Table of contents

We present a general framework to treat the evolution of one-point probability distribution functions (PDFs) for cosmic density δ and velocity divergence fields θ. In particular, we derive an evolution equation for the one-point PDFs and consider the stochastic nature associated with these quantities. Under the local approximation that the evolution of cosmic fluid fields can be characterized by the Lagrangian local dynamics with finite degrees of freedom, the evolution equation for PDFs becomes a closed form, and consistent formal solutions are constructed. Adopting this local approximation, we explicitly evaluate the one-point PDFs P(δ) and P(θ) from the spherical and ellipsoidal collapse models as the representative Lagrangian local dynamics. In a Gaussian initial condition, while the local density PDF from the ellipsoidal model almost coincides with that of the spherical model, differences between spherical and ellipsoidal collapse models are found in the velocity divergence PDF. These behaviors have also been confirmed from the perturbative analysis of higher order moments. Importantly, the joint PDF of local density, P(δ, t; δ', t'), evaluated at the same Lagrangian position but at the different times t and t' from the ellipsoidal collapse model, exhibits a large amount of scatter. The mean relation between δ and δ' does fail to match the one-to-one mapping obtained from the spherical collapse model. Moreover, the joint PDF P(δ; θ) from the ellipsoidal collapse model shows a similar stochastic feature, both of which are indeed consistent with the recent result from N-body simulations. Hence, the local approximation with the ellipsoidal collapse model provides a simple but more physical model than the spherical collapse model of cosmological PDFs, consistent with the leading-order results of exact perturbation theory.

Exotic dark matter, together with the vacuum energy (associated with the cosmological constant), seems to dominate the universe. Thus, its direct detection is central to particle physics and cosmology. Supersymmetry provides a natural dark matter candidate, the lightest supersymmetric particle (LSP). One essential ingredient in obtaining the direct detection rates is the density and velocity distribution of the LSP. The detection rate is proportional to this density in our vicinity. Furthermore, since this rate is expected to be very low, one should explore the two characteristic signatures of the process, namely, the modulation effect, i.e., the dependence of the event rate on the Earth's motion, and the correlation of the directional rate with the motion of the Sun. Both of these crucially depend on the LSP velocity distribution. In the present paper we study simultaneously density profiles and velocity distributions based on the Eddington theory.

We show that the observed upper bound on the line-of-sight velocity dispersion of the stars in an early-type galaxy, σ_e ≲ 400 km s^-1, may have a simple dynamical origin within the ΛCDM cosmological model, under two main hypotheses. The first is that most of the stars now in the luminous parts of a giant elliptical galaxy formed at redshift z ≳ 6. Subsequently, the stars behaved dynamically just as an additional component of the dark matter. The second hypothesis is that the mass distribution characteristic of a newly formed dark matter halo forgets such details of the initial conditions as the stellar "collisionless matter" that was added to the dense parts of earlier generations of halos. We also assume that the stellar velocity dispersion does not evolve much at z ≲ 6, because a massive host halo grows mainly by the addition of material at large radii well away from the stellar core of the galaxy. These assumptions lead to a predicted number density of elliptical galaxies as a function of stellar velocity dispersion that is in promising agreement with the Sloan Digital Sky Survey data.

We conduct a comprehensive investigation of the detectability of the first stars and their enrichment signatures in galaxy clusters. As the initial mass function (IMF) of these Population III stars is unknown and likely to be biased to high masses, we base our study on analytical models that parameterize these uncertainties and allow us to make general statements. We show that the mean metallicity of outflows from Population III objects containing these stars is well above the critical transition metallicity (Z_cr ~ 10^-4Z_☉) that marks the formation of normal stars. Thus, the fraction of Population III objects formed as a function of redshift is heavily dependent on the distribution of metals and fairly independent of the mean metallicity of the universe, or the precise value of Z_cr. Using an analytic model of inhomogeneous structure formation, we study the evolution of Population III objects as a function of the star formation efficiency, IMF, and efficiency of outflow generation. For all models, Population III objects tend to be in the 10^6.5-10^7.0M_☉ mass range, just large enough to cool within a Hubble time, but small enough that they are not clustered near areas of previous star formation. Although the mean metallicity exceeds Z_cr at z ~ 15 in all models, the peak of Population III star formation occurs at z ~ 10, and such stars continue to form well into the observable range. We discuss the observational properties of these objects, some of which may have already been detected in ongoing surveys of high-redshift Lyα emitters. Finally, we combine our Population III distributions with the yield models of Heger & Woosley to study their impact on the intracluster medium (ICM) in galaxy clusters. We find that Population III stars can contribute no more than 20% of the iron observed in the ICM, but if they form with characteristic masses ~200-260 M_☉, their peculiar elemental yields help to reconcile theoretical models with the observed Fe and Si/Fe abundances. However, these stars tend to overproduce S/Fe and can account only for the O/Fe ratio in the inner regions of poor clusters. Additionally, the associated supernova heating falls far short of the observed level of ~1 keV per ICM gas particle. Thus, the properties of the first objects may be best determined by direct observation.

A prediction of standard inflationary cosmology is that the elemental composition of the medium out of which the earliest stars and galaxies condensed consisted primarily of hydrogen and helium ⁴He, with small admixtures of deuterium, lithium ⁷Li, and ³He. The most redshifted quasars, galaxies, and Lyα absorbers currently observed, however, all exhibit at least some admixture of heavier elements, as do the most ancient stars in the Galaxy. Here we examine ways in which the abundance of these same elements, if present before the epoch of Population III formation, might be observationally established or ruled out.

We discuss point-source foregrounds for the Wilkinson Microwave Anisotropy Probe (WMAP) experiment. We consider several possible strategies for removing them and we assess how the statistics of the cosmic microwave background (CMB) signal are affected by the residual sources. Assuming a power-law distribution for the point sources, we propose a method aimed at determining the slope of the distribution from the analysis of the moments of the observed maps. The same method allows for a determination of the underlying CMB variance. We conclude that the best strategy for finding point sources is the simultaneous thresholding of the filtered map at all frequencies, with a relatively low threshold. With this strategy, we expect to find 70% (95%) of the sources above 3 (4) σ. Assuming the most conservative case for point-source detection, the recovered slope of the point-source distribution is 2.55 ± 0.15, for a fiducial n = 2.5 value. The recovered CMB plus noise map variance is within 0.2% from the real one, with a standard deviation of 0.31%, while cosmic variance contributes 2.2% to the same CMB plus noise map.

We have simulated the interferometric observation of the cosmic microwave background (CMB) temperature and polarization fluctuations. We have constructed data pipelines from the time-ordered raw visibility samples to the CMB power spectra that utilize the methods of data compression, maximum likelihood analysis, and optimal subspace filtering. They are customized for three observational strategies: the single pointing, the mosaicking, and the drift-scanning. For each strategy, derived are the optimal strategy parameters that yield band power estimates with minimum uncertainty. The results are general and can be applied to any close-packed array on a single platform such as the CBI and the forthcoming AMiBA experiments. We have also studied the effect of rotation of the array platform on the band power correlation by simulating the CBI single-pointing observation. It is found that the band power anticorrelations can be reduced by rotating the platform and thus densely sampling the visibility plane. This enables us to increase the resolution of the power spectrum in the l-space down to the limit of the sampling theorem (Δl = 226 ≈ π/θ), which is narrower by a factor of about than the resolution limit (Δl ≈ 300) used in the recent CBI single-pointing observation. The validity of this idea is demonstrated for a two-element interferometer that samples visibilities uniformly in the uv-annulus. From the fact that the visibilities are the Fourier modes of the CMB field convolved with the beam, a fast unbiased estimator (FUE) of the CMB power spectra is developed and tested. It is shown that the FUE gives results very close to those from the quadratic estimator method without requiring large computer resources even though uncertainties in the results increase.

We report a detection of a galaxy-QSO cross-correlation w_GQ in the Sloan Digital Sky Survey early data release over 02-30' scales. We cross-correlate galaxy samples of different mean depths r' = 19-22 (_G = 0.15-0.35) with the main QSO population (i < 19.1) at_Q ≃ 1.6. We find significant positive correlation in all cases except for the faintest QSOs, as expected if the signal were due to weak-lensing magnification. The amplitude of the signal on arcminute scales is about 20% at_G = 0.15, decreasing to 10% at _G = 0.35. This is a few times larger than currently expected from weak lensing in the ΛCDM models but confirms, at a higher significance, previous measurements by several groups. When compared to the galaxy-galaxy correlation w_GG, a weak-lensing interpretation indicates a strong and steep nonlinear amplitude for the underlying matter fluctuations: σ ≃ 400 on scales of 0.2 Mpc h^-1, in contradiction with nonlinear modeling of ΛCDM fluctuations. We also detect a normalized skewness (galaxy-galaxy-QSO correlation) of S₃ ≃ 21 ± 6 at ≃ 0.15 (S₃ ≃ 14 ± 4 at ≃ 0.35), which is several standard deviations low compared to standard ΛCDM expectations. These observational trends can be reconciled with lensing in a flat Λ universe with σ₈ ≃ 1, provided that the linear spectrum is steeper (n ≃ 1) than in the ΛCDM model on small (cluster) scales. Under this interpretation, the galaxy distribution traces the matter variance with an amplitude that is 100 times smaller; i.e., galaxies are antibiased with b ≃ 0.1 on small scales, increasing to b ≃ 1 at ≃10 Mpc h^-1.

We present the observations of the gravitationally lensed system QSO 2237+0305 (Einstein Cross) performed with the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory on 2000 September 6 and on 2001 December 8 for 30.3 and 9.5 ks, respectively. Imaging analysis resolves the four X-ray images of the Einstein Cross. A possible fifth image is detected; however, the poor signal-to-noise ratio of this image combined with contamination produced by a nearby brighter image make this detection less certain. We investigate possible origins of the additional image. Fits to the combined spectrum of all images of the Einstein Cross assuming a simple power law with Galactic and intervening absorption at the lensing galaxy yields a photon index of 1.90 consistent with the range of Γ measured for large samples of radio-quiet quasars. For the first Chandra observation of the Einstein Cross this spectral model yields a 0.4-8.0 keV X-ray flux of 4.6 × 10^-13 ergs cm^-2 s^-1 and a 0.4-8.0 keV lensed luminosity of 1.0 × 10⁴⁶ ergs s^-1. The source exhibits variability over both long and short timescales. The X-ray flux has dropped by 20% between the two observations, and the Kolmogorov-Smirnov test showed that image A is variable at the 97% confidence level within the first observation. Furthermore, a possible time delay of 2.7 hr between images A and B with image A leading is detected in the first Chandra observation. The X-ray flux ratios of the images are consistent with the optical flux ratios that are affected by microlensing, suggesting that the X-ray emission is also microlensed. A comparison between our measured column densities and those inferred from extinction measurements suggests a higher dust-to-gas ratio in the lensing galaxy than the average value of our Galaxy. Finally, we report the detection at the 99.99% confidence level of a broad emission feature near the redshifted energy of the Fe Kα line in only the spectrum of image A. The rest frame energy, width, and equivalent width of this feature are E_line = 5.7 keV, σ_line = 0.87 keV, and EW = 1200 eV, respectively.

High-resolution optical (HIRES/Keck) and UV (STIS/Hubble Space Telescope) spectra, covering a large range of chemical transitions, are analyzed for three single-cloud weak Mg II absorption systems along the line of sight toward the quasar PG 1634+706. Weak Mg II absorption lines in quasar spectra trace metal-enriched environments that are rarely closely associated with the most luminous galaxies (>0.05L*). The two weak Mg II systems at z = 0.81 and 0.90 are constrained to have at least solar metallicity, while the metallicity of the z = 0.65 system is not as well constrained, but is consistent with more than 1/10 solar. These weak Mg II clouds are likely to be local pockets of high metallicity in a lower metallicity environment. All three systems have two phases of gas, a higher density region that produces narrower absorption lines for low-ionization transitions, such as Mg II, and a lower density region that produces broader absorption lines for high-ionization transitions, such as C IV. The C IV profile for one system (at z = 0.81) can be fitted with a single broad component (b ~ 10 km s^-1), but those for the other two systems require one or two additional offset high-ionization clouds. Two possible physical pictures for the phase structure are discussed: one with a low-ionization, denser phase embedded in a lower density surrounding medium and the other with the denser clumps surrounding more highly ionized gas.

Rotation measure observations of nine quasars, four BL Lacertae objects, and three radio galaxies are presented. The rest-frame rotation measures in the cores of the quasars and the jets of the radio galaxy M87 are several thousand rad m^-2. The BL Lac objects and the jets of the quasars have rest-frame rotation measures of a few hundred rad m^-2. A core rotation measure of 500 rad m^-2 in the rest frame is suggested as the dividing line between quasars and BL Lac objects. The substantial rotation measures of the BL Lac objects and quasars cast doubt on the previous polarization position angle investigations of these objects at frequencies of 15 GHz or less. BL Lac itself has a rotation measure that varies in time, similar to the behavior observed for the quasars 3C 273 and 3C 279. A simple model with magnetic fields of 40 μG or less can account for the observed rotation measures.

We investigate the color-magnitude distribution in the rich cluster AC 118 at z = 0.31. The sample is selected by the photometric redshift technique, allowing us to study a wide range of properties of stellar populations, and is complete in the K band, allowing us to study these properties up to a given galaxy mass. We use galaxy templates based on population synthesis models to translate the physical properties of the stellar populations—formation epoch, timescale of star formation, and metallicity—into observed magnitudes and colors. The distributions of galaxies in color-magnitude space thus map into distributions in physical parameter space. This is achieved by means of a statistical procedure that constrains the photometric properties of AC 118 galaxies to reproduce those of a nearby rich cluster once evolved at z ~ 0. In this way we show that a sharp luminosity-metallicity relation is inferred without any assumption on the galaxy formation scenario (either monolithic or hierarchical). Our data exclude significant differences in formation redshift along the color-magnitude relation and therefore confirm a pure metallicity interpretation for its origin, with an early (z ~ 5) formation epoch for the bulk of stellar populations. The dispersion in the color-magnitude diagram implies that fainter galaxies in our sample (K ~ 18) ceased to form stars as late as z ~ 0.5, in agreement with the picture that these galaxies were recently accreted into the cluster environment. The trend with redshift of the total stellar mass shows that half of the luminous mass in AC 118 was already formed at z ~ 2 and also that 20% of the stars formed at z < 1.

We present new N-body, hydrodynamical simulations of the interaction between the starburst galaxy NGC 7714 and its poststarburst companion NGC 7715, focusing on the formation of the collisional features, including (1) the gas-rich star-forming bridge, (2) the large gaseous loop (and stellar tails) to the west of the system, (3) the very extended H I tail to the west and north of NGC 7714, and (4) the partial stellar ring in NGC 7714. Our simulations confirm the results of earlier work that an off-center inclined collision between two disk galaxies is almost certainly responsible for the peculiar morphologies of this system. However, we have explored a wider set of initial galaxy and collisional encounter parameters than previously and have found a relatively narrow range of parameters that reproduce all the major morphologies of this system. The simulations suggest specific mechanisms for the development of several unusual structures. We find that the complex gas bridge has up to four distinct components, with gas contributed from two sides of NGC 7715, as well as from NGC 7714. The observed gas-star offset in this bridge is accounted for in the simulations by the dissipative evolution of the gas. The models suggest that the most recently formed gas bridge component from NGC 7715 is interacting with gas from an older component. This interaction may have stimulated the band of star formation on the north side of the bridge. The models also indicate that the low surface brightness H I tail to the far west of NGC 7714 is the end of the NGC 7715 countertail, curved behind the two galaxies. The sensitivity of the tidal structures to collision parameters is demonstrated by comparisons between models with slightly different parameter values. Comparison of model and observational (H I) kinematics provides an important check that the morphological matches are not merely fortuitous. Line-of-sight velocity and dispersion fields from the model are found to match those of the observations reasonably well at current resolutions. Spectral evolutionary models of the NGC 7714 core by Lançon et al. suggest the possibility of multiple starbursts in the last 300 Myr. Our hydrodynamic models suggest that bursts could be triggered by induced ringlike waves and a postcollision buildup of gas in the core of the galaxy.

We use the results of realistic N-body simulations to investigate the appearance of the white dwarf population in dense star clusters. We show that the presence of a substantial binary population in a star cluster, and the interaction of this population with the cluster environment, has serious consequences for the morphology of the observed white dwarf sequence and the derived white dwarf cooling age of the cluster. We find that over time the dynamical evolution of the cluster—mass segregation, stellar interactions, and tidal stripping—hampers the use of white dwarfs as tracers of the initial mass function and also leads to a significant enhancement of the white dwarf mass fraction. Future observations of star clusters should be conducted slightly interior to the half-mass radius of the cluster in order to best obtain information about the cluster age and initial mass function from the white dwarf luminosity function. The evolution of binary stars and the cluster environment must necessarily be accounted for when the white dwarf populations of dynamically evolved star clusters are studied.

Interferometers offer multiple methods for studying microlensing events and determining the properties of the lenses. We investigate the study of microlensing events with optical interferometers, focusing on narrow-angle astrometry, visibility, and closure phase. After introducing the basics of microlensing and interferometry, we derive expressions for the signals in each of these three channels. For various forecasts of the instrumental performance, we discuss which method provides the best means of measuring the lens angular Einstein radius θ_E, a prerequisite for determining the lens mass. If the upcoming generation of large-aperture, AO-corrected long-baseline interferometers (e.g., VLTI or Keck) perform as well as expected, θ_E could be determined with signal-to-noise ratios greater than 10 for all bright events. We estimate that ~5 events per year will be sufficiently bright and have long enough durations to allow the measurement of the lens mass and distance from the ground. We also consider the prospects for a VLTI survey of all bright lensing events using a Fisher matrix analysis, and find that even without individual masses, interesting constraints can be placed on the bulge mass function, although large numbers of events would be required.

We consider the effects of the corotation resonance of the Galactic spiral structure on the stellar orbits. It is shown that because of resonant interaction with the spiral gravitation field, stars can wander in the radial direction over a large part of the Galactic disk, moving over distances ~2-3 kpc in a short time, on the order of 1 billion years or even much less. This mechanism of radial stellar wandering is much faster than other stellar diffusion mechanisms that have been suggested in the literature. Corotation resonance also influences the Galactic distribution of heavy elements that are derived from old stellar-like objects. If at the initial time there is a simple linear distribution of metallicity in the disk, this is broken in ~3 billion years. In the framework of the model for the spiral density wave pattern with the corotation resonance close to the solar position (supposed to be 8.5 kpc from the center), the bimodal abundance pattern with a gradient in the inner part of the Galaxy (R ≤ 7.5 kpc) and a plateau for R between ~7.5 and about 10-11 kpc forms under the influence of the corotation resonance.

The spallative production rates of lithium, beryllium, and boron (LiBeB) are a necessary component in any calculation of the evolution of these nuclei in the Galaxy. Previous calculations of these rates relied on two assumptions relating to the nuclear physics aspects: the straight-ahead approximation that describes the distribution of fragment energies and the assumption that the major contributor to the production rate arises from single-step reactions between primary cosmic-ray projectiles and interstellar medium targets. We examine both assumptions by using a semiempirical description for the spall's energy distribution and by including the reactions that proceed via intermediary fragments. After relaxing the straight-ahead approximation, we find that the changes in the production rates and emerging fluxes are small and do not warrant rejection of this approximation. In contrast, we discover that two-step reactions can alter the production rate considerably, leading to noticeable increases in the efficiency of producing the LiBeB nuclei. Motivated by this result, we introduce a cascade technique to compute the production rates exactly and find that the results differ only slightly from those of our two-step calculations. We thus conclude that terminating the reaction network at the two-step order is sufficiently accurate for current studies of spallation.

We present a catalog of 2357 point sources detected during 590 ks of Chandra observations of the 17' × 17' field around Sgr A*. This field encompasses a physical area of 40 × 40 pc at a distance of 8 kpc. The completeness limit of the sample at the Galactic center is 10³¹ ergs s^-1 (2.0-8.0 keV), while the detection limit is an order of magnitude lower. The 281 sources detected below 1.5 keV are mainly in the foreground of the Galactic center, while comparisons to the Chandra deep fields at high Galactic latitudes suggest that only about 100 of the observed sources are background AGNs. The surface density of absorbed sources (not detected below 1.5 keV) falls off as 1/θ away from Sgr A*, in agreement with the distribution of stars in infrared surveys. This demonstrates the X-ray sources trace the general stellar population at the Galactic center. Point sources brighter than our completeness limit produce 10% of the flux previously attributed to diffuse emission. The log N- log S distribution of the Galactic center sources is extremely steep (power-law slope α = 1.7). If this distribution extends down to a flux of 10^-17 ergs cm^-2 s^-1 (10²⁹ ergs s^-1 at 8 kpc, 2.0-8.0 keV) with the same slope, then point sources would account for all of the previously reported diffuse emission. However, there are numerous diffuse, filamentary structures in the field that also contribute to the total flux, so the 2.0-8.0 keV luminosity distribution must flatten between 10²⁹ and 10³¹ ergs s^-1. Many types of stellar systems should be present in the field at the luminosities to which we are sensitive. However, the spectra of more than half of the Galactic center sources are very hard and can be described by a power law (E^-Γ) with photon index Γ < 1. Such hard spectra have been seen previously only from magnetically accreting white dwarfs (polars and intermediate polars) and wind-accreting neutron stars (pulsars), suggesting that there are large numbers of these systems in our field.

We present spectra of optical filaments associated with the X-ray knot D in the Vela supernova remnant. It has been suggested that knot D is formed by a bullet of supernova ejecta, that it is a breakout of the shock front of the Vela supernova remnant, and also that it is an outflow from the recently discovered remnant RX J0852.0-4622. We find that knot D is a bow shock propagating into an interstellar cloud with normal abundances and typical cloud densities (n_H ~ 4-11 cm^-3). Optical long-slit spectra show that the [S II] λλ6716, 6731 to Hα line ratio is greater than unity, proving that the optical filaments are shock excited. The analysis of far-ultraviolet spectra obtained with the Hopkins Ultraviolet Telescope and with the Far Ultraviolet Spectroscopic Explorer (FUSE) LWRS aperture show that slower shocks (~100 km s^-1) produce most of the low-ionization lines such as O III] λ1662, while faster shocks (~180 km s^-1) produce the O VI λλ1032, 1038 and other high-ionization lines. C III and O VI lines are also detected in the FUSE MDRS aperture, which was located on an X-ray-bright region away from the optical filaments. The lines have two velocity components consistent with ~150 km s^-1 shocks on the near and far sides of the knot. The driving pressure in the X-ray knot, P/k_B ~ 1.8 × 10⁷ cm^-3 K, is derived from the shock properties. This is over an order of magnitude larger than the characteristic X-ray pressure in the Vela supernova remnant. The velocity distribution of the emission and the overpressure support the idea that knot D is a bow shock around a bullet or cloud that originated near the center of the Vela remnant.

We found diffuse hard X-ray sources G11.0+0.0, G25.5+0.0, and G26.6-0.1 in the ASCA Galactic plane survey data. The X-ray spectra are featureless with no emission line and are fitted with both models of a thin thermal plasma in nonequilibrium ionization and a power-law function. The source distances are estimated to be 1-8 kpc, using the best-fit N_H values under the assumption that the mean density in the line of sight is 1 H cm^-3. The source sizes and luminosities are then 4.5-27 pc and (0.8-23) × 10³³ ergs s^-1. Although the source sizes are typical for supernova remnants (SNR) with young-to-intermediate ages, the X-ray luminosity, plasma temperature, and weak emission lines in the spectra are all unusual. This suggests that these objects are either shell-like SNRs dominated by X-ray synchrotron emission, like SN 1006 or, alternatively, plerionic SNRs. The total number of these classes of SNRs in our Galaxy is also estimated.

This paper reports the first study of the O VI resonance line emission (λλ1032, 1038) originating in the Local Bubble (or Local Hot Bubble) surrounding the solar neighborhood. In spite of the fact that O VI absorption within the Local Bubble has been observed, no resonance line emission was detected during our 230 ks Far Ultraviolet Spectroscopic Explorer observation of a "shadowing" filament in the southern Galactic hemisphere. As a result, tight 2 σ upper limits are set on the intensities in the 1032 and 1038 Å emission lines: 500 and 530 photons cm^-2 s^-1 sr^-1, respectively. These values place strict constraints on models and simulations. They suggest that the O VI-bearing plasma and the X-ray emissive plasma reside in distinct regions of the Local Bubble and are not mixed in a single plasma, whether in equilibrium with T ~ 10⁶ K or highly overionized with T ~ 4 to 6 × 10⁶ K. If the line of sight intersects multiple cool clouds within the Local Bubble, then the results also suggest that hot/cool transition zones differ from those in current simulations. With these intensity upper limits, we establish limits on the electron density, thermal pressure, path length, and cooling timescale of the O VI-bearing plasma in the Local Bubble. Furthermore, the intensity of O VI resonance line doublet photons originating in the Galactic thick disk and halo is determined (3500-4300 photons cm^-2 s^-1 sr^-1), and the electron density, thermal pressure, path length, and cooling timescale of its O VI-bearing plasma are calculated. The pressure in the Galactic halo's O VI-bearing plasma (3100-3800 K cm^-3) agrees with model predictions for the total pressure in the thick disk/lower halo. We also report the results of searches for the emission signatures of interstellar C I, C II, C III, N I, N II, N III, Mg II, Si II, S II, S III, S IV, S VI, Fe II, and Fe III.

Compact high-velocity clouds (CHVCs) are the most distant of the HVCs in the Local Group model, and, at d ~ 1 Mpc, they have H I volume densities of ~3 × 10^-4 cm^-3. Clouds with these volume densities and the observed column densities N ~ 10¹⁹ cm^-2 will be largely ionized, even if exposed only to the extragalactic ionizing radiation field. Here we examine the implications of this process for models of CHVCs. We have modeled the ionization structure of spherical clouds (with and without dark matter halos) for a large range of densities and sizes, appropriate to CHVCs over the range of suggested distances, exposed to an extragalactic ionizing photon flux ϕ_i ~ 10⁴ photons cm^-2 s^-1. Constant-density cloud models in which the CHVCs are at Local Group distances have total (ionized plus neutral) gas masses ~20-30 times larger than the neutral gas masses, implying that the gas mass alone of the observed population of CHVCs is ~4 × 10¹⁰M_☉. With a realistic (10 : 1) dark matter to gas mass ratio, the total mass in such CHVCs is a significant fraction of the dynamical mass of the Local Group, and their line widths would greatly exceed the observed ΔV. Self-consistent models of gas in dark matter halos fare even more poorly; they must lie within approximately 200 kpc of the Galaxy and (for a given distance) are much more massive than the corresponding uniform density models. We also show that exponential neutral hydrogen column density profiles are a natural consequence of an external source of ionizing photons and argue that these profiles cannot be used to derive model-independent distances to the CHVCs. These results argue strongly that the CHVCs are not cosmological objects and are instead associated with the Galactic halo.

The recent detections of a large population of faint submillimeter sources, an excess halo γ-ray background, and the extreme scattering events observed for extragalactic radio sources have been explained as being due to baryonic dark matter in the form of small, dark gas clouds. In this paper, we present the results of a search for the transient stellar obscurations such clouds are expected to cause. We examine the MACHO project light curves of 48 × 10⁶ stars toward the Galactic bulge, Large Magellanic Cloud, and Small Magellanic Cloud for the presence of dark cloud extinction events. We find no evidence for a population of dark gas clouds with A_V > 0.2 in the masses range from ~10^-4 to 2 × 10^-2M_☉, in either the Galactic disk or halo. However, it is possible that such dark cloud populations could exist if they are clustered in regions away from the observed lines of sight.

In the previous papers in this series, we found that radiative torques can play a major role in the alignment of grains with the interstellar magnetic field. Since the radiative torques can drive the grains to suprathermal rotational speeds, in previous work we made the simplifying assumption that the grain principal axis of greatest moment of inertia is always parallel to the grain angular momentum. This enabled us to describe many of the features of the grain dynamics. However, this assumption fails when the grains enter periods of thermal rotation, which occur naturally in the radiative torque alignment scenario. In the present paper we relax this assumption and explore the consequences for the grain dynamics. We develop a treatment to follow the grain dynamics, including thermal fluctuations and "thermal flipping," and show results for one illustrative example. By comparing with a treatment without thermal fluctuations, we see that inclusion of thermal fluctuations can lead to qualitative changes in the grain dynamics. In future installments in this series, we will use the more complete dynamical treatment developed here to perform a systematic study of grain alignment by radiative torques and test the model against observations of starlight polarization and polarized thermal emission from dust.

High-quality archival spectra of interstellar absorption from C I toward nine stars, taken with the Goddard High Resolution Spectrograph on the Hubble Space Telescope, were analyzed. Our sample was supplemented by two sight lines, 23 Ori and β¹ Sco, for which the C I measurements of Federman, Welty, & Cardelli were used. Directions with known CH⁺ absorption, but only upper limits on absorption from C₂ and CN, were considered for our study. This restriction allows us to focus on regions where CH⁺ chemistry dominates the production of carbon-bearing molecules. Profile synthesis of several multiplets yielded column densities and Doppler parameters for the C I fine-structure levels. Equilibrium excitation analyses, using the measured column densities as well as the temperature from H₂ excitation, led to values for gas density. These densities, in conjunction with measurements of CH, CH⁺, C₂, and CN column densities, provided estimates for the amount of CH associated with CH⁺ production, which in turn set up constraints on the present theories for CH⁺ formation in this environment. We found for our sample of interstellar clouds that on average 30%-40% of the CH originates from CH⁺ chemistry, and in some cases it can be as high as 90%. A simple chemical model for gas containing nonequilibrium production of CH⁺ was developed for the purpose of predicting column densities for CH, CO, HCO⁺, CH, and CH generated from large abundances of CH⁺. Again, our results suggest that nonthermal chemistry is necessary to account for the observed abundance of CH and probably that of CO in these clouds.

The Parker instability, which has been considered as a process governing the structure of the interstellar medium, is induced by the buoyancy of magnetic fields and cosmic rays. In previous studies, while the magnetic field has been fully incorporated in the context of isothermal magnetohydrodynamics, cosmic rays have normally been treated with the simplifying assumption of infinite diffusion along magnetic field lines but no diffusion across them. The cosmic-ray diffusion is, however, finite. In this work, we fully take into account the diffusion process of cosmic rays in a linear stability analysis of the Parker instability. Cosmic rays are described with the diffusion-convection equation. With realistic values of cosmic-ray diffusion coefficients expected in the interstellar medium, we show that the result of previous studies with the simplifying assumption about cosmic-ray diffusion applies well. The finiteness of the parallel diffusion decreases the growth rate of the Parker instability, while the relatively smaller perpendicular diffusion has no significant effect. We discuss the implication of our result on the role of the Parker instability in the interstellar medium.

We present new wide-field optical and ultraviolet images of the Pleiades reflection nebula that allow a more thorough evaluation of the dust scattering than any prior data set. Vacuum-UV images were taken at 1650 and 2200 Å during the first flight of the Wide-Field Imaging Survey Polarimeter (WISP), a sounding rocket-borne telescope. WISP captured the brighter parts of the nebula at both wavelengths, with 3 σ sensitivities of 22.5 and 23.4 UV mag arcsec^-2, respectively. The 50 × 17 WISP field was also mapped at 4400 Å with a mosaic of 40 Burrell Schmidt CCD frames using a broadband B_J filter. The Schmidt mosaic shows extensive and intricate nebulosity down to a 5 σ sensitivity limit of 27.6 B mag arcsec^-2, including features undetected by photographic surveys. We explore the intensity and color behavior of the nebula in our UV and optical images and far-infrared IRAS data. We find that the photometric structure near bright stars is more complex than previous studies have implied, but general trends are still apparent. The color gradients around the stars are caused by phase function effects rather than internal reddening. The greater concentration of scattered light versus thermal emission indicates that most of the observed scattering is from foreground dust. A somewhat greater concentration of UV versus optical light suggests grain scattering is more forward-directed at shorter wavelengths. The UV nebula is much fainter than expected from the stellar photometry and interstellar reddening. Explaining this UV faintness requires either more reddening than is measured or significant alterations to current dust property estimates.

We have used wide-field ultraviolet, optical, and far-infrared photometric images of Pleiades reflection nebulosity to analyze dust properties and the three-dimensional nebular geometry. Scattered light data were taken from 1650 and 2200 Å Wide-Field Imaging Survey Polarimeter images and a large 4400 Å mosaic of Burrell Schmidt CCD frames. Dust thermal emission maps were extracted from IRAS data. The scattering geometry analysis is complicated by the blending of light from many stars and the likely presence of more than one scattering layer. Despite these complications, we conclude that most of the scattered light comes from dust in front of the stars in at least two scattering layers, one far in front and extensive, the other nearer the stars and confined to areas of heavy nebulosity. The first layer can be approximated as an optically thin, foreground slab whose line-of-sight separation from the stars averages ~0.7 pc. The second layer is also optically thin in most locations and may lie at less than half the separation of the first layer, perhaps with some material among or behind the stars. The association of nebulosities peripheral to the main condensation around the brightest stars is not clear. Models with standard grain properties cannot account for the faintness of the scattered UV light relative to the optical. Some combination of significant changes in grain model albedo and phase function asymmetry values is required. Our best-performing model has a UV albedo of 0.22 ± 0.07 and a scattering asymmetry of 0.74 ± 0.06. Hypothetical optically thick dust clumps missed by interstellar sight line measurements have little effect on the nebular colors but might shift the interpretation of our derived scattering properties from individual grains to the bulk medium.

The ³P₁-³P₀ fine-structure line of the neutral carbon atom ([C I]) has been mapped over the 18 × 13 area of the L1688 cloud in the ρ Ophiuchi region with the Mount Fuji submillimeter-wave telescope. The ³P₂-³P₁ line of [C I] has also been observed toward two representative positions to evaluate the excitation temperature of the [C I] lines. The overall extent of the [C I] distribution generally resembles that of the ¹³CO distribution. The [C I] distribution has two major peaks; one (peak I) is at ρ Oph A, and the other (peak II) is toward the east side of the C¹⁸O core in the southern part of L1688. Peak II is located beyond the C¹⁸O core with respect to the exciting star HD 147889. The C⁰ column density is 5.0 × 10¹⁷ cm^-2 toward peak II. The spatial distribution of the [C I] emission is compared with plane-parallel photodissociation region (PDR) models, which suggest that peak II is associated with a lower density PDR front, adjacent to the dense cloud cores observed in the C¹⁸O line emission. Alternatively, peak II is in the early stage of chemical evolution, where C⁰ has not been completely converted to CO. In this case, the difference in the [C I] and C¹⁸O distributions represents an evolutionary sequence. This is consistent with a picture of a shock-compressed formation of the dense cores in this region due to influences from the Sco OB2 association.

We report results of continuum (1.3 and 3.6 cm) and H₂O maser line high angular resolution observations, made with the Very Large Array (VLA) in the A configuration, toward the star-forming region AFGL 2591. Three radio continuum sources (VLA 1, VLA 2, and VLA 3) were detected in the region at 3.6 cm, and one source (VLA 3) at 1.3 cm. VLA 1 and VLA 2 appear resolved and their spectral indices suggest free-free emission from optically thin H II regions. VLA 3 is elongated in the east-west direction, along the axis of the bipolar molecular outflow observed in the region. Its spectral energy distribution is consistent with it being a ~200 AU optically thick disk plus a photoionized wind. In addition, we detected 85 water maser spots toward the AFGL 2591 region, which are distributed in three main clusters. Two of these clusters are spatially associated with VLA 2 and VLA 3, respectively. The third cluster of masers, including the strongest water maser of the region, does not coincide with any known continuum source. We suggest that this third cluster of masers is excited by an undetected protostar that we predict to be located ≃05 (500 AU) north from VLA 3. The maser spots associated with VLA 3 are distributed along a shell-like structure of 001 size, showing a peculiar velocity-position helical distribution. We propose that VLA 3 is the powering source of the observed molecular outflow in this region. Finally, we support the notion that the AFGL 2591 region is a cluster of B0-B3 type stars.

We study the interaction between a dipolar magnetic field rooted in the central star and the circumstellar accretion disk in a classical T Tauri system. The MHD equations, including radiative energy transport, are solved for an axisymmetric system with a resistive, turbulent gas. A Shakura-Sunyaev-type eddy viscosity and a corresponding eddy magnetic diffusivity are assumed for the disk. The computations cover the disk and its halo in a radial interval from 1.7 to 20 stellar radii. The initial magnetic field configuration is unstable. Because of magnetocentrifugal forces caused by the rotational shear between star and disk, the magnetic field is stretched outward and part of the field lines open. For a solar-mass pre-main-sequence star and an accretion rate of 10^-7M_☉ yr^-1, a dipolar field of 1 kG (on the stellar surface) is not sufficient to disrupt the disk. The outer, slowly rotating parts of the disk become disconnected, and about 1/10 of the accretion flow is lost because of an outflow at midlatitudes. The critical field strength for the disruption of the disk lies between 1 and 10 kG. Outflows occur at midlatitudes, with mass fluxes of the order of 10% of the accretion rate of the disk. We find solutions in which the magnetic field tends to spin down the stellar rotation without disk disruption, but in these cases the accretion torque is dominant, and the star is still spun-up.

We have obtained Hubble Space Telescope (HST) WFPC2 images of the HV Tauri young triple system. The tertiary star appears as a compact bipolar nebula at visual wavelengths as already known in the near-infrared. New, deeper adaptive optics observations made at the Canada-France-Hawaii Telescope show no point source in the nebula to a limiting magnitude of K > 15. The results therefore confirm that HV Tau C is an optically thick circumstellar disk seen close to edge-on. Clear evidence for small, chromatic dust particles in the outer disk is provided by the color structure of the nebula: the thickness of the central dust lane shrinks by 30% between 0.55 and 2.2 μm. Bipolar jets extending 03-07 perpendicular to the dust lane are seen in HST narrowband [S II] and [O I] images. The continuum images are compared to multiple scattering models, with optimal density model parameters derived through χ² minimization. A disk density distribution provides a reasonable fit to the K-band image but is unable to reproduce the vertical extent of the nebula at I band without resorting to an unreasonably large scale height. Adding an envelope structure around the disk results in a much better fit to the HST image, and with a physically reasonable disk scale height. Our preferred model has a disk outer radius of 50 AU, inclination of 6°, and scale height of 6.5 AU at r = 50 AU. The thickness of the dark lane establishes a disk mass near 2 × 10^-3M_☉ (~2M_Jup) of dust and gas, if the dust grains have interstellar properties and remain fully mixed vertically. The envelope, with a much smaller mass ~4 × 10^-5M_☉, would be very short-lived unless replenished by new material from the star or surrounding medium.

We present medium-resolution 3 μm spectroscopy of the carbon-rich proto-planetary nebula IRAS 22272+5435. Spectroscopy with the Subaru Telescope adaptive optics system revealed a spatial variation of hydrocarbon molecules and dust surrounding the star. The rovibrational bands of acetylene (C₂H₂) and hydrogen cyanide (HCN) at 3.0 μm are evident in the central star spectra. The molecules are concentrated in the compact region near the center. The 3.3 and 3.4 μm emission of aromatic and aliphatic hydrocarbons is detected at 600-1300 AU from the central star. The separation of spatial distribution between gas and dust suggests that the small hydrocarbon molecules are indeed the source of solid material and that the gas left over from the grain formation is being observed near the central star. The intensity of aliphatic hydrocarbon emission relative to the aromatic hydrocarbon emission decreases with distance from the central star. The spectral variation is well matched to that of a laboratory analog thermally annealed with different temperatures. We suggest that either the thermal process after the formation of a grain or the variation in the temperature in the dust-forming region over time determines the chemical composition of the hydrocarbon dust around the proto-planetary nebula.

We report the detection of the 1665 and 1667 MHz main lines of OH (hydroxyl) and upper limits on the 1612 MHz satellite line of OH toward the carbon-rich AGB star IRC +10216. We find a beam-averaged fractional abundance x(OH) ~ 4 × 10^-8. This detection supports the identification by Melnick et al. of the 1₁₀-1₀₁ transition of water vapor with a 556.936 GHz rest-frequency emission feature detected toward IRC +10216, since OH is the expected photodissociation product of water vapor. The shape of the OH lines, however, differs significantly from the shape expected on the basis of the observations of Melnick et al. Possible explanations for the anomalous shapes of the 1665 and 1667 MHz lines are discussed. The most likely explanations for the unexpected OH line shapes are either masing or an asymmetric distribution of OH molecules around IRC +10216.

We revisit Chandra observations of planetary nebulae NGC 7027 and BD +30°3639 in order to address the question of abundance anomalies in the X-ray-emitting gas. Enhanced abundances relative to solar of magnesium (Mg) for NGC 7027 and neon (Ne) for BD +30°3639 are required to fit their X-ray spectra, whereas observations at optical and infrared wavelengths show depleted Mg and Ne in these systems. We attribute the enhancement of Mg in NGC 7027 in the X-ray, relative to the optical, to the depletion of Mg onto dust grains within the optical nebula. For BD +30°3639, we speculate that the highly enhanced Ne comes from a white dwarf companion, which accreted a fraction of the wind blown by the asymptotic giant branch progenitor and went through a novalike outburst that enriched the X-ray-emitting gas with Ne.

We describe a conservative, shock-capturing scheme for evolving the equations of general relativistic magnetohydrodynamics. The fluxes are calculated using the Harten, Lax, & van Leer scheme. A variant of constrained transport, proposed earlier by Tóth, is used to maintain a divergence-free magnetic field. Only the covariant form of the metric in a coordinate basis is required to specify the geometry. We describe code performance on a full suite of test problems in both special and general relativity. On smooth flows we show that it converges at second order. We conclude by showing some results from the evolution of a magnetized torus near a rotating black hole.

This paper describes the development and testing of a general relativistic magnetohydrodynamic (GRMHD) code to study ideal MHD in the fixed background of a Kerr black hole. The code is a direct extension of the hydrodynamic code of Hawley, Smarr, & Wilson and uses Evans & Hawley constrained transport (CT) to evolve the magnetic fields. Two categories of test cases were undertaken. A one-dimensional version of the code (Minkowski metric) was used to verify code performance in the special relativistic limit. The tests include Alfvén wave propagation, fast and slow magnetosonic shocks, rarefaction waves, and both relativistic and nonrelativistic shock tubes. A series of one- and two-dimensional tests were also carried out in the Kerr metric: magnetized Bondi inflow, a magnetized inflow test due to Gammie, and two-dimensional magnetized constant-l tori that are subject to the magnetorotational instability.

The magnetospheres of accreting neutron stars develop electrostatic gaps with huge potential drops. Protons and ions, accelerated in these gaps along the dipolar magnetic field lines to energies greater than 100 TeV, can impact onto the surrounding accretion disk. A proton-induced cascade develops, and charged pion decays produce ν emission. With extensive disk shower simulations using DPMJET and GEANT4, we have calculated the resulting ν spectrum. We show that the spectrum produced out of the proton beam is a power law. We use this result to propose accretion-powered X-ray binaries (with highly magnetized neutron stars) as a new population of pointlike ν sources for kilometer-scale detectors such as ICECUBE. As a particular example, we discuss the case of A0535+26. We show that ICECUBE should find A0535+26 to be a periodic ν source, one for which the formation and loss of its accretion disk can be fully detected. Finally, we comment briefly on the possibility that smaller telescopes such as AMANDA could also detect A0535+26 by folding observations with the orbital period.

We have made a multiwavelength study of the overlapping error boxes of the unidentified γ-ray sources TeV J2032+4130 and 3EG J2033+4118 in the direction of the Cygnus OB2 association (d = 1.7 kpc) in order to search for a point-source counterpart of the first unidentified TeV source. Optical identifications and spectroscopic classifications for the brighter X-ray sources in ROSAT PSPC and Chandra ACIS images are obtained, without finding a compelling counterpart. The classified X-ray sources are a mix of early- and late-type stars, with one exception. The brightest source in the Chandra observation is a new, hard absorbed source that is both transient and rapidly variable. It lies 7' from the centroid of the TeV emission, which places it outside of the claimed 2 σ location (r ≈ 48). A possible eclipse or "dip" transition is seen in its light curve. With a peak 1-10 keV luminosity of ≈7 × 10³²(d/1.7 kpc)² ergs s^-1, this source could be a quiescent low-mass X-ray binary that lies beyond the Cyg OB2 association. A coincident, reddened optical object of R = 20.4, J = 15.4, H = 14.2, and K = 13.4 is observed but not yet classified as a result of the lack of obvious emission or absorption features in its spectrum. Alternatively, this Chandra and optical source might be a considered a candidate for a "proton blazar," a long hypothesized type of radio-weak γ-ray source. More detailed observations will be needed to determine the nature of this variable X-ray source and to assess the possibility of its connection with TeV J2032+4130.

The pulsars B1534+12 and B1913+16 are two unique neutron star binaries exhibiting a wide range of relativistic phenomena that are impossible to detect in other systems. They constitute an exquisite observational ground on which theories can be tested. To date, the timing observations of B1534+12 and B1913+16 have been successfully used to test the strong field regime of relativistic gravity by measuring and then comparing with theory the evolution of the orbital elements of the pulsars. In this paper we develop a method that allows us to detect the timing signature of yet another relativistic phenomenon, the geodetic spin precession, and derive the misalignment angle between the orbital angular momentum and the spin vector of the pulsar, an important quantity that can be used to assess the degree of asymmetry of the supernova explosion that created the pulsar. Although we demonstrate that observations of PSR B1534+12—using the Penn State Pulsar Machine and the Mark III system—do not yet have a sufficient time span to detect precessional effects in the timing, we show that in about 10-25 years we will be able to get a good grasp on the misalignment angle of this pulsar. This may seem a long time to wait but in fact is typical for timing relativistic binary pulsars and, as in the case of PSR B1913+16, patient observing will eventually turn out to be very rewarding.

We detect a significant broadening in the wings of the 401 Hz peak in the power spectrum of the accreting millisecond binary pulsar SAX J1808.4-3658. This feature is consistent with the convolution of the red noise present in the power spectrum with the harmonic line. We conclude that the flux modulated by the spin period shows aperiodic variability similar to the red noise in the overall flux, suggesting that such variability also originates at the magnetic caps close to the neutron star surface. This is analogous to the results found in some longer period, higher magnetic field pulsators in high-mass X-ray binaries.

The region around the η Carinae Nebula has three OB associations, which contain a Wolf-Rayet star and several massive O3 stars. An early Chandra ACIS-I image was centered on η Car and includes Trumpler 16 and part of Trumpler 14. The Chandra image confirms the well-known result that O and very early B stars are X-ray sources with L_X ≃ 10^-7L_bol over an X-ray luminosity range of about 100. Two new, anomalously strong X-ray sources have been found among the hot star population: Tr 16-244, a heavily reddened O3 I star, and Tr 16-22, a heavily reddened O8.5 V star. Two stars have an unusually large L_X/L_bol: HD 93162, a Wolf-Rayet star (and possible binary), and Tr 16-22, a possible colliding-wind binary. In addition, a population of sources associated with cool stars is detected. In the color-magnitude diagram, these X-ray sources sit above the sequence of field stars in the Carina arm. The OB stars are on average more X-ray-luminous than the cool star X-ray sources. X-ray sources among A stars have X-ray luminosities similar to those of cooler stars and may be due to cooler companions. Upper limits are presented for B stars that are not detected in X-rays. These upper limits are also the upper limits for any cool companions that the hot stars may have. Hardness ratios are presented for the most luminous sources in bands 0.5-0.9, 0.9-1.5, and 1.5-2.04 keV. The available information on the binary nature of the hot stars is discussed, but binarity does not correlate with X-ray strength in a simple way.

We have used the Far Ultraviolet Spectroscopic Explorer to conduct a snapshot survey of O VI variability in the winds of 66 OB-type stars in the Galaxy and the Magellanic Clouds. These time series consist of two or three observations separated by intervals ranging from a few days to several months. Although these time series provide the bare minimum of information required to detect variations, this survey demonstrates that the O VI doublet in the winds of OB-type stars is variable on various scales in both time and velocity. For spectral types from O3 to B1, 64% vary in time. At spectral types later than B1, no wind variability is observed. In view of the limitations of this survey, this fraction represents a lower limit on the true incidence of variability in the O VI wind lines, which is very common and probably ubiquitous. In contrast, for S IV and P V, only a small percentage of the whole sample shows wind variations, although this may be principally due to selection effects. The observed variations extend over several hundreds of kilometers per second of the wind profile and can be strong. The width over which the wind O VI profile varies is only weakly correlated with the terminal velocity (v_∞), but a significant correlation (close to a 1 : 1 relationship) is derived between the maximum velocity of the variation and v_∞. High-velocity O VI wind absorption features (possibly related to the discrete absorption components seen in other wind lines) are also observed in 46% of the cases for spectral types from O3 to B0.5. These features are variable, but the nature of their propagation cannot be determined from this survey. If X-rays can produce sufficient O VI by Auger ionization of O IV and the X-rays originate from strong shocks in the wind, this study suggests that stronger shocks occur more frequently near v_∞, causing an enhancement of O VI near v_∞.

The processes of planet formation and migration depend intimately on the interaction between planetesimals and the gaseous disks in which they form. The formation of gaps in the disk can severely limit the mass of the planet and its migration toward the protostar. We investigate the process of gap formation through magnetohydrodynamic simulations in which internal stress arises self-consistently from turbulence generated by the magnetorotational instability. The simulations investigate three different planetary masses and two disk temperatures to bracket the tidal (thermal) and viscous gap opening conditions. The results are in general qualitative agreement with previous simulations of gap formation but show significant differences. In the presence of MHD turbulence, the gaps produced are shallower and asymmetrically wider than those produced with pure hydrodynamics. The rate of gap formation is also slowed, with accretion occurring across the developing gap. Viscous hydrodynamics does not adequately describe the evolution, however, because planets capable of producing gaps also may be capable of affecting the level of MHD turbulence in different regions of the disk.

We analyze the formation and migration of an already formed proto-Jovian companion embedded in a circumstellar disk. We use two-dimensional (r,θ) hydrodynamic simulations using a piecewise parabolic method code to model the evolutionary period in which the companion makes its transition from type I migration to type II migration. The results of our simulations show that spiral waves extending several wavelengths inward and outward from the planet are generated by the gravitational torque of the planet on the disk. Their effect on the planet causes it to migrate inward toward the star, and their effect on the disk causes it to form a deep (low surface density) gap near the planet. We study the sensitivity of the planet's migration rate to the planet's mass and to the disk's mass. Until a transition to slower type II migration, the migration rate of the planet is of order 1 AU per 10³ yr and varies by less than a factor of 2 with a factor of 20 change in planet mass, but it depends near linearly on the disk mass. Although the disk is stable to self-gravitating disk perturbations (Toomre Q > 5 everywhere), implying that the effects of gravity should be insignificant, migration is faster by a factor of 2 or more when disk self-gravity is suppressed. Migration is equally sensitive to the disk's mass distribution within 1-2 Hill radii of the planet, as demonstrated by our simulations' sensitivity to the planet's assumed gravitational softening parameter, which also crudely models the effect of the disk's extent into the third (z) dimension. Deep gaps form within ~500 yr after the beginning of the simulations, but migration can continue much longer: the formation of a deep gap and the onset of type II migration are not equivalent. The gap is several AU in width and displays very nearly the M proportionality predicted by theory. Beginning from an initially unperturbed 0.05 M_☉ disk, planets of mass M_pl > 0.3M_J can open a gap that is deep and wide enough to complete the transition to slower type II migration. Lower mass objects continue to migrate rapidly for the duration of the simulation, eventually impacting the inner boundary of our grid. This transition mass is much larger than that predicted as the "Shiva mass" discussed in Ward & Hahn, making the survival of forming planets even more precarious than they would predict.

We continue our numerical study of the migration of an already formed proto-Jovian companion embedded in a circumstellar disk. We first study the sensitivity of the planet's migration to its mass accretion rate, finding that the disk can supply a forming planet with mass at an essentially infinite rate (~1M_J per 25 yr) so that a gap could form very quickly via further dynamical interactions between the planet and remaining disk matter. The accreted matter has less orbital angular momentum than the planet and exerts an effective inward torque, so that inward migration is slightly accelerated. However, if a partial gap is formed prior to rapid accretion, the effective torque is small and its contribution to the migration is negligible. Although the disk can supply mass at a high rate, we show that mass accretion rates faster than ~10^-4M_J yr^-1 are not physically reasonable in the limit of either a thin, circumplanetary disk or of a spherical envelope. Planet growth and ultimately survival are therefore limited to the planet's ability to accept additional matter, not by the disk in which it resides. Large gravitational torques are produced both at Lindblad resonances and at corotation resonances. We compare the torques in our simulations to analytic theories at Lindblad resonances and find that common approximations to the theories predict torques that are a factor of ~10 or more larger than those obtained from the simulations. Accounting for the disk's vertical structure (crudely modeled in our simulations and the theory with a gravitational softening parameter) and small shifts in resonance positions due to pressure gradients, disk self-gravity, and inclusion of non-WKB terms in the analysis (Artymowicz) can reduce the difference to a factor of ~3-6 but do not account for the full discrepancy. Torques from the corotation resonances that are positive in sign, slowing the migration, contribute 20%-30% or more of the net torque on the planet, but are not well resolved and vary from simulation to simulation. A more precise accounting of the three-dimensional mass distribution and flow pattern near the planet will be required to accurately specify the torques from both types of resonances in the simulations. We show that the assumption of linearity underlying theoretical analyses of the interactions at Lindblad resonances is recovered in the simulations with planets with masses below 0.5M_J, but the assumption that interactions occur only at the resonances may be more difficult to support. Angular momentum transfer occurs over a region of finite width near both Lindblad and corotation resonances. The shape of the disk's response there (due, e.g., to local variations in epicyclic frequency) varies from pattern to pattern, making the true position of the resonance less clear. We speculate that the finite width allows for overlap and mixing between resonances and may be responsible for the remainder of the differences between torques from theory and simulation, but whether accounting for such overlap in a theory will improve the agreement with the simulations is not clear.

The exosolar planet HD 80606b has a highly eccentric (e = 0.93) and tight (a = 0.47 AU) orbit. We study how it might arrive at such an orbit and how it has avoided being tidally circularized until now. The presence of a stellar companion to the host star suggests the possibility that the Kozai mechanism and tidal dissipation combined to draw the planet inward well after it formed: Kozai oscillations produce periods of extreme eccentricity in the planet orbit, and the tidal dissipation that occurs during these periods of small pericenter distances leads to gradual orbital decay. We call this migration mechanism the "Kozai migration." It requires that the initial planet orbit be highly inclined relative to the binary orbit. For a companion at 1000 AU and an initial planet orbit at 5 AU, the minimum relative inclination required is ~85°. We discuss the efficiency of tidal dissipation inferred from the observations of exoplanets. Moreover, we investigate possible explanations for the velocity residual (after the motion induced by the planet is removed) observed on the host star: a second planet in the system is excluded over a large extent of semimajor axis space if Kozai migration is to work, and the tide raised on the star by HD 80606b is likely too small in amplitude. Last, we discuss the relevance of Kozai migration for other planetary systems.

Using a self-consistent atmosphere code, we construct a new model of the atmosphere of the transiting extrasolar giant planet HD 209458b to investigate the disparity between the observed strength of the sodium absorption feature at 589 nm and the predictions of previous models. For the atmospheric temperature-pressure profile we derive, silicate and iron clouds reside at a pressure of several millibars in the planet's atmosphere. These clouds have significant vertical extent and optical depth because of our slant viewing geometry and lead to increased absorption in bands directly adjacent to the sodium line core. Using a non-LTE sodium ionization model that includes photoionization by stellar UV flux, collisional processes with H₂, and radiative recombination, we show that the ionization depth in the planet's atmosphere reaches ~ mbar at the day/night terminator. Ionization leads to a slight weakening of the sodium feature. We present our baseline model, including ionization and clouds, which falls near the observational error bars. The sensitivity of our conclusions to the derived atmospheric temperature-pressure profile is discussed.

The Doppler dimming technique is used for the first time to study ultraviolet polar plumes in the height range of 1.05-1.35 R_☉, using observations from the spectrometer SUMER on the Solar and Heliospheric Observatory. It is found that, contrary to a number of published suggestions, outflow velocities in the plumes exceed those in the interplume regions. Plume velocities are in excess of 60 km s^-1 and are approximately constant throughout this height region. They tend to converge with the velocity of the accelerating interplume material at some height above our region of study. The analysis suggests that plume material makes a substantial contribution to the total line of sight, favoring either a "curtain" model for plumes or a chance alignment of a number of elementary cylindrical plumes. The intrinsic local density of plume material is some 20%-50% in excess of the interplume regions. Estimation of the total mass outflow indicates that approximately half of the fast solar wind at 1.1 R_☉ arises from plumes, with the remainder from interplume material. This result validates the published electron temperature profile of David et al. for the fast wind onset, which had been questioned over the suggestion that the flow velocity might be negligible in solar plumes.

In this paper we first present a series of pickup proton solar wind solutions following the fluid motion in the upwind direction to show that the wind speed V and temperature T, at a given r outside 10 AU, are primarily functions of the 1 AU wind speed V₀. This relationship is attributed to the accumulated effects of the pickup proton process on the heating and deceleration of the solar wind. Because pickup protons are expected to have similar effects on the solar wind at all latitudes in the upwind side of the heliosphere, in the second part of the paper, the two formulae V(r, V₀) and T(r, V₀) are extended to study the termination shock at 35° latitude. Wang and Sheeley have an empirical model for calculating the 1 AU wind speed V₀ from the observed photospheric field. We use the simulated wind speed V₀ to calculate V and T outside 60 AU following the fluid motion; then we can study the solar cycle variation of the termination shock. The shock location near 35° is unambiguously dependent on the solar cycle, with a period of approximately 1 solar cycle; the amplitude for variation of the shock location is greater than 50 AU. The new result supports the idea that the first encounter of Voyager 1 with the termination shock may occur during the declining phase of cycle 23. After the first encounter, the spacecraft will cross the shock two more times over a period of 8 years.

Magnetostatic solutions describing magnetic flux ropes in realistic geometry are used to study solar coronal structures observed to have sigmoidal forms in soft X-rays. These solutions are constructed by embedding a rope of helically symmetric force-free magnetic fields in an external field such that force balance is assured everywhere. The two observed sigmoidal shapes, the S shapes and the mirror-reflected S shapes referred to as Z shapes in this paper, are found in both hemispheres of the solar corona, but observations made over the last two solar cycles suggest that the Z and S shapes occur preferentially in the northern and southern solar hemispheres, respectively. Our study makes an identification of the sigmoidal high-temperature coronal plasmas with heating by the spontaneous formation of current sheets described by the theory of Parker. This process involves a tangential discontinuity developing across a ribbon-like, twisted flux surface through an interaction between a magnetic flux rope and the photosphere, under conditions of high electrical conductivity. In this identification, Z- and S-shaped sigmoids are associated with flux ropes with negative and positive magnetic helicities, respectively. This association is physically consistent with the conclusion, based independently on measurements of prominence magnetic fields, that magnetic flux ropes occur preferentially with negative and positive helicities in the northern and southern solar hemispheres, respectively.

NASA/National Solar Observatory Spectromagnetograph (SPM) data are compared with spacecraft measurements of total solar irradiance (TSI) variations for 8 yr beginning with the declining phase of solar cycle 22 and extending into the maximum of cycle 23. Previously reported conclusions based on a similar comparison for a shorter time period appear to be robust: three factors (sunspots, strong unipolar regions, and strong mixed-polarity regions) describe most of the variation in the SPM record, but only the first two are associated with TSI. Additionally, the residuals of a linear multiple regression of TSI against SPM observations over the entire 8 yr period show an unexplained, increasing, linear time variation with a rate of about 0.05 W m^-2 yr^-1. Separate regressions for the periods before and after 1996 January 1 show no unexplained trends but differ substantially in regression parameters. This behavior may reflect a solar source of TSI variations beyond sunspots or uncompensated nonsolar effects in one or both of the TSI and SPM data sets.

Sunspots appear on the Sun in two bands on either side of the equator that drift toward lower latitudes as each sunspot cycle progresses. We examine the drift of the centroid of the sunspot area toward the equator in each hemisphere from 1874 to 2002 and find that the drift rate slows as the centroid approaches the equator. We compare the drift rate at sunspot cycle maximum with the period of each cycle for each hemisphere and find a highly significant anticorrelation: hemispheres with faster drift rates have shorter periods. These observations are consistent with a meridional counterflow deep within the Sun as the primary driver of the migration toward the equator and the period associated with the sunspot cycle. We also find that the drift rate at maximum is significantly correlated with the amplitude of the following cycle, a prediction of dynamo models that employ a deep meridional flow toward the equator. Our results indicate an amplitude of about 1.2 m s^-1 for the meridional flow velocity at the base of the solar convection zone.

We propose a self-consistent and self-explanatory picture of the acceleration/collimation process of magnetohydrodynamic (MHD) outflows in the asymptotic domain. With the criticality and current-closure conditions properly taken into account in the global solutions, the most crucial step is the correct usage of the transfield force balance equation, which determines the curvature radius R of poloidal field lines. The sign of R determines the collimation or decollimation of each field streamline, and without referring to the current-closure condition, magnetic self-collimation cannot be discussed—the "anticollimation theorem." The location of the fast magnetosonic points are fixed at the innermost distances of the asymptotic domain, and hence, the asymptotic domain is nothing but the superfast, accelerating domain. The curvature 1/R has so far mistakenly been regarded as negligibly small, leading to the pseudo-force-free state in the far asymptotic domain and thereby producing the wrong concept that full MHD acceleration—meaning the conversion of all the Poynting flux into kinetic energy flux—is implausible. It cannot be stressed enough that this global picture has been reached by combining Sakurai's numerical results with Heyvaerts & Norman's analytic asymptotic formalism.

We report on the discovery of a candidate cluster of galaxies at redshift z = 1.47 based on Chandra observations in the field of quasars UM 425A and UM 425B. We detect with high significance diffuse emission due to the intracluster hot gas around the quasar pair. This is the second highest redshift cluster candidate after 3C 294 at z = 1.786. The diffuse emission is elliptical in shape with about 17'' extent. If indeed at z = 1.47, this corresponds to a physical size of 140 h kpc and 2-10 keV luminosity of ~3 × 10⁴³ ergs s^-1. The cluster is unlikely to be the long-sought gravitational lens invoked to explain the unusual brightness of UM 425A and the close quasar pair. Coexistence of the quasars with the cluster suggests a link of activity to cluster environment. The unusually high luminosity of UM 425A may then be due to a higher accretion rate. We also comment briefly on the X-ray spectra of UM 425A and UM 425B, both of which exhibit broad absorption line optical spectra. The present evidence suggests that the quasars are just a pair and not lensed images of the same quasar.

Pairs of radio-emitting jets with lengths up to several hundred kiloparsecs emanate from the central region (the "core") of radio-loud active galaxies. In the most powerful of them, these jets terminate in the "hot spots," compact high-brightness regions, where the jet flow collides with the intergalactic medium (IGM). Although it has long been established that in their inner (~1 pc) regions these jet flows are relativistic, it is still not clear if they remain so at their largest (hundreds of kiloparsecs) scales. We argue that the X-ray, optical, and radio data of the hot spots, despite their at-first-sight disparate properties, can be unified in a scheme involving a relativistic flow upstream of the hot spot that decelerates to the subrelativistic speed of its inferred advance through the IGM and viewed at different angles to its direction of motion. This scheme, besides providing an account of the hot spot spectral properties with jet orientation, also suggests that the large-scale jets remain relativistic all the way to the hot spots.

We report a change in the trajectory of a well-studied jet component of the quasar 3C 279. The component changes in apparent projected speed and direction, and we find it to be moving with a Lorentz factor γ ≳ 15 at an initial angle of ≲1° to the line of sight. The new trajectory of the component has approximately the same speed and direction as an earlier superluminal feature, originally seen in the early 1970s. The new direction for the component is also much better aligned with larger scale Very Large Baseline Array and Very Large Array structure out to 01. We suggest that the trajectory change is a collimation event occurring at ≳1 kpc (deprojected) along the jet. While the change in trajectory on the sky appears to be 26°, the intrinsic change is ≲1°. We estimate the Doppler factor prior to the change in direction to be δ ≳ 28 and after the change to be δ ≳ 23. A comparison with independent constraints on the Doppler factor suggests that the energy in the radiating particles cannot greatly exceed the energy in the magnetic field unless the volume filling factor is very much less than 1.

In this Letter, we present evidence suggesting that the absence or presence of hidden broad-line regions (HBLRs) in Seyfert 2 galaxies is regulated by the rate at which matter accretes onto a central supermassive black hole, in units of the Eddington rate. Evidence is based on data from a subsample of type 2 active galactic nuclei extracted from the Tran spectropolarimetric sample and made up of all those sources that also have good-quality X-ray spectra available and for which a bulge luminosity can be estimated. We use the intrinsic (i.e., unabsorbed) X-ray luminosities of these sources and their black hole masses (estimated by using the well-known relationship between nuclear mass and bulge luminosity in galaxies) to derive the nuclear accretion rate in Eddington units. We find that virtually all HBLR sources have accretion rates larger than a threshold value of _thres ≃ 10^-3 (in Eddington units), while non-HBLR sources lie at ≲ _thres. These data nicely fit predictions from a model proposed by Nicastro in which the broad-line regions (BLRs) are formed by accretion disk instabilities occurring in proximity of the critical radius at which the disk changes from gas pressure dominated to radiation pressure dominated. This radius diminishes with decreasing ; for low enough accretion rates (and therefore luminosities), the critical radius becomes smaller than the innermost stable orbit and BLRs cannot form.

We have studied the average 3-200 keV spectra of Seyfert galaxies of type 1 and 2, using data obtained with BeppoSAX. The average Seyfert 1 spectrum is well fitted by a power-law continuum with photon spectral index Γ ~ 1.9, a Compton reflection component R ~ 0.6-1 (depending on the inclination angle between the line of sight and the reflecting material), and a high-energy cutoff at around 200 keV; there is also an iron line at 6.4 keV characterized by an equivalent width of 120 eV. Seyfert 2 galaxies, on the other hand, show stronger neutral absorption [N_H = × 10²² atoms cm^-2], as expected, but are also characterized by an X-ray power law that is substantially harder (Γ ~ 1.75) and with a cutoff at lower energies (E_c ~ 130 keV); the iron line parameters are instead substantially similar to those measured in type 1 objects. There are only two possible solutions to this problem: to assume more reflection in Seyfert 2 galaxies than observed in Seyfert 1 galaxies or more complex absorption than estimated in the first instance. The first possibility is ruled out by the Seyfert 2 to Seyfert 1 ratio, while the second provides an average Seyfert 2 intrinsic spectrum very similar to that of the Seyfert 1. The extra absorber is likely an artifact due to summing spectra with different amounts of absorption, although we cannot exclude its presence in at least some individual sources. Our result argues strongly for a very similar central engine in both types of galaxies, as expected under the unified theory.

We present new accurate near-infrared (NIR) spheroid (bulge) structural parameters obtained by a two-dimensional image analysis of all galaxies with a direct black hole (BH) mass determination. As expected, NIR bulge luminosities L_bul and BH masses are tightly correlated, and if we consider only those galaxies with a secure BH mass measurement and an accurate L_bul (27 objects), the spread of M_BH-L_bul is similar to M_BH-σ_e, where σ_e is the effective stellar velocity dispersion. We find an intrinsic rms scatter of ≃0.3 dex in log M_BH. By combining the bulge effective radii R_e measured in our analysis with σ_e, we find a tight linear correlation (rms ≃ 0.25 dex) between M_BH and the virial bulge mass (∝R_eσ), with ⟨M_BH/M_bul⟩ ~ 0.002. A partial correlation analysis shows that M_BH depends on both σ_e and R_e and that both variables are necessary to drive the correlations between M_BH and other bulge properties.

We present a comparison between the observational data on the kinematical structure of G1 in M31, obtained with the Hubble Space Telescope Wide Field Planetary Camera 2 and Space Telescope Imaging Spectrograph instruments, and the results of dynamical simulations carried out using the special purpose computer GRAPE-6. We have obtained good fits for models starting from single-cluster King model initial conditions and even better fits when starting our simulations with a dynamically constructed merger product of two star clusters. In the latter case, the results from our simulations are in excellent agreement with the observed profiles of luminosity, velocity dispersion, rotation, and ellipticity. We obtain a mass-to-light ratio of M/L = 4.0 ± 0.4 and a total cluster mass of M = (8 ± 1) × 10⁶M_☉. Given that our dynamical model can fit all available observational data very well, there seems to be no need to invoke the presence of an intermediate-mass black hole in the center of G1.

We explore an accretion origin for ω Cen by N-body modeling of the orbital decay and disruption of a Milky Way dwarf satellite. This work is focused on studying a particular satellite model that aims to reproduce the present orbit of ω Cen, as recently determined from absolute proper motions. The model satellite is launched from 58 kpc from the Galactic center, on a radial low-inclination orbit. We find that a capture scenario can produce an ω Cen-like object with the current low-energy orbit of the cluster. Our best model is a nucleated dwarf galaxy with a Hernquist density profile that has a mass of 8 × 10⁹M_☉ and a half-mass radius of 1.4 kpc.

We have analyzed Rossi X-Ray Timing Explorer data of the neutron star transient Aquila X-1 obtained during its outbursts in 1999 May/June and 2000 September/October. We find that in the early rise of these outbursts, a hard flare in the energy range above 15 keV preceded the soft X-ray peak. The hard X-ray flux of the hard flares at maximum was more than a factor of 3 stronger than at any other point in the outbursts. The rise of the hard X-ray flare to this maximum was consistent with a monotonically brightening low-/hard-state spectrum. After the peak of the hard flare, a sharp spectral transition occurred with spectral pivoting in the range 8-12 keV. Our timing analysis shows that during the hard flare, the power spectra were composed mainly of band-limited noise and a ~1-20 Hz quasi-periodic oscillation (QPO), which correlate in frequency. Immediately after the hard flare, the power spectra turned into power-law noise. The spectral and timing properties during and after the hard flares are very similar to those in black hole transients during the early rise of an outburst. We suggest that these hard flares and spectral transitions in Aql X-1 are of the same origin as those observed in black hole transients. This leads to the association of the 1-20 Hz QPOs and band-limited noise in Aql X-1 with those in black hole transients. We discuss the impact of this discovery on our understanding of soft X-ray transient outbursts, state transitions, and variability in X-ray binaries.

We estimate the observed distribution of chirp masses of compact object binaries for gravitational-wave detectors. The stellar binary evolution is modeled using the StarTrack population synthesis code. The distribution of the predicted "observed" chirp masses varies with the variation of the different parameters describing stellar binary evolution. We estimate the sensitivity of the observed distribution to the variation of these parameters, and we show which of the parameters can be constrained after observing 20, 100, and 500 compact object mergers. As a general feature of all our models, we find that the population of observed binaries is dominated by the double black hole mergers.

We present a detailed study of the Hα and He I spectral features of COM J1740-5340 (the companion to PSR J1740-5340 in the Galactic globular cluster NGC 6397), exploiting a series of high-resolution spectra obtained at different orbital phases. The Hα absorption line shows a complex two-component structure, revealing that optically thin hydrogen gas resides outside the Roche lobe of COM J1740-5340. The line morphology precludes the existence of any residual disk around the millisecond pulsar and suggests the presence of a stream of material going from the companion toward the neutron star. This material never reaches the neutron star surface, being driven back by the pulsar radiation far beyond COM J1740-5340. By analyzing the He I absorption lines as a function of orbital phase, we infer the presence of an overheated longitudinal strip (about 150 times narrower than it is long) on the COM J1740-5340 surface facing the radio pulsar.

We present subarcsecond J, H, and K_s images (FWHM ~ 05) of an unbiased 5' × 5' (16 × 16 pc) survey of the densest region of the W49 giant molecular cloud. The observations reveal four massive stellar clusters (with stars as massive as ~120 M_☉), the largest (cluster 1) about 3 pc east of the well-known Welch ring of ultracompact H II regions. Cluster 1 is (i) extincted by at least A_V > 20 mag of foreground (unrelated and local) extinction, (ii) has more than 30 mag of internal inhomogeneous extinction, implying that it is still deeply buried in its parental molecular cloud, and (iii) is powering a 6 pc diameter giant H II region seen at both the near-infrared and the radio continuum. We also identify the exciting sources of several ultracompact H II regions. The census of massive stars in W49A agrees or is slightly overabundant when compared with the number of Lyman continuum photons derived from radio observations. We argue that although the formation of the Welch ring could have been triggered by cluster 1, the entire W49A starburst region seems to have been multiseeded instead of resulting from a coherent trigger.

We report the discovery of a nearby star with a very large proper motion of 505 ± 003 yr^-1. The star is called SO 025300.5+165258 and referred to herein as a high proper motion star (HPMS). The discovery came as a result of a search of the SkyMorph database, a sensitive and persistent survey that is well suited for finding stars with high proper motions. There are currently only seven known stars with proper motions greater than 5'' yr^-1. The spectrum and measured tangential velocity indicate that the HPMS is a main-sequence star with spectral type M6.5. Trigonometric and photometric parallaxes have been determined, yielding distance estimates of 2.4 pc (lower limit) and 3.6 ± 0.4 pc, respectively. If the former is correct, the HPMS ranks third in the list of nearest stellar systems. If the latter is correct, it is 17th. A more precise trigonometric parallax measurement is expected to be completed near the end of the year.

Observations of the central radian of the Galaxy by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) have yielded a high-resolution measurement of the 1809 keV line from ²⁶Al, detected at 11 σ significance in 9 months of data. The RHESSI result for the width of the cosmic line is 2.03 keV FWHM. The best-fit line width of 5.4 keV FWHM reported by Naya et al. using the Gamma-Ray Imaging Spectrometer balloon instrument is rejected with high confidence.