Table of contents

We formulate a general method for perturbative evaluations of statistics of smoothed cosmic fields and provide useful formulae for application of the perturbation theory to various statistics. This formalism is an extensive generalization of the method used by Matsubara, who derived a weakly nonlinear formula of the genus statistic in a three-dimensional density field. After describing the general method, we apply the formalism to a series of statistics, including genus statistics, level-crossing statistics, Minkowski functionals, and a density extrema statistic, regardless of the dimensions in which each statistic is defined. The relation between the Minkowski functionals and other geometrical statistics is clarified. These statistics can be applied to several cosmic fields, including three-dimensional density field, three-dimensional velocity field, two-dimensional projected density field, and so forth. The results are detailed for second-order theory of the formalism. The effect of the bias is discussed. The statistics of smoothed cosmic fields as functions of rescaled threshold by volume fraction are discussed in the framework of second-order perturbation theory. In CDM-like models, their functional deviations from linear predictions plotted against the rescaled threshold are generally much smaller than that plotted against the direct threshold. There is still a slight meatball shift against rescaled threshold, which is characterized by asymmetry in depths of troughs in the genus curve. A theory-motivated asymmetry factor in the genus curve is proposed.

Using a simple analytic approach we address the question of whether radiative cooling, nongravitational heating, and cooling plus heating models can simultaneously explain the observed global X-ray properties (entropy and X-ray luminosity distributions) of groups and clusters and the residual soft X-ray background (XRB) after discrete sources are removed. Within the framework of typical cold dark matter structure formation characterized by an amplitude of matter power spectrum σ₈ = 0.9, it is argued that while radiative cooling alone is able to marginally reproduce the entropy floor detected in the central regions of groups and clusters, it is insufficient to account for the steepening of the X-ray luminosity-temperature relation for groups and the unresolved soft XRB. A phenomenological preheating model, in which either an extra specific energy budget or an entropy floor is added to the hot gas in groups and clusters, fails in the recovery of at least one of the X-ray observed features. Finally, the soft XRB predicted by our combined model of cooling plus heating exceeds the observational upper limits by a factor of ~2, if the model is required to reproduce the observed entropy and X-ray luminosity-temperature relationships of groups and clusters. Inclusion of the cosmic variation of metallicity and the self-absorption of the cooled gas as a result of radiative cooling in groups and clusters, or exclusion of the contribution of nearby massive clusters to the XRB, does not significantly alter the situation. If the discrepancy is not a result of the oversimplification of our analytic models, this implies that either our current understanding of the physical processes of the hot gas is still incomplete, or the normalization of the present power spectrum has been systematically overestimated. For the latter, both the X-ray properties of groups and clusters and the XRB predicted by the preheating model and the cooling plus heating model can be reconciled with the X-ray observations if a lower value of the normalization parameter σ₈ ≈ 0.7 is assumed.

We present the first results from a survey of the relative spatial distributions of galaxies, intergalactic neutral hydrogen, and intergalactic metals at high redshift. We obtained high-resolution spectra of eight bright quasars (QSOs) at 3.1 < z < 4.1 and spectroscopic redshifts for 431 Lyman break galaxies (LBGs) at slightly lower redshifts. Comparing the locations of galaxies to the absorption lines in the QSO spectra shows that the intergalactic medium contains less neutral hydrogen than the global average within r ≲ 0.5 h^-1 comoving Mpc of LBGs and more than average at slightly larger distances 1 h^-1 comoving Mpc ≲ r ≲ 5 h^-1 comoving Mpc. The intergalactic medium within the largest overdensities at z ~ 3, which will presumably evolve into the intracluster medium by z ~ 0, is rich in neutral hydrogen and C IV. The lack of H I absorption at small distances from LBGs appears unlikely to be produced solely by the Lyman continuum radiation they emit; it may show that the galaxies' supernova-driven winds maintain their measured outflow velocities of ~600 km s^-1 for a few hundred million years and drive away nearby intergalactic gas. We present correlation functions of galaxies with Lyα forest flux decrements, with C IV systems, and with other galaxies. We describe the association of galaxies with damped Lyα systems and with intergalactic He II opacity. A strong observed correlation of galaxies with intergalactic metals supports the idea that LBG winds have enriched their surroundings.

We use a sample of seven starburst galaxies at intermediate redshifts (z ~ 0.4 and 0.8) with observations ranging from the observed ultraviolet to 1.4 GHz, to compare the star formation rate (SFR) estimators that are used in the different wavelength regimes. We find that extinction-corrected Hα underestimates the SFR, and the degree of this underestimation increases with the infrared luminosity of the galaxies. Galaxies with very different levels of dust extinction as measured with SFR_IR/SFR(Hα, uncorrected for extinction) present a similar attenuation A[Hα], as if the Balmer lines probed a different region of the galaxy than the one responsible for the bulk of the IR luminosity for large SFRs. In addition, SFR estimates derived from [O II] λ3727 match very well those inferred from Hα after applying the metallicity correction derived from local galaxies. SFRs estimated from the UV luminosities show a dichotomic behavior, similar to that previously reported by other authors in galaxies at z ≲ 0.4. Here we extend this result up to z ~ 0.8. Finally, one of the studied objects is a luminous compact galaxy (LCG) that may be suffering similar dust-enshrouded star formation episodes. These results highlight the relevance of quantifying the actual L_IR of LCGs, as well as that of a much larger and generic sample of luminous infrared galaxies, which will be possible after the launch of SIRTF.

We present a multiwavelength analysis of high-resolution observations of the quadruple lens B1608+656 from the Hubble Space Telescope archive, acquired with WFPC2 through filters F606W (V band) and F814W (I band) and with NIC1 in filter F160W (H band). In the three bands, the observations show extended emission from the four images of the source in a ringlike configuration that surrounds the two resolved, ensing galaxies. B1608+656 was discovered as a double-lobed radio source and later identified as a poststarburst galaxy in the optical. Based on photometry and optical spectroscopy we estimate that the stellar population of the source has an age of ~500 Myr. This provides a model for the spectrum of the source that extends over spectral regions where no observations are available and is used to generate Tiny Tim point-spread functions (PSFs) for the filters. Deconvolutions performed with the Lucy-Richardson method are presented, and the limitations of these restorations are discussed. V-I and I-H color maps show evidence of extinction by dust associated with one of the lensing galaxies, a late-type galaxy presumably disrupted after its close encounter with the other lens, an elliptical galaxy. The extinction affects the two lens galaxies and two of the four multiple images. The diagnostic of wavelength-dependent effects in the images shows that corrections for contamination with light from the lenses, extinction, and PSF convolution need to be applied before using the extended structure in the images as a constraint on lens models. We will present the restoration of the images in a subsequent paper.

We determine the evolution of the faint, high-redshift, optical luminosity function (LF) of active galactic nuclei (AGNs) implied by several observationally motivated models of the ionizing background from 3 < z < 5. Our results depend crucially on whether we use the total ionizing rate measured by the proximity effect technique or the lower determination favored by the flux decrement distribution of Lyα forest lines. Assuming a faint-end LF slope of 1.58 and the SDSS estimates of the bright-end slope and normalization, we find that the LF must break at M = -24.2, -22.3, -20.8 at z = 3, 4, 5 if we adopt the lower ionization rate and assume no stellar contribution to the background. The breaks must occur at M = -20.6, -18.7, -18.7 for the proximity effect estimate. Since stars may also contribute to the background, these values are lower limits on the break luminosity, and they brighten by as much as ~2 mag if the escape fraction of ionizing photons from high-z galaxies is consistent with recent estimates: f_esc = 0.16. By comparing our expectations to faint AGN searches in the Hubble Deep Field and high-z galaxy fields, we find that typically quoted proximity effect estimates of the background imply an overabundance of AGNs compared to the faint counts (even with f_esc = 1). Even adopting the lower bound on proximity effect measurements, the stellar escape fraction must be high: f_esc ≳ 0.2. Conversely, the lower flux decrement-derived background requires a smaller number of ionizing sources, and faint AGN counts are consistent with this estimate only if there is a limited stellar contribution, f_esc ≲ 0.05. Our derived LFs together with the locally estimated black hole density suggest that the efficiency of converting mass to light in optically unobscured AGNs is somewhat lower than expected, ≲ 0.05 (all models). Comparison with similar estimates based on X-ray counts suggests that more than half of all AGNs are obscured in the UV/optical. We also derive lower limits on typical AGN lifetimes and obtain ≳10⁷ yr for favored cases.

We present Chandra observations of the enigmatic galaxy IRAS 00317-2142, which is classified as a star-forming galaxy on the basis of the ionization level of its emission lines. However, a weak broad Hα wing and a high X-ray luminosity give away the presence of an active nucleus. The Chandra image reveals a nuclear point source (L_{(2-10 keV)} ~ 6 × 10⁴¹ergs s^-1), contributing over 80% of the galaxy X-ray counts in the 0.3-8 keV band. This is surrounded by some fainter nebulosity extending up to 6 kpc. The nucleus does not show evidence for short-term variability. However, we detect long-term variations between the ROSAT, ASCA, and Chandra epochs. Indeed, the source has decreased its flux by over a factor of 25 in a period of about 10 yr. The nuclear X-ray spectrum is well represented by a power law with a photon index of Γ = 1.91, while the extended emission is represented by a Raymond-Smith component with a temperature of ~0.6 keV. We find no evidence for the presence of an Fe line. The nucleus is absorbed by an intrinsic column density of N_H ~ 8 × 10²⁰ cm^-2. Thus, the Chandra observations suggest that at least the X-ray emission is dominated by a type 1 active galactic nucleus (AGN). Then the observed optical spectrum can be explained by the masking of the nucleus by the powerful star-forming component. These, together with previous X-ray observations of galaxies with no clear signs of AGN activity in the optical (e.g., NGC 6240), cast doubt on the optical classification scheme and have implications for the nature of the "normal" galaxies detected in deep Chandra X-ray surveys.

We present the results of a multiepoch Very Long Baseline Array study of the powerful radio source 4C +12.50 (PKS 1345+125) at a wavelength of 2 cm. This compact radio source is associated with a hidden quasar whose host galaxy shows signs of a recent merger. It has been classified as a compact symmetric source (CSO) as a result of its small overall size (~220 pc) and twin-jet morphology, although it also has faint extended emission that may be a relic of previous activity. We report the detection of exceedingly high linear fractional polarization in isolated features of the southern jet (up to 60%), which is highly unusual for a CSO. Given the large amount of gas present in the host galaxy, we would expect significant Faraday depolarization across the whole source, unless the depolarizing gas is fairly clumpy. The southern jet also contains two features that are moving outward from the core at apparent speeds of v/c = 1.0 ± 0.3 and 1.2 ± 0.3. These represent the first positive detections of superluminal motion in a CSO and, taken together with the jet-to-counterjet flux ratio, suggest an intrinsic flow speed of v/c = 0.84 ± 0.12. The apparent ridgeline of the jet and counterjet is consistent with a conical helix of wavelength 280 pc that is the result of Kelvin-Helmholtz instabilities driven by a slow precession of the jet nozzle. A fit to the data implies that the nozzle is precessing around a cone with half-angle 23°, whose axis lies at an angle of 82° to the line of sight. We suggest that the S-shaped jet morphologies commonly seen in recent active galactic nucleus outflows such as 4C +12.50 may simply reflect the fact that their black hole spin axes are still precessing and have not had sufficient time to align with their accretion disks.

We report the identification of a very radio loud, narrow-line quasar, SDSS J094857.3+002225, discovered in the Sloan Digital Sky Survey (SDSS) early data release (EDR). With FWHM (Hβ) ≈ 1500 km s^-1 and undetected [O III] λλ 4959, 5007 SDSS J094857.3+002225 is consistent with the conventional definition of narrow-line Seyfert 1 galaxies (NLS1s). Its strong optical Fe II emission [R₄₅₇₀ ≡ f(Fe II λ4570)/f(Hβ) ≈ 1.59] is also typical of NLS1s. With a radio loudness RL ≡ (f_{ν,4.85 GHz}/f_ν,B) ≳ 1000 and radio power of P_{4.85 GHz} ~ (3.6-5.2) × 10²⁶ W Hz^-1, it is the first bona fide, very radio loud NLS1. The inverted radio spectrum and very high brightness temperature (≳10¹³ K) derived from variation of the radio flux suggest the presence of a relativistic jet beaming toward the observer in this NLS1.

The recent long-look X-ray observations of TeV blazars have revealed many important new features concerning their time variability. In this paper we suggest a physical interpretation for those features based on the framework of the internal and external shock scenarios. We present a simplified model applicable to TeV blazars and investigate through simulations how each of the model parameters would affect the observed light curve or spectrum. In particular, we show that the internal shock scenario naturally leads to all the observed variability properties, including the structure function, but for it to be applicable, the fractional fluctuation of the initial bulk Lorentz factors must be small, σ ≡ σ_Γ/Γ_avg ≪ 0.01. This implies very low dynamical efficiency of the internal shock scenario. We also suggest that several observational quantities—such as the characteristic timescale, the relative amplitude of flares as compared to the steady ("offset") component, and the slope of the structure function—can be used to probe the inner jet. The results are applied to the TeV blazar Mrk 421, and this, within the context of the model, leads to the determination of several physical parameters: the ejection of a shell with average thickness of ~10¹³ cm occurs on average every 10 minutes, and the shells collide ~10¹⁷ cm away from the central source.

The nucleus of M87 displays a LINER spectrum at optical wavelengths, with a nuclear disk of nebulosity that is resolved by the Hubble Space Telescope (HST). We present new results from optical and ultraviolet spectra of the central ~40 pc as measured by HST. In contrast with previous results for the extended disk, the emission-line spectrum of the central region is best reproduced by a multicomponent photoionization scenario, rather than shock heating. The nebular properties as well as energetic considerations suggest a transition on scales of several tens of parsecs, from predominantly photoionization by a central accretion source to shock ionization within the disk. If this source is at all representative, it suggests that many LINERs may be composite in terms of the energetic processes that give rise to the emission spectrum. We also report measurements of resonance-line absorption for the nucleus. The absorption spectrum, like the emission lines, is characterized by low ionization. The absorption-line measurements coupled with independent constraints suggest a total hydrogen column density of 10¹⁹-10²⁰ cm^-2, outflowing from the galaxy center with a velocity of ~126 km s^-1. The kinematic signature of an outflow, along with evidence that the absorber covers the power-law continuum source but not the emission-line clouds, suggests that the absorbing matter is related to accretion phenomena in the nucleus. The presence of such an outflow resembles similar behavior in luminous active galactic nuclei, although the low ionization that characterizes LINERs is probably indicative of a different mode of accretion in these sources.

We present a study of the flux and spectral variability of the two broad-line radio galaxies (BLRGs) 3C 390.3 and 3C 120, observed almost daily with the Rossi X-Ray Timing Explorer for nearly 2 months each in 1996 and 1997, respectively. Our original motivation for this study was to search for systematic differences between BLRGs and their radio-quiet counterparts, the Seyfert galaxies, whose temporal and spectral behavior is better studied. We find that both 3C 390.3 and 3C 120 are highly variable but in different ways, and we quantify this difference by means of a structure function analysis. 3C 390.3 is significantly more variable than 3C 120, despite its jet's larger inclination angle, implying either that the X-ray variability is not dominated by the jet or that two different variability processes are simultaneously at work in 3C 390.3. We perform an energy-selected and time-resolved analysis based on the fractional variability amplitude and find that the variability amplitude of both objects is strongly anticorrelated with the energy. This last result, along with the correlated change of the photon index with the X-ray continuum flux, can be qualitatively explained within the scenario of thermal Comptonization, generally invoked for radio-quiet active galaxies. Moreover, the time-resolved and energy-selected fractional variability analysis shows a trend opposite to that observed in jet-dominated active galactic nuclei (blazars), suggesting only a minor contribution of the jet to the X-ray properties of BLRGs. Time-averaged spectral analysis indicates the presence of a strong resolved Fe Kα line with a centroid at 6.4 keV and a weak (Ω/2π ≃ 0.1-0.4) reflection component in both objects. The overall PCA+HEXTE spectra are best fitted with the constant density ionization model of Ross & Fabian, but with a modest ionization parameter. We perform a time-resolved spectral analysis of 3C 390.3 with the aim of constraining the delay between Fe Kα line and continuum variability; however, the limited signal-to-noise ratio of the line flux hampers a thorough study of the line variability.

We consider nonthermal emission from clusters of galaxies produced by particle acceleration by resonant scattering of Alfvén waves driven by fluid turbulence through the Lighthill mechanism in the intracluster medium. We assume that the turbulence is driven by cluster mergers. We find that the resonant Alfvén waves can accelerate electrons up to γ ~ 10⁵ through resonant scattering. We also find that the turbulent resonant acceleration can give enough energy to electrons to produce the observed diffuse radio relic emission from clusters if the clusters have a pool of electrons with γ ~ 10³. This mechanism can also explain the observed hard X-ray emission from clusters if the magnetic field in a cluster is small enough (≲μG) or the fluid turbulence spectrum is flatter than the Kolmogorov law. The fluid turbulence could be observed with Astro-E2 in the regions where diffuse radio emission is observed. Although nongravitational heating before cluster formation (preheating) steepens a relation between radio luminosity and X-ray temperature, our predicted relation is still flatter than the observed one.

We present the K-band local luminosity function derived from a sample of 1056 bright (K < 15) K-selected galaxies from the Hawaii+Anglo-Australian Observatory (AAO) K-band redshift survey. The Hawaii+AAO K-band redshift survey covers four equatorial fields with a total area of 8.22 deg². We derive both the nonparametric and Schechter luminosity function from our data and determine M*(K) = -23.70 ± 0.08 + 5 log₁₀(h), α = -1.37 ± 0.10, and ϕ* = 0.013 ± 0.003 h³ Mpc^-3 for a universe with Ω_m = 0.3 and Ω_Λ = 0.7. We also measure the K-band luminosity function for the early- and later type galaxies from our morphologically classified subsample. It appears that later type galaxies have a fainter M* and a steep slope, while early-type galaxies have a much brighter M* and a quite flat slope in their K-band luminosity functions. This is consistent with what has been found in optical-type dependent luminosity function. The K-band luminosity density derived using our luminosity function is now measured at a similar redshift depth to optical luminosity densities in the Sloan Digital Sky Survey redshift survey. It is 2 times higher than the previous measurement from the shallower 2MASS sample and resolves the previously reported discrepancies between optical and near-infrared luminosity densities.

We present in this paper a detailed analysis of the effect of environment on the star formation activity of galaxies within the Early Data Release (EDR) of the Sloan Digital Sky Survey (SDSS). We have used the Hα emission line to derive the star formation rate (SFR) for each galaxy within a volume-limited sample of 8598 galaxies with 0.05 ≤ z ≤ 0.095 and M(r*) ≤ -20.45. We find that the SFR of galaxies is strongly correlated with the local (projected) galaxy density, and thus we present here a density-SFR relation that is analogous to the density-morphology relation. The effect of density on the SFR of galaxies is seen in three ways. First, the overall distribution of SFRs is shifted to lower values in dense environments compared with the field population. Second, the effect is most noticeable for the strongly star-forming galaxies (Hα EW > 5 Å) in the 75th percentile of the SFR distribution. Third, there is a "break" (or characteristic density) in the density-SFR relation at a local galaxy density of ~1 h Mpc^-2. To understand this break further, we have studied the SFR of galaxies as a function of clustercentric radius from 17 clusters and groups objectively selected from the SDSS EDR data. The distribution of SFRs of cluster galaxies begins to change, compared with the field population, at a clustercentric radius of 3-4 virial radii (at the >1 σ statistical significance), which is consistent with the characteristic break in density that we observe in the density-SFR relation. This effect with clustercentric radius is again most noticeable for the most strongly star-forming galaxies. Our tests suggest that the density-morphology relation alone is unlikely to explain the density-SFR relation we observe. For example, we have used the (inverse) concentration index of SDSS galaxies to classify late-type galaxies and show that the distribution of the star-forming (EW Hα > 5 Å) late-type galaxies is different in dense regions (within 2 virial radii) compared with similar galaxies in the field. However, at present, we are unable to make definitive statements about the independence of the density-morphology and density-SFR relation. We have tested our work against potential systematic uncertainties including stellar absorption, reddening, SDSS survey strategy, SDSS analysis pipelines, and aperture bias. Our observations are in qualitative agreement with recent simulations of hierarchical galaxy formation that predict a decrease in the SFR of galaxies within the virial radius. Our results are in agreement with recent 2dF Galaxy Redshift Survey results as well as consistent with previous observations of a decrease in the SFR of galaxies in the cores of distant clusters. Taken together, these works demonstrate that the decrease in SFR of galaxies in dense environments is a universal phenomenon over a wide range in density (from 0.08 to 10 h Mpc ^-2) and redshift (out to z ≃ 0.5).

We present the analysis of the H I survey of isolated galaxies described in the first two papers in this series. This survey found 13 gas-rich companions around 10 of the 41 galaxies observed. In this paper, we present numerous pieces of evidence that the number of companions detected is less than expected by cold dark matter models of galaxy formation or H I observations of the population of galaxies in the field. We discuss the implications of these data for galaxy formation and the evolution of the properties of these companions. A statistical analysis of the observed distribution of the ensemble of companions with respect to the primary galaxies implies that, in general, these companions are on circular orbits and not eccentric orbits. While individual companions may be on eccentric orbits, the majority of the orbits are circular and, therefore, fairly stable. In fact, on the basis of the observed separations, both spatial and in velocity space, only those companions currently interacting with the galaxy they orbit should be accreted in the next few Gyr, and only half of the companies will be accreted in less than a Hubble time. Of the close companions, only UGC 260A and UGC 11152A appear to be significantly affecting the morphology of the primary galaxy. The remaining companions have too little mass and are too far away from the main galaxies to have any current effect on their morphology. None of these companions are massive enough to destroy the disk of the isolated galaxy when they are accreted, but instead will represent a minor merger that will supply a large amount of H I to fuel future star formation in these galaxies and produce small effects on the morphology. All of which implies that we are seeing nothing more than the tail end of the galaxy formation process. We have also compared the cumulative velocity distribution function of our companions with the predictions of cold dark matter and other galaxy formation models. Our data are only consistent with cold dark matter models when generous, but not unreasonable, correction factors are applied for nondetections of gas-poor galaxies. Models that suppress observable satellite galaxies, such as warm dark matter models or those that prevent neutral gas from accumulating in dark matter halos, would better explain our observed population of companions. Finally, we compare the H I mass function (MF) for companions to isolated galaxies with the H I MF from the work of others. Our H I MF is consistent with a variety of faint-end slopes up to α ~ -1.3. Such a slope argues against the Blitz et al. model for high-velocity clouds.

We present here optical spectroscopy and BVRJHK_s photometry of the recently discovered ultracompact group of galaxies CG J1720-67.8. This work represents a considerable extension of the preliminary results we presented in a previous paper. Despite the complicated morphology of the group, a quantitative morphological classification of the three brightest members of the group is attempted based on photometric analysis. We find that one galaxy is consistent with a morphological type S0, while the other two are most probably late-type spiral galaxies that are already losing their identity because of the interaction process. Information on the star formation activity and dust content derived from both spectroscopic data and optical and near-infrared colors is complemented with a reconstruction of far-infrared maps from IRAS raw data. Enhanced star formation activity is revealed in all the group's members, including the early-type galaxy and the extended tidal tail, along which several tidal dwarf galaxy candidates are identified. The metallicity of the gaseous component is investigated, and photoionization models are applied to the three main galaxies of the group, while a detailed study of the tidal dwarf candidates will appear in a companion paper. Subsolar metal abundances are found for all the three galaxies, the highest values being shown by the early-type galaxy (Z ~ 0.5 Z_☉).

Lenticular galaxies remain remarkably mysterious as a class. Observations to date have not led to any broad consensus about their origins, properties, and evolution, although they are often thought to have formed in one big burst of star formation early in the history of the universe and to have evolved relatively passively since then. In that picture, current theory predicts that stellar evolution returns substantial quantities of gas to the interstellar medium; most is ejected from the galaxy, but significant amounts of cool gas might be retained. Past searches for that material, though, have provided unclear results. We present results from a survey of molecular gas in a volume-limited sample of field S0 galaxies selected from the Nearby Galaxies Catalog. CO emission is detected from 78% of the sample galaxies. We find that the molecular gas is almost always located inside the central few kiloparsecs of a lenticular galaxy, meaning that in general it is more centrally concentrated than in spirals. We combine our data with H I observations from the literature to determine the total masses of cool and cold gas. Curiously, we find that, across a wide range of luminosity, the most gas-rich galaxies have ~10% of the total amount of gas ever returned by their stars. That result is difficult to understand within the context of either monolithic or hierarchical models of evolution of the interstellar medium.

Microlensing surveys derive the microlensing optical depth toward various directions such as the Galactic center from the distribution of observed Einstein radius crossing times. I show that the formula that is being used is invalid for "exotic" lensing events. The corrected formula is derived. The "parallax" effect (Earth motion) requires no correction. Corrections for the finite sizes of sources and wide binary lenses are small (typically less than 1%), except for blended events. Corrections for intermediate-type binaries such as MACHO LMC-9 can be substantial.

Numerical simulations of the interaction between supernova ejecta and a stellar wind are presented. We follow the temporal evolution of the shock fronts that are formed through such an interaction and determine the velocities, temperatures, and densities. We model the X-ray emission from the supernova remnant-stellar wind collision region, and we compare it with recent results from X-ray observations carried out with the Chandra satellite of the SMC supernova remnant SNR 0057-7226, which could be interacting with the wind of the Wolf-Rayet system HD 5980. The simulations predict the presence of shell-like regions of enhanced X-ray emission that are consistent with the presence of X-ray-emitting arcs in the Chandra image. Also, the observed X-ray luminosity is comparable to the X-ray luminosities we obtain from the simulations for a supernova with an initial energy in the (1–5) × 10⁵⁰ ergs range.

The turbulent energy spectrum of molecular clouds in a variety of environments is measured via principal component analysis (PCA) of spectral line imaging observations at millimeter wavelengths. Molecular clouds with known distances have been previously shown to accurately obey a universal scale dependence of turbulent velocity dispersion over spatial scales of 1-50 pc, via both standard object-based analysis and, more recently, PCA. The PCA-based spectrum is accurately obeyed such that it may be used as a distance estimator for molecular clouds with ~30% accuracy, where the error budget is contributed to strongly by input H II region distances used for the calibration. The use of ¹³CO spectral line data for distance estimation is examined and compared to the distance calibration established for ¹²CO observations. We show that distances estimated using ¹³CO are in good agreement with those obtained using ¹²CO, with a possible ~10% distance overestimation for ¹³CO relative to the ¹²CO calibration. Several molecular clouds with known distances are subjected to PCA, and we demonstrate that the universal spectrum is closely respected by all clouds; PCA-based distances estimated under the assumption of exact adherence to the universal spectrum are derived and are shown to be in excellent agreement with optically estimated distances. We examine the possibility that the PCA distance estimation method may be used to solve the kinematic distance ambiguity in the inner Galaxy. We establish how PCA may be used to diagnose severe blending of near/far emission and, in cases of little or no blending, to accurately establish the near or far distance. The inner Galaxy results provide initial support for the global validity of the universal PCA spectrum previously demonstrated for the outer Galaxy only. In conjunction with the accurate velocity information provided by millimeter wavelength spectral line data, PCA can provide useful information for studies of Galactic structure and kinematics.

We have modeled the states of hydrogenation and charge of polycyclic aromatic hydrocarbons (PAHs) in diffuse clouds for molecules ranging from benzene up to species containing 200 carbon atoms. It is found that the hydrogenation state of PAHs strongly depends on the size of the molecule. Small PAHs with fewer than about 15-20 carbon atoms are destroyed in most environments. Intermediate-size PAHs in the range of 20-30 carbon atoms are stripped of most of their peripheral hydrogen atoms, but may be able to survive in the interstellar medium because of the relative stability of their carbon skeleton upon UV photon absorption. Larger PAHs primarily have normal hydrogen coverage (i.e., with each peripheral carbon atom bearing a single hydrogen), with competition between this form and PAHs containing an additional hydrogen. Very large PAHs may be fully hydrogenated, with every peripheral carbon atom bearing two hydrogen atoms. Our finding that extremely dehydrogenated PAH neutrals or positively charged C_mH, with m ranging from 15 to 30 and n ≤ 2, can survive in the interstellar medium contrasts with previous work, where it was generally assumed that PAHs losing their hydrogen coverage were quickly destroyed. A mechanism is proposed for the selective growth of these small dehydrogenated PAHs in diffuse clouds with respect to larger PAHs. Finally, our results are compared to previous studies on the hydrogenation and charge states of PAHs.

Because H₂ formation on dust-grain surfaces completely dominates gas-phase H₂ formation in local molecular clouds, it is often assumed that gas-phase formation is never important. In fact, it is the dominant mechanism in a number of cases. In this paper, I briefly summarize the chemistry of gas-phase H₂ formation and show that it dominates for dust-to-gas ratios less than a critical value D_cr. I also show that D_cr is simple to calculate for any given astrophysical situation and illustrate this with a number of examples, ranging from H₂ formation in warm atomic gas in the Milky Way to the formation of protogalaxies at high redshift.

We have investigated the correlations between the equivalent widths of 21 selected diffuse interstellar bands (DIBs) and the corresponding interstellar column densities N(C₂), N(CN), and N(CH), toward 53 stars with color excesses 0.11 ≤ E(B-V) ≤ 1.99. The observational data were derived primarily from echelle spectra acquired at R = 38,000 as part of our extensive, continuing survey of the bands. All but six of the 53 final spectra show signal-to-noise ratios ≥800 at 5780 Å. The principal result presented here is that seven of the 21 bands prove to be examples of "the C₂ DIBs," a class of weak, narrow bands whose normalized equivalent widths W_λ(X)/W_λ (λ6196) are well correlated specifically with N(C₂)/E(B-V) via power laws. In contrast, the similarly normalized equivalent widths of the 14 other, well-known DIBs analyzed here are uncorrelated, or weakly anticorrelated, with N(C₂)/E(B-V), to within the observational uncertainties. Thus, the polyatomic molecule(s) presumed to cause these seven C₂ DIBs may bear a direct chemical relation to C₂ that is not shared by the polyatomic molecules putatively responsible for the other 14 bands. The C₂ DIBs also show positive correlations with N(CN)/E(B-V) and N(CH)/E(B-V) in our particular sample of light paths, although generally with shallower slopes in the case of N(CN) and with greater scatter in the case of N(CH). Eleven additional C₂ DIBs are also identified but are not analyzed here. Among the 18 C₂ DIBs identified, four apparently have not been previously detected. The λ4963 band is generally the strongest of the 18 C₂ DIBs, while the λ4734 band shows the most sensitive correlation with N(C₂).

We have searched for CO molecular outflows associated with the cloud A and B2 of the ρ Ophiuchi star-forming region. On the basis of single -dish ¹²CO (J = 3-2) and interferometric ¹²CO (J = 1-0) observations, we have identified three molecular outflows in the ρ Oph A cloud and one in the ρ Oph B2 cloud. Out of the four outflows observed in these regions, one is the known outflow driven by the Class 0 source, VLA 1623, and the other three are newly discovered ones. The inclination and opacity corrected momentum flux F_CO of newly detected outflows are roughly comparable to those of Class I objects observed by Bontemps et al. (1996). All the newly discovered outflows are likely to be driven by Class II sources or near-infrared sources, and none of them is associated with the cold submillimeter-millimeter sources without infrared counter parts. These results indicate that protostars associated with outflows whose mass and momentum flux exceed our detection limit are not formed yet in the high-density ridge of the ρ Oph A region and the northeastern condensation in the ρ Oph B2 region, suggesting that they are considered to be pre-protostellar cores.

We have made imaging and spectroscopic observations of the ultracompact H II region K3-50A with a spatial resolution of 04 using a new mid-infrared instrument, the Cooled Mid-Infrared Camera and Spectrometer, on the 8.2 m Subaru Telescope. The spectra show thermal dust emission, 9.7 μm silicate absorption, and fine-structure line emissions of [Ne II] at 12.8 μm, [Ar III] at 8.99 μm, and [S IV] at 10.51 μm. From the maps of the continuum, line emissions, and/or derived dust parameters, we identify eight mid-infrared sources in K3-50A. Especially the central [Ne II] emission has been resolved into two peaks clearly. The ionization condition is investigated with the line flux ratios I([Ar III])/I([Ne II]) and I([S IV])/I([Ne II]). It is suggested that the spectral types of the ionizing stars in K3-50A correspond to B0-O8 V, which is much later than O5.5 V, the type estimated from radio continuum observations under a single-star assumption. The three line fluxes suggest a number of Ne⁺ ions greater than what is ionized by a single star of any spectral type, but the numbers of Ar²⁺ and S³⁺ are similar to that formed by a single O8-O9 V star and that by a single O7-O9 V star, respectively. From these features as well as the dust temperature and the correspondence of each identified source with the near-infrared source, we propose that K3-50A is excited by at least two (possibly three) ionizing stars. This is the first convincing example that a massive stellar cluster is ionizing an ultracompact H II region.

Recent R-matrix calculations of electron impact excitation rates among the 3s²3p⁴ levels of Cl II are used to derive the nebular emission-line intensity ratios R₁ = I(6161.8 Å)/I(8578.7 Å) and R₂ = I(6161.8 Å)/I(9123.6 Å) as a function of electron temperature (T_e) and density (N_e). The ratios are found to be very sensitive to changes in T_e but not N_e for densities lower than 10⁵ cm^-3. Hence, they should, in principle, provide excellent optical T_e diagnostics for planetary nebulae. The observed values of R₁ and R₂ for the planetary nebulae NGC 6741 and IC 5117, measured from spectra obtained with the Hamilton echelle spectrograph on the 3 m Shane Telescope, imply temperatures in excellent agreement with those derived from other diagnostic lines formed in the same region of the nebula as [Cl II]. This provides some observational support for the accuracy of the [Cl II] line ratio calculations and hence the atomic data on which they are based. The [Cl II] 8578.7 and 9123.6 Å lines are identified for the first time (to our knowledge) in a high-resolution spectrum of the symbiotic star RR Telescopii, obtained with the University College London Echelle Spectrograph on the 3.9 m Anglo-Australian Telescope. However, the 6161.8 Å feature is unfortunately too weak to be identified in the RR Telescopii observations, consistent with its predicted line strength.

We show that the collapsar model of gamma-ray bursts results in a series of successive shocks and rarefaction waves propagating in the "cork" of stellar material being pushed ahead of the jet, as it emerges from the massive stellar progenitor. Our results are derived from analytical calculations assuming a hot, ultrarelativistic one-dimensional flow with an initial Lorentz factor Γ_j ~ 100. The shocks result in a series of characteristic, increasingly shorter and harder thermal X-ray pulses, as well as a nonthermal γ-ray pulse, which precede the usual nonthermal MeV γ-rays. We consider jets escaping from both compact (CO or He dwarf) and blue supergiant stellar progenitors. The period, fluence, and hardness of the pulses serves as a diagnostic for the size and density of the outer envelope of the progenitor star.

We report results of a comprehensive study of the soft γ-ray (30 keV to 1.7 MeV) emission of GRO J0422+32 during its first known outburst in 1992. These results were derived from the BATSE earth-occultation database with the JPL data analysis package, EBOP (Enhanced BATSE Occultation Package). Results presented here focus primarily on the long-term temporal and spectral variability of the source emission associated with the outburst, which complement those reported earlier by BATSE, OSSE, COMPTEL, and SIGMA. The light curves with 1 day resolution in six broad energy bands (e.g., 35-100, 100-200, 200-300, 300-400, 400-700, and 700-1000 keV) show that the high-energy flux (>200 keV) led the low-energy flux (<200 keV) by ~5 days in reaching the primary peak, but lagged the latter by ~7 days in starting the declining phase. We confirm the "secondary maximum" of the low-energy (<200 keV) flux at ~TJD 8970-8981, ~120 days after the first maximum, reported earlier by the BATSE team. Our data show that the secondary maximum was also prominent in the 200-300 keV band, but became less pronounced at higher energies. During this 200 day period, the spectrum evolved from a power law with photon index of 1.75 on TJD 8839, to a shape that can be described by a Comptonized model or an exponential power law below 300 keV, with a variable power-law tail above 300 keV. The spectrum remained roughly in this two-component shape until around November 9 (TJD 8935), when the 35-429 keV luminosity dropped to below ~20% of its peak value observed on TJD 8848. It then returned to the initial power-law shape with an index of ~2 and stayed in this shape until the end of the period. The correlation of the two spectral shapes (e.g., Compton/power law tail vs. power law) with the high and low luminosities of the soft γ-ray emission is strongly reminiscent of that seen in Cyg X-1, suggesting that similar processes are at work in both systems. We also observed four separate episodes of high-energy (400-1000 keV) emission during the first 84 days of the event. We interpret these results in terms of the advection-dominated accretion flow (ADAF) model with possibly a "jetlike" region that persistently produced the nonthermal power-law γ-rays observed throughout the event.

A Chandra X-ray observation has detected an unresolved source at the center of the supernova remnant Kes 79. The best single-model fit to the source spectrum is a blackbody with an X-ray luminosity of L_X(0.3-8.0 keV) = 7 × 10³³ ergs s^-1. There is no evidence for a surrounding pulsar wind nebula. There are no cataloged counterparts at other wavelengths, but the absorption is high. The source properties are similar to the central source in Cas A even though the Kes 79 remnant is considerably older.

We have conducted a detailed analysis of the emission geometry of a handful of radio pulsars that have prominent, multiple-component profiles at meter wavelengths. From careful determination of the total number of emission components and their locations in pulse longitude, we find that all of the six pulsars show clear evidence for retardation and aberration effects in the conal emission beams. Using this information, coupled with a dipolar field geometry, we obtain estimates of the height and transverse location in the magnetosphere for each of the emitting cones in these pulsars. These results support our earlier conclusions for PSR B0329+54 in that we find successive outer cones (in cases of multiple-cone pulsars) being emitted at higher altitudes in the magnetosphere. The range of inferred heights is from ~200 to 2200 km. The set of "active" field lines from which the conal emissions originate are located in the region from ~0.22 to ~0.74 times the polar cap radius. At the neutron star surface, these conal rings map to radii of a few to several tens of meters, and the separation between successive rings is about 10-20 m. We discuss the implications of these findings for the understanding of the pulsar emission geometry and for current theories and models of the emission mechanism.

We reconstruct a 3 + 1 formalism of general relativistic electromagnetism and derive the equations of motion of charged particles in a pulsar magnetosphere, taking account of the inclination between the rotation axis and the magnetic axis. Unlike previous works in which space charge is evaluated by assuming the flow velocity to be light speed, we analyze particle motion in the polar cap, finding that gravity significantly changes its dynamics and the condition for acceleration.

We present far-ultraviolet (905-1182 Å), time-series spectroscopy of the eclipsing, nova-like cataclysmic variable UX UMa acquired with Far Ultraviolet Spectroscopic Explorer (FUSE). The time-averaged spectrum is complex and is dominated by overlapping spectral features. The most prominent features are emission lines of C III, N III, N IV, and O VI. They are broad (FWHM ≥ 1800 km s^-1) and double-peaked, with a central absorption at zero velocity. During eclipse, the spectrum is simpler: the emission lines remain bright, but the absorption components of the lines and the weaker features between the emission lines disappear entirely, leaving a flat continuum. This behavior is also evident in Goddard High-Resolution Spectrograph (1149-1660 Å) spectra that we retrieved from the Hubble Space Telescope (HST) archive. The FUV spectra show flickering on timescales of several minutes. The flickering is seen primarily in the continuum and/or the weaker lines rather than in the prominent emission lines. The orbital light curve has a dip in the FUV flux between orbital phases 0.45-0.65, similar to a preeclipse dip detected in HST observations. The equivalent widths of the line absorption features decrease during the dip. We have detected a systematic wavelength shift of spectral features on the orbital period, but with a phase lag of ≃20°, a phenomenon that has been reported at optical wavelengths. We discuss the implications of our results in the context of models of an accretion disk with a chromosphere between the disk and the extended wind. Finally, we note that the observed FUV flux is too low to be consistent with the temperature and radius of the WD derived in 1995 by Baptista et al., suggesting that their remaining binary parameters, including a mass ratio of 1, ought to be viewed with skepticism.

V1494 Aql (Nova Aql 1999 No. 2) was discovered on 1999 December 2. We obtained Chandra ACIS-I spectra on 2000 April 15 and June 7 which appear to show only emission lines. Our third observation, on August 6, showed that its spectrum had evolved to that characteristic of a Super Soft X-ray Source. We then obtained Chandra LETG+HRC-S spectra on September 28 (8 ks) and October 1 (17 ks). We analyzed the X-ray light curve of our grating observations and found both a short timescale "burst" and oscillations. Neither of these phenomena has previously been seen in the light curve of a nova in outburst. The "burst" was a factor of ~10 rise in X-ray counts near the middle of the second observation, and which lasted about 1000 s; it exhibited at least two peaks, in addition to other structure. Our time series analysis of the combined 25 ks observation shows a peak at ~2500 s which is present in independent analyses of both the zeroth-order image and the dispersed spectrum and is not present in similar analyses of grating data for HZ 43 and Sirius B. Further analyses of the V1494 Aql data find other periods present which implies that we are observing nonradial g⁺ modes from the pulsating, rekindled white dwarf.

Using the adaptive optics system, Hokupa`a, at Gemini North, we have directly imaged a companion around the UKIRT faint standard M8 star LHS 2397a (FS 129) at a separation of 2.96 AU. Near-infrared photometry of the companion has shown it to be an L7.5 brown dwarf and confirmed the spectral type of the primary to be M8. We also derive a substellar mass of the companion of 0.068 M_☉, although masses in the range 0.061-0.069 M_☉ are possible, and the primary mass is 0.090 M_☉ (0.089-0.094 M_☉). Reanalysis of archival imaging from the Hubble Space Telescope has confirmed the secondary as a common proper motion object. This binary represents the first clear example of a brown dwarf companion within 4 AU of a low-mass star and should be one of the first late-L dwarfs to have a dynamical mass. As part of a larger survey of M8-L0 stars, this object may indicate that there is no "brown dwarf desert" around low-mass primaries.

The opacity due to photodissociation of ²⁴MgH in the atmospheres of cool stars is investigated. The lowest two electronic transitions A²Π ← X²Σ⁺ and B' ²Σ⁺ ← X²Σ⁺ are considered, where the cross sections for the latter were published previously (Weck, Stancil, & Kirby) and the former are presented in this work. Model atmospheres calculated with the PHOENIX code are used to investigate the effect of the photodissociation opacity on spectra of cool stars. The A²Π ← X²Σ⁺ photodissociation cross sections are obtained using a combination of ab initio and experimentally derived potential curves and dipole transition moments. Partial cross sections have been evaluated over the accessible wavelength range 1770- 4560 Å for all rotational transitions from the vibrational levels v'' = 0-11. Assuming a Boltzmann distribution of the rovibrational levels of the X²Σ⁺ state, LTE photodissociation cross sections are presented for temperatures between 1000 and 5000 K. Shape resonances, arising from rotational predissociation of quasi-bound levels of the A²Π state near threshold, characterize the LTE photodissociation cross sections. A sum rule is proposed as a check on the accuracy of the photodissociation calculations.

Divergent migration of planets within a viscous circumstellar disk can engender resonance crossings and dramatic excitation of orbital eccentricities. We provide quantitative criteria for the viability of this mechanism. For the orbits of two bodies to diverge, a ring of viscous material must be shepherded between them. As the ring diffuses in radius by virtue of its intrinsic viscosity, the two planets are wedged farther apart. The ring mass must be smaller than the planetary masses so that the crossing of an individual resonance lasts longer than the resonant libration period. At the same time, the resonance crossing cannot be of such long duration that the disk's direct influence on the bodies' eccentricities interferes with the resonant interaction between the two planets. This last criterion is robustly satisfied because resonant widths are typically tiny fractions of the orbital radius. We evaluate our criteria not only for giant planets within gaseous protoplanetary disks but also for shepherd moons that bracket narrow planetary rings in the solar system. A shepherded ring of gas orbiting at a distance of 1 AU from a solar-type star and having a surface density of less than 500 g cm^-2, a dimensionless alpha viscosity of 0.1, and a height-to-radius aspect ratio of 0.05 can drive two Jupiter-mass planets through the 2 : 1 and higher order resonances so that their eccentricities amplify to values of several tenths. Because of the requirement that the disk mass in the vicinity of the planets be smaller than the planet masses, divergent resonance crossings may figure significantly into the orbital evolution of planets during the later stages of protoplanetary disk evolution, including the debris disk phase.

It is well known that coronagraphic observations of halo coronal mass ejections (CMEs) are subject to projection effects. Viewing in the plane of the sky does not allow us to determine the crucial parameters that define the geoeffectiveness of CMEs, such as the space speed, width, or source location. Assuming that halo CMEs have constant velocities, are symmetric, and propagate with constant angular widths, at least in their early phase, we have developed a technique that allows us to obtain the required parameters. This technique requires measurements of sky-plane speeds and the moments of the first appearance of the halo CMEs above opposite limbs. We apply this technique to obtain the parameters of all the halo CMEs observed by the Solar and Heliospheric Observatory (SOHO) mission's Large Angle and Spectrometric Coronagraph experiment until the end of 2000. We also present a statistical summary of these derived parameters of the halo CMEs.

This paper treats the reconfiguration of a twisted magnetic field, from an initial two-flux system containing a current sheet to a minimum-energy state, under the conservation of total relative magnetic helicity. In the specific model presented, we assume that a fresh magnetic field of the opposite polarity has emerged into a corona containing a preexisting magnetic field, both represented by constant-α force-free fields with the same constant α. The magnetic reconnection that takes place between the two twisted magnetic flux systems during a relaxation is assumed to take the field to a minimum-energy state that keeps the total relative magnetic helicity conserved. Our calculations suggest that this kind of relaxation may result in the formation of magnetic flux ropes and may change the twist directions in flux ropes in situations where flux ropes exist in the emerging or preexisting fields. These effects are all due to the interplays between the internal magnetic helicities of the two flux systems and their mutual magnetic helicity, with redistribution of these helicities through magnetic reconnection. In the absence of an interior current sheet, the lowest α force-free field always has the minimum magnetic energy for a given magnetic helicity, as Berger has shown. When an interior current sheet is present, this result breaks down. The lowest α force-free magnetic field with an interior equilibrium current sheet does not always have the minimum magnetic energy for a given total magnetic helicity. Implications of our results for flux emergence in the solar corona are also addressed.

The origin of the energetic particles associated with solar flares is a challenge to solar plasma theory. In this paper, we investigate the feature of stochastic acceleration of electrons by lower hybrid (LH) turbulence, which is a possible mechanism for the energization of relativistic electrons. The transport and acceleration parameters are studied with the Fokker-Planck coefficients D_μμ, D_μp, and D_pp, obtained from the quasi-linear method under the test particle assumption. Using the analytic expressions of the diffusion coefficients, we obtain the aspects of electron acceleration by LH turbulence, such as the acceleration and scattering timescales. The results show that the acceleration time of an original thermal electron over 20 keV is less than 1 s, which is required by hard X-ray observations during impulsive flares. However, the isotropy of the pitch-angle distribution cannot be maintained, and high-energy (~100 MeV) acceleration is inefficient. Some results of different plasma cases are compared, which are helpful in the understanding of particle acceleration in solar flares.

We study the Evershed flow in the photosphere and the reverse Evershed flow in the chromosphere from simultaneous observations, giving emphasis to the temporal evolution of the phenomena. We compute the components of the velocity vector as a function of distance from the center of the sunspot, assuming an axial symmetry of both the spot and the flow. A five-minute oscillatory pattern is obvious in the penumbra at photospheric level. Our results verify that the velocity of the Evershed flow has a maximum above the penumbra in the photosphere and well outside the penumbra in the chromosphere. We find evidence of temporal variations prominent in the radial component of the average photospheric velocity with a characteristic timescale of 25 minutes. We consider a transient siphon flow or a wave superimposed on a steady flow as possible explanations for the temporal behavior of the photospheric Evershed flow. The radial component of the chromospheric reverse Evershed flow shows a repetitive temporal variation with a typical timescale of 15 minutes. The variation consists of enhanced velocity amplitudes that propagate to an opposite direction from the flow with a velocity of about 5-6 km s^-1. This behavior cannot be easily explained in the frame of a transient flow and strongly suggests that it is related to the propagation of a wave. We examine the possibility of its being associated with the propagation of running penumbral waves in the superpenumbra. The temporal evolution of the line-of-sight velocity across superpenumbral fibrils presents alterations that can be associated with a time-dependent flow. However, we also observe propagating velocity packets that can be associated with a wave.

Recently Dikpati & Gilman have shown, using a shallow-water model of the solar tachocline that allows the top surface to deform, that a tachocline with the observed broad differential rotation and a strong toroidal field is prolate. A strong toroidal field ring requires extra mass on its poleward side to provide a hydrostatic latitudinal pressure gradient to balance the poleward curvature stress. In a parallel study using a different approach, Rempel, Schüssler, & Tóth have shown that such a latitudinal pressure gradient is found in a strongly subadiabatic stratification, whereas a weakly subadiabatic stratification leads to a complementary equilibrium state of the overshoot tachocline in which the magnetic curvature stress is balanced by a prograde rotational jet inside the toroidal ring. We show that the shallow-water model with height deformation is a first-order approach to the equilibrium state found by Rempel, Schüssler, & Tóth for a strongly subadiabatic stratification. We also show that the shallow-water model can be generalized to allow for the equilibrium state found for a weakly subadiabatic stratification by suppressing the shell deformation associated with the toroidal field and allowing the differential rotation to be modified.

A kinetic equation for Compton scattering is given that differs from the Kompaneets equation in several significant ways. By using an inverse differential operator, this equation allows treatment of problems for which the radiation field varies rapidly on the scale of the width of the Compton kernel. This inverse operator method describes, among other effects, the thermal Doppler broadening of spectral lines and continuum edges and automatically incorporates the process of Compton heating/cooling. It is well adapted for inclusion into a numerical iterative solution of radiative transfer problems. The equivalent kernel of the new method is shown to be a positive function and with reasonable accuracy near the initial frequency, unlike the Kompaneets kernel, which is singular and not wholly positive. It is shown that iterations of the inverse operator kernel can be easily calculated numerically, and a simple summation formula over these iterations is derived that can be efficiently used to compute Comptonized spectra. It is shown that the new method can be used for initial-value and other problems with no more numerical effort than the Kompaneets equation and that it more correctly describes the solution over times comparable to the mean scattering time.

Subarcsecond lensing statistics depend sensitively on the inner mass profiles of low-mass objects and the faint-end slopes of the Schechter luminosity function and the Press-Schechter mass function. By requiring the luminosity and mass functions to give consistent predictions for the distribution of image separation below 1'', we show that dark matter halos with masses below 10¹²M_☉ cannot have a single type of profile, be it the singular isothermal sphere (SIS) or the shallower "universal" dark matter profile. Instead, consistent results are achieved if we allow a fraction of the halos at a given mass to be luminous with the SIS profile and the rest to be dark with an inner logarithmic slope shallower than -1.5 to compensate for the steeper faint-end slope of the mass function compared with the luminosity function. We quantify how rapidly the SIS fraction must decrease with decreasing halo mass, thereby providing a statistical measure for the effectiveness of feedback processes on the baryon content in low-mass halos.

We report the first results of the Chandra temporal monitoring of the ultraluminous X-ray sources (ULXs) in the Antennae galaxies (NGC 4038/4039). Observations at four different epochs, covering timescales of 2 yr to 2 months, show variability in seven out of nine ULXs, confirming that they are likely to be accreting compact X-ray binaries. The seven variable ULXs exhibit a variety of temporal and spectral behaviors: one has harder X-ray colors with decreasing luminosity, similar to the black hole binary Cyg X-1, but four other ULXs show the opposite behavior. We suggest that the latter may be black hole binaries accreting at very high rates.

Near-infrared Fabry-Perot imaging has revealed H₂ emission extended to about 130 pc from the disk of NGC 253. It is closely related to the hot plasma observed in soft X-rays: filamentary H₂ features are found at the edges of the hot plasma. These are the places of direct interaction between a superwind and its surrounding molecular gas. We suggest that the filamentary features actually trace a more or less conical shell-like structure, whose tangential line of sight to us is intensely observed. The H₂ emission shell is most likely from the molecular gas blown out or swept to the side by the hot plasma outflow. Dust is associated with this molecular gas structure. The outflow is tilted with respect to the disk, possibly suggesting the inhomogeneous nature of the interstellar medium in which the starburst takes place.

By means of high-resolution spectra, we have measured radial velocities of the companion (hereafter COM J1740-5340) to the eclipsing millisecond pulsar PSR J1740-5340 in the galactic globular cluster NGC 6397. The radial velocity curve fully confirms that COM J1740-5340 is orbiting the pulsar, and this enables us to derive the most accurate mass ratio (M_PSR/M_COM = 5.85 ± 0.13) for any nonrelativistic binary system containing a neutron star. Assuming a pulsar mass in the range 1.3-1.9 M_☉, the mass of COM J1740-5340 spans the interval 0.22-0.32 M_☉, the inclination of the system is constrained within 56° ≳ i ≳ 47°, and the Roche lobe radius is r_RL ~ 1.5-1.7 R_☉. A preliminary chemical abundance analysis confirms that COM J1740-5340 has a metallicity compatible with that measured for other stars in this metal-poor globular, but the unexpected detection of strong He I absorption lines implies the existence of regions at T > 10,000 K, which are significantly warmer than the rest of the star. The intensity of this line correlates with the orbital phase, suggesting the presence of a region on the companion surface, heated by the millisecond pulsar flux.

A great deal of evidence has recently been gathered in favor of the picture that soft gamma repeaters and anomalous X-ray pulsars are powered by ultrastrong magnetic fields (B > 10¹⁴ G; i.e., magnetars). Nevertheless, present determination of the magnetic field in such magnetar candidates has been indirect and model-dependent. A key prediction concerning magnetars is the detection of ion-cyclotron resonance features, which would offer a decisive diagnostic of the field strength. Here we present the detection of a 5 keV absorption feature in a variety of bursts from the soft gamma repeater SGR 1806-20, confirming our initial discovery (Ibrahim et al.) and establishing the presence of the feature in the source's burst spectra. The line feature is well explained as proton-cyclotron resonance in an ultrastrong magnetic field, offering a direct measurement of SGR 1806-20's magnetic field (B ≈ 10¹⁵ G) and clear evidence of a magnetar. Together with the source's spin-down rate, the feature also provides the first measurement of the gravitational redshift, mass, and radius of a magnetar.

We present results from the analysis of X-ray power density spectra and coherence when GRS 1915+105 is in soft states. We use three data sets that belong to μ, ϕ, and δ classes as found in the work of Belloni et al. We find that the power density spectra appear to be complex, with several features between 0.01 and 10 Hz. The coherence deviates from unity above a characteristic frequency. We discuss our results from different models. The corona size in the sphere-disk model implied by this break frequency is on the order of 10⁴GM/c², which is unphysical. Our results are more consistent with the prediction of the model of a planar corona sustained by magnetic flares, in which the characteristic frequency is associated with the longest timescale of an individual flare, which is about 8 s.

We present Keck images of the dust disk around β Pictoris at λ = 17.9 μm that reveal new structure in its morphology. Within 1'' (19 AU) of the star, the long axis of the dust emission is rotated by more than 10° with respect to that of the overall disk. This angular offset is more pronounced than the warp detected at 35 by the Hubble Space Telescope (HST) and is in the opposite direction. By contrast, the long axis of the emission contours ~15 from the star is aligned with the HST warp. Emission peaks between 15 and 4'' from the star hint at the presence of rings similar to those observed in the outer disk at ~25'' with the HST Space Telescope Imaging Spectrograph. A deconvolved image strongly suggests that the newly detected features arise from a system of four noncoplanar rings. Bayesian estimates based on the primary image lead to ring radii of 14 ± 1, 28 ± 3, 52 ± 2, and 82 ± 2 AU, with orbital inclinations that alternate in orientation relative to the overall disk and decrease in magnitude with increasing radius. We believe these new results make a strong case for the existence of a nascent planetary system around β Pic.

Long-slit spectra probe the southwest side of the disk in the 8-13 μm region. Dust within 08 of the star shows silicate emission features, including amorphous and crystalline species. Farther out, the disk spectra are featureless and are entirely dominated by dust thermal continuum emission. Mid-infrared imaging of β Pictoris reveals spatial asymmetries in its dusty circumstellar disk at a spatial resolution of 05. Visible for the first time is an inner (r < 20 AU) warp in the disk that is aligned differently from a larger scale warp observed in scattered light by the Hubble Space Telescope. At 12 μm, as has been observed before, the southwest extension of the disk is brighter than the northeast side. This asymmetry appears to be not as significant at 18 μm. The spatially resolved spectra show that this asymmetry cannot be accounted for by small dust grains that have a spectral feature.

The orbital inclinations of a bounded, self-gravitating particle population subject to secular perturbations from an inclined planet are studied. The particle disk is shown to have discrete modes and pattern speeds in which the wave behavior is very pronounced compared with the nonwave behavior. The response is in the form of a standing wave pattern produced by wave reflection at the finite outer edge of the disk. In the absence of dissipation, the perpendicular torque between disk and perturber vanishes, with equal incoming and outgoing angular momentum fluxes carried by the wave trains. This resonant cavity theory is applied to Neptune perturbations of the primordial Kuiper belt. If the nodal precession rate of Neptune was near one of the disk's mode frequencies, very high inclinations could be produced for this population. There are a number of mechanisms in the early solar system that could have tuned the system to pass through one or more resonant states.