Table of contents

We develop median statistics that provide powerful alternatives to χ² likelihood methods and require fewer assumptions about the data. Application to astronomical data demonstrates that median statistics lead to results that are quite similar and almost as constraining as χ² likelihood methods but with somewhat more confidence since they do not assume Gaussianity of the errors or that their magnitudes are known.

Applying median statistics to Huchra's compilation of nearly all estimates of the Hubble constant, we find a median value H₀ = 67 km s^-1 Mpc^-1. Median statistics assume only that the measurements are independent and free of systematic errors. This estimate is arguably the best summary of current knowledge because it uses all available data and, unlike other estimates, makes no assumption about the distribution of measurement errors. The 95% range of purely statistical errors is ±2 km s^-1 Mpc^-1. The high degree of statistical accuracy of this result demonstrates the power of using only these two assumptions and leads us to analyze the range of possible systematic errors in the median, which we estimate to be roughly ±5 km s^-1 Mpc^-1 (95% limits), dominating over the statistical errors.

Using a Bayesian median statistics treatment of high-redshift Type Ia supernovae (SNe Ia) apparent magnitude versus redshift data from Riess et al., we find the posterior probability that the cosmological constant Λ > 0 is 70% or 89%, depending on the prior information we include. We find the posterior probability of an open universe is about 47%, and the probability of a spatially flat universe is 51% or 38%. Our results generally support the observers' conclusions but indicate weaker evidence for Λ > 0 (less than 2 σ). Median statistics analysis of the Perlmutter et al. high-redshift SNe Ia data shows that the best-fit flat-Λ model is favored over the best-fit Λ = 0 open model by odds of 366 : 1; the corresponding Riess et al. odds are 3 : 1 (assuming in each case prior odds of 1 : 1).

A scalar field with a potential energy with a "tail" behaves like a time-variable Λ. Median statistics analyses of the SNe Ia data do not rule out such a time-variable Λ and may even favor it over a time-independent Λ and a Λ = 0 open model.

We apply the hierarchical clustering model and nonlinear perturbation theory to the cosmological density and temperature fields. This allows us to calculate the intergalactic gas-pressure power spectrum, Sunyaev-Zeldovich (SZ) anisotropy power spectrum, skewness, and related statistics. Then we show the effect of the nongravitational heating. Our model confirms recent simulations yielding a mass-weighted gas temperature_g ~ 0.35 keV and reproduces the power spectra found in these simulations. While the SZ effect contains only angular information, we show that it is possible to extract the full time-resolved gas-pressure power spectrum when combined with galaxy photometric redshift surveys by using a variation on the cross-correlation. This method further allows the disentanglement of the gravitational and the nongravitational heating.

We show that the winds identified with high-redshift low-mass galaxies may strongly affect the formation of stars in more massive galaxies that form later. With three-dimensional realizations of a simple nonlinear growth model, we track gas shocking, metal enrichment, and cooling, together with dark halo formation. We show that outflows typically strip baryonic material out of previrialized intermediate-mass halos, suppressing star formation. More massive halos can trap the heated gas but collapse later, leading to a broad bimodal redshift distribution, with a larger characteristic mass and metallicity associated with the lower redshift peak. This scenario accounts for the observed bell-shaped luminosity function of early-type galaxies, explains the small number of Milky Way satellite galaxies relative to standard cold dark matter prescriptions, and provides a reasonable explanation for the lack of metal-poor stars in the solar neighborhood and the more general lack of low-metallicity stars in other massive galaxies relative to "closed-box" models of chemical enrichment. Heating of the intergalactic medium by early outflows should produce spectral distortions in the cosmic microwave background that will be measurable with the next generation of experiments.

Recently it has been suggested that the majority of dark matter in the universe resides in the form of Jupiter-mass black holes distributed cosmologically. This population makes itself apparent by microlensing high-redshift quasars and introducing pronounced variability into their observed light curves. While several arguments dismissing this hypothesis have been presented, a conclusive observational test is, alas, sadly lacking. In this paper we investigate the effect of a cosmologically distributed population of microlensing masses on galaxies at low to intermediate redshift. The magnification of bright stars in these galaxies leads to small, but observable, fluctuations in their surface brightness. The variability timescale for Jupiter-mass lensing objects is of the order of a few months, and this population can be detected through a future space-based monitoring campaign of a field containing z ~ 0.5 galaxies. The monitoring of galactic surface brightness will provide an effective test of the nature of dark matter on cosmological scales.

We have measured the Li abundance of 18 stars with -2 ≲ [Fe/H] ≲ -1 and 6000 ≲ T_eff ≲ 6400 K, a parameter range that was poorly represented in previous studies. We examine the Galactic chemical evolution (GCE) of this element, combining these data with previous samples of turnoff stars over the full range of halo metallicities. We find that A(Li) increases from a level of ~2.10 at [Fe/H] = -3.5 to ~2.40 at [Fe/H] = -1.0, where A(Li) =(n(Li)/n(H)) + 12.00. We compare the observations with several GCE calculations, including existing one-zone models and a new model developed in the framework of inhomogeneous evolution of the Galactic halo. We show that Li evolved at a constant rate relative to iron throughout the halo and old disk epochs but that during the formation of young disk stars, the production of Li relative to iron increased significantly. These observations can be understood in the context of models in which postprimordial Li evolution during the halo and old disk epochs is dominated by Galactic cosmic-ray fusion and spallation reactions, with some contribution from the ν-process in supernovae. The onset of more efficient Li production (relative to iron) in the young disk coincides with the appearance of Li from novae and asymptotic giant branch (AGB) stars. The major challenge facing the models is to reconcile the mild evolution of Li during the halo and old disk phases with the more efficient production (relative to iron) at [Fe/H] > -0.5. We speculate that cool-bottom processing (production) of Li in low-mass stars may provide an important late-appearing source of Li, without attendant Fe production, that might explain the Li production in the young disk.

If weakly interacting massive particles (WIMPs) in bound solar orbits are systematically driven into the Sun by solar system resonances (as Farinella and coworkers have shown is the case for many Earth-crossing asteroids), then the capture of high-mass WIMPs by the Earth would be affected dramatically because high-mass WIMPs are captured primarily from bound orbits. WIMP capture would be eliminated for M_x > 630 GeV and would be highly suppressed for M_x ≳ 150 GeV. Annihilation of captured WIMPs and anti-WIMPs is expected to give rise to neutrinos coming from the Earth's center. The absence of such a neutrino signal has been used to place limits on WIMP parameters. At present, one does not know whether typical WIMP orbits are in fact affected by these resonances. Until this question is investigated and resolved, one must (conservatively) assume that they are. Hence, limits on high-mass WIMP parameters are significantly weaker than was previously believed.

We have obtained follow-up observations of the quasar pair LBQS 0107-025A and B and new observations of the nearby quasar LBQS 0107-0232 with the Hubble Space Telescope Faint Object Spectrograph. Extended wavelength coverage of LBQS 0107-025A and B using the G270H grating was also obtained. This triple system is unique in providing sensitivity to coherent Lyα absorption on transverse scales of approximately 1 Mpc at z < 1. Monte Carlo simulations were used to establish the confidence level for matches between absorption features in different lines of sight as a function of velocity separation. Pairwise, there are 8, 9, and 12 lines that match between spectra. Three instances of matches between all three lines of sight were found with velocity separations of less than 550 km s ^-1. Two of the pairings have coincident lines within |Δv| < 200 km s^-1 that would occur with less than 10% probability by chance. Taking into account the equivalent widths of the lines, one of these triple coincidences is significant at the 99.99% confidence level based on Monte Carlo simulations with random line placements. Matches with strong lines preferentially have small velocity separations. These same simulations are used to demonstrate that the distribution of matches for a population of absorbers randomly distributed in velocity space is peaked toward |Δv| = 0, which has implications for the statistical significance of matches. One of the triple coincidences appears to be a strong absorber with a sheetlike, but inhomogeneous, geometry and a coherence length approaching or exceeding 1 Mpc.

The UV resonance transitions of neutral argon, Ar I λ1066, and of singly ionized phosphorus, P II λ963, originated in the damped Lyα system (DLA) at z_abs = 3.3901 toward QSO 0000-2620, have been detected by means of the Ultraviolet-Visual Echelle Spectrograph (UVES) spectrograph at the 8.2 m ESO KUEYEN telescope. So far, this is the first measurement of Ar I and the second of P II ever performed in damped galaxies and in high-redshift objects. This DLA is well known for having one of the lowest metal abundances and dust content and the lowest fractional abundance of molecular hydrogen H₂. The measured Ar abundance is [Ar/H] = -1.91 ± 0.09, which is equal to the abundances of the other α-chain elements (O, S, and Si). The similarity of the Ar abundance with the other α-chain elements implies the absence of significant photoionization by either UV background or stellar sources along the sight line throughout the damped Lyα system. Both log(Ar/O) and log(Ar/S) ratios are found close to those measured in the extragalactic H II regions and in blue compact galaxies where O is more abundant by at least 1 order of magnitude. This strengthens the universality of the Ar/O and Ar/S ratios and lends support to the existence of a universal IMF. The abundance of the nonrefractory element phosphorus [P/H] = -2.31 ± 0.10 confirms the low amount of chemical evolution in the DLA. This is the measurement of P in the most metal-poor material and shows a subsolar [P/Fe] = -0.27 value. The measured ratios [P/Si] = -0.40 ± 0.13 and [P/S] = -0.40 ± 0.13 provide evidence for a mild odd-even effect. Finally, a stringent upper limit to the population of the ³P₁ level in the ground state of O I is derived, which provides a lower limit to the physical dimensions of the z_abs = 3.3901 system of L > 7 pc.

The advection-dominated accretion flow (ADAF) has been quite successful in explaining a wide variety of accretion-powered astronomical sources. The physical characteristics of ADAF complement the classical thin disk flow quite nicely, and extensive work has been done on it. However, all high-temperature accretion solutions including ADAF are physically thick, so outgoing radiation interacts with the incoming flow. Thus, ADAF solutions share as much or more resemblance with classical spherical accretion flows as with disk flows. This interaction, which has been neglected by most authors, is primarily through the Compton heating process that will typically limit the steady solutions to L ≲ 10^-2L_Edd, where L_Edd is the Eddington luminosity. We examine this interaction for the popular ADAF case, but similar conclusions, we expect, would apply for other high-temperature, geometrically thick disks as well. We study the global thermal nature of the flow, with special consideration given to various cooling and, especially, preheating by Comptonizing hot photons produced at smaller radii. We find that without allowance for Compton preheating a very restricted domain of ADAF solution is permitted and with Compton preheating included a new high-temperature PADAF branch appears in the solution space. In the absence of preheating, high-temperature flows do not exist when the mass accretion rate ≡ c²/L_Edd ≳ 10^-1.5. Below this mass accretion rate, a roughly conical region around the hole cannot sustain high-temperature ions and electrons for all flows having ≳ 10^-4, which may lead to a funnel possibly filled with a tenuous hot outgoing wind. If the flow starts at large radii with the usual equilibrium temperature ~10⁴ K, the critical mass accretion rate is much lower, ~ 10^-3.7, above which level no self-consistent ADAF (without preheating) can exist. However, above this critical mass accretion rate, the flow can be self-consistently maintained at high temperature if Compton preheating is considered. These solutions constitute a new branch of solutions as in spherical accretion flows. High-temperature PADAF flows can exist above the critical mass accretion rate in addition to the usual cold thin disk solutions. We also find solutions where the flow near the equatorial plane accretes normally while the flow near the pole is overheated by Compton preheating, possibly becoming a polar wind, solutions which we designate WADAF.

This paper examines whether a fractal cloud geometry can reproduce the emission-line spectra of active galactic nuclei (AGNs). The nature of the emitting clouds is unknown, but many current models invoke various types of magnetohydrodynamic confinement. Recent studies have argued that a fractal distribution of clouds, in which subsets of clouds occur in self-similar hierarchies, is a consequence of such confinement. Whatever the confinement mechanism, fractal cloud geometries are found in nature and may be present in AGNs too. We first outline how a fractal geometry can apply at the center of a luminous quasar. Scaling laws are derived that establish the number of hierarchies, typical sizes, column densities, and densities. Photoionization simulations are used to predict the integrated spectrum from the ensemble. Direct comparison with observations establishes all model parameters so that the final predictions are fully constrained. Theory suggests that denser clouds might form in regions of higher turbulence and that larger turbulence results in a wider dispersion of physical gas densities. An increase in turbulence is expected deeper within the gravitational potential of the black hole, resulting in a density gradient. We mimic this density gradient by employing two sets of clouds with identical fractal structuring but different densities. The low-density clouds have a lower column density and large covering factor similar to the warm absorber. The high-density clouds have high column density and smaller covering factor similar to the broad-line region (BLR). A fractal geometry can simultaneously reproduce the covering factor, density, column density, BLR emission-line strengths, and BLR line ratios as inferred from observation. Absorption properties of the model are consistent with the integrated line-of-sight column density as determined from observations of X-ray absorption, and when scaled to a Seyfert galaxy, the model is consistent with the number of multiple UV absorption components observed in them. Rough estimates show that about one in 100 of the galaxies that harbor a supermassive black hole will show activity, assuming that material needs to be within its EUV continuum emitting radius for activity to occur. This is close to the observationally determined duty cycle. Stochastic feeding of the central engine of fractal cloud distribution of material may therefore account for continuum variations and long-term activity. The total cloud mass is much larger than that measured in ionized gas alone since the clouds are mutually self-shielding.

We present an exhaustive statistical analysis of the associated (Δv_abs < 5000 km s^-1), high-ionization (C IV, N V, O VI) narrow absorption line (NAL) systems in a sample of 59 QSOs defined from the Hubble Space Telescope (HST) QSO Absorption Line Key Project. The goals of the research were twofold: (1) to determine the frequency of associated NALs at low redshift and in low luminosity QSOs, and (2) to address the question of what QSO properties either encourage or inhibit the presence of associated NAL gas. To that end, we have compiled the QSO rest-frame luminosities at 2500 Å, 5 GHz, and 2 keV, spectral indices at 2500 Å and 5 GHz, the Hβ emission-line FWHM, and the radio core fraction at observed 5 GHz. In addition, we have measured the C IV emission-line FWHM. We find 17 associated NALs (16 selected by C IV and one selected by O VI) toward 15 QSOs, of which ~10 are statistically expected to be intrinsic. From a multivariate clustering analysis, we find that the QSOs group together (in parameter space) based primarily on radio luminosity, followed (in order of importance) by radio spectral index, C IV emission-line FWHM, and soft X-ray luminosity. We find that radio-loud QSOs that have compact radio morphologies, flat radio spectra [α(5 GHz) > -0.5], and mediocre C IV FWHM (≲6000 km s^-1) do not have detectable associated NALs, down to W_r(C ) = 0.35 Å. We also find that broad absorption line (BAL) QSOs have an enhanced probability of hosting detectable NAL gas. In addition, we find that the velocity distribution of associated NALs is peaked around the emission redshifts rather than the systemic redshifts of the QSOs. Finally, we find only one strong NAL [W_r(C ) ≳ 1.5 Å] in our low-redshift sample. A comparison with previous higher redshift surveys reveals evolution in the number of strong NAL systems with redshift. We interpret these results in the context of an accretion disk model. We propose that NAL gas hugs the streamlines of the faster, denser, low-latitude wind, which has been associated with BALs. In the framework of this scenario, we can explain the observational clues as resulting from differences in orientation and wind properties, the latter presumably associated with the QSO radio properties.

The emission-line flux ratio of [O III] λ4363/[O III] λ5007 (R_{O III}) is a useful diagnostic for the ionization mechanism and physical properties of emission-line regions in active galactic nuclei (AGNs). However, it is known that simple photoionization models underpredict the [O III] λ4363 intensity, being inconsistent with observations. In this paper we report on several pieces of evidence that a large fraction of the [O III] λ4363 emission arises from the dense gas obscured by putative tori: (1) the visibility of high-R_{O III} regions is correlated to that of broad-line regions, (2) higher R_{O III} objects show hotter mid-infrared colors, (3) higher R_{O III} objects show stronger highly ionized emission lines such as [Fe VII] λ6087 and [Fe X] λ6374, and (4) higher R_{O III} objects have broader line width of [O III] λ4363 normalized by that of [O III] λ5007. To estimate how such a dense component contributes to the total emission-line flux, dual-component photoionization model calculations are performed. It is shown that the observed values of R_{O III} of type 1 AGNs may be explained by introducing a 5%-20% contribution from the dense component, while those of type 2 AGNs may be explained by introducing a 0%-2% contribution. We also discuss the [O III] λ4363 emitting regions in LINERs in the framework of our dual-component model.

We determine the quantitative morphology and star formation properties of galaxies in six nearby X-ray-detected, poor groups using multiobject spectroscopy and wide-field R imaging. The mean recessional velocities of the galaxy groups range from 2843 to 7558 km s^-1. Each group has 15-38 confirmed members ranging in luminosity from dwarfs to giants (-13.7 ≥ M_R - 5 log h ≥ -21.9). We measure structural parameters for each galaxy by fitting a PSF-convolved, two-component model to their surface brightness profiles. To compare the samples directly, we fade, smooth, and rebin each galaxy image so that we effectively observe each galaxy at the same redshift (9000 km s^-1) and physical resolution (0.87 h^-1 kpc). The structural parameters are combined with [O II] measurements to test for correlations between morphological characteristics and current star formation in these galaxies. We compare results for the groups to a sample of field galaxies. We find that: (1) Galaxies spanning a wide range in morphological type and luminosity are fit well by a de Vaucouleurs bulge with exponential disk profile. (2) Morphologically classifying these nearby group galaxies by their bulge fraction (B/T) is fairly robust on average, even when their redshift has increased by up to a factor of 4 and the effective resolution of the images is degraded by up to a factor of 5. (3) The fraction of bulge-dominated systems in these groups is higher than in the field (~50% versus ~20%). (4) The fraction of bulge-dominated systems in groups decreases with increasing radius, similar to the morphology-radius (~density) relation observed in galaxy clusters. (5) Current star formation in group galaxies is correlated with significant morphological asymmetry for disk-dominated systems (B/T < 0.4). (6) The group galaxies that are most disk dominated (B/T < 0.2) are less star forming and asymmetric on average than their counterparts in the field.

Orbital motion in triaxial nuclei with central point masses, representing supermassive black holes, is investigated. The stellar density is assumed to follow a power law ρ ∝ r^-γ, with γ = 1 or γ = 2. At low energies the motion is essentially regular; the major families of orbits are tubes and pyramids. Pyramid orbits are similar to box orbits but have their major elongation parallel to the short axis of the figure. A number of regular orbit families associated with resonances also exist, most prominently the banana orbits, which are also elongated parallel to the short axis. At a radius where the enclosed stellar mass is a few times the black hole mass, the pyramid orbits become stochastic. The energy of transition to this "zone of chaos" is computed as a function of γ and of the shape of the stellar figure; it occurs at lower energies in more elongated potentials. Our results suggest that supermassive black holes may place tight constraints on departures from axisymmetry in galactic nuclei by both limiting the allowed shapes of regular orbits and inducing chaos.

Recent work on the gas dynamics in the Galactic center has improved our understanding of the accretion processes in galactic nuclei, particularly with regard to properties such as the specific angular momentum distribution, density, and temperature of the inflowing plasma. With the appropriate extrapolation of the physical conditions, this information can be valuable in trying to determine the origin of the broad-line region (BLR) in active galactic nuclei (AGNs). In this paper, we explore various scenarios for cloud formation based on the underlying principle that the source of plasma is ultimately that portion of the gas trapped by the central black hole from the interstellar medium. Based on what we know about the Galactic center, it is likely that in highly dynamic environments such as this, the supply of matter is due mostly to stellar winds from the central cluster. Winds accreting onto a central black hole are subjected to several disturbances capable of producing shocks, including a Bondi-Hoyle flow, stellar wind-wind collisions, and turbulence. Shocked gas is initially compressed and heated out of thermal equilibrium with the ambient radiation field; a cooling instability sets in as the gas is cooled via inverse-Compton and bremsstrahlung processes. If the cooling time is less than the dynamical flow time through the shock region, the gas may clump to form the clouds responsible for broad-line emission seen in many AGN spectra. Clouds produced by this process display the correct range of densities and velocity fields seen in broad emission lines. Very importantly, the cloud distribution agrees with the results of reverberation studies, in which it is seen that the central line peak (due to infalling gas at large radii) responds more slowly to continuum changes than the line wings, which originate in the faster moving, circularized clouds at smaller radii. Finally, we provide an example of fitting an observed line profile using the parameters specified by our model.

We present a new phenomenological model for the spectral energy distribution of normal star-forming galaxies between 3 and 1100 μm. A sequence of realistic galaxy spectra are constructed from a family of dust emission curves assuming a power-law distribution of dust mass over a wide range of interstellar radiation fields. For each interstellar radiation field heating intensity, we combine emission curves for large and very small grains and aromatic feature carriers. The model is constrained by IRAS and ISOCAM broadband photometric and ISOPHOT spectrophotometric observations for our sample of 69 normal galaxies; the model reproduces well the empirical spectra and infrared color trends. These model spectra allow us to determine the infrared energy budget for normal galaxies and in particular to translate far-infrared fluxes into total (bolometric) infrared fluxes. The 20-42 μm range appears to show the most significant growth in relative terms as the activity level increases, suggesting that the 20-42 μm continuum may be the best dust emission tracer of current star formation in galaxies. The redshift dependence of infrared color-color diagrams and the far-infrared-to-radio correlation for galaxies are also explored.

The temperature distribution of the intracluster medium (ICM) in the Virgo Cluster of galaxies has been derived from extensive mapping observations with ASCA covering an area of 19 deg². In the spectral analysis, the inner region within a radius of ~60' from M87 is characterized by an ICM temperature of kT ~ 2.5 keV with little variation. On the other hand, the outer regions indicate significant variation of the temperature with an amplitude of about 1 keV. The temperature map was produced from the hardness ratio (HR) values with a resolution of about 5'. Besides the previously reported hot region with kT > 4 keV between M87 and M49, several hot regions with kT = 3-4 keV are detected in the cluster outskirts. The autocorrelation function for the HR variation shows that the temperature variation is correlated within a size of about 300 kpc, suggesting that gas blobs falling in the Virgo Cluster have a typical size of groups of galaxies. A correlation with the velocity dispersion of member galaxies shows that only the northwest region indicates an unusually large β_spec value of 2-4. The upper limit for extended nonthermal emission in the Virgo Cluster is obtained to be L_X ~ 9 × 10⁴¹ ergs s^-1 in the 2-10 keV band. We discuss that these features consistently indicate that the Virgo Cluster is in a relatively early stage of the cluster evolution.

Using N-body numerical simulations, we examine possible mechanisms for the origin of the intracluster starlight features recently discovered in the Coma Cluster of galaxies. We show that tidal interactions of a "normal" elliptical galaxy with a symmetrically distributed dark matter potential do not produce the observed intracluster starlight features. A head-on collision of two normal ellipticals does explain, however, the origin of the intracluster features with a surface brightness of about 26 mag arcsec^-2. Another possible explanation for the intracluster starlight features is galactic tidal interactions with massive intracluster substructures. The presence of substructures in Coma with a density of ~0.1 times the density in the center of the galaxy and with a total mass about 3 orders of magnitude larger than the mass of a normal elliptical galaxy would account for the observations.

We have made BeppoSAX observations of the Seyfert 2/1.9 galaxy ESO 103-G35, which contains a nuclear maser source and is known to be heavily absorbed in the X-rays. Analysis of the X-ray spectra observed by BeppoSAX in 1996 October and 1997 October yields a spectral index α_E = 0.74 ± 0.07 (F_ν ∝ ν), which is typical of Seyfert galaxies and consistent with earlier observations of this source. The strong, soft X-ray absorption has a column density N_H of 1.79 ± 0.09 × 10²³ cm^-2, again consistent with earlier results. The best-fitting spectrum is that of a power law with a high-energy cutoff at 29 ± 10 keV, a cold (E = 6.3 ± 0.1 keV, rest frame), marginally resolved (σ = 0.35 ± 0.14 keV, FWHM ~ 31 ± 12 × 10³ km s^-1) Fe Kα line with EW 290 eV (1996), and a mildly ionized Fe K edge at 7.37 keV, τ = 0.24. The Kα line and cold absorption are consistent with origin in an accretion disk/torus through which our line of sight passes at a radial distance of ~50 pc. The Fe K edge is mildly ionized, suggesting the presence of ionized gas, probably in the inner accretion disk close to the central source or in a separate warm absorber. The data quality is too low to distinguish between these possibilities, but the edge-on geometry implied by the water maser emission favors the former. Comparison with earlier observations of ESO 103-G35 shows little or no change in spectral parameters while the flux changes by factors of a few on timescales of a few months. The 2-10 keV flux decreased by a factor of ~2.7 between 1996 October and 1997 October with no detectable change in the count rate greater than 20 keV (i.e., the Phoswich Detector System data). Spectral fits to the combined data sets indicate either a significant hardening of the spectrum (α_E ~ 0.5) or an approximately constant or delayed response reflection component. The high-energy cutoff (29 ± 10 keV) is lower than the typical ~300 keV values seen in Seyfert galaxies. A significant subset of similar sources would affect current models of the active galactic nucleus contribution to the cosmic X-ray background which generally assume a high-energy cutoff of ~300 keV.

Near-infrared (NIR) high angular resolution speckle imaging and imaging spectroscopy of the nuclear region (~10'' ≈ 840 pc) of the Seyfert 1 galaxy NGC 3227 are presented. The images reveal an unresolved nuclear source in the K band in addition to a nuclear stellar cluster that is slightly resolved in the J and H bands. The contribution of this stellar cluster to the NIR continuum is increasing from the K to the J bands. The stellar absorption lines are extended compared to the neighboring continuum suggesting a nuclear stellar cluster size of ~70 pc FWHM. Analysis of the stellar absorption lines suggests that the stars are contributing about 65% (40%) of the total continuum emission in the H (K) band in a 36 aperture. The dominant stellar type is cool M-type stars. Population synthesis in conjunction with NIR spectral synthesis indicates that the age of the mapped nuclear stellar cluster is in the range of 25-50 Myr when red supergiants contribute most to the NIR light. This is supported by published optical data on the Mg I b line and the Ca II triplet. Although a higher age of ~0.5 Gyr where asymptotic giant branch (AGB) stars dominate the NIR light cannot be excluded, the observed parameters are at the limit of those expected for a cluster dominated by AGB stars. However, in either case the resolved stellar cluster contributes only about ~15% of the total dynamical mass in the inner 300 pc. This immediately implies at least one other much older stellar population that contributes to the mass but not the NIR luminosity. Pure constant star formation over the last 10 Gyr can be excluded based on the observational fact that in such a scenario the total observed (spatially unresolved and spectrally resolved) Brγ flux would be of stellar origin that is spatially extended. Therefore, at least two star formation/starburst events took place in the nucleus of NGC 3227. Since such sequences in the nuclear star formation history are also observed in the nuclei of other galaxies a link between the activity of the star formation and the AGN itself seems likely.

We investigate current problems in obtaining reliable ages for old stellar systems based on stellar population synthesis modeling of their integrated spectra. In particular, we address the large ages derived for the globular cluster 47 Tuc, which is at odds with its color-magnitude diagram (CMD) age. Using a new age indicator, Hγ_σ<130, which is particularly effective at breaking the degeneracy between age and metallicity, we confirm the discrepancy between the spectroscopic age and the CMD age of 47 Tuc, in that the spectroscopic age is much older. Nebular emission appears unlikely to be a source for weakening the observed Balmer lines. We then explore a number of key parameters affecting the temperature of turnoff stars, which are the main contributors to the Balmer lines for old metal-rich stellar populations. We find that α-enhanced isochrones with atomic diffusion included not only provide a good fit to the CMD of 47 Tuc but also lead to a spectroscopic age in better agreement with the CMD age.

We present an analysis of interstellar and intergalactic absorption lines in the Far-Ultraviolet Spectroscopic Explorer (FUSE) spectrum of the low-redshift quasar PG 0804+761 (z_em = 0.100) at intermediate resolution (FWHM~ 25 km s^-1) in the direction l = 1383, b = 310. With a good signal-to-noise ratio (S/N) and the presence of several interesting Galactic and extragalactic absorption components along the sight line, this spectrum provides a good opportunity to demonstrate the ability of FUSE to do both interstellar and extragalactic science. Although the spectrum of PG 0804+761 is dominated by strong absorption from local Galactic gas at 0 km s^-1, we concentrate our study on absorption by molecular hydrogen and neutral and ionized metals related to an intermediate-velocity cloud (IVC) in the lower Galactic halo at -55 km s^-1, and on absorption from O VI extended to negative velocities. In the IVC, weak molecular hydrogen absorption is found in six lines for rotational levels 0 and 1, leading to a total H₂ column density of log N = 14.71 ± 0.30. We derive an O I gas-phase abundance for the IVC of 1.03 solar. Lower abundances of other elements (Fe, Si) imply depletion onto dust grains or the presence of higher, undetected ionization states. The presence of N II and Fe III absorption at -55 km s^-1 indicates that a fraction of the hydrogen is ionized. From the relative abundances of O I and P II we estimate a degree of ionization H⁺/(H⁰+H⁺) of ~19%. Absorption by O VI is found at velocities as negative as -110 km s^-1, but no absorption from any species is found at velocities of ~-180 km s^-1, where absorption from the nearby high-velocity cloud complex A would be expected. We suggest that the extended O VI absorption traces hot gas situated above the Perseus spiral arm. Finally, we find intergalactic absorption by an intervening H I Lyβ absorber at z_abs = 0.019 and absorption by H I, C III, and O VI in an associated system at z_abs = 0.102. No intervening O VI absorbers are seen in the spectrum of PG 0804+761.

Maps of the Galactic electron-positron annihilation radiation show evidence for three distinct and significant features: (1) a central bulge source, (2) emission in the Galactic plane, and (3) an enhancement of emission at positive latitudes above the Galactic center. In this paper, we explore the possibility that Sgr A East, a very prominent radio structure surrounding the Galactic nucleus, may be a significant contributor to the central bulge feature. The motivation for doing so stems from a recently proposed link between this radio object and the EGRET γ-ray source 2EG J1746-2852. If this association is correct, then Sgr A East is also expected to be a source of copious positron production. The results presented here show that indeed Sgr A East must have produced a numerically significant population of positrons, but also that most of them have not yet had sufficient time to thermalize and annihilate. As such, Sgr A East by itself does not appear to be the dominant current source of annihilation radiation, but it will be when the positrons have cooled sufficiently and they have become thermalized. This raises the interesting possibility that the bulge component may be a result of the relics of earlier explosive events like the one that produced Sgr A East.

We present predictions of the evolution of the light elements, Li, Be, and B, in the early epochs of the Galactic halo, using a model of supernova-induced chemical evolution based on contributions from supernovae (SNe) and cosmic rays (CRs), as recently proposed by Tsujimoto et al. and Suzuki et al. This model has the great advantage of treating various elements self-consistently, even under inhomogeneous conditions, as might arise from stochastic star formation processes triggered by SN explosions. The most important prediction from our model is that the abundances of light elements in extremely metal-poor stars might be used as age indicators in the very early stages of an evolving halo population, at times when the abundances of heavy elements ("metallicity") in most stars are dominated by local metal enrichment due to nearby SN events, and is poorly correlated with age. Plots of the expected frequency distribution of stars in the age versus elemental abundance diagram show that the best "cosmic clock" is the ⁶Li abundance. We have derived relationships among various cosmic-ray parameters such as energy input to CRs by SNe, the spectral shape of the CRs, and the chemical composition in CRs, and find that we can reproduce very well recent observations of ⁶Li, Be, and B in metal-poor stars. Although our model is successful for certain sets of cosmic-ray parameters, larger energy should be absorbed by energetic particles from each SN than required to the current situation of Galactic disk. We discuss an alternative hypothesis of active galactic nuclei activity in the early Galaxy as another possible accelerator of CRs.

Present data on cosmic-ray elemental and isotopic relative abundances are shown to be unable to distinguish between various models of cosmic-ray sources and their composition. For example, the model of freshly nucleosynthesized material from supernova explosions as the cosmic-ray source is unable to account for some measured, key cosmic-ray elemental abundances. This and two other models are evaluated here in light of recent isotopic and elemental measurements. It is shown that model-dependent preferential injection, acceleration, and reacceleration do not allow a clear distinction of one model against the others. Future measurements of critical elements and isotopes are suggested, which should afford us the ability to do that. We base our suggestions on measurements and a quantitative comparison between the predictions of the standard leaky-box model for the Galactic propagation of cosmic rays and one in which reacceleration is taken into account.

We develop a method for recovering the global density distribution of the ancient Galactic stellar halo prior to disk formation based on the present orbits of metal-poor stars observed in the solar neighborhood. The method relies on the adiabatic invariance of the action integrals of motion for the halo population during the slow accumulation of a disk component subsequent to earlier halo formation. The method is then applied to a sample of local stars with [Fe/H] ≤ -1.5, which are likely to be dominated by the halo component, taken from Beers et al.'s recently revised and supplemented catalog of metal-poor stars selected without kinematic bias. We find that even if the Galactic potential is made spherical by removing the disk component in an adiabatic manner, the halo density distribution in the inner halo region (R ≤ 15 kpc) remains moderately flattened, with an axial ratio of about 0.8 for stars in the abundance range [Fe/H] ≤ -1.8 and about 0.7 for the more metal-rich interval -1.8 < [Fe/H] ≤ -1.5. The outer halo remains spherical for both abundance intervals. We also find that this initial flattening of the inner halo is caused by the anisotropic velocity dispersions of the halo stars. These results suggest that the two-component nature of the present-day stellar halo, characterized by a highly flattened inner halo and nearly spherical outer halo, is a consequence of both an initially two-component density distribution of the halo (perhaps a signature of dissipative halo formation) and of the adiabatic flattening of the inner part by later disk formation. Further implications of our results for the formation of the Galaxy are also discussed, in particular in the context of the hierarchical clustering scenario of galaxy formation.

We present a three-component mixing model for the evolution of O abundance relative to Fe, taking into account the contributions of the first very massive stars (with masses of ≳100 M_☉) formed from Big Bang debris. We show that the observations of O and Fe abundances in metal-poor stars in the Galaxy by Israelian et al. in 1998 and by Boesgaard et al. in 1999 can be well represented both qualitatively and quantitatively by this model. We use the representation of the number ratios (O/Fe) versus 1/(Fe/H). In this representation, if there is only a single source with a fixed production ratio of O to Fe beyond a certain point, the subsequent evolution of (O/Fe) is along a straight line segment. Under the assumption of an initial Fe ([Fe/H] ~ -3) and O inventory caused by the prompt production by the first very massive stars, the data of Israelian et al. and Boesgaard et al. at -3 ≲ [Fe/H] ≲ -1 are interpreted to result from the addition of O and Fe only from Type II supernovae (SNII) to the prompt inventory. At [Fe/H] ≳ -1, SNII still contribute O while both SNII and Type Ia supernovae contribute Fe. During this later stage, (O/Fe) sharply drops off to an asymptotic value of ~0.8 (O/Fe)_☉. The value of (O/Fe) for the prompt inventory at [Fe/H] ~ -3 is found to be (O/Fe) ~ 20 (O/Fe)_☉. This result suggests that protogalaxies with low "metallicities" should exhibit high values of (O/Fe). The C/O ratio produced by the first very massive stars is expected to be ≪ 1 so that all the C should be tied up as CO and that C dust and hydrocarbon compounds should be quite rare at epochs corresponding to [Fe/H] ≲ -3.

The enrichment of Pb in the Galaxy is followed in the framework of a detailed model of Galactic chemical evolution that already proved adequate to reproduce the chemical enrichment of O and of the elements from Ba to Eu. The stellar yields are computed through nucleosynthesis calculations in the asymptotic giant branch (AGB) phase of low- and intermediate-mass stars covering a wide range of metallicities. The physical parameters of the stellar structure were derived from full stellar evolutionary models computed previously. We show that low-mass AGB stars are the main producers of Pb in the Galaxy, with a complex dependence on metallicity and a maximum efficiency at [Fe/H] ~ -1. Our calculations succeed in reproducing the abundances of Pb isotopes in the solar system: the role attributed by the classical analysis of the s-process to the strong component, in order to explain more than 50% of solar ²⁰⁸Pb, is actually played by the high production of Pb in low-mass and low-metallicity AGB stars. We then follow the Galactic chemical evolution of Pb isotopes and give our expectations on the s-process contribution to each of them at the epoch of the solar system formation. Finally, we present new spectroscopic estimates of Pb abundance on a sample of field stars and compare them, together with a few other determinations available, with the predicted trend of [Pb/Fe] in the Galaxy.

This paper describes some interesting properties of waves in, and oscillations of, the interstellar medium (ISM) in the direction normal to the plane of the Galaxy. Our purpose is to examine possible reasons for four observed phenomena: the falling sky in the northern hemisphere; the apparent presence of clouds in absorption spectra when a sight line is occupied primarily only by warm intercloud gas; the peculiar structuring of spiral arms involving clumps, spurs, and feathering; and the existence of an abundance of high-stage ions far off the plane of the Galaxy. We explored the reaction of the interstellar medium—in the vertical direction only—to large imposed disturbances (initial displacements, expansive velocities, and compressions) and to the introduction of small-amplitude waves via oscillation of the midplane. Our findings included (1) the anticipated growth in amplitude of high-frequency waves with height; (2) the four lowest normal modes for the oscillation of the atmosphere as a whole, as functions of the height of the outer boundary; (3) the time for material to "bounce" from one unusually dense state to the next as a function of height; and (4) the tendency for the disk to develop a hot outer halo, either after the passage of a single shock from a large event or in response to a continuous stream of small-amplitude waves. We discovered that three of the four observed phenomena targeted are likely to be closely connected. Following a large expansion, material near the plane falls back first, with material at higher z then falling in upon it. This provides precisely the sort of velocity segregation observed in the northern sky, at about 50 Myr after the event. In addition, this bounce time (and/or the period of the subsequent smaller oscillations, which is about twice the bounce time) may be linked to structure in the spiral arms, with vertical oscillations having been provoked by initial compressions in the arms. Oscillations of the fundamental symmetric (breathing) mode of the ISM also produce substantial disturbances in the outer atmosphere. This can result in the production of an extensive layer of hot gas overlying the cooler disk material, i.e., a hot Galactic halo with a significant population of high-stage ions. Hence, three of the four phenomena may be natural results of the simple existence of strong local compressions at the spiral arms and the associated vertical motions in a thick Galactic disk. Finally, the somewhat mysterious appearance of clouds in some absorption spectra can be produced by small-amplitude waves in the ISM. Under the right conditions, clouds will seem to appear through "velocity crowding," when in fact there are no density concentrations in space.

The strengths of magnetic fields in interstellar gas clouds are obtained through observations of the circular polarization of spectral line radiation. Irregularities in this magnetic field may be present because of turbulence, waves, or perhaps other causes, and may play an essential role in the structure and evolution of the gas clouds. To infer information about these irregularities from the observational data, we develop statistical relationships between the rms values of the irregular component of the magnetic field and spatial variations in the circular polarization of the spectral line radiation. The irregularities are characterized in analogy with descriptions of turbulence—by a sum of Fourier waves having a power spectrum with a slope similar to that of Kolmogorov turbulence. For comparison, we also perform computations in which turbulent magnetic and velocity fields from representative MHD simulations by others are utilized. Although the effects of the variations about the mean value of the magnetic field along the path of a ray tend to cancel, a significant residual effect in the polarization of the emergent radiation remains for typical values of the relevant parameters. A map of observed spectra of the 21 cm line toward Orion A is analyzed and the results are compared with our calculations in order to infer the strength of the irregular component of the magnetic field. The rms of the irregular component is found to be comparable in magnitude to the mean magnetic field within the cloud. Hence, the turbulent and Alfvén velocities should also be comparable.

Using hydrodynamic simulations, we investigate the time evolution and fragmentation of regions within molecular clouds that have lost their turbulent support, leading to gravitational contraction. The initial density distributions are described by random Gaussian fluctuations with varying slopes ν of the power spectrum P(k) ∝ k^-ν, covering the range from flat (ν = 0) to very steep (ν = 3) spectra. We consider molecular cloud volumes containing different masses relative to the average Jeans mass M_J, from 1M_J to 222M_J. This parameter study extends a previous detailed analysis of systems with, initially, P(k) ∝ k^-2 and mass 222M_J. The dynamical evolution of the simulated molecular cloud regions is insensitive to the slope of the initial density fluctuation spectrum. The system evolves into a complex network of intersecting filaments and collapsing clumps, leading to the formation of a compact cluster of accreting and interacting embedded protostellar cores. The cluster builds up as a bound entity but dissolves later due to collisional effects. In all simulations, the mass spectrum of collapsed cores is very broad, has approximately log-normal shape, and peaks roughly at the average Jeans mass. This supports the hypothesis that the average Jeans mass is the main parameter determining the peak in the stellar spectrum and suggests that the interplay between self-gravity on the one side and thermal and turbulent pressure on the other side is the dominant process that regulates the formation of stellar clusters.

Parallel thermal conduction along stochastic magnetic field lines may be reduced because the heat-conducting electrons become trapped and detrapped between regions of strong magnetic field (magnetic mirrors). The problem reduces to a simple but realistic model for diffusion of monoenergetic electrons based on the fact that when there is a reduction of diffusion, it is controlled by a subset of the mirrors, the principal mirrors. The diffusion reduction can be considered as equivalent to an enhancement of the pitch angle scattering rate. Therefore, in deriving the collision integral, we modify the pitch angle scattering term. We take into account the full perturbed electron-electron collision integral, as well as the electron-proton collision term. Finally, we obtain the four plasma transport coefficients and the effective thermal conductivity. We express them as reductions from the classical values. We present these reductions as functions of the ratio of the magnetic field decorrelation length to the electron mean free path at the thermal speed V_T = ^1/2. We briefly discuss an application of our results to clusters of galaxies.

We present the results of a Far Ultraviolet Spectroscopic Explorer observation of an X-ray-selected knot in the Vela supernova remnant. Spectra were obtained through the 30'' × 30'' low-resolution aperture and the 4'' × 20'' medium-resolution aperture. O VI λλ1032, 1038 and C III λ977 are detected strongly in both spectra, and S VI λλ933, 944 is detected weakly only in the larger aperture spectrum. We also report the first detection of C II λ1037 emission in a supernova remnant. The spectra show the presence of two kinematic components along the line of sight—one with both low- and high-excitation emission centered at a velocity of -50 km s^-1 and another with only low-excitation emission centered at a velocity of +100 km s^-1. We associate the -50 km s^-1 component with the observed X-ray knot and find a dynamical pressure of 3.7 × 10^-10 dyne cm^-2 driving the shock. We compare our results with data obtained using the Hopkins Ultraviolet Telescope at nearby locations and find that differences in the spectra imply the existence of two emitting components in the X-ray knot. Based on the X-ray morphology seen in a ROSAT HRI image, we identify two distinct regions that can be associated with these two components which have dramatically different ultraviolet emission. These observations demonstrate the importance of high spectral resolution in understanding the proper physical relationships between the various emitting components in supernova remnants.

We have observed the G9.62+0.19 region of massive star formation with the IRAM 30 m single-dish telescope in the HCO⁺(1-0), SO(4₃-3₂), and SiO(5-4) spectral lines, and with the IRAM Plateau de Bure interferometer in the 3 mm continuum and the HCO⁺(1-0) and CH₃OH(15₃-14₄) A^- spectral lines. We detect a high-velocity molecular ouflow with a total velocity extent of at least 60 km s^-1. The orientation of the outflow axis is very close to the line of sight. Our estimates of the mass and energetics of the outflowing gas indicate that a massive object must be responsible for driving the flow, with the hot core region G9.62+0.19 F as the most likely candidate. We also detect emission from hot methanol coincident with the position of continuum component E, but CH₃OH(15₃-14₄) A^- is not detected at the position of the G9.62+0.19 F hot core region.

The isolated neutron star RX J185635-3754 is the closest known neutron star to the Sun. Based on Hubble Space Telescope Wide Field Planetary Camera 2 observations over a 3 yr baseline, I report its proper motion (332 ± 1 mas yr^-1 at a position angle of 1003 ± 01) and parallax (16.5 ± 2.3 mas, 61 pc). This proper motion brings the neutron star from the general vicinity of the Sco-Cen OB association. For an assumed neutron star radial velocity between -55 and -60 km s^-1, the runaway O star ζ Oph, the Upper Sco OB association, and the neutron star come into spatial coincidence between 0.9 × 10⁶ and 1.0 × 10⁶ yr ago. RX J185635-3754 may be the remnant of the original primary of the ζ Oph system. If so, the space velocity suggests that the neutron star received a kick of about 200 km s^-1 at birth.

We used the High-Energy Transmission Grating Spectrometer on board the Chandra X-Ray Observatory to perform two observations, separated by 3 weeks, of the Orion Trapezium region. The zeroth-order images on the Advanced CCD Imaging Spectrometer provide spatial resolution of 05 and moderate energy resolution. Within a 160'' × 140'' region around the Orion Trapezium, we resolve 111 X-ray sources with luminosities between 7 × 10²⁸ and 2 × 10³² ergs s^-1. We do not detect any diffuse emission. All but six sources are identified. From spectral fits of the three brightest stars in the Trapezium, we determine the line-of-sight column density to be N_H = (1.93 ± 0.29) × 10²¹ cm^-2. Many sources appear much more heavily absorbed, with N_H in the range of 10²²-10²³ cm^-2. A large fraction of sources also show excursions in luminosity by more than a factor 5 on timescales greater than 50 ks; many are detected only in one of the observations. The main objective of this paper is to study the Orion Trapezium and its close vicinity. All five Trapezium stars are bright in X-rays, with θ¹ Ori C accounting for about 60% of the total luminosity of the Trapezium. The CCD spectra of the three very early-type members can be fitted with a two-temperature thermal spectrum with a soft component of kT ~ 0.8 keV and a hard component of kT ~ 2-3 keV. θ¹ Ori B is an order of magnitude fainter than θ¹ Ori E and shows only a hard spectrum of kT ~ 3 keV. θ¹ Ori D is another order of magnitude fainter than θ¹ Ori B, with only a kT ~ 0.7 keV component. We discuss these results in the context of stellar wind models. We detect eight additional, mostly variable X-ray sources in the close vicinity of the Trapezium. They are identified with thermal and nonthermal radio sources, as well as infrared and optical stars. Five of these X-ray sources are identified with proplyds, and we argue that the X-ray emission originates from class I, II, and III protostars at the cores of the proplyds.

We present an analysis of high-quality spectra of members of the young cluster M35. By using a multifiber spectrograph, we are able to collect high signal-to-noise ratio, high-resolution spectra of a sample of photometric candidate members. Accurate radial velocities are used to establish the membership status, and rotational velocities are measured using cross-correlation. We also derive the metal content of the cluster, [Fe/H]_M35 = -0.21 ± 0.10, based on spectral synthesis. Finally, we derive the lithium abundances of the bona fide cluster members and compare the results with members of other clusters. For example, M35 shows a smaller range in both rotation rates and lithium abundances as compared to the Pleiades. We discuss possible roles of various parameters. Our high-quality M35 database of lithium abundances and rotational velocities is perfectly suited to be used as a laboratory to test theoretical models dealing with the lithium depletion phenomenon. We discuss the role of stellar inhomogeneities and rotation on the lithium depletion phenomenon.

A new semianalytical model of a star evolving in a tidal field is proposed. The model is a generalization of the so-called affine stellar model. In our model the star is composed of elliptical shells with different parameters and different orientations depending on time and on the radial Lagrangian coordinate of the shell. The evolution equations of this model are derived from the virial relations under certain assumptions, and the integrals of motion are identified. It is shown that the evolution equations can be deduced from a variational principle. The evolution equations are solved numerically and compared quantitatively with the results of three-dimensional numerical computations of the tidal interaction of a star with a supermassive black hole. The comparison shows very good agreement between the main "integral" characteristics describing the tidal interaction event in our model and in the three-dimensional computations. Our model is effectively a one-dimensional Lagrangian model from the point of view of numerical computations, and therefore it can be evolved numerically 10²-10³ times faster than the three-dimensional approach allows. This makes our model well suited for intensive calculations covering the whole parameter space of the problem.

The γ-ray background from supernovae is calculated on the basis of the reconstructed efficiency of supernova explosions from star formation at various redshifts. The calculations presented here show how Type Ia supernova (SN) rates compatible with the results of optical supernova searches give a background emission in the MeV range that can explain the extragalactic emission measured by COMPTEL and the Solar Maximum Mission (SMM). Star formation histories reaching maximum values of 0.3-0.4 M_☉ yr^-1 Mpc^-3 at z ~ 1.5 (and/or possibly keeping that value to higher z) give a γ-ray background compatible with observations, while they also predict the observed supernova rates per comoving volume. While supernova rates are sensitive to the cosmology and details of star formation history along z, the γ-ray background in the MeV range is very weakly dependent on the cosmology and compatible with a range of star formation histories. It is mostly sensitive to the star formation rate at z ~ 1-2, but the activity in forming stars at even higher z has an influence as well on the absolute level of the cosmic γ-ray background; the time elapsed by the SNe Ia progenitors until they explode has a broad distribution and the rates of SNe Ia at z ~ 1.5-2 depend on the formation frequencies of their progenitor stars at earlier epochs. The mutual consistency of predictions of optical rates and integrated emission in the γ-ray domain place on firm ground the Type Ia supernova identification as the astrophysical source responsible for this cosmic background.

We study the γ-ray emissions from an outer magnetospheric potential gap around a rotating neutron star. Migratory electrons and positrons are accelerated by the electric field in the gap to radiate copious γ-rays via a curvature process. Some of these γ-rays materialize as pairs by colliding with the X-rays in the gap, leading to a pair production cascade. Imposing the closure condition that a single pair produces one pair in the gap on average, we explicitly solve the strength of the acceleration field and demonstrate how the peak energy and the luminosity of the curvature-radiated, GeV photons depend on the strength of the surface blackbody and the power-law emissions. Some predictions on the GeV emission from 12 rotation-powered pulsars are presented. We further demonstrate that the expected pulsed TeV fluxes are consistent with their observational upper limits. An implication of high-energy pulse phase width versus pulsar age, spin, and magnetic moment is discussed.

Rather than accreting via a disk, some white dwarfs (WDs) in quiescent dwarf novae (DN) could accrete via an advection-dominated accretion flow (ADAF) possibly responsible for the X-ray bremsstrahlung emission observed. Such a hot accretion flow is also expected to produce characteristic thermal line emission. Using SS Cyg as a specific example, we show that knowing a priori the inclination and the WD mass in quiescent DN makes X-ray line diagnostics powerful probes of the flow structure in these systems. Current X-ray instruments can discriminate, from their width, between lines emitted from a flow with a Keplerian rotation rate and those emitted at a substantially sub-Keplerian rate. This could be used to observationally test the property of energy advection, which is at the origin of the partial radial pressure support by the hot gas and the sub-Keplerian rotation rate in an ADAF.

We report on 9 years of timing observations of PSR J2019+2425, a millisecond pulsar in a wide 76.5 day orbit with a white dwarf. We measure a significant change over time of the projected semimajor axis of the orbit, /x = × 10^-15 s^-1, where x ≡ (a₁ sin i)/c. We attribute this to the proper motion of the binary. This constrains the inclination angle to i < 72^°, with a median likelihood value of 63°. A similar limit on the inclination angle arises from the lack of a detectable Shapiro delay signal. These limits on inclination angle, combined with a model of the evolution of the system, imply that the neutron star mass is at most 1.51 M_☉; the median likelihood value is 1.33 M_☉. In addition to these timing results, we present a polarization profile of this source. Fits of the linear polarization position angle to the rotating vector model indicate that the magnetic axis is close to alignment with the rotation axis, α < 30^°.

We present results from multiwavelength observations of the Hercules X-1 system during a short high state (SHS) and during an anomalous low state (ALS). The magnitude of deviation from spin-up appears to be positively correlated with duration of the ALS. Such a correlation is consistent with an interpretation of the ALS in terms of a change in mass accretion rate that causes the disk to tilt and twist beyond the normal deviations that cause the 35 day cycle. A larger deviation from the average results in a stronger disruption of the disk and causes the disk to take longer to settle back to its "normal" 35 day behavior. Our model—which includes X-ray heating of the disk and companion star, shadowing of the X-ray flux by the disk, and a contribution to the continuum emission from the accretion stream or hot spot—can consistently explain the observed changes in X-ray, ultraviolet (UV), and optical continuum light curves for both the SHS and ALS. The Hubble Space Telescope (HST) and Space Telescope Imaging Spectrograph (STIS) observations presented here are the first UV observations of sufficient spectral and temporal resolution to construct Doppler tomograms of the line emission. Doppler tomograms of the UV emission lines during SHS and ALS show the majority of the emission coming from the surface of the companion star rather than from the accretion disk. Tomograms made after separating the N V emission lines into broad and narrow components suggests that while the narrow component is associated with emission from the companion star, the broad component may be associated with emission from a distorted disk. The Doppler maps also show that heating over the inner face of HZ Her is not uniform and imply partial eclipse of the UV line emission by an accretion stream and/or hot spot.

We present disk wind model calculations for the broad emission lines seen in the ultraviolet spectra of the X-ray binary Hercules X-1. Recent HST/STIS observations of these lines suggest that they are kinematically linked to the orbital motion of the neutron star and exhibit a redshifted to blueshifted evolution of the line shape during the progression of the eclipse from ingress to egress that is indicative of disk emission. Furthermore, these lines are single peaked, which implies that they may be formed in a disk wind similar to those we have proposed as producing the broad emission lines seen in the UV spectra of active galactic nuclei. We compute line profiles as a function of eclipse phase and compare them to the observed line profiles. Various effects may modify the appearance of the lines including resonant scattering in the wind itself, self-shadowing of the warped disk from the central continuum, and self-obscuration of parts of the disk along the observer's line of sight. The latter two effects can cause orbital and precessional phase-dependent variations in the emission lines. Hence, examination of the line profiles as a function of these phases can, in principle, provide additional information on the characteristics of the disk warp.

In mid-September of 1999, a multiwavelength campaign was carried out on the coronally active RS Canum Venaticorum binary HR 1099 (K1 IV + G5 V; P = 2.84 days), during commissioning of the High Energy Transmission Grating Spectrometer (HETGS) of the Chandra X-ray Observatory (CXO). The coordinated program involved the Extreme Ultraviolet Explorer (EUVE), the Hubble Space Telescope Imaging Spectrograph (STIS), and the Very Large Array (VLA). The objective was to study the system in quiescence, across as much of the electromagnetic spectrum as practical, and to catch any flares that might occur.

The EUVE 80-180 Å light curve of HR 1099, covering the period September 13-22, showed only a single impulsive outburst, which occurred at the very end of the 9 day pointing. The 3-25 Å bremsstrahlung continuum displayed an overall decay during the 1.5 day Chandra observation on September 14-16, with a few superimposed mild enhancements. The VLA 3.5 cm and 20 cm radio light curves, obtained during a 10 hr period on September 15 immediately before the HST pointing, similarly revealed normal quiescent gyrosynchrotron emission and an absence of impulsive events. In contrast, the 7 hr STIS time series later on September 15 contained two distinct flares. The first was accompanied by intensification of the preexisting broad wings of the medium excitation species (e.g., Si IV λ1393 and C IV λ1548), while the second involved primarily the narrow cores of the lines, and even cooler temperatures. The Fe XXI λ1354 forbidden line showed little response to either flare, consistent with the contemporaneous soft X-ray and EUV light curves. The lack of coronal counterparts to the ultraviolet flares is unusual and suggests that they belong to a separate class of outbursts, sharing some similarities with the "transition zone explosive events" seen on the Sun.

The density sensitive O IV λ1400 multiplet was not affected by either flare. The density sensitive Si III λ1300 multiplet showed little response to the first flare, but a dramatic brightening in the second, likely due to the lower peak temperature of that event. The O IV line ratios were near their low-density limits and suggest n_e ~ 10¹⁰ cm^-3 for the duration of the HST observations. The Si III ratios during the second flare rise indicate n_e ~ 7 × 10¹⁰ cm^-3. The far-UV diagnostics jointly imply electron pressures of n_eT ~ 2 × 10¹⁵ K cm^-3, if formed close to their respective ionization equilibrium temperatures. The helium-like triplets of O VII, Ne IX, and Mg XI in the HETGS spectra have forbidden-to-intercombination line ratios consistent with average coronal electron densities of ≲10¹¹ cm^-3 at T ~ 0.3-1 × 10⁷ K over the duration of the Chandra pointing. Fe XXI λ102/λ128 and λ142/λ128 ratios from EUVE suggest n_e ≲ 10¹²-10¹³ cm^-3, near 10⁷ K. Thus, the coronal electron pressures could be as much as several orders of magnitude larger than those of the lower atmosphere.

We constructed time-resolved spectra from the HETGS event lists and measured the centroids of the three brightest X-ray lines of HR 1099 in 60 minute bins. In high-S/N Ne X λ12.1, we believe that we can see the changing radial velocity of the K1 IV star over the half an orbital cycle recorded by Chandra. The amplitude of the velocity change is only ~100 km s^-1 over the 1.5 day interval, a small fraction of the 300-500 km s^-1 resolution of the HETGS at 12 Å. Ne X λ12.1 also displayed a transient blueshift of ~60 km s^-1 immediately prior to the first STIS flare, during which the far-UV lines briefly exhibited blueshifts of similar magnitude. The apparent displacement of Ne X appears independently in the -1 and +1 orders of the medium energy band (MEG), but there was no confirmation in the redundant high energy band (HEG), nor in other X-ray lines. Fe XXI λ1354 showed a blueshift about 30 minutes later at the beginning of the second HST orbit, but only at the 20 km s^-1 level. Analogous HETGS time series of the brightest lines of comparison star Capella (α Aurigae; G8 III + G1 III) exhibited steady behavior, consistent with measurement uncertainties, without the transient Doppler "bursts" seen in HR 1099. Although the reality of the Ne X Lyα blueshift is in doubt, there is no question that the Chandra HETGS velocity scales are stable and free from large systematic errors.

H-R diagrams are presented for the very young galactic clusters Trumpler 14 and Trumpler 16, which are the two most populous clusters in the region of vigorous star formation surrounding η Carinae. Point spread function photometry of UBV CCD images is presented down to V ≈ 19 for over 560 stars in Tr 16 and 290 stars in Tr 14. We have also obtained similar data for a local background field. After determining cluster membership through proper motions from a previous work, we find that the reddening of cluster members is significantly lower than that of the local background stars. Thus, we are able to use individual reddenings to identify likely members at far deeper levels than possible with proper motions. This work has revealed a significant population of pre-main-sequence (PMS) stars in both clusters. The location of the PMS stars in the H-R diagram indicates that the theoretical "stellar birthline" of Palla & Stahler follows the locus of stars far better than that of Beech & Mitalas. Comparison with both pre- and post-main-sequence isochrones also reveals that although intermediate-mass stars have been forming continuously over the last 10 Myr, the high-mass stars formed within the last 3 Myr. There is no evidence that the formation of the intermediate-mass stars was truncated by the formation of the high-mass stars.

This paper presents diffraction-limited 1-18 μm images of the young quadruple star system HD 98800 obtained with the W. M. Keck 10 m telescopes using speckle and adaptive optics imaging at near-infrared wavelengths and direct imaging at mid-infrared wavelengths. The two components of the visual binary, A and B, both themselves spectroscopic binaries, were separable at all wavelengths, allowing us to determine their stellar and circumstellar properties. Combining these observations with spectroscopic data from the literature, we derive an age of ~10⁷ yr, masses of 0.93 and 0.64 M_☉, and an inclination angle of 58° for the spectroscopic components of HD 98800 B and an age of ~10⁷ yr and a mass of 1.1 M_☉ for HD 98800 Aa. Our data confirm that the large mid-infrared excess is entirely associated with HD 98800 B. This excess exhibits a blackbody temperature of 150 K and a strong 10 μm silicate emission feature. The theoretical equilibrium radius of large, perfectly absorbing, 150 K grains around HD 98800 B is 2.4 AU, suggesting a circumspectroscopic binary distribution. Our observations set important upper limits on the size of the inner dust radius of ~2 AU (from the mid-infrared data) and on the quantity of scattered light of less than 10% (from the H-band data). For an inner radius of 2 AU, the dust distribution must have a height of at least 1 AU to account for the fractional dust luminosity of ~20% L_B. Based on the scattered-light limit, the dust grains responsible for the excess emission must have an albedo of less than 0.33. The presence of the prominent silicate emission feature at 10 μm implies dust grain radii of ≳2 μm. The total mass of the dust is greater than 0.002 M_⊕. We conclude that the dust is located in a circumbinary disk around the HD 98800 B spectroscopic binary with an inner gap of ~2 AU and a height of ≳1 AU, and we speculate that the A-Bc orbital dynamics are responsible for the characteristics of the observed dust in the system.

We report here the flux densities of the evolving radio source SN 1987A at 843 MHz measured from observations made with the Molonglo Observatory Synthesis Telescope between 1994 September and 2000 May. The radio light curve shows that the rate of increase of the flux density jumped markedly around days 2800-3000 (i.e., in the first half of 1995) and that since then the radio evolution has been remarkably well fitted by a simple linear increase of 62.7 ± 0.5 μJy day^-1. We discuss in detail the relationship between the radio light curve and the recent brightening of the system at optical wavelengths.

This paper is intended to study the evolution of a magnetic flux tube that rises from the upper convection zone to the solar atmosphere by means of a 2.5-dimensional MHD simulation with the focus on the cross section of the flux tube. A cylindrical flux tube placed horizontally in the convection zone starts rising by magnetic buoyancy. When the top of the tube reaches the photosphere, the cross section of the tube changes from the circular shape to horizontally extended shape, forming a magnetic layer under the contact surface between the tube and the photosphere. As the plasma inside that magnetic layer is squeezed out to both sides of the layer, the contact surface is locally subject to the Rayleigh-Taylor instability because the lighter magnetic layer is overlain by the heavier photospheric layer. The wavelength of the undulating magnetic layer at the contact surface increases as the flattening of the tube proceeds, and after it becomes longer than the critical wavelength for the Rayleigh-Taylor instability, the tube can emerge through the photosphere. The emergence part of the tube starts expanding into the atmosphere if it has a sufficiently strong magnetic pressure compared to the surrounding gas pressure. We find that this expansion process is characterized by a self-similar behavior, that is, both the plasma and the magnetic field have a steady distribution in the expanding area. On the basis of those results, we try to clarify several important features of emerging flux tubes expected from observations. We focus on two solar phenomena, the birth of emerging flux tubes and the formation of filaments, and discuss the physical processes related to these phenomena.

Spectra of comet Hyakutake at 80-700 Å observed with a resolving power of 10 reveal for the first time the emission lines of multiple-charged ions that are brought to the comet by the solar wind and excited in charge exchange with cometary neutral species. The most prominent lines are O⁴⁺ λ215, C⁴⁺ λ249, and He⁺ λ304. Some other lines, which are of comparable strength, are blended. The results convincingly prove that the charge exchange mechanism is the dominant process in excitation of X-ray and EUV emissions from comets. The He⁺ line at 304 Å is emitted in a similar process by the solar wind α-particles. The quantum yield of charge exchange is ≈4 photons per heavy ion in collisionally thick parts of comae, and the photon luminosity of charge exchange at energy below 100 eV exceeds that above 100 eV by a factor of 2. However, the energetic luminosity below 100 eV is smaller than that above 100 eV by a factor of 2. The O⁺ lines at 538/539, 617, and 430/442 Å are formed by photoionization of atomic oxygen similarly to those in Earth's dayglow. The observed depletion of neon relative to the solar abundance by more than a factor of 2600 confirms the current view that Oort cloud comets formed in the Jupiter-Neptune region of the solar nebula.

We present 1.2-18.3 μm photometry of comet 1995 01 (Hale-Bopp). Our observations spanned nearly 4 months pre- and post-perihelion. The dust grain size-dependent parameters of superheat, 10 μm silicate excess, and albedo are used to make determinations about the Hale-Bopp coma grain population. The evolution of the coma grain size distribution is tracked by measuring these parameters over a range of heliocentric distances. We also observed several regions in the Hale-Bopp coma revealing significant gradients in the measured color temperature suggesting differences in the dust grain size distribution. Finally, we compare the grain size-dependent parameters to those of 11 other bright comets. The extreme values of these parameters displayed by Hale-Bopp suggest it had a smaller mean dust grain size than those associated with the other comets.

The theoretical response of long-baseline optical interferometers and Fourier transform spectrometers, including polarization effects, is derived. The formalism, employing the Jones and Mueller calculi as well as the relationship between them, was adapted from previous work in radio interferometry and tailored specifically for optical wavelengths. Expressions for Jones and Mueller matrices corresponding to specific optical components and effects are stated. It was determined that the system squared visibility depends on the instrument, atmosphere, and normalized intrinsic polarization of sources under observation. A sample algebraic calculation was performed to highlight the typical functional form for the instrumental system squared visibility, demonstrating that reductions from unity can be determined directly from differential measurements of polarization quantities between the arms. Monte Carlo simulations were performed using two trains of identical mirrors with random relative orientation offsets, yielding results consistent with the algebraic example. Four mathematical appendices are provided for reference.This series of papers is dedicated to the late Dr. William Blitzstein, astronomer and friend.

We present the first results of observations of the intrinsic anisotropy of the cosmic microwave background radiation with the Cosmic Background Imager from a site at 5080 m altitude in northern Chile. Our observations show a sharp decrease in C_l in the range l = 400-1500. The broadband amplitudes we have measured are δT_band = 58.7 μK for l = 603 and δT_band = 29.7 μK for l = 1190, where these are half-power widths in l. Such a decrease in power at high l is one of the fundamental predictions of the standard cosmological model, and these are the first observations which cover a broad enough l range to show this decrease in a single experiment. The C_l we have measured enables us to place limits on the density parameter, Ω_tot ≤ 0.4 or Ω_tot ≥ 0.7 (90% confidence).

We present multicolor light curves of the optical afterglow of gamma-ray burst (GRB) 000926. Beginning ~1.5 days after the burst, the light curves of this GRB steepen measurably. The existence of such achromatic breaks is usually taken to be an important observational signature that the ejecta are not expanding isotropically but rather have a collimated jetlike geometry. If we interpret the data in this context, we derive an opening angle of 5°, which reduces the energy release compared with an isotropic model by a factor of ~275, to 1.7 × 10⁵¹ ergs. To fit the data with a simple jet model requires extinction along the line of sight. The derived A_V is in the range 0.11-0.82 mag, depending on the adopted extinction law and whether the electrons giving rise to the optical emission are undergoing synchrotron cooling or not. Since this is in excess of the expected extinction from our Galaxy, we attribute this to the GRB host. We note that this extinction is typical of a galactic disk, and therefore the event likely took place in the disk of its host.

We use the redshift evolution of the Lyα forest mean transmitted flux at z ≳ 2.5 to infer the evolution of the intensity of the ionizing background, based on theoretical predictions for the density distribution of the intergalactic gas giving rise to the Lyα forest. The proper background intensity declines gradually with redshift, decreasing by a factor of ~3 between z = 3 and z = 5. Detailed predictions of the flux distribution at high redshift are presented. If the Lyα forest is due to the gravitational evolution of structure originating from Gaussian primordial fluctuations, the highest values of the transmitted flux should reflect the lowest gas densities of voids, except near the epoch of reionization, when large fluctuations in the ionizing background can alter the distribution of transmitted flux. For the highest redshift quasar known (with a mean Lyα forest redshift z = 5.2), and given the observed mean transmitted flux of = 0.09, we predict that only about 3% of the Lyα spectrum should have a flux higher than 50% of the continuum and less than 0.5% of pixels should have flux above 75% of the continuum, assuming a uniform background intensity. We show that this is consistent with the spectrum shown in Fan et al. The result found here for the evolution of the ionizing background implies that the comoving ionizing emissivity decreases from z = 3 to z = 5 by a factor of ~2, which is much less than the reduction in the number of quasars, suggesting that a different population of sources dominates the background at z = 5.

The feasibility of the geometric test as a probe of the cosmic equation of state of the dark energy is discussed assuming the future Two Degree Field QSO sample. We examine the sensitivity of the QSO two-point correlation functions, which are theoretically computed incorporating the light cone effect and the redshift distortions, as well as the nonlinear effect, to a bias model whose evolution is phenomenologically parameterized. It is shown that the correlation functions are sensitive to a mean amplitude of the bias and not to the speed of the redshift evolution. We will also demonstrate that an optimistic geometric test could suffer from the possibility that a signal from the cosmological model can be confused with that from a stochastic character of the bias.

Formation of subgalactic halos is suppressed in a warm dark matter (WDM) model due to thermal motion of WDM particles. This may provide a natural resolution to some puzzles in standard cold dark matter (CDM) theory such as the cusped density profiles of virialized dark halos and the overabundance of low-mass satellites. One of the observational tests of the WDM model is to measure the gravitationally lensed images of distant quasars below subarcsecond scales. In this Letter, we report a comparison of the lensing probabilities of multiple images between CDM and WDM models using a singular isothermal sphere model for the mass density profiles of dark halos and the Press-Schechter mass function for their distribution and cosmic evolution. It is shown that the differential probability of multiple images with small angular separations down to ~10 mas should allow one to set useful constraints on the WDM particle mass. We discuss briefly the feasibility and uncertainties of this method in future radio surveys (e.g., VLBI) for gravitational lensing.

The statistics of wide-separation (6'' < θ < 15'') gravitational lenses constrain the amount of mass in the cores of dark matter halos on group and cluster mass scales. For a family of halo models with a central cusp ρ ∝ r^-α (1.0 ≤ α ≤ 1.9), the lack of wide-separation lenses in the large Cosmic Lens All-Sky Survey yields an upper limit on the fraction of the halo mass that is contained within ~4% of the virial radius, f_core < 0.023 (95% confidence level, Λ cold dark matter). This limit offers an important test of the cold dark matter paradigm. While the halo profiles derived from numerical simulations appear to be consistent with this upper limit, larger surveys currently underway such as the Two-degree Field and Sloan Digital Sky Survey should detect wide-separation lenses and thus provide a measurement of the core mass fraction in massive dark matter halos.

This paper presents data from the BIMA interferometer showing spatially resolved absorption spectra of the gravitationally lensed quasar PKS 1830-211. High-resolution (1.2 km s^-1) spectra were taken in two spectral windows centered on the redshifted frequencies of the HCO⁺(2 ← 1) and HCN(2 ← 1) molecular transitions. There is no molecular absorption in the northeast image, but the southwest image reveals optically thick absorbing gas at these transition frequencies. Further analyses conclude that the spectra are consistent with completely saturated absorption in the southwest image, and the line profiles suggest that the absorbing medium is complex, perhaps containing multiple components and small-scale structure. The absorption occurs along a pencil beam through the lensing galaxy which is thought to be a late-type spiral oriented almost face-on. However, the spectra show absorption spanning more than 60 km s^-1, which is difficult to explain for this scenario.

Most gravitational lens systems consist of two or four observable images. The absence of detectable odd images allows us to place a lower limit on the power-law slope of the inner mass profile of lensing galaxies. Using a sample of six two-image radio-loud lens systems and assuming a singular power-law surface density (Σ ∝ r^-β) for the inner several kiloparsecs of the mass distribution, we find that there is less than a 10% probability that the data are consistent with profile slopes β < 0.80. Furthermore, individual mass modeling yields β > 0.85 for B0739+366 and β > 0.91 for B1030+074. Modeling central black holes as additional point masses changes the constraints in these systems to β > 0.84 and β > 0.83, respectively. The inner mass profiles of lensing galaxies are therefore not much shallower than isothermal.

Sensitive images of low-level, megaparsec-sized radio cocoons offer new opportunities to probe large-scale intergalactic gas flows outside clusters of galaxies. New radio images of high surface brightness sensitivity at strategically chosen wavelengths of the giant radio galaxy NGC 315 reveal significant asymmetries and particularities in the morphology, radio spectrum, and polarization of the ejected radio plasma. We argue that the combination of these signatures provides a sensitive probe of an environmental shock wave. Analysis of optical redshifts in NGC 315's vicinity confirms its location to be near, or at, a site of large-scale flow collisions in the 100 Mpc sized Pisces-Perseus supercluster region. NGC 315 resides at the intersection of several galaxy filaments, and its radio plasma serves there as a "weather station," probing the flow of the elusive and previously invisible intergalactic medium gas. If our interpretation is correct, this is the first indication for a shock wave in flows caused by the cosmological large-scale structure formation, which is located in a filament of galaxies. The possibility that the putative shock wave is a source of gamma rays and ultrahigh-energy cosmic rays is briefly discussed.

The Chandra X-Ray Observatory obtained a 50 ks observation of the central region of M81 using the ACIS-S in imaging mode. The global properties of the 97 X-ray sources detected in the inner 83 × 83 field of M81 are examined. Roughly half the sources are concentrated within the central bulge. The remainder are distributed throughout the disk, with the brightest disk sources lying preferentially along spiral arms. The average hardness ratios of both bulge and disk sources are consistent with power-law spectra of index Γ ~ 1.6, indicative of a population of X-ray binaries. A group of much softer sources is also present. The background-source-subtracted log N-log S distribution of the disk follows a power law of index ~-0.5 with no change in slope over three decades in flux. The log N-log S distribution of the bulge follows a similar shape but with a steeper slope above ~4 × 10³⁷ ergs s^-1. There is unresolved X-ray flux from the bulge with a radial profile similar to that of the bulge sources. This unresolved flux is softer than the average of the bulge sources, and extrapolating the bulge log N-log S distribution toward weaker sources can account for only 20% of the unresolved flux. No strong time variability was observed for any source with the exception of one bright, soft source.

We use the Chandra observation of the cold front in the intracluster gas of A3667 to estimate the magnetic field strength near the front. The front is seen in the Chandra data as a sharp discontinuity in the gas density that delineates a large body of dense cool gas moving with near-sonic velocity through the less dense, hotter gas. Without a magnetic field, the front should be quickly disturbed by the Kelvin-Helmholtz instability arising from tangential motion of gas layers. However, the Chandra image shows that the front is stable within a ±30° sector in the direction of the cloud motion, beyond which it gradually disappears. We suggest that the Kelvin-Helmholtz instability within the ±30° sector is suppressed by the surface tension of the magnetic field whose field lines are parallel to the front. The required field strength is B ~ 10 μG. The magnetic field near the front is expected to be stronger and to have a very different structure compared with the bulk of the intergalactic medium because the field lines are stretched by the tangential gas motions. Such a magnetic configuration, once formed, would effectively stop the plasma diffusion and heat conduction across the front and may inhibit gas mixing during the subcluster merger. We note that even the increased magnetic field near the front contributes only 10%-20% to the total gas pressure, and therefore magnetic pressure is unimportant for hydrostatic cluster mass estimates.

We report preliminary results from an arcsecond-resolution X-ray survey of nearby galaxies using the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory. The total sample consists of 41 low-luminosity active galactic nuclei (AGNs), including Seyfert galaxies, LINERs, and LINER/H II transition objects. In the initial subsample of 24 objects observed thus far, we detect in ~62% of the objects a compact, pointlike source astrometrically coincident with either the optical or radio position of the nucleus. The high detection rate strongly suggests that the majority of the objects do contain weakly active, AGN-like cores, presumably powered by central massive black holes. The 2-10 keV luminosities of the nuclear sources range from less than 10³⁸ to 10⁴¹ ergs s^-1, with a median value of 2 × 10³⁸ ergs s^-1. Our detection limit corresponds to L_X(2-10 keV) ≈ 8 × 10³⁷ ergs s^-1 for the typical sample distance of 12 Mpc; this limit is 2 orders of magnitude fainter than the weakest sources of this kind previously studied using ASCA or BeppoSAX. The new data extend toward lower luminosities the known linear correlation between hard X-ray and Hα luminosity for broad-line AGNs. Many narrow-line objects do contain X-ray cores, consistent with either weak AGNs or X-ray binary systems, but they have X-ray luminosities a factor of 10 below the L_X-L_Hα relation of the broad-line sources. Their distributions of photon energies show no indication of exceptionally high absorption. The optical line emission in these nuclei is likely powered, at least in part, by stellar processes.

We have used VLBI Space Observatory Programme (VSOP) observations to measure the brightness temperature distribution of a well-defined subset of the Pearson-Readhead sample of extragalactic radio sources. VLBI, which is restricted to Earth-diameter baselines, is not generally sensitive to emitting regions with brightness temperatures greater than approximately 10¹² K, coincidentally close to theoretical estimates of brightness temperature limits, 10¹¹-10¹² K. We find that a significant proportion of our sample have brightness temperatures greater than 10¹² K; many have unresolved components on the longest baselines, and some remain completely unresolved. These observations begin to bridge the gap between the extended jets seen with ground-based VLBI and the microarcsecond structures inferred from intraday variability, evidenced here by the discovery of a relationship between intraday variability and VSOP-measured brightness temperature, likely due to the effects of relativistic beaming. Also, lower limits on jet Lorentz factors, estimated from space VLBI observations, are starting to challenge numerical simulations that predict low Lorentz factor jets.

We report velocity-resolved spectroscopy of infrared hydrogen recombination lines in the interacting region of the Antennae galaxies (NGC 4038/4039). It generally has been assumed that the active star formation found there is due to the interaction of the disks of the two galaxies and indeed two molecular cloud complexes, separated in velocity by ~100 km s^-1, have been observed in the southern part of this region. Our measurements imply that the two cloud complexes are moving away from each other. This result poses interesting questions about the physical mechanisms leading to starbursts in Arp 244 and other interacting galaxies.

We report the detection of main-sequence and blue horizontal-branch stars of the Ursa Minor dwarf spheroidal galaxy beyond its tidal radius, indicating the existence of a possible tidal extension in this satellite of the Milky Way. This tidal extension could spread out well beyond the area covered in our survey (R > 80'), as suggested by the presence of a "break" to a shallower slope observed in its density profile. The V-band surface brightness for this possible tidal extension ranges from 29.8 to 31.5 mag arcsec^-2. The area covered in our survey (~1.65 deg²) is not enough to discriminate if this extratidal population is part of a tidal tail or an extended halo around the galaxy. The existence of this tidal extension in Ursa Minor indicates that this satellite is currently undergoing a tidal disruption process by the Milky Way. We discuss the possibility of a tidal origin for the high mass-to-light ratio observed in this galaxy on the basis of our result and recent theoretical simulations of the tidal disruption of dwarf satellites in the Galactic halo.

The idea that the TeV air showers, thought to be produced by gamma rays greater than 10 TeV from Markarian 501, can be mimicked by coherent bunches of sub-TeV photons is reexamined, focusing on fundamental considerations. In particular, it is shown that the minimum spot size of a beam of photons arriving at Earth is on the order of a few kilometers unless a lens with certain characteristics is placed between the TeV laser and Earth. The viability of the production mechanism of coherent bunches of TeV photons proposed by M. Harwit et al. is also reassessed.

We report the discovery of nearly coherent oscillations with a frequency of ~567 Hz during type I X-ray bursts from the X-ray transient and eclipsing binary X1658-298. If these oscillations are directly related to the neutron star rotation, then the spin period of the neutron star in X1658-298 is ~1.8 ms. The oscillations can be present during the rise or decay phase of the bursts. Oscillations during the decay phase of the bursts show an increase in frequency of ~0.5-1 Hz. However, in one particular burst the oscillations reappear at the end of the decay phase at about 571.5 Hz. This represents an increase in oscillation frequency of about 5 Hz, which is the largest frequency change seen so far in a burst oscillation. It is unclear if such a large change can be accommodated by present models used to explain the frequency evolution of the oscillations. The oscillations at 571.5 Hz are unusually soft compared to the oscillations found at 567 Hz. We also observed several bursts during which the oscillations are detected at much lower significance or not at all. Most of these bursts happen during periods of X-ray dipping behavior, suggesting that the X-ray dipping might decrease the amplitude of the oscillations (although several complications exist with this simple picture). We discuss our discovery in the framework of the neutron star spin interpretation.

The ultraluminous X-ray sources (ULXs) are unique in exhibiting moderately bright X-ray luminosities, L_X ~ 10³⁸-10⁴⁰ ergs s^-1, and relatively high blackbody temperatures, T_in ~ 1.0-2.0 keV. From the constraint that L_X cannot exceed the Eddington luminosity L_E, we require relatively high black hole masses, M ~ 10-100 M_☉; however, for such large masses the standard disk theory predicts lower blackbody temperatures, T_in < 1.0 keV. To understand a cause of this puzzling fact, we carefully calculate the accretion flow structure shining at ~L_E, fully taking into account the advective energy transport in the optically thick regime and the transonic nature of the flow. Our calculations show that at high accretion rate ( ≳ 30L_E/c²) an apparently compact region with a size of R_in ≃ (1-3)r_g (with r_g being the Schwarzschild radius) is shining with a blackbody temperature of T_in ≃ 1.8(M/10 M_☉)^-1/4 keV even for the case of a nonrotating black hole. Furthermore, R_in decreases as increases, contrary to the canonical belief that the inner edge of the disk is fixed at the radius of the marginally stable last circular orbit. Accordingly, the loci of a constant black hole mass on the H-R diagram (representing the relation between L_X and T_in both on the logarithmic scales) are not straight but bent toward the lower M-direction in the frame of the standard disk relation.

We also plot the ASCA data of some ULXs on the same H-R diagram, finding that they all fall on the regions with relatively high masses, M ~ 10-30 M_☉, and high accretion rates, ≳ 10L_E/c². Interestingly, IC 342 source 1, in particular, was observed to move along the constant M line (not constant R_in line) in our simulations. This provides firm evidence that at least some ULXs are shining at ≳L_E and contain black holes with M ≃ 10-100 M_☉.

We report time-resolved spectroscopic observations of the recently discovered dwarf nova, IY Ursae Majoris, over 2 days during its 2000 January superoutburst. The spectra show prominent, broad, and double-peaked emission lines of Hα, Hβ, Hγ, Hδ, and He II and weaker He I lines. The radial velocity curve of the peaks of Hα and Hβ has an amplitude of 93 ± 7 km s^-1 and a phase offset of 0.12 ± 0.03. From clear evidence for a precessing disk, we obtain a solution to an eccentric outer disk consistent with theoretical works, which demonstrates the validity of the relation between superhumps and tidal effects. The γ velocity of the binary is derived as -4 ± 32 km s^-1 according to the solution. The inner part of the disk is also eccentric as evidenced by asymmetric wings in the lines. Therefore, the whole disk is eccentric. Finally, we find that a simple model can explain all asymmetric features of the emission lines.

We report the discovery of a highly coherent oscillation in a type I X-ray burst observed from 4U 1916-053 by the Rossi X-Ray Timing Explorer (RXTE). The oscillation was most strongly detected ≈1 s after the burst onset at a frequency of 269.4 Hz, and it increased in frequency over the following 4 s of the burst decay to a maximum of ≃272 Hz. The total measured drift of 3.58 ± 0.41 Hz (1 σ) represents the largest fractional change in frequency (1.32% ± 0.15%) yet observed in any burst oscillation. If the asymptotic frequency of the oscillation is interpreted in terms of a decoupled surface burning layer, the implied neutron star spin period is around 3.7 ms. However, the expansion of the burning layer required to explain the frequency drift during the burst is around 80 m, substantially larger than expected theoretically (assuming rigid rotation). The oscillation was not present in the persistent emission before the burst, nor in the initial rise. When detected, its amplitude was 6%-12% (rms) with a roughly sinusoidal profile. The burst containing the oscillation showed no evidence for photospheric radius expansion, while at least five of the other nine bursts observed from the source by RXTE during 1996 and 1998 did. No comparable oscillations were detected in the other bursts. A pair of kilohertz quasi-periodic oscillations (kHz QPOs) has been previously reported from this source with a mean separation of 348 ± 12 Hz. 4U 1916-053 is the first example of a source where the burst oscillation frequency is significantly smaller than the frequency separation of the kHz QPOs.

We report on the detection of an extreme flarelike event on DF Tauri during a spectroscopic monitoring program of some classical T Tauri stars known to show hot spots on their surfaces. These observations were performed on a nightly basis within roughly one rotational period of DF Tau between 2000 January 7 and 14. The estimated amplitude of variation during this time reached up to 6 mag in the B band. Significant inverse P Cygni profiles in high Balmer lines—evidence of mass accretion—dominated the spectrum obtained on January 8, when the brightness of DF Tau was approaching its maximum. The flux in the ultraviolet dropped dramatically on the following day. Ca II H and K emission lines, the indicator of chromospheric activity, turned into deep, wide absorption, and all other emission lines in the blue end were also absent (filled in or weak absorption), suggesting severe absorption in the line of sight possibly caused by a transient envelope produced in the process of a violent accretion shock. The effect of veiling decreased rapidly from January 10, and normal P Cygni profiles, indicating the presence of strong wind, began to appear. Based on the unique features displayed, we propose that the spectroscopic record of this flarelike brightening on DF Tau, believed to be due to episodic mass accretion accompanied by the release of gravitational energy, might actually be taken as an isolated, snapshot view of the YY Orionis phenomenon.

Self-similar gravitational collapse of a globular cluster is studied using the nonlinear kinetic standard model of stellar dynamics consisting of the Fokker-Planck-Vlasov transport equation coupled self-consistently to Poisson's equation for the Newtonian gravitational potential. It is shown rigorously that any locally integrable self-similar solution to these equations must approach a mass density profile ρ(r, t) ∝ r^-α, α = 3, in the final stage of the collapse. The discrepancy between the exact value α = 3 and previous results in the range 2 < α < 2.5 obtained from the orbit-averaged approximation to the kinetic model raises some questions about the validity of this popular approximation.

We have made high-resolution, high-sensitivity dynamic spectra of a sample of strong pulsars at 430 MHz with the Arecibo radio telescope. For four pulsars we find faint but sharply delineated features in the secondary spectra. These are examples of the previously observed "crisscross" or "multiple drift slope" phenomenon presumed to be due to multiple imaging of the pulsar by the interstellar medium (ISM). The unprecedented resolution and dynamic range of our observations allow a deeper level of analysis. Distances to the dominant scattering screen along the line of sight are determined and are shown to agree well with those inferred from other scintillation phenomena. Multiple imaging of the pulsar by the ISM is required. A compact central image surrounded by a faint scattering halo, roughly circularly symmetric, is consistent with the data. Scattering from filaments may also be consistent. The angular extent of the scattering material parallel to the direction of the pulsar velocity is ~5 mas, corresponding to a linear extent of ~2 AU. Further observations of these features should allow better discrimination between models and an identification of the scattering structures.

We present high time resolution observations of single pulses from the Vela pulsar (PSR B0833-45) made with a baseband recording system at observing frequencies of 660 and 1413 MHz. We have discovered two startling features in the 1413 MHz single-pulse data. The first is the presence of giant micropulses that are confined to the leading edge of the pulse profile. One of these pulses has a peak flux density in excess of 2500 Jy, more than 40 times the integrated pulse peak. The second new result is the presence of a large-amplitude Gaussian component on the trailing edge of the pulse profile. This component can exceed the main pulse in intensity but is switched on only relatively rarely. Fluctuation spectra reveal a possible periodicity in this feature of 140 pulse periods. Unlike the rest of the profile, this component has low net polarization and emits predominantly in the orthogonal mode. This feature appears to be unique to the Vela pulsar. We have also detected microstructure in the Vela pulsar for the first time. These same features are present in the 660 MHz data. We suggest that the full width of the Vela pulse profile might be as large as 10 ms but that the conal edges emit only rarely.

We have made a clear detection of excess power, providing strong evidence for solar-like oscillations in the G2 subgiant β Hyi. We observed this star over five nights with the University College London Echelle Spectrograph on the 3.9 m Anglo-Australian Telescope, using an iodine absorption cell as a velocity reference. The time series of 1196 velocity measurements shows an rms scatter of 3.30 m s^-1, and the mean noise level in the amplitude spectrum at frequencies above 0.5 mHz is 0.11 m s^-1. We see a clear excess of power centered at 1.0 mHz, with peak amplitudes of about 0.5 m s^-1, in agreement with expectations for this star. Fitting the asymptotic relation to the power spectrum indicates the most likely value for the large separation is 56.2 μHz, also in good agreement with the known properties of β Hyi.

Early data taken during commissioning of the Sloan Digital Sky Survey (SDSS) have resulted in the discovery of a very cool white dwarf. It appears to have stronger collision-induced absorption from molecular hydrogen than any other known white dwarf, suggesting it has a cooler temperature than any other. While its distance is presently unknown, it has a surprisingly small proper motion, making it unlikely to be a halo star. An analysis of white dwarf cooling times suggests that this object may be a low-mass star with a helium core. The SDSS imaging and spectroscopy also recovered LHS 3250, the coolest previously known white dwarf, indicating that the SDSS will be an effective tool for identifying these extreme objects.

Calculations are performed for the linear polarization of starlight due to extinction by aligned dust grains when the starlight traverses a medium with irregular magnetic fields. This medium is intended to represent the optically thick components of interstellar clouds that are observed to make little, if any, contribution to the polarization of starlight. In agreement with the observations, we find that the polarization properties of the starlight—the average fractional polarization and the dispersion in position angles—can be essentially unchanged. For this, the rms of the irregular component must be greater than the average magnetic field, which in turn tends to imply that the turbulent velocities in these interstellar clouds are super-Alfvénic.

One of the major outstanding problems in hot star wind theory is an understanding of the observed X-ray emissions from the early-type B, O, and Wolf-Rayet (WR) stars. The latest X-ray satellites Chandra and XMM-Newton are providing key new observations of resolved emission profiles to advance that understanding. This study presents a derivation of the expected emission-line profiles, assuming optically thin line emission and spherical symmetry, with a proper treatment of the attenuation of X-rays by the dense cool wind component. Examples of line profile variability for a narrow outflowing shell are presented. Then the case of embedded hot gas existing throughout the wind flow is discussed. It is shown that for the special case of constant expansion, emission profile shapes can be derived analytically, and in the limit of strong wind attenuation, the profile achieves a self-similar form. The results of this Letter provide a framework in which to model X-ray line profiles and analytic results to serve as a benchmark for more sophisticated numerical evaluations.

A survey with the Heinrich Hertz Submillimeter Telescope of HCN emission from mass-losing carbon stars has revealed masers in the J = 3-2 and 4-3 transitions of the (01^1c0) vibrational bending mode. These lines have not previously been known to show maser action. Five stars—R Scl, V384 Per, R Lep, Y CVn, and V Cyg—out of 12 observed were detected as masers. Allowing for evidence of variability, this detection rate suggests that these HCN lines are masers at least some of the time in the majority of mass-losing carbon stars. The line widths and velocities imply that the maser action occurs in gas close to the star, where the circumstellar envelope is just being accelerated outward.

Analysis of the current solar cycle 23 shows a greater increase in total solar irradiance (TSI) for the early phase of this cycle than expected from measurements of the total magnetic flux and traditional solar activity indices, which indicate that cycle 23 is weaker than cycle 22. In contrast, space observations of TSI from the Solar and Heliospheric Observatory/VIRGO and the Upper Atmospheric Research Satellite/ACRIMII show an increase in TSI of about 0.8-1.0 W m^-2 from solar minimum in 1996 to the end of 1999. This is comparable to the TSI increase measured by Nimbus 7/ERB from 1986 to 1989 during the previous cycle. Thus, solar radiative output near the maximum of the 11 yr cycle has been relatively constant despite a factor of 2 smaller amplitude increase for cycle 23 in sunspot and facular areas determined from ground-based observations. As a result, empirical models of TSI based on sunspot deficit and facular/network excess in cycle 22 underestimate the TSI measurements in 1999. This suggests either a problem in the observations or a change in the sources of radiative variability on the Sun.

Duvall's law is shown to be equivalent to the phase time-distance curve. The two are connected through a simple transformation. Thus, like Duvall's law, the phase time-distance curve can be analytically inverted to obtain the radial sound speed. The use of the phase information of the time-distance technique makes the inversion of the time-distance Duvall law particularly advantageous over the traditional inversion of Duvall's law. The radial sound speed can also be obtained numerically through the radial limit of the global or local tomography of the Sun. With global tomography, one can take advantage of the higher order skips of wave packets to obtain local tomograms of any section of the Sun, including the interior of the farside, that currently cannot be directly observed.

We report here on the discovery of supersonic Evershed downflows in the penumbra of a sunspot. These flows are shown to occur along spatially unresolved, very cold magnetic flux tubes whose downflowing footpoints are found from the middle penumbra outward. Evershed flows along magnetic field lines returning to the solar surface were discovered by Westendorp Plaza and coworkers, but only in the outer parts of the penumbra and beyond its visible boundary; on the other hand, no supersonic flows of any type have ever been reported in the photosphere of sunspots, except for the very different case of the delta spot analyzed by Martínez Pillet and coworkers. We present unequivocal evidence of such supersonic motions, already predicted theoretically by the siphon-flow model, from the interpretation of infrared spectropolarimetric observations of a sunspot with unprecedented spatial resolution.

We investigate the height of shock formation in coronal plumes for slow magnetosonic waves. The models take into account plume geometric spreading, heat conduction, and radiative damping. The wave parameters as well as the spreading functions of the plumes and the base magnetic field strength are given by empirical constraints mostly from the Solar and Heliospheric Observatory/Ultraviolet Coronagraph Spectrometer. Our models show that shock formation occurs at low coronal heights, i.e., within 1.3 R_☉, depending on the wave parameters. The shock formation is calculated using the well-established wave-breaking condition given by the intersection of C⁺ characteristics in the space-time plane. Our models show that shock heating by slow magnetosonic waves is expected to be relevant at most heights in solar coronal plumes, although slow magnetosonic waves are most likely not a solely operating energy supply mechanism.

Laboratory measurements of the interaction of low-energy, bare, and hydrogen-like ions with neutral gases are presented. The measurements demonstrate that charge-exchange-induced cometary K-shell X-ray spectra represent rich spectral diagnostics for determining the speed of the solar wind and the collision dynamics within the coma. We show that the K-shell spectrum observed from low-energy ion-neutral collisions (≤ 50 km s^-1) has a distinct high-energy component that is suppressed in high-energy collisions (≥800 km s^-1). As a result, the hardness ratio of the K-shell spectrum increases by as much as a factor of 4 as the ions decelerate in the coma. The change in spectral shape can be observed even with low-resolution energy dispersive solid-state detectors, opening the possibility of spatial imaging of the solar wind heavy-ion velocity profile in the coma. Our results clearly show that energy-dependent data are needed to fully describe charge-exchange-induced X-ray production in the heliosphere.