Table of contents

We report on the strength of the seed magnetic flux of the accretion disk surrounding Population III stars. Themagnetic field in accretion disks might play an important role in the transport of angular momentum because of the turbulence induced by magnetorotational instability (MRI). On the other hand, since the primordial star-forming clouds contain no heavy elements or grains, they experience a very different thermal history and magnetic-field-dissipation history in the course of their gravitational contraction from those in the present-day star-forming molecular clouds. In order to assess the magnetic field strength in the accretion disk of Population III stars, we calculate the thermal history of the primordial collapsing clouds and investigate the coupling of the magnetic field with the primordial gas. As a result, we find that the magnetic field strongly couples with the primordial gas cloud throughout the collapse, i.e., the magnetic field is frozen to the gas, as long as the initial field strength satisfies B ≲ 10^-5(N_H/10³ cm^-3)^0.55 G.

We show that various milestones of high-redshift galaxy formation, such as the formation of the first stars or the complete reionization of the intergalactic medium, occurred at different times in different regions of the universe. The predicted spread in redshift, caused by large-scale fluctuations in the number density of galaxies, is at least an order of magnitude larger than previous expectations, which argued for a sharp end to reionization. This cosmic scatter in the abundance of galaxies introduces new features that affect the nature of reionization and the expectations for future probes of reionization and may help explain the present properties of dwarf galaxies in different environments. The predictions can be tested by future numerical simulations and may be verified by upcoming observations. Current simulations, limited to relatively small volumes and periodic boundary conditions, largely omit cosmic scatter and its consequences. In particular, they artificially produce a sudden end to reionization, and they underestimate the number of galaxies by up to an order of magnitude at redshift 20.

Hierarchical cold dark matter (CDM) models predict that Milky Way-sized halos contain several hundred dense low-mass dark matter satellites (the substructure), an order of magnitude more than the number of observed satellites in the Local Group. If the CDM paradigm is correct, this prediction implies that the Milky Way and Andromeda are filled with numerous dark halos. To understand why these halos failed to form stars and become galaxies, we need to understand their history. We analyze the dynamical evolution of the substructure halos in a high-resolution cosmological simulation of Milky Way-sized halos in the ΛCDM cosmology. We find that about 10% of the substructure halos with the present masses ≲10⁸-10⁹M_☉ (circular velocities V_m ≲ 30 km s^-1) had considerably larger masses and circular velocities when they formed at redshifts z ≳ 2. After the initial period of mass accretion in isolation, these objects experience dramatic mass loss because of tidal stripping. Our analysis shows that strong tidal interaction is often caused by actively merging massive neighboring halos, even before the satellites are accreted by their host halo. These results can explain how the smallest dwarf spheroidal galaxies of the Local Group were able to build up a sizable stellar mass in their seemingly shallow potential wells. We propose a new model in which all the luminous dwarf spheroidals in the Local Group are descendants of the relatively massive (≳10⁹M_☉) high-redshift systems, in which the gas could cool efficiently by atomic line emission, and which were not significantly affected by the extragalactic ultraviolet radiation. We present a simple galaxy formation model based on the trajectories extracted from the simulation, which accounts for the bursts of star formation after strong tidal shocks and the inefficiency of gas cooling in halos with virial temperatures T_vir ≲ 10⁴ K. Our model reproduces the abundance, spatial distribution, and morphological segregation of the observed Galactic satellites. The results are insensitive to the redshift of reionization.

Two years of microwave background observations with the Cosmic Background Imager (CBI) have been combined to give a sensitive, high-resolution angular power spectrum over the range 400 < l < 3500. This power spectrum has been referenced to a more accurate overall calibration derived from the Wilkinson Microwave Anisotropy Probe (WMAP). The data cover 90 deg ², including three pointings targeted for deep observations. The uncertainty on the l > 2000 power previously seen with the CBI is reduced. Under the assumption that any signal in excess of the primary anisotropy is due to a secondary Sunyaev-Zeldovich anisotropy in distant galaxy clusters, we use CBI, Arcminute Cosmology Bolometer Array Receiver, and Berkeley-Illinois-Maryland Association array data to place a constraint on the present-day rms mass fluctuation on 8 h^-1 Mpc scales, σ₈. We present the results of a cosmological parameter analysis on the l < 2000 primary anisotropy data that show significant improvements in the parameters as compared to WMAP alone, and we explore the role of the small-scale cosmic microwave background data in breaking parameter degeneracies.

We present our results on the cross-correlation of Lyman break galaxies (LBGs) around three damped Lyα absorbers (DLAs) at z_abs ≃ 3 from deep [μ_I,AB(sky) ≃ 27.6 mag arcsec^-2] UBVI KPNO 4 m/MOSAIC images. The large area of the MOSAIC images, 0.31 deg² or ~65 × 65 h Mpc comoving at redshift z = 3, allows us to probe the clustering of LBGs on scales up to 20 Mpc comoving. Our survey covers a total of 1 deg² and contains ~3000 LBGs with photometric redshifts between 2.8 and 3.5. Using the redshift likelihood distributions with m_I as a prior, we selected LBGs within a redshift slice of width W_z = 0.15 (corresponding to σ_z, the uncertainty in photometric redshifts) centered on the redshift of the absorbers. Within that redshift slice, we find that the DLA-LBG cross-correlation w_dg is w_dg = (1.62 ± 1.32) × w_gg, where w_gg is the LBG autocorrelation. This corresponds to a correlation length of r₀ = 5 ± 4.5 h^-1 (comoving) (or r₀ = 7 ± 6.8 h Mpc). The cross-correlation is most significant on scales 5-10 Mpc. Through Monte Carlo simulations, we find that w_dg is significantly greater than zero at the greater than 95% level. In three other redshift slices that do not contain a DLA, we do not find any evidence of clustering. A larger sample will enable us to discriminate between w_dg/w_gg < 1 and w_dg/w_gg > 1, i.e., to test whether DLA halos are more or less massive than LBG halos.

We measure the two-point correlation function ξ(r_p, π) in a sample of 2219 galaxies between z = 0.7 and 1.35 to a magnitude limit of R_AB = 24.1 from the first season of the DEEP2 Galaxy Redshift Survey. From ξ(r_p, π) we recover the real-space correlation function, ξ(r), which we find can be approximated within the errors by a power law, ξ(r) = (r/r₀)^-γ, on scales ~0.1-10 h^-1 Mpc. In a sample with an effective redshift of z_eff = 0.82, for a ΛCDM cosmology we find r₀ = 3.53 ± 0.81 h^-1 Mpc (comoving) and γ = 1.66 ± 0.12, while in a higher redshift sample with z_eff = 1.14 we find r₀ = 3.12 ± 0.72 h^-1 Mpc and γ = 1.66 ± 0.12. These errors are estimated from mock galaxy catalogs and are dominated by the cosmic variance present in the current data sample. We find that red, absorption-dominated, passively evolving galaxies have a larger clustering scale length, r₀, than blue, emission-line, actively star-forming galaxies. Intrinsically brighter galaxies also cluster more strongly than fainter galaxies at z ≃ 1. Our results imply that the DEEP2 galaxies have an effective bias b = 0.96 ± 0.13 if σ_8DM = 1 today or b = 1.19 ± 0.16 if σ_8DM = 0.8 today. This bias is lower than that predicted by semianalytic simulations at z ≃ 1, which may be the result of our R-band target selection. We discuss possible evolutionary effects within our survey volume, and we compare our results with galaxy-clustering studies at other redshifts, noting that our star-forming sample at z ≃ 1 has selection criteria very similar to the Lyman break galaxies at z ≃ 3 and that our red, absorption-line sample displays a clustering strength comparable to the expected clustering of the Lyman break galaxy descendants at z ≃ 1. Our results demonstrate that galaxy-clustering properties as a function of color, spectral type, and luminosity seen in the local universe were largely in place by z ≃ 1.

We report the results of a 15 GHz (2 cm) multiepoch Very Long Baseline Array (VLBA) program, begun in 1994 to study the outflow in radio jets ejected from quasars and active galaxies. The observed flow of 208 distinct features measured in 110 quasars, active galaxies, and BL Lac objects shows highly collimated relativistic motion with apparent transverse velocities typically between zero and about 15c, with a tail extending up to about 34c. Within individual jets, different features appear to move with a similar characteristic velocity that can represent an underlying continuous jet flow, but we also see some stationary and even apparently inward-moving features that coexist with the main features. Comparison of our observations with published data at other wavelengths suggests that there is a systematic decrease in apparent velocity with increasing wavelength, probably because the observations at different wavelengths sample different parts of the jet structure.

The observed distribution of linear velocities is not consistent with any simple ballistic model. Either there is a rather broad range of Lorentz factors, a significant difference between the velocity of the bulk relativistic flow and the pattern speed of underlying shocks, or a combination of these options. Assuming a ballistic flow, comparison of observed apparent velocities and Doppler factors computed from the timescale of flux density variations is consistent with a steep power-law distribution of intrinsic Lorentz factors, an isotropic distribution of orientations of the parent population, and intrinsic brightness temperatures about an order of magnitude below the canonical inverse Compton limit. It appears that the parent population of radio jets is not dominated by highly relativistic flows, and contrary to the assumption of simple unified models, not all sources have intrinsic speeds close to c.

Usually, the observed jet flow is in the general direction of an established jet. However, many jets show significant bends and twists, where the observed motions are nonradial but are aligned with the local jet direction, suggesting that the jet flow occurs along preexisting bent channels. In a few cases we have observed a clear change in the direction of a feature as it flows along the jet. Radio jets that are also strong gamma-ray sources detected by EGRET appear to have significantly faster speeds than the non-EGRET sources, consistent with the idea that gamma-ray sources have larger Doppler factors than non-gamma-ray sources. Sources at high redshift have systematically lower angular speeds than low-redshift jets, consistent with standard cosmologies.

Supplementing existing survey data with Very Large Array (VLA) observations, we have extended γ-ray counterpart identifications down to decl. = -40° using our "figure-of-merit" approach. We find blazar counterparts for ~70% of EGRET sources above decl. = -40° away from the Galaxy. Spectroscopic confirmation is in progress, and spectra for ~24 sources are presented here. We find evidence that increased exposure in the bulge region allowed EGRET to detect relatively faint blazars; a clear excess of nonblazar objects in this region, however, argues for an additional (new) source class.

BL Lacertae was the target of an extensive multiwavelength monitoring campaign in the second half of 2000. The campaign had revealed optical and X-ray intraday variability on timescales of ~1.5 hr and evidence for significant spectral variability both at optical and X-ray frequencies. During the campaign, BL Lacertae was observed in two different activity states: a quiescent state with relatively low levels of optical and X-ray fluxes and a synchrotron cutoff at energies below the X-ray regime, and a flaring state with high levels of optical and X-ray emission and a synchrotron cutoff around or even beyond ~10 keV. In this paper, we are using both leptonic and hadronic jet models to fit the broadband spectra and spectral variability patterns observed in both activity states in 2000. We start with global spectral models of both activity states. Subsequently, we investigate various flaring scenarios for comparison with the observed short-term variability of BL Lacertae in 2000. For our leptonic jet model, we find that the short-term variability, in particular the optical and X-ray spectral variability, can be best represented with a flaring scenario dominated by a spectral index change of the spectrum of ultrarelativistic electrons injected into the jet. Based on this result, a detailed model simulation of such a flaring scenario, reproducing the observed optical and X-ray spectral variability and broadband spectral energy distribution (SED) of BL Lacertae during the BeppoSAX pointing around 2000 November 1 simultaneously, is presented. Our leptonic modeling results are compared to fits using the hadronic synchrotron-proton blazar model. That model can reproduce the observed SEDs of BL Lacertae in a scenario with μ synchrotron-dominated high-energy emission. It requires a significantly higher magnetic field than the leptonic model (~40 vs. ~2 G in the leptonic model) and a lower Doppler factor associated with the bulk motion of the emission region (D ~ 8 vs. D ~ 18 in the leptonic model). The hadronic model predicts a significantly larger (≳100 GeV) flux than the leptonic models, well within the anticipated capabilities of VERITAS and MAGIC.

The strongest spectroscopic dust extinction feature in the Milky Way, the broad absorption bump at 2175 Å, is generally believed to be caused by aromatic carbonaceous materials, very likely a mixture of polycyclic aromatic hydrocarbon molecules, the most abundant and widespread organic molecules in the Milky Way. In this paper, we report identifications of this absorption feature in three galaxies at 1.4 ≲ z ≲ 1.5 that produce intervening Mg II absorption toward quasars discovered by the Sloan Digital Sky Survey. The observed spectra can be fitted using Galactic-type extinction laws, characterized by parameters [R_V,E(B - V)] ≃ [0.7,0.14], [1.9, 0.13], and [5.5, 0.23], respectively, where R_V ≡ A_V/E(B - V) is the total-to-selective extinction ratio and E(B - V) ≡ A_B - A_V is the color excess. These discoveries imply that the dust in these distant quasar absorption systems is similar in composition to that of the Milky Way, but with a range of different grain size distributions. The presence of complex aromatic hydrocarbon molecules in such distant galaxies is important for both astrophysical and astrobiological investigations.

We present the far-ultraviolet spectrum of the Seyfert 1 galaxy 2MASX J21362313-6224008 obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE). The spectrum features absorption from Galactic O VI at two velocities and redshifted H I Lyβ and γ, C II, C III, and O VI. The redshifted absorption features represent a single kinematic component blueshifted by ~310 km s^-1 relative to the active galactic nucleus. We use photoionization models to derive constraints on the physical parameters of the absorbing gas. An alternative interpretation for the absorption lines is also proposed, wherein the absorbing gas is associated with an intervening galaxy cluster.

We measure the X-ray temperature (and luminosity) with ASCA of all but one cluster in the Einstein Extended Medium-Sensitivity Survey (EMSS) high-redshift (z ≥ 0.3) sample. We compare these data to a complete sample of low-redshift clusters that also has temperature measurements, thereby providing cosmological constraints. Improvements over our previous work include (1) an enlarged high-redshift sample; (2) temperatures for the low-redshift comparison sample that come from the same instrument as the high-redshift sample; (3) the elimination of three EMSS clusters with the same redshift as the target (i.e., not truly serendipitous) and a fourth with an ASCA flux well below the completeness limit; (4) using a theoretical cluster mass function that more closely matches N-body simulations (the Sheth-Torman function); (5) using a cold dark matter power spectrum instead of a power law; (6) using a general cosmology with arbitrary matter density and cosmological constant; (7) using a cosmology that generalizes the cosmological constant to quintessence; (8) including the effects of temperature measurement errors and scatter in the cluster luminosity-temperature relation; and (9) marginalizing over the poorly known normalization of the mass-temperature relation. We find an allowed band in the Ω_m0-Ω_Λ0 plane of different orientation to the band of constraints provided by the supernovae Ia Hubble diagram and the cosmic microwave background fluctuations. All three bands intersect at the same place: Ω_m0 ≈ 0.3, Ω_Λ0 ≈ 0.7. We measure the quintessence equation-of-state parameter to be w = -(0.42 ± 0.21) (68% confidence for one interesting parameter), consistent with previously determined upper limits. We measure the normalization of the mass fluctuation power spectrum to be σ₈ = 0.66 ± 0.16 (68% confidence for three interesting parameters). Systematic errors are larger than the statistical errors only for σ₈ with our sample; thus the errors for it depend on the details of the marginalization over the temperature-mass normalization.

We report the search for intracluster light in four Abell type II-III (non-cD) galaxy clusters: A801, A1234, A1553, and A1914. We find that on average these clusters contain ~10% of their detected stellar luminosity in a diffuse component. We show that for two of the clusters the intracluster light closely follows the galaxy distribution, but in the other two cases, there are noticeable differences between the spatial distribution of the galaxies and the intracluster light. We report the results of a search for intracluster tidal debris in each cluster and note that A1914 in particular has a number of strong tidal features likely due to its status as a recent cluster merger. One of the A1914 features appears to be spatially coincident with an extension seen in weak lensing maps, implying that the feature traces a large amount of mass. We compare these results with numerical simulations of hierarchically formed galaxy clusters and find good general agreement between the observed and simulated images, although we also find that our observations sample only the brightest features of the intracluster light. Together, these results suggest that intracluster light can be a valuable tool in determining the evolutionary state of galaxy clusters.

The crisis of the standard cooling flow model brought about by Chandra and XMM-Newton observations of galaxy clusters has led to the development of several models that explore different heating processes in order to assess whether they can quench the cooling flow. Among the most appealing mechanisms are thermal conduction and heating through buoyant gas deposited in the intracluster medium (ICM) by active galactic nuclei (AGNs). We combine Virgo/M87 observations of three satellites (Chandra, XMM-Newton, and BeppoSAX) to inspect the dynamics of the ICM in the center of the cluster. Using the spectral deprojection technique, we derive the physical quantities describing the ICM and determine the extra heating needed to balance the cooling flow, assuming that thermal conduction operates at a fixed fraction of the Spitzer value. We assume that the extra heating is due to buoyant gas, and we fit the data using the model developed by Ruszkowski and Begelman. We derive a scale radius for the model of ~5 kpc, which is comparable with the M87 AGN jet extension, and a required luminosity of the AGN of a few × 10⁴² ergs s^-1, which is comparable to the observed AGN luminosity. We discuss a scenario in which the buoyant bubbles are filled with relativistic particles and magnetic field, which are responsible for the radio emission in M87. The AGN is supposed to be intermittent and to inject populations of buoyant bubbles through a succession of outbursts. We also study the X-ray-cool component detected in the radio lobes and suggest that it is structured in blobs that are tied to the radio buoyant bubbles.

The mass discrepancy in disk galaxies is shown to be well correlated with acceleration, increasing systematically with decreasing acceleration below a critical scale a₀ ≈ 3700 km² s^-2 kpc^-1 = 1.2 × 10^-10 m s^-2. For each galaxy, there is an optimal choice of stellar mass-to-light ratio that minimizes the scatter in this mass discrepancy-acceleration relation. The same mass-to-light ratios also minimize the scatter in the baryonic Tully-Fisher relation and are in excellent agreement with the expectations of stellar population synthesis. Once the disk mass is determined in this fashion, the dark matter distribution is specified. The circular velocity attributable to the dark matter can be expressed as a simple equation that depends only on the observed distribution of baryonic mass. It is a challenge to understand how this very fine-tuned coupling between mass and light comes about.

We study global star formation thresholds in the outer parts of galaxies by investigating the stability of disk galaxies embedded in dark halos. The disks are self-gravitating, contain metals and dust, and are exposed to UV radiation. We find that the critical surface density for the existence of a cold interstellar phase depends only weakly on the parameters of the model and coincides with the empirically derived surface density threshold for star formation. Furthermore, it is shown that the drop in the thermal velocity dispersion associated with the transition from the warm to the cold gas phase triggers gravitational instability on a wide range of scales. The presence of strong turbulence does not undermine this conclusion if the disk is self-gravitating. Models based on the hypothesis that the onset of thermal instability determines the star formation threshold in the outer parts of galaxies can reproduce many observations, including the threshold radii, the column densities, and the sizes of stellar disks as a function of disk scale length and mass. Finally, prescriptions are given for implementing star formation thresholds in (semi-)analytic models and three-dimensional hydrodynamical simulations of galaxy formation.

We select E+A candidates from a spectroscopic data set of ~800 field galaxies and measure the E+A fraction at 0.3 < z < 1 to be 2.7% ± 1.1%, a value lower than that in galaxy clusters at comparable redshifts (11% ± 3%). HST WFPC2 imaging for five of our six E+A's shows that they have a heterogeneous parent population: these E+A's span a range in half-light radius (0.8 h^-1 kpc < r_1/2 < 8 h^-1 kpc) and estimated internal velocity dispersion (50 km s^-1 ≲ σ_est ≲ 220 km s^-1), and they include luminous systems (-21.6 ≤ M - 5 log h ≤ -19.2). Despite their diversity in some aspects, the E+A's share several common characteristics that indicate that the E+A phase is an important link in the evolution of star-forming galaxies into passive systems: the E+A's are uniformly redder than the blue, star-forming galaxies that make up the majority of the field, they are more likely to be bulge-dominated than the average field galaxy, and they tend to be morphologically irregular. We find that E+A's make up ~9% of the absorption-line systems in this redshift range and estimate that ≳25% of passive galaxies in the local field had an E+A phase at z ≲ 1.

The central regions of many interacting and early-type spiral galaxies are actively forming stars. This process affects the physical and chemical properties of the local interstellar medium, as well as the evolution of the galaxies. We observed near-infrared H₂ emission lines: v = 1-0 S(1), 3-2 S(3), 1-0 S(0), and 2-1 S(1) from the central ~1 kpc regions of the archetypical starburst galaxies M82 and NGC 253 and the less dramatic but still vigorously star-forming galaxies NGC 6946 and IC 342. Like the far-infrared continuum luminosity, the near-infrared H₂ emission luminosity can directly trace the amount of star formation activity because the H₂ emission lines arise from the interaction between hot and young stars and nearby neutral clouds. The observed H₂ line ratios show that both thermal excitation and nonthermal excitation are responsible for the emission lines but that the great majority of the near-infrared H₂ line emission in these galaxies arises from energy states excited by ultraviolet fluorescence. The derived physical conditions, e.g., far-ultraviolet radiation field and gas density, from [C II] and [O I] lines and far-infrared continuum observations when used as inputs to photodissociation models also explain the luminosity of the observed H₂ 1-0 S(1) line. The ratio of the H₂ 1-0 S(1) line to far-IR continuum luminosity is remarkably constant over a broad range of galaxy luminosities: L/L_FIR ≃ 10^-5, in normal late-type galaxies (including the Galactic center), in nearby starburst galaxies, and in luminous IR galaxies (LIRGs: L_FIR > 10¹¹L_☉). Examining this constant ratio in the context of photodissociation region models, we conclude that it implies that the strength of the incident UV field on typical molecular clouds follows the gas density at the cloud surface.

We apply a uniform procedure to select very soft sources from point sources observed by Chandra in four galaxies. This sample includes one elliptical galaxy (NGC 4967), two face-on spiral galaxies (M101 and M83), and an interacting galaxy (M51). We report on some intriguing results, including the following:

1. We have found very soft X-ray sources (VSSs) in every galaxy. Some of these fit the criteria for canonical supersoft sources (SSSs), while others are somewhat harder. These latter have characteristic values of kT ≲ 300 eV; we refer to them as quasi-soft sources (QSSs). We found a combined total of 149 VSSs in the four galaxies we considered; 77 were SSSs and 72 were QSSs.

2. The data are consistent with the existence of a large VSS population, most of whose members we cannot observe because of the effects of distance and obscuration. The total VSS population of sources with L > 1037 ergs s-1 in each galaxy could be on the order of 1000.

3. Whereas in M31 only ∼10% of all X-ray sources detected by Chandra are VSSs, more than 35% of all detectable X-ray sources in the face-on galaxy M101 fit the phenomenological definition of VSSs. This difference may be due to differences in N_H between typical lines of sight to sources in each galaxy.

4. SSSs can be super-Eddington for Chandrasekhar-mass objects.

5. We find evidence for SSSs and QSSs with luminosities of 1036 ergs s^-1 < L < 10³⁷ ergs s^-1. These sources have luminosities lower than those of the ∼30 soft sources used to establish the class of SSSs.

6. In the spiral galaxies M101, M83, and M51, a large fraction of the SSSs and QSSs appear to be associated with the spiral arms. This may indicate that some SSSs are young systems, possibly younger than 10₈ yr.

7. In addition to finding hot white dwarfs and soft X-ray binaries, our method should also be efficient at selecting supernova remnants (SNRs). A small fraction of the VSSs in the spiral arms of M101 appear to be associated with SNRs

We report the results of spectral fits to four bright ultraluminous X-ray sources (ULXs) in the Antennae galaxies (NGC 4038/4039) observed for 41 ks with XMM-Newton. Although emission regions are not resolved as well as in prior Chandra observations, at least four ULXs (X-11, X-16, X-37, and X-44 in the Zezas and Fabbiano scheme) are sufficiently bright and well separated with XMM-Newton that reliable extractions and spectral analyses are possible. We find that the single-component multicolor disk blackbody models cannot describe any of the spectra. Sources X-11 and X-16 are acceptably fitted with simple power-law models. A thermal bremsstrahlung model provides a better fit to the spectrum of X-44. Including a disk blackbody component to the spectrum of X-37 improves the fit and reveals an apparently cool disk (kT = 0.13 ± 0.02 keV). This would suggest a parallel to cool disks recently found in other very luminous ULXs, which may contain intermediate-mass black holes; however, the complex diffuse emission of the Antennae demands that this finding be regarded cautiously.

We have obtained 17 epochs of Chandra High Resolution Camera (HRC) snapshot images, each covering most of the M31 disk. The data cover a total baseline of ~2.5 yr and contain a mean effective exposure of 17 ks. We have measured the mean fluxes and long-term light curves for 166 objects detected in these data. At least 25% of the sources show significant variability. The cumulative luminosity function (CLF) of the disk sources is well fitted by a power law with a slope comparable to those observed in typical elliptical galaxies. The CLF of the bulge is a broken power law similar to measurements made by previous surveys. We note several sources in the southwestern disk with L_X > 10³⁷ ergs s^-1. We cross-correlate all of our sources with published optical and radio catalogs, as well as new optical data, finding counterpart candidates for 55 sources. In addition, 17 sources are likely X-ray transients. We analyze follow-up Hubble Space Telescope (HST) WFPC2 data of two X-ray transients, finding F336W (U-band equivalent) counterparts. In both cases, the counterparts are variable. In one case, the optical counterpart is transient with F336W = 22.3 ± 0.1 mag. The X-ray and optical properties of this object are consistent with a ~10 M_☉ black hole X-ray nova with an orbital period of 23 days. In the other case, the optical counterpart varies between F336W = 20.82 ± 0.06 and 21.11 ± 0.02 mag. Ground-based and HST observations show that this object is bright (V = 18.8 ± 0.1) and slightly extended. Finally, the frequency of bright X-ray transients in the M31 bulge suggests that the ratio of neutron star to black hole primaries in low-mass X-ray binaries (NS/BH) is ~1.

We report on the Chandra X-Ray Observatory ACIS-S3 imaging observation of the Galactic globular cluster M4 (NGC 6121). We detect 12 X-ray sources inside the core and 19 more within the cluster half-mass radius. The limiting luminosity of this observation is L_X ≈ 10²⁹ ergs s^-1 for sources associated with the cluster, the deepest X-ray observation of a globular cluster to date. We identify six X-ray sources with known objects and use ROSAT observations to show that the brightest X-ray source is variable. Archival data from the Hubble Space Telescope allow us to identify optical counterparts to 16 X-ray sources. Based on the X-ray and optical properties of the identifications and the information from the literature, we classify two (possibly three) sources as cataclysmic variables, one X-ray source as a millisecond pulsar, and 12 sources as chromospherically active binaries. Comparison of M4 with 47 Tuc and NGC 6397 suggests a scaling of the number of active binaries in these clusters with the cluster (core) mass.

Using the deepest and finest resolution images of the universe acquired with the Hubble Space Telescope and a similar image taken 7 yr later for the Great Observatories Origins Deep Survey, we have derived proper motions for the point sources in the Hubble Deep Field-North. Two faint blue objects, HDF 2234 and HDF 3072, are found to display significant proper motion, 10.0 ± 2.5 and 15.5 ± 3.8 mas yr^-1. Photometric distances and tangential velocities for these stars are consistent with disk white dwarfs located at ~500 pc. The faint blue objects analyzed by Ibata et al. and Mendez & Minniti do not show any significant proper motion; they are not halo white dwarfs, and they do not contribute to the Galactic dark matter. These objects are likely to be distant active galactic nuclei.

We present an analysis of rotation measure (RM) fluctuations from the test region of the Southern Galactic Plane Survey (SGPS), along with emission measure (EM) fluctuations in the same field taken from the Southern Hα Sky Survey Atlas. The structure function of RM fluctuations shows a relatively steep slope at small scales (1'-5'), a break in slope to a flatter structure function at intermediate scales (5'-60'), and a systematic variation of the strength of fluctuations as a function of position angle on the sky at the largest scales (60'-200'). The structure function of EM fluctuations shows similar behavior, although the lower resolution of the data prevents detection of a possible break in the spectrum. We interpret the anisotropy in RM/EM structure on large scales as resulting from a large-scale gradient in electron density (and possibly magnetic field) across the region. The break in the slope of the RM structure function at scales of ~5' can be explained by contributions from two spatially distinct magnetoionized screens, most likely in the Local and Carina spiral arms. The observed structure function then implies that the outer scale of RM fluctuations in these screens is ~2 pc. Such behavior is in striking contrast to the expectation that interstellar turbulence forms an unbroken spectrum from kiloparsec down to AU scales. We conclude that we have identified an additional source of enhanced turbulence, injected on scales of a few pc, possibly seen only in the Galactic plane. The most likely source of such turbulence is individual H II regions from relatively low mass stars, whose characteristic scale size is similar to the outer scale of turbulence inferred here. These sources may be the dominant source of density and velocity fluctuations in warm ionized gas in the Galactic plane.

At the early, ejecta-dominated stage of supernova remnant (SNR) expansion, a fraction of the swept-up circumstellar magnetic field is dynamically compressed to approximate equipartition at the contact discontinuity separating the SN progenitor's wind (or the interstellar medium) and the ejecta. We propose that the thin nonthermal X-ray filaments observed by the Chandra satellite in several young SNRs are associated with such "pile-up" of the magnetic field. We use a one-dimensional diffusion-convection transport equation to describe the propagation of nonthermal electrons near the contact discontinuity of a young SNR and to calculate spatially resolved emission spectra in the X-ray and TeV bands. The results suggest that the high-energy electrons are possibly accelerated at the forward shock and emit efficiently only when they diffuse into regions of high magnetic field near the contact discontinuity. Much more likely, though, is that they are locally accelerated at the contact discontinuity, in which case the acceleration cannot be related to Fermi-type processes and should occur because of other plasma mechanisms. As a consequence, the forward shock in young SNRs is inconspicuous and often unobservable, similar to that in the Crab Nebula.

Spectroscopy of the Crab Nebula along different slit directions reveals the three-dimensional structure of the optical nebula. On the basis of the linear radial expansion result first discovered by V. Trimble in 1968, we make a three-dimensional model of the optical emission. Results from a limited number of slit directions suggest that optical lines originate from a complicated array of wisps that are located in a rather thin shell pierced by a jet. The jet is certainly not prominent in optical emission lines, but the direction of the piercing is consistent with the direction of the X-ray and radio jet. The shell's effective radius is ≈79'', its thickness is about a third of the radius, and it is moving out with an average velocity of 1160 km s^{- 1}.

The decay of Alfvén waves in a filamentary molecular cloud is investigated through three-dimensional numerical simulations. We have considered a filamentary molecular cloud supported in part by the Alfvén wave against the self-gravity. Our attention has been focused on the basic physical mechanism for the decay. The decay rate is obtained as a function of the wavelength and amplitude. It is found that when the wave is circularly polarized, the decay e-folding timescale is several times the fast wave crossing timescale for the filament and independent of the wavelength, whereas when the wave is linearly polarized, the amplitude of the wave decreases inversely proportional to time. It is also found that the decay of Alfvén waves induces rotation and shear flow in the filamentary cloud. The propagation of two Alfvén waves in the medium results in the excitation of daughter waves due to nonlinear coupling between mother waves. The wavenumber of the daughter waves is the sum or difference between those of the mother waves, and below a critical wavenumber of the daughter wave, the filamentary cloud fragments as a result of Jeans instability. The fragments collapse to form high-density rotating magnetized disks. In contrast, below a critical wavenumber of the mother wave, the cloud becomes a dense helical filament after the decay of the Alfvén waves. The present models are compared with previous simulations and observations with regard to the rotation, fragmentation, and helical structure of filamentary clouds.

Infrared spectroscopy provides a direct handle on the composition and structure of interstellar dust. We have studied the dust along the line of sight toward the Galactic center using Short Wavelength Spectrometer data obtained with the Infrared Space Observatory (ISO). We focused on the wavelength region from 8 to 13 μm, which is dominated by the strong silicate absorption feature. Using the absorption profiles observed toward Galactic center sources 3 and 4, which are C-rich Wolf-Rayet Stars, as reference objects, we are able to disentangle the interstellar silicate absorption and the silicate emission intrinsic to the source, toward Sgr A* and derive a very accurate profile for the intrinsic 9.7 μm band. The interstellar absorption band is smooth and featureless and is well reproduced using a mixture of 15.1% amorphous pyroxene and 84.9% of amorphous olivine by mass, all in spherical submicron-sized grains. There is no direct evidence for substructure due to interstellar crystalline silicates. By minimizing χ² of spectral fits to the absorption feature, we are able to determine an upper limit to the degree of crystallinity of silicates in the diffuse interstellar medium (ISM) and conclude that the crystalline fraction of the interstellar silicates is 0.2% ± 0.2% by mass. This is much lower than the degree of crystallinity observed in silicates in the circumstellar environment of evolved stars, the main contributors of dust to the ISM. There are two possible explanations for this discrepancy. First, an amorphization process occurs in the ISM on a timescale significantly shorter than the destruction timescale, possibly caused by particle bombardment by heavyweight ions. Second, we consider the possibility that the crystalline silicates in stellar ejecta are diluted by an additional source of amorphous silicates, in particular supernovae. We also compare our results with a study on silicate presolar grains found in interplanetary dust particles.

We analyze interstellar absorption observed toward two subdwarf O stars, JL 9 and LS 1274, using spectra taken by the Far Ultraviolet Spectroscopic Explorer (FUSE). Column densities are measured for many atomic and molecular species (H I, D I, C I, N I, O I, P II, Ar I, Fe II, and H₂), but our main focus is on measuring the D/H ratios for these extended lines of sight, as D/H is an important diagnostic for both cosmology and Galactic chemical evolution. We find D/H = (1.00 ± 0.37) × 10^-5 toward JL 9 and D/H = (0.76 ± 0.36) × 10^-5 toward LS 1274 (2 σ uncertainties). With distances of 590 ± 160 and 580 ± 100 pc, respectively, these two lines of sight are currently among the longest Galactic lines of sight with measured D/H. With the addition of these measurements, we see a significant tendency for longer Galactic lines of sight to yield low D/H values, consistent with previous inferences about the deuterium abundance from D/O and D/N measurements. Short lines of sight with H I column densities of log N(H ) < 19.2 suggest that the gas-phase D/H value within the Local Bubble is (D/H)_LBg = (1.56 ± 0.04) × 10^-5. However, the four longest Galactic lines of sight with measured D/H, which have d > 500 pc and log N(H ) > 20.5, suggest a significantly lower value for the true local disk gas-phase D/H value, (D/H)_LDg = (0.85 ± 0.09) × 10^-5. One interpretation of these results is that D is preferentially depleted onto dust grains relative to H and that longer lines of sight that extend beyond the Local Bubble sample more depleted material. In this scenario, the higher Local Bubble D/H ratio is actually a better estimate than (D/H)_LDg for the true local disk D/H, (D/H)_LD. However, if (D/H)_LDg is different from (D/H)_LBg simply because of variable astration and incomplete interstellar medium mixing, then (D/H)_LD = (D/H)_LDg.

We present an analysis of high-resolution optical spectra for a sample of very young, mid- to late-M, low-mass stellar and substellar objects: 11 in the Upper Scorpius association, and two (GG Tau Ba and Bb) in the Taurus star-forming region. Effective temperatures and surface gravities are derived from a multiple-feature spectral analysis using TiO, Na I, and K I, through comparison with the latest synthetic spectra. We show that these spectral diagnostics complement each other, removing degeneracies with temperature and gravity in the behavior of each. In combination, they allow us to determine temperature to within 50 K and gravity to within 0.25 dex, in very cool young objects. Our high-resolution spectral analysis does not require extinction estimates. Moreover, it yields temperatures and gravities independent of theoretical evolutionary models (although our estimates do depend on the synthetic spectral modeling). We find that our gravities for most of the sample agree remarkably well with the isochrone predictions for the likely cluster ages. However, discrepancies appear in our coolest targets: these appear to have significantly lower gravity (by up to 0.75 dex) than our hotter objects, even though our entire sample covers a relatively narrow range in effective temperature (~300 K). This drop in gravity is also implied by intercomparisons of the data alone, without recourse to synthetic spectra. We consider, and argue against, dust opacity, cool stellar spots, or metallicity differences leading to the observed spectral effects; a real decline in gravity is strongly indicated. Such gravity variations are contrary to the predictions of the evolutionary tracks, causing improbably low ages to be inferred from the tracks for our coolest targets. Through a simple consideration of contraction timescales, we quantify the age errors introduced into the tracks through the particular choice of initial conditions and demonstrate that they can be significant for low-mass objects that are only a few megayears old. However, we also find that these errors appear insufficient to explain the magnitude of the age offsets in our lowest gravity targets. We venture that this apparent age offset may arise from evolutionary model uncertainties related to accretion, deuterium burning and/or convection effects. Finally, when combined with photometry and distance information, our technique for deriving surface gravities and effective temperatures provides a way of obtaining masses and radii for substellar objects independent of evolutionary models; radius and mass determinations are presented in Paper II.

We present mass and radius derivations for a sample of very young, mid- to late-M, low-mass stellar and substellar objects in Upper Scorpius and Taurus. In a previous paper we determined effective temperatures and surface gravities for these targets from an analysis of their high-resolution optical spectra and comparisons to the latest synthetic spectra. We now derive extinctions, radii, masses, and luminosities by combining our previous results with observed photometry, surface fluxes from the synthetic spectra, and the known cluster distances. These are the first mass and radius estimates for young, very low mass bodies that are independent of theoretical evolutionary models (although our estimates do depend on spectral modeling). We find that for most of our sample, our derived mass-radius and mass-luminosity relationships are in very good agreement with the theoretical predictions. However, our results diverge from the evolutionary model values for the coolest, lowest mass targets: our inferred radii and luminosities are significantly larger than predicted for these objects at the likely cluster ages, causing them to appear much younger than expected. We suggest that uncertainties in the evolutionary models—e.g., in the choice of initial conditions and/or treatment of interior convection—may be responsible for this discrepancy. Finally, two of our late-M objects (USco 128 and 130) appear to have masses close to the deuterium-fusion boundary (~9M_J-14M_J within a factor of 2). This conclusion is primarily a consequence of their considerable faintness compared to other targets with similar extinction, spectral type, and temperature (difference of ~1 mag). Our result suggests that the faintest young late-M or cooler objects may be significantly lower in mass than current theoretical tracks indicate.

We present an analysis of recent near-infrared, high-resolution spectra of the variable FU Ori objects. During a phase of rapid fading in optical brightness during 1997, V1057 Cyg exhibited shell absorption in first-overtone (v''-v' = 2-0) CO lines, blueshifted by about 50 km s^-1 from the system velocity. This shell component had not been seen previously, nor was it present in 1999, although some blueshifted absorption asymmetry is seen at the latter epoch. The appearance of this CO absorption shell is connected with the roughly contemporaneous appearance of blueshifted, low-excitation optical absorption lines with comparable low velocities; we suggest that this shell was also responsible for some of the peculiar emission features seen in red-optical spectra of V1057 Cyg. FU Ori continues to exhibit broad CO lines, with some evidence for the double-peaked profiles characteristic of an accretion disk; the line profiles are consistent with previous observations. Both FU Ori and V1057 Cyg continue to exhibit lower rotational broadening at 2.3 μm than at optical wavelengths, in agreement with the prediction of differentially rotating disk models; we have a marginal detection of the same effect in V1515 Cyg. The relative population of the first-overtone CO rotational levels in the FU Ori objects suggests low excitation temperatures. We compare disk models to the observations and find agreement with overall line strengths and rotational broadening, but the observed line profiles are generally less double-peaked than predicted. We suggest that the discrepancy in line profiles is due to turbulent motions in FU Ori disks, an effect qualitatively predicted by recent simulations of the magnetorotational instability in vertically stratified accretion disks.

We present an analysis of candidate members of the η Cha and MBM 12A young associations. For an area of 0.7 deg² toward η Cha, we have performed a search for members of the association by combining JHK_s photometry from the Two Micron All Sky Survey and i photometry from DENIS with follow-up optical spectroscopy at Magellan Observatory. We report the discovery of three new members with spectral types of M5.25-M5.75, corresponding to masses of 0.13-0.08 M_☉ by theoretical evolutionary models. Two and three of these members were found independently by Lyo and coworkers and Song and coworkers, respectively. Meanwhile, no brown dwarfs were detected in η Cha down to the completeness limit of 0.015 M_☉. For MBM 12A, we have obtained spectra of three of the remaining candidate members that lacked spectroscopy at the end of the survey by Luhman, all of which are found to be field M dwarfs. Ogura and coworkers have recently presented four "probable" members of MBM 12A. However, two of these objects were previously classified as field dwarfs by the spectroscopy of Luhman. In this work, we find that the other two objects are field dwarfs as well.

We present an analysis of the Chandra High Energy Transmission Grating Spectrometer observation of the rapidly rotating (P_rot = 0.94 days) post-T Tauri (~20 Myr old) star PZ Telescopii, in the Tucana association. Using two different methods, we have derived the coronal emission measure distribution EM(T) and chemical abundances. The EM(T) peaks at log T = 6.9 and exhibits a significant emission measure at temperatures log T > 7. The coronal abundances are generally ~0.5 times the solar photospheric values, which are presumed fairly representative of the composition of the underlying star. A minimum in abundance is seen at a first ionization potential (FIP) of 7-8 eV, with evidence for higher abundances at both lower and higher FIP, similar to patterns seen in other active stars. From an analysis of the He-like triplet of Mg XI, we have estimated electron densities of ~10¹²-10¹³ cm^-3. All the coronal properties found for PZ Tel are much more similar to those of AB Dor, which is slightly older than PZ Tel, than to those of the younger T Tauri star TW Hya. These results support earlier conclusions that the soft X-ray emission of TW Hya is likely dominated by accretion activity rather than by a magnetically heated corona. Our results also suggest that the coronae of pre-main-sequence stars rapidly become similar to those of older active main-sequence stars soon after the accretion stage has ended.

We estimate a gamma-ray burst (GRB) formation rate based on the new relation between the spectral peak energy (E_p) and the peak luminosity. The new relation is derived by combining the data of E_p and the peak luminosities by BeppoSAX and BATSE, and it looks considerably tighter and more reliable than the relations suggested by the previous works. Using the new E_p-luminosity relation, we estimate redshifts of the 689 GRBs without known distances in the BATSE catalog and derive a GRB formation rate as a function of the redshift. For the redshift range of 0 ≤ z ≤ 2, the GRB formation rate increases and is well correlated with the star formation rate, while it keeps constant toward z ~ 12. We also discuss the luminosity function and the redshift dependence of the intrinsic luminosity (luminosity evolution).

We systematically reanalyzed all gamma-ray burst (GRB) afterglow data published through the end of 2002 in an attempt to detect the predicted supernova light component and to gain statistical insight into its phenomenological properties. We fit the observed photometric light curves as the sum of an afterglow, an underlying host galaxy, and a supernova component. The latter is modeled using published multicolor light curves of SN 1998bw as a template. The total sample of afterglows with established redshifts contains 21 bursts (GRB 970228-GRB 021211). For nine of these GRBs a weak supernova excess (scaled to SN 1998bw) was found, which is what makes this one of the first samples of high-z core-collapse supernovae. Among this sample are all bursts with redshifts less than ~0.7. These results strongly support the notion that in fact all afterglows of long-duration GRBs contain light from an associated supernova. A statistics of the physical parameters of these GRB-supernovae shows that SN 1998bw was at the bright end of its class, while it was not special with respect to its light-curve shape. Finally, we have searched for a potential correlation of the supernova luminosities with the properties of the corresponding bursts and optical afterglows, but we have not found such a relation.

We present optical and near-infrared follow-up observations of the X-ray flash (XRF) of 2003 July 23. Our observations in the R band cover the temporal range from 4.2 hr to 64 days after the high-energy event. We also present the results of multicolor imaging extending to the K band on three epochs. The light curve of the R-band afterglow the first week after the burst is similar to the light curve for long-duration gamma-ray bursts (GRBs), i.e., a broken power law with a late time slope of α ≈ 2.0 (F_ν ∝ t^-α). Furthermore, the spectral energy distribution (SED) has a power-law (F_ν ∝ ν^-β) shape with slope β ≈ 1.0. However, the decay slope at t < 1 day is shallow, consistent with zero. This is in qualitative agreement with the prediction that XRFs are off-axis classical GRBs. After the first week there is a strong bump in the light curve, which peaks at around 16 days. The SED after the peak becomes significantly redder. We discuss the possible interpretations of this bump and conclude that an underlying supernova is the most likely explanation since no other model appears consistent with the evolution of the SED. Finally, we present deep spectroscopy of the burst both in the afterglow and in the bump phase. A firm upper limit of z = 2.3 is placed on the redshift of XRF 030723 from the lack of Lyα forest lines in the spectrum of the afterglow. The lack of significant absorption and emission lines in either of the two spectra excludes a spectroscopic redshift determination.

Although thermal disk emission is suppressed or absent in the hard state of X-ray binaries, the presence of a cold, thin disk can be inferred from signatures of reprocessing in the ~2-50 keV band. The strength of this signature is dependent on the source spectrum and flux impinging on the disk surface and is thus very sensitive to the system geometry. The general weakness of this feature in the hard state has been attributed to either a truncation of the thin disk, large ionization, or beaming of the corona region away from the disk with β ~ 0.3. This latter velocity is comparable to jet nozzle velocities, so we explore whether a jet can account for the observed reflection fractions. It has been suggested that jets may contribute to the high-energy spectra of X-ray binaries, via either synchrotron from around (100-1000)r_g along the jet axis or from inverse Compton (synchrotron self-Compton and/or external Compton) from near the base. Here we calculate the reflection fraction from jet models wherein either synchrotron or Compton processes dominate the emission. Using as a guide a data set for GX 339-4, for which the reflection fraction previously has been estimated as ~10%, we study the results for a jet model. We find that the synchrotron case gives less than 2% reflection, while a model with predominantly synchrotron self-Compton in the base gives ~10%-18%. This shows for the first time that an X-ray binary jet is capable of significant reflection fractions and that extreme values of the reflection may be used as a way of discerning the dominant contributions to the X-ray spectrum.

We present the long-term X-ray light curves and detailed spectral and timing analyses of XTE J1908+094 using the Rossi X-Ray Timing Explorer Proportional Counter Array observations covering two outbursts in 2002 and early 2003. At the onset of the first outburst, the source was found in a spectrally low/hard state lasting for ~40 days, followed by a 3 day long transition to the high/soft state. The source flux (in 2-10 keV) reached ~100 mcrab on 2002 April 6, then decayed rapidly. In power spectra, we detect strong band-limited noise and varying low-frequency quasi-periodic oscillations that evolved from ~0.5 to ~5 Hz during the initial low/hard state of the source. We find that the second outburst closely resembled the spectral evolution of the first. The X-ray transient's overall outburst characteristics led us to classify XTE J1908+094 as a black hole candidate. Here we also derive precise X-ray position of the source using Chandra observations that were performed during the decay phase of the first outburst and following the second outburst.

Using a high-resolution spectrum of the secondary star in the black hole binary A0620-00, we have derived the stellar parameters and veiling caused by the accretion disk in a consistent way. We have used a χ² minimization procedure to explore a grid of 800,000 LTE synthetic spectra computed for a plausible range of both stellar and veiling parameters. Adopting the best model parameters found, we have determined atmospheric abundances of Fe, Ca, Ti, Ni, and Al. The Fe abundance of the star is [Fe/H] = 0.14 ± 0.20. Except for Ca, we found the other elements moderately overabundant as compared with stars in the solar neighborhood of similar iron content. Taking into account the small orbital separation, the mass transfer rate, and the mass of the convection zone of the secondary star, a comparison with element yields in supernova explosion models suggests a possible explosive event with a mass cut comparable to the current mass of the compact object. We have also analyzed the Li abundance, which is unusually high for a star of this spectral type and relatively low mass.

We present calculations of magnetic field evolution by the Hall effect and ohmic decay in the crust of neutron stars (NSs). In accreting NSs, ohmic decay is always the dominant effect because of the large resistivity. In isolated NSs with relatively pure crusts, the Hall effect dominates ohmic decay after a time t_switch ≃ 10⁴ yr B, where B₁₂ is the magnetic field strength in units of 10¹² G. We compute the evolution of an initial field distribution by ohmic decay and give approximate analytic formulae for both the surface and interior fields as a function of time. Because of the strong dependence of t_switch on B₁₂, early ohmic decay can alter the currents down to the base of the crust for B ~ 10¹¹ G, neutron drip for B ~ 10¹² G, and near the top of the crust for B ≳ 10¹³ G. We then discuss magnetic field evolution by the Hall effect. Several examples are given to illustrate how an initial field configuration evolves. Hall-wave eigenfunctions are computed, including the effect of the large density change across the crust. We estimate the response of the crust to the magnetic stresses induced by Hall waves and give a detailed discussion of the boundary conditions at the solid-liquid interface. Finally, we discuss the implications for the Hall cascade proposed by Goldreich & Reisenegger.

The Hanle effect has been proposed as a new diagnostic of circumstellar magnetic fields for early-type stars, for which it is sensitive to field strengths in the 1-300 G range. In this paper we compute the polarized P Cygni line profiles that result from the Hanle effect. For modeling the polarization, we employ a variant of the "last scattering approximation." For cases in which the Sobolev optical depths are greater than unity, the emergent line intensity is assumed to be unpolarized, while for smaller optical depths, the Stokes source functions for the Hanle effect with optically thin line scattering are used. For a typical P Cygni line, the polarized emission forms in the outer wind, because the Sobolev optical depth is large at the inner wind. For low surface field strengths, weak P Cygni lines are needed to measure the circumstellar field. For high values of the surface fields, both the Zeeman and Hanle diagnostics can be used, with the Zeeman effect probing the photospheric magnetic fields and the Hanle effect measuring the magnetic field in the wind flow. Polarized line profiles are calculated for a self-consistent structure of the flow and the magnetic geometry based on the WCFields model, which is applicable to slowly rotating stellar winds with magnetic fields drawn out by the gas flow. For surface fields of a few hundred gauss, we find that the Hanle effect can produce line polarizations in the range of a few tenths of a percent up to about 2%.

We discuss in detail the evolutionary properties of low-mass stars (M ≤ 1 M_☉) having metallicity lower than Z = 10^-6 from the pre-main sequence up to (almost) the end of the early asymptotic giant branch phase. We also discuss the possibility that the large [C/Fe], [N/Fe] observed on the surface of the most iron-poor star currently known, HE 0107-5240, could be attributed to the autopollution induced by the penetration of the He convective shell into the H-rich mantle during the He core flash of a low-mass, very low metallicity star. On the basis of a quite detailed analysis, we conclude that the autopollution scenario cannot be responsible for the observed chemical composition of HE 0107-5240.

We carry out a large set of very high resolution, three-dimensional, smoothed particle hydrodynamics simulations describing the evolution of gravitationally unstable gaseous protoplanetary disks. We consider a broad range of initial disk parameters. Disk masses out to 20 AU range from 0.075 to 0.125 M_☉, roughly consistent with the high end of the mass distribution inferred for disks around T Tauri stars. Minimum outer temperatures range from 30 to 100 K, as expected from studies of the early protosolar nebula and suggested by the modeling of the spectra of protoplanetary disks. The mass of the central star is also varied, although it is usually assumed to be equal to that of the Sun. Overall, the initial disks span minimum Q-parameters between 0.8 and 2, with most models having Q ~ 1.4. The disks are evolved assuming either a locally isothermal equation of state or an adiabatic equation of state with varying γ. Heating by (artificial) viscosity and shocks is included when the adiabatic equation of state is used. When condensations above a specific density threshold appear as a result of gravitational instability in a locally isothermal calculation, the equation of state is switched to adiabatic to account for the increased optical depth. We show that when a disk has a minimum Q-parameter less than 1.4, strong trailing spiral instabilities, typically three- or four-armed modes, form and grow until fragmentation occurs along the arms after about 5 mean disk orbital times. The resulting clumps contract quickly to densities several orders of magnitude higher than the initial disk density, and the densest of them survive even under adiabatic conditions. These clumps are stable to tidal disruption and merge quickly, leaving two to three protoplanets on fairly eccentric orbits (the mean eccentricity being around 0.2) after ~10³ yr. Fragmentation is not strongly dependent on whether the disk starts from a marginally unstable state or gradually achieves it; we show that if the disk is allowed to grow in mass from a very light, very stable state over tens of orbital times, it still fragments at roughly the same mass and temperature as in the standard disk models. We show that the first stages of the instability, until the appearance of the overdensities, can be understood in terms of the maximum unstable Toomre wavelength and the local Jeans length. A high mass and force resolution are needed to correctly resolve both scales and follow the fragmentation process appropriately. Varying disk mass and temperature affects such physical scales and hence the typical masses of the protoplanets that form. Objects smaller than Saturn or a couple of times bigger than Jupiter can both be produced by fragmentation. Their final masses will then depend on the subsequent interactions and mergers with other clumps and on the accretion of disk material. The accretion rate depends on the disk thermodynamics and is negligible with adiabatic conditions. After ~10³ yr the masses range from just below 1M_Jup to more than 7M_Jup, well in agreement with those of detected extrasolar planets.

We investigate the conditions for planetesimal accretion in a circumbinary disk. Until recently, it had been believed that only single solar-type stars might harbor planetary systems. On the other hand, circumbinary disks have been detected by infrared or radio observation. Planets may be formed in such disks. Binary systems give stronger gravitational perturbation against planetesimals orbiting nearer to the binary. Therefore, the relative velocities between planetesimals will be larger and when they exceed the escape velocity it is impossible for the planetesimals to accumulate into planets. We performed long-term numerical integrations of binary and planetesimal orbital motions in the framework of the coplanar elliptic restricted three-body problem and have found the upper limit of the orbital radius inside of which the relative velocity between the planetesimals exceeds the escape velocity. One of our results is that planetesimal accretion cannot occur in a zone within 13 AU from the barycenter of the binary system when the binary semimajor axis is 1 AU, the binary eccentricity 0.1, the total mass m₁ + m₂ = 1 M_☉, and the mass ratio m₂/(m₁ + m₂) = 0.2. In regions farther out than 13 AU, planetesimals can accrete. We also derive analytic expressions of the eccentricity of a planetesimal pumped up by the gravitational perturbation of the binary and the inner boundary radius of the planetesimal accretion zone according to the secular perturbation theory.

We report new spectroscopic observations with the Keck HIRES instrument of the recently discovered transiting planet OGLE-TR-56b. Our radial velocity measurements with errors of ~100 m s^-1 show clear variations that are in excellent agreement with the phasing (period and epoch) derived from the OGLE transit photometry. Careful analysis of the spectral line bisectors, along with numerical simulations of blend scenarios, supports the argument for the planetary nature of the companion. The new data, combined with measurements from the previous season, allow an improved determination of the mass of the planet, M_p = M_Jup. All available OGLE photometry, including new measurements made during the 2003 season, have also been analyzed to derive an improved value for the planetary radius of R_p = R_Jup. We discuss the implications of these results for the theory of extrasolar planets.

Numerical simulations of charged particle trajectories in a Parker field with model turbulence are used to study the transport mechanisms of solar energetic particles (SEPs): magnetic focusing, pitch-angle diffusive transport due to magnetic fluctuations, and adiabatic cooling. The results of the simulations are compared with analytical formulae for the transport effects. We find that adiabatic cooling is significant for SEPs traveling from the Sun to 1 AU. The simulation results agree much better with the anisotropic adiabatic cooling effect theory than with the isotropic one. Also, we show that the simulation results agree well with the theory of magnetic focusing with pitch-angle scattering by magnetic fluctuations. In addition, in order for quasilinear theory to be valid, the average particle's Larmor radius must be much smaller than the correlation scale of the two-dimensional component of turbulence.

We revisit the impulsive beamlike particle events detected in situ from 1997 to 2000 by the Electron, Proton, and Alpha Monitor (EPAM) experiment on the Advanced Composition Explorer spacecraft. We study in detail a subset of events for which there are radio coronal observations from the Nançay Radioheliograph. EPAM measures electrons in the energy range from 40 to 300 keV over a wide range of look directions and with better than 1 minute time resolution, while the Nançay radioheliograph provides images of the solar corona at five different frequencies with time cadence of eight images per second and per frequency. The radio images are complemented with spectral information from a series of radiospectrographs over a wide frequency range (from dm to km wavelengths), white-light coronagraphic images from the Large Angle Spectroscopic Coronagraph (LASCO) on the the Solar and Heliospheric Observatory (SOHO) spacecraft, and EUV images from the Extreme Ultraviolet Imaging Telescope (EIT), also on SOHO. We separate the particle events according to their associated radio emissions in the meter to decameter wavelengths, in radio-simple (only type III bursts) and radio-complex (also type II bursts and/or continua). The electron events in the radio-simple category have rather short durations, are very weak, and show essentially no delay between the onset of type III emission and the inferred release time for the energetic electrons. The electron events in the radio-complex category present variable delays between the onset of type III emission and the inferred release time for the energetic electrons. The inferred release time for the particles in the radio-complex category always coincides with the onset or major changes in the complex radio emissions; this good association suggests that the coronal processes involved in the radio emissions are at the origin of the electron acceleration. The timing and spectral characteristics of the radio emissions, when compared with the properties of the particles seen at EPAM, and the white-light information from LASCO, strongly support an acceleration process in the corona, at variable heights and below the leading edge of the associated coronal mass ejection. The coronal restructuring put in evidence by the radio signatures is the simplest explanation for the origin of those energetic particles.

Analytical solutions describing equilibrium magnetic fields in the solar corona, deformed by prominence-like plasma condensations in the Cartesian plane, are treated in this paper. The equilibrium equations for this class of problems usually take different forms in distinct subdomains, separated by free boundaries to be solved as one of the unknowns, across which solutions on the two sides are matched by suitable jump conditions. By idealizing the plasma condensations as a horizontal, circular cylinder whose weight in a uniform gravitational field is supported by an external magnetic field, we avoid solving free-boundary problems and present a method to directly construct solutions presenting prominence magnetic fields in the so-called normal and inverse configurations. The solutions illustrate the morphologies of plasma and vector magnetic field distributions in the cylindrical condensations in relation to the magnetic fields beneath the condensations. These solutions provide theoretical magnetic field properties that may be relevant to the current renewed interest in observing solar prominence magnetic fields by polarimetric spectroscopy.

Recently, we suggested that the source of ion heating in solar coronal holes is small-scale reconnection events (microflares) at the coronal base. The microflares launch intermittent heat flux up into the corona exciting ion cyclotron waves through a plasma microinstability. The ions are heated by these waves during the microflare bursts and then evolve with no energy input between the bursts. In this paper, we show that the structure of the proton distribution in the relatively long time periods between the microflares is determined by collisions at small heliocentric distances. At greater distances, the collisional processes can be replaced by similar processes due to secondary instabilities. These are excited by the distortion of the distribution under the action of the mirror force. At the same time, the heating during the microflare bursts is not affected by either the collisions or the secondary instabilities because of the short duration of the bursts. We demonstrate that in each intermittent heating event the protons diffuse approximately along one-dimensional curves in the phase space and can develop a quasi-plateau. The corresponding temperature increase can then be calculated without solving the diffusion equations. The overall coronal heating by this mechanism is a summed effect of all microflare bursts during the expansion time of the solar wind and adiabatic cooling between the microflares. The calculations for the collision-dominated region suggest that the overall heating is efficient enough to account for the acceleration of the fast solar wind in this region.

We present the results of MHD simulations in the low-β regime of the evolution of the three-dimensional coronal magnetic field as an arched, twisted magnetic flux tube emerges into a preexisting coronal potential magnetic arcade. We find that the line-tied emerging flux tube becomes kink-unstable when a sufficient amount of twist is transported into the corona. For an emerging flux tube with a left-handed twist (which is the preferred sense of twist for active region flux tubes in the northern hemisphere), the kink motion of the tube and its interaction with the ambient coronal magnetic field lead to the formation of an intense current layer that displays an inverse-S shape, consistent with the X-ray sigmoid morphology preferentially seen in the northern hemisphere. The position of the current layer in relation to the lower boundary magnetic field of the emerging flux tube is also in good agreement with the observed spatial relations between the X-ray sigmoids and their associated photospheric bipolar magnetic regions. We argue that the inverse-S-shaped current layer formed is consistent with being a magnetic tangential discontinuity limited by numerical resolution and thus may result in the magnetic reconnection and significant heating that causes X-ray sigmoid brightenings.

A number of methods of flare prediction rely on classification of physical characteristics of an active region, in particular optical classification of sunspots, and historical rates of flaring for a given classification. However, these methods largely ignore the number of flares the active region has already produced, in particular the number of small events. The past history of occurrence of flares (of all sizes) is an important indicator of future flare production. We present a Bayesian approach to flare prediction, which uses the flaring record of an active region together with phenomenological rules of flare statistics to refine an initial prediction for the occurrence of a big flare during a subsequent period of time. The initial prediction is assumed to come from one of the extant methods of flare prediction. The theory of the method is outlined, and simulations are presented to show how the refinement step of the method works in practice.

The latest SOHO VIRGO total solar irradiance (TSI) time series is analyzed using new solar variability measures obtained from full-disk solar images made at the San Fernando Observatory and the Mg II 280 nm index. We discuss the importance of solar cycle 23 as a magnetically simpler cycle and a variant from recent cycles. Our results show the continuing improvement in TSI measurements and surrogates containing information necessary to account for irradiance variability. Use of the best surrogate for irradiance variability due to photospheric features (sunspots and faculae) and chromospheric features (plages and bright network) allows fitting the TSI record to within an rms difference of 130 ppm for the period 1986 to the present. Observations show that the strength of the TSI cycle did not change significantly despite the decrease in sunspot activity in cycle 23 relative to cycle 22. This points to the difficulty of modeling TSI back to times when only sunspot observations were available.

We examine the intensity correlations among lower transition region emission lines of N III, N IV, N V, O II, O III, O IV, O V, S IV, and S V. We find strong intensity correlations, with 1 σ deviations in line intensity ratios less than about 35% on spatial scales of 1''. For strong lines the percent deviations are significantly less than this. The line intensities were obtained from the Solar Ultraviolet Measurement of Emitted Radiation (SUMER) spectrometer on the Solar and Heliospheric Observatory (SOHO). We find a dependence of the O II/O III and O V/N V ratios with intensity. The degree of correlation we obtain on arcsecond spatial scales is consistent with size scales for the basic transition region structures that are significantly less than 1'' (730 km).

We present observations of the pure rotational transition 1₁₀ → 1₀₁ of ortho-H₂O vapor at 556.936 GHz made with the Submillimeter Wave Astronomy Satellite (SWAS) for comet C/1999 T1 (McNaught-Hartley). The emission was monitored during the postperihelion phase from (UT) 2001 February 2.00 to 11.07 and February 23.01 to April 5.95 for a Sun-comet distance, r_h, ranging from 1.41 to 2.05 AU. The water production rate, Q, derived from the observations depends on the assumptions regarding the electron abundance in the coma. We obtain Q ~ 4 × 10²⁸ s^-1 for the observations made in the first week of February (r_h ~ 1.4 AU) using a model in which the electron abundance has been normalized to the in situ measurements obtained in 1P/Halley. The value of Q decreases for larger r_h, and we derive ~1.5 × 10²⁸ s^-1 for the observations made at r_h ~ 2 AU in the last week of 2001 March. The inferred water production rate is larger by ~40% for a radiative transfer model with the electron abundance reduced by 80% because of the reduced excitation of water by electron collisions in this model. A comparison to literature data suggests that the SWAS results derived with the smaller electron abundance are in better agreement with independent measurements of Q obtained from observations of H₂O photodissociation products. A χ² fit to the variation of Q with the heliocentric distance gives a power-law index of -3.0 ± 0.5 (statistical) ±0.1 (systematic).

Recently, deuterium has been the focus of a high level of experimental and theoretical activity, sparked by a disagreement on the experimental value of the maximum compression along the principal Hugoniot. The behavior of deuterium at megabar pressures is not well understood. It is of great interest to understand how the current uncertainty on the hydrogen/deuterium equation of state (EOS) affects the inferred structures of Jupiter and Saturn. In particular, the mass of a core of heavy elements (other than H and He) and the total mass of those heavy elements in these two planets are quite sensitive to the EOS of hydrogen and constitute important clues to their formation process. We present a study of the range of structures allowed for Jupiter and Saturn by the current uncertainty in the hydrogen EOS and astrophysical observations of the two planets. An improved experimental understanding of hydrogen at megabar pressures and better determinations of the gravitational moments of both planets are necessary to put tight bounds on their internal structure.

We discuss aspects of non-LTE line formation for hydrogen in early-type stars. We evaluate the effect of variations in the electron-impact excitation cross sections in model atoms of differing complexity by comparison with observation. While the Balmer lines are basically unaffected by the choice of atomic data, the Paschen, Brackett, and Pfund series members allow us to discriminate between the different models. Non-LTE calculations based on the widely used approximation formulae of Mihalas, Heasley, & Auer and of Johnson fail to simultaneously reproduce the optical and IR spectra over the entire parameter range. The use of data from ab initio calculations up to principal quantum number n ≤ 7 largely solves the problem. We recommend a reference model using the available data. This model is of general interest because of the ubiquity of the hydrogen spectrum.

The fragmentation pathways of C₆H are studied using density functional theory. All of the transition states that were found, excluding the rearrangement to the C_5v structure, are significantly above the six-membered ring structure. This is consistent with the soft ionization experiments that find mostly dication production instead of fragmentation. Since the C_5v structure is probably of limited importance for multiple-ring polycyclic aromatic hydrocarbons, the computed barriers suggest that Coulomb explosion is not a significant channel under most astrophysical conditions, and therefore, the dications are long-lived and should be considered in modeling of the interstellar medium.

We present estimates of CN and Mg overabundances with respect to Fe for early-type galaxies in eight clusters over a range of richness and morphology. Spectra were taken from the Sloan Digital Sky Survey Data Release 1 and from William Hershel Telescope and Centro Astronómico Hispano-Alemán observations. Abundances were derived from absorption lines and single-burst population models, by comparing galaxy spectra with appropriately broadened synthetic model spectra. We detect correlations between the [Mg/CN] and [CN/Fe] values and the cluster X-ray luminosity. No correlation is observed for [Mg/Fe]. We also see a clear trend in the richness and morphology of the clusters. This is interpreted as giving varying formation timescales for CN, Mg, and Fe and a varying star formation history in early-type galaxies as a function of their environment: intermediate-mass early-type galaxies in more massive clusters are assembled on shorter timescales than in less massive clusters, with an upper limit of ~1 Gyr.

We report the detection of a 4 h Mpc long large-scale filament leading into the massive galaxy cluster MACS J0717.5+3745. The extent of this object well beyond the cluster's nominal virial radius (~2.3 Mpc) rules out prior interaction between its constituent galaxies and the cluster and makes it a prime candidate for a genuine filament as opposed to a merger remnant or a double cluster. The structure was discovered as a pronounced overdensity of galaxies selected to have V-R colors close to the cluster red sequence. Extensive spectroscopic follow-up of over 300 of these galaxies in a region covering the filament and the cluster confirms that the entire structure is located at the cluster redshift of z = 0.545. Featuring galaxy surface densities of typically 15 Mpc^-2 down to luminosities of 0.13L, the most diffuse parts of the filament are comparable in density to the clumps of red galaxies found around A851 in the only similar study carried out to date (Kodama et al.). Our direct detection of an extended large-scale filament funneling matter onto a massive distant cluster provides a superb target for in-depth studies of the evolution of galaxies in environments of greatly varying density and supports the predictions from theoretical models and numerical simulations of structure formation in a hierarchical picture.

We report the discovery of two damped Lyα systems (DLAs) near redshift z = 1 along a single quasar sight line (Q1727+5302) with neutral hydrogen column densities of N = (1.45 ± 0.15) × 10²¹ and (2.60 ± 0.20) × 10²¹ atoms cm^-2. Their sight line velocity difference of 13,000 km s^-1 corresponds to a proper separation of 106 h Mpc if interpreted as the Hubble flow (Ω_m = 0.3, Ω_Λ = 0.7). The random probability of such an occurrence is significantly less than 3%. Follow-up spectroscopy reveals neutral gas-phase Zn abundances of [Zn/H] = -0.58 ± 0.15 (26.5% solar) and -1.32 ± 0.28 (4.7% solar), respectively. The corresponding Cr abundances are [Cr/H] = -1.26 ± 0.15 (5.5% solar) and -1.77 ± 0.28 (1.7% solar), respectively, which is evidence for depletion onto grains. Follow-up IR images show the two most likely DLA galaxy candidates to have impact parameters of ≈22 and ≈32 h kpc if near z = 1. They are significantly underluminous relative to the galaxy population at z = 1. To investigate the possibility of additional high-N absorbers, we have searched the Sloan Digital Sky Survey database for z > 1 quasars within 30' of the original sight line. Five were found, and two show strong Mg II-Fe II absorption near z = 1, consistent with classical damped Lyα absorption ≈37% of the time, but almost always N > 10¹⁹ atoms cm^-2. Consequently, this rare configuration of four high-N absorbers with a total sight line velocity extent of 30,600 km s^-1 may represent a large filament-like structure stretching over a proper distance of 241 h Mpc along our sight line, and a region in space capable of harboring excessive amounts of neutral gas. Future studies of this region of the sky are encouraged.

At z = 0.1055, the gamma-ray burst GRB 031203 is the second nearest GRB known. Using observations from the Very Large Array and the Chandra X-Ray Observatory, we derive subarcsecond localizations of the radio and X-ray afterglow of this GRB. We present near-infrared observations of the supernova SN 2003lw, which exploded in the host galaxy of GRB 031203. Our deep high-resolution Magellan data establish that this SN is spatially coincident with the radio and X-ray localizations of the afterglow of GRB 031203 to subarcsecond precision and is thus firmly associated with the GRB. We use image differencing to subtract the bright emission from the host galaxy and measure the SN flux at ~5, ~7, and ~50 days after the GRB. Our J-band measurements are inconsistent with predictions derived by placing SN 1998bw (associated with GRB 980425) at z = 0.1055. In particular, our early data points show that before peak, SN 2003lw was significantly fainter in rest frame ~1.13 μm (observed J band) than SN 1998bw. We measure similar fluxes at ~7 and ~50 days after the GRB, suggesting that SN 2003lw had a light-curve shape that is quite different from that of SN 1998bw, the best-studied GRB-associated SN so far.

Recently, twin-peak quasi-periodic oscillations (QPOs) have been observed in a 3 : 2 ratio for three Galactic black hole microquasars with frequencies that have been shown to scale as 1/M, as expected for general relativisitic motion near a black hole. It may be possible to extend this result to distinguish between the following two disparate models that have been proposed for the puzzling ultraluminous X-ray sources (ULXs): (1) an intermediate-mass black hole (M ~ 10³M_☉) radiating very near the Eddington limit and (2) a conventional black hole (M ~10 M_☉) accreting at a highly super-Eddington rate with its emission beamed along the rotation axis. We suggest that one could discriminate between these models by detecting the counterpart of a Galactic twin-peak QPO in a ULX: the expected frequency for the intermediate-mass black hole model is only about 1 Hz, whereas for the conventional black hole model the expected frequency would be the ~100 Hz value observed for the Galactic microquasars.

We report on continued monitoring of the anomalous X-ray pulsar (AXP) 1E 1048.1-5937 using the Rossi X-R ay Timing Explorer. We confirm that this pulsar has exhibited significant pulsed flux variability. The principal features of this variability are two pulsed X-ray flares. Both flares lasted several months and had well-resolved few-week-long rises. The long rise times of the flares are a phenomenon not previously reported for this class of object. The epochs of the flare peaks were MJD 52,218.8±4.5 and 52,444.4±7.0. Both flares had shorter rise than fall times. The flares had peak fluxes of 2.21 ± 0.16 and 3.00 ± 0.13 times the quiescent value. We estimate a total 2-10 keV energy release of ~2.7 × 10⁴⁰ and ~2.8 × 10⁴¹ ergs for the flares, assuming a distance of 5 kpc. We also report large (factor of ~12) changes to the pulsar's spin-down rate on timescales of weeks to months, shorter than has been reported previously. We find marginal evidence for correlation between the flux and spin-down rate variability, with probability of nonrandom correlation 6%. We discuss the implications of our findings for AXP models.

We report on the measurement at 820 and 1400 MHz of the orbital modulation of the diffractive scintillation timescale from pulsar A in the double-pulsar system J0737-3039 using the Green Bank Telescope. With fits to this modulation, we determine the systemic velocity in the plane of the sky to be V_ISS ≃ 140.9 ± 6.2 km s^-1. The parallel and perpendicular components of this velocity with respect to the line of nodes of the pulsar's orbit are V_plane ≃ 96.0 ± 3.7 km s^-1 and V_perp ≃ 103.1 ± 7.7 km s^-1, respectively. The large V_perp implies that pulsar B was born with a kick speed of ≳100 km s^-1. Future Very Long Baseline Array determinations of the angular proper motion in conjunction with improved V_ISS measurements should provide a precise distance to the system. Using high-precision timing data and the V_ISS model, we estimate a best-fit orbital inclination of i = 887 ± 09.

We observed the isolated neutron star RX J0720.4-3125 with Chandra's Low Energy Transmission Grating Spectrometer, following the XMM-Newton discovery of the long-term spectral evolution of this source. The new observation shows that the spectrum of RX J0720.4-3125 has continued to change in the course of 5 months. It has remained hard, similar to the last XMM-Newton observation, but the strong depression observed with XMM-Newton at long wavelengths has disappeared. Contrary to the XMM-Newton observations, the new Chandra observation shows that the flux increase at short wavelength and the decrease at long wavelength do not necessarily occur simultaneously.

An analysis of Lyman series lines arising from hydrogen-like oxygen and neon ions in the coronae of the active RS CVn-type binaries II Peg and IM Peg, observed using the Chandra High Resolution Transmission Grating Spectrograph, shows significant decrements in the Lyα/Lyβ ratios as compared with theoretical predictions and with the same ratios observed in similar active binaries. We interpret these decrements in terms of resonance scattering of line photons out of the line of sight; these observations present the first strong evidence of this effect in active stellar coronae. The net line photon loss implies a nonuniform and asymmetric surface distribution of emitting structures on these stars. Escape probability arguments, together with the observed line ratios and estimates of the emitting plasma density, imply typical line-of-sight sizes of the coronal structures that dominate the X-ray emission of 10¹⁰ cm at temperatures of 3 × 10⁶ K and of 10⁸ cm at 10⁷ K. These sizes are an order of magnitude larger than predicted by simple quasi-static coronal loops models but are still very small compared to the several 10¹¹ cm radii of the underlying stars.

We investigate numerically the combined effects of supersonic turbulence, strong magnetic fields, and ambipolar diffusion on cloud evolution leading to star formation. We find that in clouds that are initially magnetically subcritical, supersonic turbulence can speed up star formation, through enhanced ambipolar diffusion in shocks. The speedup overcomes a major objection to the standard scenario of low-mass star formation involving ambipolar diffusion, since the diffusion timescale at the average density of a molecular cloud is typically longer than the cloud lifetime. At the same time, the strong magnetic field can prevent the large-scale supersonic turbulence from converting most of the cloud mass into stars in one (short) turbulence crossing time and thus alleviate the high efficiency problem associated with the turbulence-controlled picture for low-mass star formation. We propose that relatively rapid but inefficient star formation results from supersonic collisions of somewhat subcritical gas in strongly magnetized turbulent clouds. The salient features of this shock-accelerated, ambipolar diffusion-regulated scenario are demonstrated with numerical experiments.

We study the internal structure and the convective motions of giant extrasolar planets in order to calculate their dynamo-generated surface magnetic field and dipolar magnetic moment. Using evolutionary models, we investigate the existence of magnetism in planets with masses ranging from 0.3M_J to 10M_J and with rotation periods ranging from synchronism in "hot Jupiters" (with periods of 3-4 days) to the breakdown centrifugal limit (with periods of 2-5 hr). We find that the high Rayleigh and low Ekman numbers in the interior metallic-hydrogen region guarantee convective motions even for low-mass and evolved (aged) planets. The convective velocities estimated from mixing-length theory and from "magnetostrophic" balances yield high Reynolds and magnetic Reynolds numbers (~10³-10⁶), permitting the self-excited dynamo mechanism. Strong magnetism occurs in young massive and rapidly rotating extrasolar planets (with surface magnetic fields of ~30-60 G), but older or orbitally synchronized planets should have fields of ~1 G. We discuss the implications of these results for the detection of magnetic fields in extrasolar planets.

High-resolution G-band images of the interior of a supergranulation cell show ubiquitous bright points (BPs; some 0.3 BPs per Mm²). They are located in intergranular lanes and often form chains of elongated blobs whose smallest dimension is at the resolution limit (135 km on the Sun). Most of them live for a few minutes, having peak intensities from 0.8 to 1.8 times the mean photospheric intensity. These BPs are probably tracing intense magnetic concentrations, whose existence has been inferred in spectropolarimetric measurements. Our finding provides a new convenient tool for the study of the internetwork magnetism, so far restricted to the interpretation of weak polarimetric signals.

We report phase-difference and travel-time analyses of propagating chromospheric oscillations in and around an equatorial coronal hole as observed by TRACE. Our results suggest a significant change in atmospheric conditions at the base of the chromosphere inside the coronal hole relative to its boundary and quiet-Sun regions.

Atmospheric loss induced by an incident plasma, often called atmospheric sputtering, can significantly alter the volatile inventories of solar system bodies. Based on the present atmospheric sputtering rate, the net loss of nitrogen from Titan in the last 4 Gyr was small, consistent with Titan retaining a component of its primordial atmosphere. However, atmospheric sputtering by the magnetospheric plasma ions and by pickup ions, even at present levels, would have caused the loss of a large, residual Titan-like atmosphere from Io and Europa and a significant fraction of such an atmosphere from Ganymede. At Callisto, higher magnetospheric plasma densities would have been required for the loss of such an atmosphere. Since higher plasma densities were probable in earlier epochs, the evolution of the volatile inventories of each of the Galilean satellites has been profoundly affected by the interaction of their atmospheres with the Jovian magnetospheric plasma.