma-ray burst (GRB) of 1994 October 17 (GRB 941017) showed a distinct high-energy spectral component extending from ≲a few to ≳200 MeV, in addition to the typical GRB emission, which peaked at ≲a few hundred keV. The high-energy component carried at least ~3 times more energy than the lower energy component. It displayed an almost constant flux with a rather hard spectrum (F_ν ∝ ν^-α with α ~ 0) from ≲20 s into the burst up to ~200 s, while the duration of the GRB, where 90% of the energy in the lower energy component was emitted, was only 77 s. Such a high-energy component was seen in only one out of ~30 GRBs in which a similar component could have been detected and thus appears to be quite rare. We examine possible explanations for this high-energy spectral component and find that most models fail. The only emission region that provides the right temporal behavior is the reverse shock that goes into the GRB ejecta as it is decelerated by the ambient medium, or possibly the very early forward shock while the reverse shock is still going on. The best candidate for the emission mechanism is synchrotron self-Compton emission from the reverse shock. Even in this model the most natural spectral slope is only marginally consistent with the observed value, and some degree of fine-tuning is required in order to improve the agreement. This might suggest that an additional or alternative emission mechanism is at work here. A prediction of this interpretation is that such a high-energy component should be accompanied by a bright optical transient, similar to the one observed in GRB 990123.

We report the Chandra discovery of an X-ray jet associated with the z = 4.3 radio-loud quasar GB 1508+5714. The jet X-ray emission peaks ~2'' to the southwest of the quasar core. We present archival Hubble Space Telescope Wide Field Planetary Camera 2 data of the quasar field that show no optical emission at the location of the X-ray jet. We discuss possible emission mechanisms and give constraints to the magnetic field and energy densities for synchrotron radiation or for Compton scattering of the cosmic microwave background radiation as the jet X-ray emission process.

The geometry of the LMC bar is studied using the dereddened mean magnitudes of the red clump stars (I₀) from the Optical Gravitational Lensing Experiment II catalog. The value of I₀ is found to vary in the east-west direction such that both the east and the west end of the bar are closer to us with respect to the center of the bar. The maximum observed variation has a statistical significance of more than 7.6 σ with respect to the maximum value of random error. The variation in I₀ indicates the presence of warp in the bar of the LMC. The warp and the structures seen in the bar indicate that the bar could be a dynamically disturbed structure.

We report on preliminary results from the recent multiepoch neutral hydrogen absorption measurements toward three pulsars, B0823+26, B1133+16, and B2016+28, using the Arecibo telescope. We do not find significant variations in optical depth profiles over periods of 0.3 and 9-10 yr or on spatial scales of 10-20 and 70-85 AU. The large number of nondetections of the tiny-scale atomic structure suggests that the AU-sized structure is not ubiquitous in the interstellar medium and could be quite a rare phenomenon.

We compare recent observations of the supernova remnant G11.2-0.3 taken with the Very Large Array (VLA) during 2001-2002 with images from VLA archives (1984-1985) to detect and measure the amount of expansion that has occurred during 17 years. The bright, circular outer shell shows a mean expansion of 0.71% ± 0.15% and 0.50% ± 0.17%, from 20 and 6 cm data, respectively, which corresponds to a rate of 0057 ± 0012 yr^-1 at 20 cm and 0040 ± 0013 yr^-1 at 6 cm. From this result, we estimate the age of the remnant to be roughly between 960 and 3400 yr old, according to theoretical models of supernova evolution. This is highly inconsistent with the 24,000 yr characteristic age of PSR J1811-1925, located at the remnant's center, but rather is consistent with the time that has passed since the observation of the historical supernova of A.D. 386. We also predict that G11.2-0.3 is currently in a pre-Sedov evolutionary state and set constraints on the distance to the remnant based on Chandra X-ray spectral results.

We report on recent near-IR observations of V4332 Sgr—the nova-like variable that erupted in 1994. Its rapid, post-outburst evolution to a cool M-type giant/supergiant, soon after its outburst, showed that it was an unusual object differing from other eruptive variables, such as classical/symbiotic novae or born-again asymptotic giant branch stars. The present study of V4332 Sgr was motivated by the keen interest in the recent eruption of V838 Mon—an object with a spectacular light echo that, along with V4332 Sgr, is believed to belong to a new class of objects (we propose that they be called "quasi novae"). Our observations show new developments in the evolution of V4332 Sgr. The most striking feature is the detection of several molecular bands of AlO—a rarely seen molecule in astronomical spectra—in the JHK spectra. Many of these bands are being detected for the first time. The only other detection of some of these AlO bands is in V838 Mon, thereby showing further spectral similarities between the two objects. JHK photometry shows the development of a new dust shell around V4332 Sgr with a temperature of ~900 K and a lower limit on the derived mass of M_dust = 3.7 × 10^-12M_☉. This dust shell does not appear to be associated with ejecta of the 1994 outburst but is due to a second mass-loss episode, which is not expected in a classical nova outburst. The cold molecular environment, suggested by the AlO emission, is also not expected in novae ejecta. We model the AlO bands and also discuss the possible formation mechanism of the AlO. These results show the need to monitor V4332 Sgr regularly for unexpected developments. The results can also be significant in predicting possible changes in the future evolution of V838 Mon.

We utilized the unique 6.5 m Multiplie Mirror Telescope deformable secondary adaptive optics (AO) system to produce high-resolution (FWHM = 03), very high Strehl mid-infrared (9.8, 11.7, and 18 μm) images of the post-asymptotic giant branch star AC Her. The very high (98% ± 2%) Strehls achieved with mid-IR AO led naturally to an ultrastable point-spread function (PSF) independent of air mass, seeing, or location on the sky. We find no significant difference between AC Her's morphology and our unresolved PSF calibration stars (μ UMa and α Her) at 9.8, 11.7, and 18 μm. Our current observations do not confirm any extended mid-IR structure around AC Her. These observations are in conflict with previously reported Keck (seeing-limited) 11.7 and 18 μm images that suggested the presence of a resolved ~06 edge-on circumbinary disk. We conclude that AC Her has no extended mid-IR structure on scales greater than 02 (R < 75 AU). These first results of mid-IR AO science are very encouraging for future high-accuracy mid-IR imaging with this technique.

We have carried out a high-sensitivity search for circumstellar disks around Herbig Be stars in the continuum at 1.4 and 2.7 mm using the IRAM interferometer at the Plateau de Bure. In this Letter, we report data on three well-studied B0 stars: MWC 1080, MWC 137, and R Mon. The two latter have also been observed in the continuum at 0.7 and 1.3 cm using the NRAO Very Large Array. We report the detection of circumstellar disks around MWC 1080 and R Mon with masses of M_d ~ 0.003 and 0.01 M_☉, respectively, while for MWC 137 we estimate a disk mass upper limit of 0.007 M_☉. Our results show that the ratio M_d/M_* is at least an order of magnitude lower in Herbig Be stars than in Herbig Ae and T Tauri stars.

We present imaging polarimetry observations of the eruptive variable V838 Monocerotis and its neighboring field obtained in 2002 October. The polarization of field stars confirms the previously determined interstellar polarization along the line of sight to V838 Mon. While V838 Mon showed intrinsic polarization shortly after its second outburst on 2002 February 8, all subsequent observations only showed a quiescent interstellar polarization component. We find that V838 Mon once again showed significant intrinsic polarization in 2002 October, suggesting the presence of an asymmetrical geometry of scattering material close to the star. Furthermore, an observed 90° position angle flip in the intrinsic polarization from 2002 February to October suggests that the distribution of nearby circumstellar material has experienced significant changes. We discuss the opacity changes in the evolving circumstellar cloud around V838 Mon that may explain these observations.

The origin of low α/Fe ratios in some metal-poor stars, the so-called low-α stars, is discussed. It is found that most of low-α stars in the Galaxy are on the main sequence. This strongly suggests that these stars suffered from external pollution. It is also found that the Zn/Fe abundance ratios of low-α stars both in the Galaxy and in dwarf spheroidal galaxies are lower than the average value of Galactic halo stars, whereas damped Lyα absorbers have higher ratios. This implies that some low-α stars accreted matter that was depleted from gas onto dust grains. To explain the features in these low-α stars, we have proposed that metal-poor stars harboring planetary systems are the origin of these low-α stars. Stars engulfing a small fraction of planetesimals enhance the surface content of Fe to exhibit low α/Fe ratios on their surfaces, while they are on the main sequence, because dwarfs have shallow surface convection zones where the engulfed matter is mixed. After the stars leave the main sequence, the surface convection zones become deeper, reducing the enhancement of Fe. Eventually, when the stars ascend to the tip of the red giant branch, they engulf giant planets to become low-α stars again as observed in dwarf spheroidal galaxies. We predict that low-α stars with low Mn/Fe ratios harbor planetary systems.

We model the infrared (IR) emission from the ringlike dust disk around the main-sequence (MS) star Eridani, a young analog to our solar system, in terms of a porous dust model previously developed for the extended wedge-shaped disk around the MS star β Pictoris and the sharply truncated ringlike disks around the Herbig Ae/Be stars HR 4796A and HD 141569A. It is shown that the porous dust model with a porosity of ~90% is also successful in reproducing the IR to submillimeter dust emission spectral energy distribution as well as the 850 μm flux radial profile of the dust ring around the more evolved MS star Eri. Predictions are made for future SIRTF observations that may allow a direct test of the porous dust model.

Numerical simulations show that the migration of growing planetary cores may be dominated by turbulent fluctuations in the protoplanetary disk, rather than by any mean property of the flow. We quantify the impact of this stochastic core migration on the formation timescale and core mass of giant planets at the onset of runaway gas accretion. For standard solar nebula conditions, the formation of Jupiter can be accelerated by almost an order of magnitude if the growing core executes a random walk with an amplitude of a few tenths of an AU. A modestly reduced surface density of planetesimals allows Jupiter to form within 10 Myr, with an initial core mass below 10 M_⊕, in better agreement with observational constraints. For extrasolar planetary systems, the results suggest that core accretion could form massive planets in disks with lower metallicities, and shorter lifetimes, than the solar nebula.

We present evidence of flaring behavior in a transequatorial loop (TEL) that lights up in soft X-rays on 2000 July 13. The large loop structure connects NOAA Active Regions 9070/9066 in the northern hemisphere and AR 9069/9068 in the southern hemisphere. We follow the loop systems for 2 days and observe several pieces of evidence strongly suggesting flare behavior of the form seen in standard flaring in active regions. These include brightenings of the loop structure, cooling of plasma that is seen both in soft X-rays and in the transition region temperatures, morphological evidence of reconnection inflow, and blueshifts around the footpoint of the TEL suggestive of chromospheric evaporation. We present, to our knowledge for the first time, observations of TEL in the O V emission line.

We report on a close relationship between the solar polarity reversal and the cessation of high-latitude coronal mass ejections (CMEs). This result holds good for individual poles of the Sun for cycles 21 and 23, for which CME data are available. The high-latitude CMEs provide a natural explanation for the disappearance of the polar crown filaments (PCFs) that rush the poles. The PCFs, which are closed field structures, need to be removed before the poles could acquire open field structure of the opposite polarity. Inclusion of CMEs along with the photospheric and subphotospheric processes completes the full set of phenomena to be explained by any solar dynamo theory.

We present the first magnetic maps of a prominence, derived from inversion of spectropolarimetric data in He I D3 using the principal component analysis of all four Stokes profiles. This prominence, along with several others, was observed in 2002 May using the Dunn Solar Telescope of the National Solar Observatory/Sacramento Peak Observatory, equipped with the High Altitude Observatory Advanced Stokes Polarimeter. The use of an unocculted instrument allowed us to map the prominence magnetic fields down to the chromospheric limb. Our analysis indicates that the average magnetic field in prominences is mostly horizontal and varies between 10 and 20 G, thus confirming previous findings. However, our maps show that fields significantly stronger than average, even as large as 60 or 70 G, can often be found in clearly organized plasma structures of the prominence.