Table of contents

Optically thin cooling gas at most temperatures above 30 K will make condensations by pressure, pushing material into cool, dense regions. This works without gravity. Cooling condensations will flatten and become planar/similarity solutions. Most star formation may start from cooling condensations, where gravity is only important in the later stages. The idea that some of the dark matter could be pristine white dwarfs that condensed slowly onto planetary-sized seeds without firing nuclear reactions is found lacking. However, recent observations indicate 50 times more halo white dwarfs than have previously been acknowledged, enough to make the halo fraction observed as MACHOs. A cosmological census shows that only 1% of the mass of the universe is of known constitution.

We study the complementarity between the cosmological information obtainable with the Planck Surveyor and the large-scale structure (LSS) redshift surveys in Λ cold+hot dark matter (ΛCHDM) cosmologies. We compute the initial full phase-space neutrino distribution function for ΛCHDM models by using numerical simulations. As initial conditions, we adopt the HDM density fluctuation power spectrum normalized on the basis of the analysis of the local cluster X-ray temperature function and derive the initial neutrino phase-space distribution at each spatial wavenumber k by using the Zel'dovich approximation. These initial neutrino phase-space distributions are implemented in the CMBFAST code for the integration of the coupled linearized Einstein, Boltzmann, and fluid equations in k-space. We find that the relative bias between the cosmic microwave background (CMB) temperature fluctuations and the underlying matter density fluctuation power spectrum in COBE Differential Microwave Radiometer normalization is given by the CDM component normalized according to the abundance of rich clusters at the present time. We use the Fisher information matrix approximation to constrain a multidimensional parameterization of the ΛCHDM model by jointly considering CMB and large-scale structure data according to the Planck and the Sloan Digital Sky Survey experimental specifications and by taking into account redshift distortions and nonlinear effects on the matter power spectrum. We found that, although the CMB anisotropy and polarization measurements tend to dominate the constraints on most of the cosmological parameters, the additional small-scale LSS data help to break the parameter degeneracies. This work has been done in the framework of the Planck Low Frequency Instrument activities.

During the recombination epoch, cosmic background photons coupled not only to free electrons through Thompson scattering, but also to neutral hydrogen through Rayleigh scattering. This latter is a ~2% effect for photons near the peak of the photon energy distribution at z = 800 and a ~0.2% effect at z = 1100. Including Rayleigh scattering in the calculation reduces Silk damping at fixed redshift, alters the position of the surface of last scattering, and alters the propagation of acoustic waves. We estimate the amplitude of these effects. For the Microwave Anisotropy Probe (MAP), Rayleigh scattering increases the anisotropy spectrum by 0.1% at the most. For the highest frequencies of the Planck Surveyor, the effects of Rayleigh scattering are much more dramatic (decreasing the anisotropy spectrum by 3% at ν ~ 550 GHz and multipole number l ~ 1000). The relative difference between the spectra of low and high frequencies is imposed by an oscillation with a function of multipole l, and the oscillation amplitude is up to 0.5% between 100 and 550 GHz. Rayleigh scattering also slows the decoupling between radiation and matter, but the effect is undetectably small.

We propose a new method for extracting non-Gaussian signatures on isotemperature statistics in the cosmic microwave background (CMB) sky, which are induced by the gravitational lensing due to the intervening large-scale structure of the universe. To develop the method, we focus on a specific statistical property of the intrinsic Gaussian CMB field: a field point in the map that has a larger absolute value of the temperature threshold tends to have a larger absolute value of the curvature parameter defined by a trace of the second-derivative matrix of the temperature field, while the ellipticity parameter similarly defined is uniformly distributed independently of the threshold because of the isotropic nature of the Gaussian field. Weak lensing then causes a stronger distortion effect on the isotemperature contours with higher thresholds and especially induces a coherent distribution of the ellipticity parameter correlated with the threshold as a result of the coupling between the CMB curvature parameter and the gravitational tidal shear in the observed map. These characteristic patterns can be statistically picked up by considering three independent characteristic functions, which are obtained from the averages of quadratic combinations of the second-derivative fields of the CMB over isotemperature contours with each threshold. Consequently, we find that the lensing effect generates non-Gaussian signatures on those functions that have a distinct functional dependence on the threshold. We test the method using numerical simulations of CMB maps and show that the lensing signals can be measured definitely, provided that we use CMB data with sufficiently low noise and high angular resolution.

By using a one-zone chemical and spectrophotometric evolution model of a disk galaxy undergoing a dusty starburst, we investigate numerically the optical spectroscopic properties in order to explore galaxy evolution in distant clusters. We adopt an assumption that the degree of dust extinction (represented by A_V) depends on the ages of starburst populations in such a way that younger stars have larger A_V (originally referred to as selective dust extinction by Poggianti & Wu). In particular, we investigate how the time evolution of the equivalent widths of [O II] λ3727 and Hδ are controlled by the adopted age dependence. This leads to the following three main results: (1) If a young stellar population (with an age of ~10⁶ yr) is more heavily obscured by dust than an old one (>10⁸ yr), the galaxy can show an "e(a)" spectrum characterized by strong Hδ absorption and relatively modest [O II] emission. (2) A dusty starburst galaxy with an e(a) spectrum can evolve into a poststarburst galaxy with an a + k (or k + a) spectrum 0.2 Gyr after the starburst and then into a passive one with a k-type spectrum 1 Gyr after the starburst. This result clearly demonstrates an evolutionary link between galaxies with different spectral classes [i.e., e(b), e(a), a + k, k + a, and k]. (3) A dusty starburst galaxy can show an a + k or k + a spectrum even in the dusty starburst phase if the age-dependence of dust extinction is rather weak; i.e., if young starburst populations with different ages (≤10⁷ yr) are uniformly obscured by dust.

Far-ultraviolet spectra obtained with FUSE show that the strong C II λ1036 interstellar absorption line is essentially black in five of the UV-brightest local starburst galaxies. Because the opacity of the neutral ISM below the Lyman edge will be significantly larger than in the C II line, these data provide strong constraints on the escape of ionizing radiation from these starbursts. Interpreted as a uniform, absorbing slab, the implied optical depth at the Lyman edge is huge (τ₀ ≥ 10²). Alternatively, the areal covering factor of opaque material is typically ≥94%. Thus, the fraction of ionizing stellar photons that escape the ISM of each galaxy is small: our conservative estimates typically yield f_esc ≤ 6%. Inclusion of extinction due to dust will further decrease f_esc. An analogous analysis of the rest-UV spectrum of the star-forming galaxy MS 1512-cB58 at z = 2.7 leads to similar constraints on f_esc. These new results agree with the constraints provided by direct observations below the Lyman edge in a few other local starbursts. However, they differ from the recently reported properties of star-forming galaxies at z ≥ 3. We assess the idea that the strong galactic winds seen in many powerful starbursts clear channels through their neutral ISM. We show empirically that such outflows may be a necessary—but not sufficient—part of the process of creating a relatively porous ISM. We note that observations will soon document the cosmic evolution in the contribution of star-forming galaxies to the metagalactic ionizing background, with important implications for the evolution of the IGM.

Diffuse γ-ray radiation in galaxies is produced by cosmic-ray interactions with the interstellar medium. With the completion of EGRET observations, the only extragalactic object from which there has been a positive detection of diffuse γ-ray emission is the Large Magellanic Cloud. We systematically estimate the expected diffuse γ-ray flux from Local Group galaxies and determine their detectability by a new generation of γ-ray observatories such as the Gamma-Ray Large Area Space Telescope (GLAST). For each galaxy, the expected γ-ray flux depends only on its total gas content and its cosmic-ray flux. We present a method for calculating cosmic-ray flux in these galaxies in terms of the observed rate of supernova explosions, in which cosmic-ray acceleration is believed to take place. The difficulty in deriving accurate supernova rates from observational data is a dominant uncertainty in our calculations. We estimate the γ-ray flux for Local Group galaxies and find that our predictions are consistent with the observations for the LMC and with the observational upper limits for the SMC and M31. Both the Andromeda galaxy, with a flux of ~1.0 × 10^-8 photons s^-1 cm^-2 above 100 MeV, and the SMC, with a flux of ~1.7 × 10^-8 photons s^-1 cm^-2 above 100 MeV, are expected to be observable by GLAST. M33 is at the limit of detectability with a flux of ~0.11 × 10^-8 s^-1 cm^-2. Other Local Group galaxies are at least 2 orders of magnitude below GLAST sensitivity.

We present results from a decimetric radio survey undertaken with the Very Large Array as part of a longer term goal to intercompare star formation and dust extinction diagnostics on a galaxy-by-galaxy basis for a representative sample of nearby galaxies. For our survey field, Selected Area 57, star formation rates derived from 1.4 GHz luminosities are compared with earlier nebular emission-line and ultraviolet (UV) continuum diagnostics. We find broad correlations, over several decades in luminosity, between the Hα, UV continuum, and 1.4 GHz diagnostics. However, the scatter in these relations is found to be larger than observational errors, with offsets between the observed relations and those expected assuming constant star formation histories and luminosity-independent extinction models. We investigate the physical origin of the observed relations and conclude that the discrepancies between different star formation diagnostics can only be partly explained by simple models of dust extinction in galaxies. These models cannot by themselves explain all the observed differences, introducing the need for temporally varying star formation histories and/or more complex models of extinction to explain the entire data set.

We examine a representative sample of 35 Seyfert 2 nuclei. Previous work has shown that nearly half (15) of these nuclei show the direct (but difficult to detect) spectroscopic signature at optical/near-UV wavelengths of the hot massive stars that power circumnuclear starbursts. In the present paper we examine a variety of more easily measured quantities for this sample, such as the equivalent widths of strong absorption features, continuum colors, emission line equivalent widths, emission line ratios and profiles, far-IR luminosities, and near-UV surface brightness. We compare the composite starburst + Seyfert 2 nuclei to "pure" Seyfert 2 nuclei, Starburst galaxies, and normal galactic nuclei. Our goals are to verify whether the easily measured properties of the composite nuclei are consistent with the expected impact of a starburst and to investigate alternative less demanding methods to infer the presence of starbursts in Seyfert 2 nuclei, applicable to larger or more distant samples. We show that starbursts do indeed leave clear and easily quantifiable imprints on the near-UV to optical continuum and emission line properties of Seyfert 2's. Composite starburst + Seyfert 2 systems can be recognized by: (1) a strong "featureless continuum" (FC), which dilutes the Ca II K line from old stars in the host's bulge to an equivalent width W_K < 10 Å; (2) emission lines whose equivalent widths are intermediate between starburst galaxies and "pure" Seyfert 2's; (3) relatively low excitation line ratios, which indicate that part of the gas ionization in these Seyfert 2's (typically ~50% of Hβ) is due to photoionization by OB stars; (4) large far-IR luminosities (≳10¹⁰L_☉); (5) high near-UV surface brightness (~10³L_☉ pc^-2). These characteristics are all consistent with the expected impact of circumnuclear starbursts on the observed properties of Seyfert 2's. Furthermore, they offer alternative empirical diagnostics of the presence of circumnuclear starbursts from a few easily measured quantities.

We have carried out a survey with the Chandra X-Ray Observatory of a sample of 10 bright broad absorption line (BAL) quasars (QSOs). Eight of 10 sources are detected. The six brightest sources have only high-ionization BALs (hiBALs), while the four faintest all show low-ionization BALs (loBALs). We perform a combined spectral fit for hiBAL QSOs (384 counts total; 0.5-6 keV) to determine the mean spectral parameters of this sample. We derive an underlying best-fit power-law slope Γ = 1.8 ± 0.35, which is consistent with the mean slope for radio-quiet QSOs from ASCA, but BAL QSOs require a (rest-frame) absorbing column of 6.5 × 10²² cm^-2, with a partial covering fraction of ~80%. The optical-to-X-ray spectral slope (α_ox from 2500 Å to 2 keV) varies from 1.7 to 2.4 across the full sample, consistent with previous results that BAL QSOs appear to be weak soft X-ray emitters. Removing the absorption component from our best-fit spectral model yields a range of α_ox from 1.55 to 2.28. All six hiBAL QSOs have deabsorbed X-ray emission consistent with non-BAL QSOs of similar luminosity. The spectral energy distributions of the hiBAL QSOs—both the underlying power-law slope and α_ox—provide the first conclusive evidence that BAL QSOs have appeared to be X-ray weak because of intrinsic absorption and that their underlying emission is consistent with non-BAL QSOs. By contrast, the removal of the best-fit absorption column detected in the hiBAL QSOs still leaves the four loBAL QSOs with values of α_ox > 2 that are unusually X-ray faint for their optical luminosities, which is consistent with other evidence that loBALs have higher column density, dustier absorbers. Important questions of whether BAL QSOs represent a special line of sight toward a QSO nucleus or rather an early evolutionary or high-accretion phase in a QSO lifetime remain to be resolved, and the unique properties of loBAL QSOs will be an integral part of that investigation.

The mini-broad absorption line (BAL) quasar RX J0911.4+0551 was observed with the Advanced CCD Imaging Spectrometer of the Chandra X-Ray Observatory for ~29 ks as part of a gravitational lens (GL) survey aimed at measuring time delays. Timing analysis of the light curve of the lensed image A2 shows a rapid flux variation with a duration of about 2000 s. A Kolmogorov-Smirnov test shows that the probability that a constant intensity source would produce the observed variability is less than ~0.2%. We discuss possible origins for the observed short-term X-ray variability. Our gravitational lens models for the RX J0911.4+0551 GL system predict a time delay of less than a day between images A1 and A2. The rapid variability combined with the predicted short time delay make RX J0911.4+0551 an ideal system to apply the GL method for estimating the Hubble constant. We describe the prospects of measuring H₀ within single X-ray observations of GL systems with relatively short time delays. Modeling of the spectrum of the mini-BAL quasar RX J0911.4+0551 suggests the presence of an intrinsic absorber. Partial covering models are slightly preferred over models that contain absorption due to intrinsic ionized or neutral gas.

We report here on multifrequency VLBA observations of three extragalactic sources within 1° of the Galactic center. These sources have been used as astrometric reference sources for VLA and VLBA determinations of the proper motion of Sagittarius A*, the compact nonthermal radio source in the Galactic center. Each source has a main component with a brightness temperature in excess of 10^7.5 K, confirming that the sources are active galactic nuclei. The sources have simple structure that can be characterized by one or two Gaussian components. The frequency dependence of the structure indicates that the positions of Sgr A* determined by the VLA astrometry of Backer & Sramek at 4.8 and 8.4 GHz should have an offset of ~2 mas. This offset is in the same direction as the 5 mas shift measured by Backer & Sramek. The structure is unlikely to bias the published 43 GHz VLBA results of Reid et al. Motions of components in the calibrator sources could lead to errors in the proper motion of Sgr A* on the order of a few km s^-1. All three sources show frequency-dependent structure consistent with scattering significantly stronger than that of the Galactic scattering model of Taylor & Cordes but significantly weaker than that of the hyperstrong Galactic center scattering. Combined with other observations, this suggests the existence of a new component of Galactic scattering located several kpc from the Galactic center.

We discuss the interstellar absorption lines found in Far Ultraviolet Spectroscopic Explorer spectra of the Wolf-Rayet binary Sk 108, which is located in the northeastern part of the main "bar" of the Small Magellanic Cloud. The spectra cover the 988-1187 Å wavelength range at a resolution of about 12,000 and a signal-to-noise ratio of 20-40. We use detailed component information from higher resolution near-UV and optical spectra to model the far-UV lines of similarly distributed species. Both the Galactic and SMC gas toward Sk 108 seem to be predominantly neutral, although a significant fraction of the SMC gas is ionized. The column densities of P II, S II, and Ar I are consistent with essentially solar ratios, relative to N(Zn II), in both the Galactic and SMC gas; the column density of N I remains somewhat uncertain. Molecular hydrogen is present in the Galactic gas, with properties similar to those found in low mean density Galactic lines of sight and in the Galactic gas toward several other LMC and SMC stars. We report a tentative detection of H₂ in the SMC gas for J = 1 and 3, with rotational level populations consistent with an excitation temperature on the order of 1000 K—similar to the H₂ found in diffuse Galactic gas toward ζ Puppis. Strong absorption from N III, S III, and Fe III has revealed a significant ionized component, particularly in the SMC; O VI is present, but relatively weak, especially in the Galactic gas. The N(C )/N(O ) ratio varies somewhat within the SMC, suggesting that several processes may contribute to the observed high ion abundances.

We present new long-slit CCD spectra of O II permitted lines and [O III] forbidden lines in the Ring Nebula NGC 6720. These observations provide spatially resolved information on both O II and [O III] over the 70'' diameter of the main shell. We find significant differences in the spatial distribution of the O II lines and [O III] λ4959. The [O III] emission follows the Hβ emission measure, peaking slightly radially inward from the Hβ peak. The O II emission peaks inside the [O III] emission. This suggests that radiative recombination may not be the primary mechanism for producing the O II lines. O⁺² abundances derived from O II lines are 5-10 times larger than those derived from [O III] in the region within 20'' of the central star. Outside of this region, however, the O II-derived and [O III]-derived abundances agree to within 0.2-0.3 dex. The electron temperature derived from [O III] lines rises smoothly from about 10,000 K in the outer shell to about 12,000 K in the center; we see no evidence for a temperature jump that would be associated with a shock. If temperature fluctuations are responsible for the discrepancy in O⁺² abundances, the average temperature would have to be approximately 6500 K in the He⁺² zone and about 9000 K in the outer shell in order to force the [O III]-derived abundance to equal that derived from O II. This would conflict with ionization models for planetary nebulae, which predict that the temperature is higher in the He⁺² region close to the ionizing star. We therefore argue that temperature fluctuations cannot explain the abundance discrepancy. A comparison of the spatial distribution of O II emission with the location of dusty knots shows that the O II recombination lines do not peak where the dense knots are located, creating difficulties for models that explain the recombination line/forbidden line discrepancy by density fluctuations. We examine the possibility that high-temperature dielectronic recombination in a central hot bubble enhances the recombination line strengths in the central part of the nebula. However, comparison of recombination rates with collisional excitation rates shows that the increase in recombination emission due to dielectronic recombination at T ≈ 10⁵ K is not sufficient to overcome the increase in collisonally excited emission. We are unable to find a completely satisfactory model to explain the discrepancy between recombination line and forbidden line abundances.

We discuss the formation of planetary nebulae (PNs) having a pair of lobes, or multilobes, in their inner region, surrounded by an elliptical or spherical shell or halo. Both elliptical and bipolar PNs are considered; when the lobes are much smaller than the main elliptical shell, the PN is termed elliptical, while when the lobes are the main structure of the nebula, the PN is termed bipolar. We suggest that most of these PNs are formed by wide binary systems with final orbital periods in the range of ~40-10⁴ yr, such that there is no strong tidal interaction. The outer, more spherical structure is formed from the early asymptotic giant branch (AGB) wind. Toward the end of the AGB, the mass-loss rate increases and wind velocity possibly decreases, making the conditions for the formation of an accretion disk around the wide companion more favorable. We assume that once a massive enough accretion disk is formed around the accreting companion, it blows jets or a collimated fast wind, which leads to the formation of a pair of lobes in the inner region. In cases of a precessing accretion disk, a multilobe structure can be formed. We conduct a population synthesis study of such systems and find that overall ~5%-20% of all PNs are formed by such binary systems. The exact percentage strongly depends on the wind velocities of stars about to leave the AGB. In about half of these systems, the initially more massive star is the AGB star and the accretor is a main-sequence star, while in the other half the initially less massive star is the AGB star and it has a white dwarf accretor. We also estimate that ~20%-40% of these systems possess observable departure from axisymmetry; e.g., the central star is not in the center of the nebula. Our population synthesis not only supports the binary model for formation of these types of PNs, e.g., Hu 2-1, He 2-113, He 2-47, and M1-37, but more generally supports the binary model for the formation of bipolar and many elliptical PNs.

Using a series of radiative transfer models of circumstellar dust shells, we explore the physical origins of the variety of shapes of the 10 μm silicate feature seen in the spectra of oxygen-rich circumstellar dust shells. In order to match the full range of observed spectral shapes, the models explore four parameters: the relative abundance of amorphous alumina and amorphous silicates, the inner dust shell radius, the optical depth, and the geometric thickness of the shell. Optically thin shells dominated by amorphous silicate grains reproduce the classic narrow silicate feature at 10 μm. Increasing the optical depth of the shell produces spectral features at 10 μm with stronger components at 11 μm, but to match the [12]-[25] IRAS colors, these optically thick shells must be geometrically thin (i.e., have a truncated outer radius). Spectra with broad, low-contrast emission features peaking at wavelengths longer than ~11 μm originate from optically thin shells composed of amorphous alumina. These findings provide a physical basis for the silicate dust sequence defined by Sloan & Price. We suggest that the [12]-[25] color is an indicator of geometric shell thickness. Thin shells can only arise if the star ejects mass and forms dust in a noncontinuous process.

We present observations of the ³P₁-³P₀ fine-structure transition of atomic carbon [C I], the J = 3-2 transition of CO, and the J = 1-0 transitions of ¹³CO and C¹⁸O toward DR 15, an H II region associated with two mid-infrared dark clouds (IRDCs). The ¹³CO and C¹⁸O J = 1-0 emissions closely follow the dark patches seen in optical wavelength, showing two self-gravitating molecular cores with masses of 2000 and 900 M_☉, respectively, at the positions of the cataloged IRDCs. Our data show a rough spatial correlation between [C I] and ¹³CO J = 1-0. Bright [C I] emission occurs in the relatively cold gas behind the molecular cores but does not occur in either highly excited gas traced by CO J = 3-2 emission or in the H II region/molecular cloud interface. These results are inconsistent with those predicted by standard photodissociation region models, suggesting an origin for interstellar atomic carbon unrelated to photodissociation processes.

We present high-resolution (R ~ 1500-2000) spectra of the 4.27 μm asymmetric stretching feature of solid CO₂ in eight lines of sight observed with the Short Wavelength Spectrometer of the Infrared Space Observatory. Two of the sources are field stars located behind the Taurus molecular cloud; the others are young stellar objects (YSOs) of predominantly low-to-intermediate mass. We find a significant source-to-source variation in the solid CO₂/H₂O abundance ratio in our sample: two lines of sight, Elias 18 and RAFGL 989, have CO₂ abundances of ~ 34%-37%, considerably higher than in other lines of sight studied to date. In agreement with a previous study of Elias 16, we confirm a substantial (~20%) abundance of solid CO₂ relative to H₂O in the quiescent intracloud medium. We compare the CO₂ profiles with laboratory spectra of interstellar ice analogs from the Leiden Observatory Laboratory database. Results show that the 4.27 μm profiles toward field stars and embedded low-mass objects are remarkably similar to each other and seem to originate mostly in cold H₂O-rich ice. In two higher mass YSOs (RAFGL 989 and S255 IRS1), the profiles are clearly different, and at least the latter source shows signs of thermal processing.

Forty-eight candidate massive protostar regions were surveyed for methyl cyanide (CH₃CN) emission using the Heinrich Hertz Telescope (HHT) on Mount Graham, AZ. CH₃CN J = 12-11 emission at 220 GHz was detected toward 25 regions. Thirteen of these are new detections, yielding a substantial number of new candidates for massive protostars. The CH₃¹³CN (J = 12-11) isotopomer was included in the bandpass of all observed sources and was detected toward eight sources in our sample. These sources are probably optically thick in some of the CH₃CN lines. CH₃CN J = 25-24 emission at 460 GHz was detected in at least one K-component toward five of 11 sources observed in this transition. Using a rotational equilibrium temperature analysis we estimate the CH₃CN rotation temperatures and beam-averaged column densities for 20 of the sources.

Observations of the HH 7-11 outflow were carried out in multiple lines of CS and NH₃ in order to trace the structure of the outflow and its interaction with the surrounding medium. The lower-J CS observations show evidence for a low-velocity (few km s^-1) CS bipolar flow in the same sense as the CO outflow. A dense (≳10⁶ cm^-3) CS and NH₃ ridge perpendicular to the outflow is seen around SVS 13 (the driving source of the outflow), VLA 3 (a recently detected 6 cm source), and SVS 13B (a class 0 protostar that is placed ~15'' southwest of SVS 13—see the 1998 work by Bachiller and colleagues). The CS observations at ambient velocities clearly show an evacuated cavity to the southeast of SVS 13, surrounding HH 10 and 11, and ending at a wide ridge of gas just downwind of HH 8. The NH₃ observations show evidence of gas heated to temperatures ≳40 K in at least three distinct regions: in the vicinity of SVS 13 and VLA 3, just downwind of HH 8, and near SVS 13B. In the case of the circumstellar material, either heating by the embedded star(s) or shock heating from wind-cloud interactions could be responsible; the gas downwind of HH 8 is either directly shock heated or is radiatively heated by the shock traced by HH 8.

We investigate a stationary pair-production cascade in the outer magnetosphere of a spinning neutron star. The charge depletion due to global flows of charged particles causes a large electric field along the magnetic field lines. Migratory electrons and/or positrons are accelerated by this field to radiate curvature gamma rays, some of which collide with the X-rays to materialize as pairs in the gap. The replenished charges partially screen the electric field, which is self-consistently solved together with the distribution functions of particles and gamma rays. If no current is injected at either of the boundaries of the accelerator, the gap is located around the conventional null surface, where the local Goldreich-Julian charge density vanishes. However, we first find that the gap position shifts outward (or inward) when particles are injected at the inner (or outer) boundary. Applying the theory to the Crab pulsar, we demonstrate that the pulsed TeV flux does not exceed the observational upper limit for moderate infrared photon density and that the gap should be located near to or outside of the conventional null surface so that the observed spectrum of pulsed GeV fluxes may be emitted via a curvature process. Some implications of the existence of a solution for a super Goldreich-Julian current are discussed.

We study the temporal and coarse spectral properties of 268 bursts from SGR 1806-20 and 679 bursts from SGR 1900+14, all observed with the Rossi X-Ray Timing Explorer/Proportional Counter Array. Hardness ratios and temporal parameters, such as T₉₀ durations and τ₉₀ emission times are determined for these bursts. We find a lognormal distribution of burst durations, ranging over more than 2 orders of magnitude: T₉₀ ~ 10^-2 to ≳1 s, with a peak at ~0.1 s. The burst light curves tend to be asymmetrical, with more than half of all events showing rise times t_r < 0.3 T₉₀. We find that there exists a correlation between the duration and fluence of bursts from both sources. We also find a significant anticorrelation between hardness ratio and fluence for SGR 1806-20 bursts and a marginal anticorrelation for SGR 1900+14 events. Finally, we discuss possible physical implications of these results within the framework of the magnetar model.

The soft gamma-ray repeater SGR 1900+14 entered a remarkable phase of activity during the summer of 1998. This activity peaked on 1998 August 27, when a giant periodic γ-ray flare resembling the famous 1979 March 5 event from SGR 0526-66 was recorded. Two days later (August 29), a strong, bright burst was detected simultaneously with the Rossi X-Ray Timing Explorer (RXTE) and the Burst and Transient Source Experiment (BATSE). This event reveals several similarities to the giant flares of August 27 and March 5 and shows a number of unique features not previously seen in SGR bursts. Unlike typically short SGR bursts (duration ~0.1 s), this event exhibits a 3.5 s burst peak that was preceded by an extended (~1 s) complex precursor, and followed by a long (~10³ s) pulsating tail modulated at the 5.16 s stellar rotation period. Spectral analysis shows a striking distinction between the spectral behavior of the precursor, main peak, and long tail. While the spectrum is uniform during the peak, a significant hard-to-soft spectral evolution is detected in both the precursor and tail emissions. Temporal behavior shows a sharp rise (~10 ms) at the precursor onset, a rapid cutoff (~17 ms) at the end of the burst peak, and a gradual decay (~17 minutes) of the pulsating tail. The tail pulsations show a simple pulse profile that did not evolve with time. The contrasted spectral and temporal signatures of the event suggest that the precursor, main peak, and extended tail are produced by different physical mechanisms, and that the observed tail represents a new emission component from SGRs. We discuss these features and their implications in the context of the magnetar model. The bright 3.5 s component is consistent with a very hot (kT ~ 1 MeV) trapped fireball, and the precursor with magnetospheric emission in which the radiating particles are heated more continuously. Less than 1% of the fireball energy will be conducted into the exposed surface of the neutron star, thereby dissociating heavy elements and even helium, and inducing rapid transformations between neutrons and protons. The extended "afterglow" tail of the August 29 burst is consistent with a cooling hot spot of small area (~13 km²), and indicates that the energy release in an SGR burst is strongly localized.

We report on long-term monitoring of the anomalous X-ray pulsar (AXP) 1E 1048.1-5937 using the Rossi X-Ray Timing Explorer (RXTE). The timing behavior of this pulsar is different from that of other AXPs being monitored with RXTE. In particular, we show that the pulsar shows significant deviations from simple spin-down such that phase-coherent timing has not been possible over time spans longer than a few months. We find that the deviations from simple spin-down are not consistent with single "glitch" type events nor are they consistent with radiative precession. We show that in spite of the rotational irregularities, the pulsar exhibits neither pulse profile changes nor large pulsed flux variations. We discuss the implications of our results for AXP models. We suggest that 1E 1048.1-5937 may be a transition object between the soft gamma-ray repeater and AXP populations and may be the AXP most likely to one day undergo an outburst.

We investigate the properties of r-modes characterized by the regular eigenvalue problem in slowly rotating, relativistic polytropes. Our numerical results suggest that discrete r-mode solutions for the regular eigenvalue problem exist only for restricted polytropic models. In particular, the r-mode associated with l = m = 2, which is considered to be the most important for gravitational radiation-driven instability, does not have a discrete mode as a solution of the regular eigenvalue problem for polytropes with polytropic index N > 1.18, even in the post-Newtonian order. Furthermore, for an N = 1 polytrope, which is employed as a typical neutron star model, discrete r-mode solutions for the regular eigenvalue problem do not exist for stars whose relativistic factor M/R is larger than about 0.1, where M and R are stellar mass and stellar radius, respectively.

We report deep near-infrared and optical observations of the X-ray point source in the Cassiopeia A supernova remnant, CXO J232327.9+584842. We have identified a J = 21.4 ± 0.3 mag and a K_s = 20.5 ± 0.3 mag source within the 1 σ error circle, but we believe this source is a foreground Population II star with T_eff = 2600-2800 K at a distance of ≈2 kpc that could not be the X-ray point source. We do not detect any sources in this direction at the distance of Cas A and therefore place 3 σ limits of R ≳ 25 mag, F675W ≳ 27.3 mag, J ≳ 22.5 mag, and K_s ≳ 21.2 mag (and roughly H ≳ 20 mag) on emission from the X-ray point source, corresponding to M_R ≳ 8.2 mag, M_F675W ≳ 10.7 mag, M_J ≳ 8.5 mag, M_H ≳ 6.5 mag, and M ≳ 8.0 mag, assuming a distance of 3.4 kpc and an extinction of A_V = 5 mag.

Using data from five years of Rossi X-Ray Timing Explorer observations, we investigate the X-ray spectral and timing properties of GRS 1915+105 during the hard steady states. The broadband energy spectrum of the source during these periods is dominated by an extended hard component with a characteristic cutoff or break at ~10-120 keV. The power density spectrum of the source's rapid aperiodic variability shows a dominant band-limited white-noise component breaking at a few hertz, accompanied by a group of strong quasi-periodic oscillation peaks and in some cases an additional high-frequency noise component with a characteristic cutoff at ~60-80 Hz. According to the results of our simultaneous X-ray spectral and timing analysis, the behavior of the source during the hard steady states can be reduced to two major, distinct types. (1) Type I states: The dominant hard component of the energy spectrum has characteristic quasi-exponential cutoff at 60-120 keV. The broadband power density spectrum of the source shows a significant high-frequency noise component with a cutoff at ~60-80 Hz. (2) Type II states: The hard spectral component has a break in its slope at ~12-20 keV. The high-frequency part of the power density spectrum fades quickly, lacking significant variability at frequencies higher than ~30 Hz. These two types of X-ray hard states are also clearly distinguished by their properties in the radio band: while during the type I observations the source tends to be "radio quiet," the type II observations are characterized by a high level of radio flux ("plateau" radio states). In this work we demonstrate the aforementioned differences using data from 12 representative hard steady state observations. We conclude that the difference between these two types can probably be explained in terms of a difference in accretion flow structure in the immediate vicinity of the compact object due to the presence of relativistic outflow of matter.

The motion and variability of the radio components in the low-mass X-ray binary system Sco X-1 have been monitored with extensive VLBI imaging at 1.7 and 5.0 GHz over 4 yr, including a 56 hr continuous VLBI observation in 1999 June. We detect one strong and one weak compact radio component, moving in opposite directions from the radio core. Their relative motion and flux densities are consistent with relativistic effects, from which we derive an average component speed of v/c = 0.45 ± 0.03 at an angle of 44° ± 6° to the line of sight. This inclination of the binary orbit suggests a mass of the secondary star that is less than 0.9 M_☉, assuming a neutron star mass of 1.4 M_☉. We suggest that the two moving radio components consist of ultrarelativistic plasma that is produced at a working surface where the energy in dual-opposing beams disrupt. The radio lobe advance velocity is constant over many hours, but differs among lobe-pairs: 0.32c, 0.46c, 0.48c, and 0.57c. A lobe-pair lifetime is less than 2 days, with a new pair formed near the core within a day. The lobe flux has flux density that is variable over a timescale of 1 hr, has a measured minimum size of 1 mas (4 × 10⁸ km), and is extended perpendicular to its motion. This timescale and size are consistent with an electron radiative lifetime of less than 1 hr. Such a short lifetime can be caused by synchrotron losses if the lobe magnetic field is 300 G or by adiabatic expansion of the electrons as soon as they are produced at the working surface. The lobes also show periods of slow expansion and a steepening radio spectrum. Two of the core flares are correlated with the lobe flares under the assumption that the flares are produced by an energy burst traveling down the beams with a speed greater than 0.95. The radio morphology for Sco X-1 differs from most other Galactic jet sources. Possible reasons for the morphology difference are that Sco X-1 is associated with a neutron star, it is a persistent X-ray source, and the source is viewed significantly away from the angle of motion. However, the lobes in Sco X-1 are similar to the hot spots found in many extragalactic radio double sources. Scaling the phenomena observed in Sco X-1 to extragalactic sources implies radio source hot-spot variability timescales of 10⁴ yr and hot-spot lifetimes of 10⁵ yr.

The statistical properties of the radio emission from the pulsars B0823+26, B0950+08, B1133+16, and B1937+21 are studied using high time resolution observations taken at the Arecibo Observatory in Puerto Rico. Temporally coherent non-Gaussian emission has been detected in three of the four observed objects. This is the first time such a phenomenon has been observed. The results have been interpreted using a generalized shot noise model, and various basic physical quantities pertaining to the magnetospheric plasma have been estimated.

Intrinsic infrared colors of stars in the Johnson 11 color system are derived. The database is a list of 3946 stars with observations in UBVRIJHKLMN, of all spectral types and luminosity classes, including carbon, T Tauri, and Wolf-Rayet stars. Intrinsic colors were derived from the zero-reddening curve that can be defined in the bluer side of temperature versus observed color diagrams. In a sample of stars of the same spectral type, the bluest stars are considered to have an observed color very near their intrinsic color. The comparison with former derivations from Johnson and Koornneef presents significant differences: new M and N colors, for all spectral types, are bluer than published values, the differences being more important for giants and supergiants; the amplitude of color values in these new results is wider.

Theoretical light curves of four recurrent novae in outburst are modeled to obtain various physical parameters. The four objects studied here are those with a red giant companion, i.e., T Coronae Borealis, RS Ophiuchi, V745 Scorpii, and V3890 Sagittarii. Our model consists of a very massive white dwarf (WD) with an accretion disk and a red giant companion. Light-curve calculation includes reflection effects of the companion star and the accretion disk together with a shadowing effect on the companion by the accretion disk. We also include a radiation-induced warping instability of the accretion disk to reproduce the second peak of T CrB outbursts. The early visual light curves are well reproduced by applying a thermonuclear runaway model to a very massive white dwarf close to the Chandrasekhar mass limit, i.e., M_WD = 1.37 ± 0.01 M_☉ for T CrB and 1.35 ± 0.01 M_☉ for RS Oph with solar metallicity (Z = 0.02), but 1.377 ± 0.01 M_☉ for RS Oph with low metallicity (Z = 0.004), 1.35 ± 0.01 M_☉ for V745 Sco, and 1.35 ± 0.01 M_☉ for V3890 Sgr. Optically thick winds, which blow from the WDs during the outbursts, play a key role in determining the nova duration and the speed of decline because the wind quickly reduces the envelope mass on the WD. The envelope mass at each optical maximum is also estimated to be ΔM ~ 3 × 10^-6M_☉ (T CrB), 2 × 10^-6M_☉ (RS Oph), 5 × 10^-6M_☉ (V745 Sco), 3 × 10^-6M_☉ (V3890 Sgr), indicating average mass accretion rates of_acc ~ 0.4 × 10^-7M_☉ yr^-1 (80 yr; T CrB), 1.2 × 10^-7M_☉ yr^-1 (18 yr; RS Oph), 0.9 × 10^-7M_☉ yr^-1 (52 yr; V745 Sco), and 1.1 × 10^-7M_☉ yr^-1 (28 yr; V3890 Sgr) during the quiescent phase before the last outburst. Although a large part of the envelope mass is blown off in the wind, each WD retains a substantial part of the envelope mass after hydrogen burning ends. Thus, we have obtained net mass-increasing rates of the WDs as_He ~ 0.1 × 10^-7M_☉ yr^-1 (T CrB), 0.12 × 10^-7M_☉ yr^-1 (RS Oph), 0.05 × 10^-7M_☉ yr^-1 (V745 Sco), 0.11 × 10^-7M_☉ yr^-1 (V3890 Sgr). These results strongly indicate that the WDs in the recurrent novae have now grown to near the Chandrasekhar mass limit and will soon explode as a Type Ia supernova if the WDs consist of carbon and oxygen. We have also clarified the reason that T CrB shows a secondary maximum but the other three systems do not.

We calculate the Type Ia supernova (SN) rate for different star formation histories in galaxies by adopting the most popular and recent progenitor models. We show that the timescale for the maximum in the SN Ia rate, which corresponds also to time of the maximum enrichment, is not unique but is a strong function of the adopted stellar lifetimes, initial mass function, and star formation rate. This timescale varies from ~ 40-50 Myr for an instantaneous starburst to ~0.3 Gyr for a typical elliptical galaxy to ~ 4.0-5.0 Gyr for a disk of a spiral galaxy like the Milky Way. We also show that the typical timescale of 1 Gyr, often quoted as the typical timescale for the SNe Ia, is just the time at which, in the solar neighborhood, the Fe production from SNe Ia starts to become important and not the time at which SNe Ia start to explode. As a consequence of this, a change in slope in the [O/Fe] ratio is expected in correspondence of this timescale. We conclude that the suggested lack of SNe Ia at low metallicities produces results at variance with the observed [O/Fe] versus [Fe/H] relation in the solar region. We also compute the SN Ia rates for different galaxies as a function of redshift and predict an extended maximum between redshift z ~ 3.6 and z ~ 1.6 for elliptical galaxies, and two maxima, one at z ~ 3 and the other at z ~ 1, for spiral galaxies, under the assumption that galaxies start forming stars at z_f ~ 5 and Ω_M = 0.3, Ω_Λ = 0.7.

R-band intensity measurements along the light curve of Type Ia supernovae (SNe Ia) discovered by the Supernova Cosmology Project (SCP) are fitted in brightness to templates allowing a free parameter the time-axis width factor w ≡ s(1 + z). The data points are then individually aligned in the time axis, normalized, and K-corrected back to the rest frame, after which the nearly 1300 normalized intensity measurements are found to lie on a well-determined common rest-frame B-band curve, which we call the "composite curve." The same procedure is applied to 18 low-redshift Calán/Tololo SNe with z < 0.11; these nearly 300 B-band photometry points are found to lie on the composite curve equally well. The SCP search technique produces several measurements before maximum light for each supernova. We demonstrate that the linear stretch factor, s, which parameterizes the light-curve timescale, appears independent of z, and applies equally well to the declining and rising parts of the light curve. In fact, the B-band template that best fits this composite curve fits the individual supernova photometry data when stretched by a factor s with χ²/dof ≈ 1—thus, as well as any parameterization can, given the current data sets. The measurement of the date of explosion, however, is model dependent and not tightly constrained by the current data. We also demonstrate the 1 + z light-curve time-axis broadening expected from cosmological expansion. This argues strongly against alternative explanations, such as tired light, for the redshift of distant objects.

The extremely helium-rich atmospheric composition determined for the halo white dwarf WD 0346+246 is reexamined. This solution is shown to be improbable from an astrophysical point of view when accretion of hydrogen and metals from the interstellar medium is taken into account. An alternate solution is proposed in which hydrogen and helium are present in the atmospheric regions in equal amounts. The best fit, at T_eff = 3780 K, log g = 8.34, and N(He)/N(H) = 1.3, is achieved by including in the model calculations a bound-free opacity from the Lyman edge associated with the so-called dissolved atomic levels of the hydrogen atom, or pseudocontinuum opacity.

Stellar models, including all effects of atomic diffusion and radiative accelerations, are evolved from the pre-main sequence to the giant branch for stars of 1.3 to 4.0 M_☉, with metallicity ranging from Z₀ = 0.01 to 0.03. It is shown that radiative accelerations lead to the accumulation of iron-peak elements around 200,000 K; this increases the opacity and causes the appearance of Fe convection zones when macroscopic motions are not rapid enough to wipe out the effects of particle transport. The behavior of Fe convection zones and conditions for their appearance are studied in detail. Iron-peak convection zones appear naturally in all solar metallicity models more massive than 1.5 M_☉. In the 1.5 M_☉ model, it is present only for a fraction of the main-sequence lifetime, but in models without turbulence of 1.7 M_☉ and more, the Fe convection zone rapidly develops after arrival on the main-sequence and remains until its end. For a metallicity of Z = 0.01, an Fe convection zone appears even in a 1.3 M_☉ model. Moreover, the interaction between the diffusion velocities of different species leads to an accumulation of heavy elements around the convective core, causing semiconvection. A detached semiconvection zone develops in the 1.5 M_☉ model. Finally, the surface abundances are calculated using a number of turbulence models and compared to observations of τ UMa in order to show how abundance anomalies may be used to test various turbulence models; the gravity at which abundance anomalies should be expected to disappear is determined. It is shown that in Am stars, the Ca underabundance should disappear during evolution at the same gravity as iron-peak overabundances.

We present two planet fits to the radial velocity measurements of the M dwarf star GJ 876 that account for the mutual perturbations between the planets, as well as their interactions with the star. We also give the results of long-term integrations testing the stability of some of our best-fit systems. Our Newtonian models fit the data much better than do Keplerian models, reducing the χ from 1.88 to as low as 1.34 for an unstable high-inclination system and 1.43 for a stable low-inclination system. Several different local minima with comparable χ are found in parameter space; thus, our results are not able to provide tight constraints on the inclinations of the orbits to the line of sight and actual planetary masses. Most sets of planetary parameters that we have derived represent systems that are stable for at least 10⁸ yr. Test particles orbiting between the two planets within our best-fit planar system are lost in less than 300 yr.

During 2000 February 9-10, the Large Angle Spectrometric Coronagraph and Ultraviolet Coronal Spectrometer (UVCS) instruments aboard the Solar and Heliospheric Observatory observed comet C/2000 C6, a member of the Kreutz family of sungrazing comets. A tail nearly 0.5 R_☉ in length was detected in Lyα emission. UVCS was able to observe the comet at four heights as it approached the Sun. A jump in the Lyα brightness between 5.71 and 4.56 R_☉ suggests that the nucleus fragmented, exposing more area to solar illumination and increasing the outgassing rate. We interpret the Lyα luminosity in terms of the outgassing rate and use this to estimate the diameter of the nucleus. The Lyα emission fades as H I is ionized, providing estimates of the density in the coronal streamer encountered by the comet.

The structure and properties of a newly emerged solar active region (NOAA Active Region 7985) are discussed using the Coronal Diagnostic Spectrometer (CDS) and the Extreme-Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory. CDS obtained high-resolution EUV spectra in the 308-381 Å and 513-633 Å wavelength ranges, while EIT recorded full-disk EUV images in the He II (304 Å), Fe IX/X (171 Å), Fe XII (195 Å), and Fe XV (284 Å) bandpasses. Electron density measurements from Si IX, Si X, Fe XII, Fe XIII, and Fe XIV line ratios indicate that the region consists of a central high-density core with peak densities of the order of 1.2 × 10¹⁰ cm^-3, which decrease monotonically to ~5.0 × 10⁸ cm^-3 at the active region boundary. The derived electron densities also vary systematically with temperature. Electron pressures as a function of both active region position and temperature were estimated using the derived electron densities and ion formation temperatures, and the constant pressure assumption was found to be an unrealistic simplification. Indeed, the active region is found to have a high-pressure core (1.3 × 10¹⁶ cm^-3 K) that falls to 6.0 × 10¹⁴ cm^-3 K just outside the region. CDS line ratios from different ionization stages of iron, specifically Fe XVI (335.4 Å) and Fe XIV (334.4 Å), were used to diagnose plasma temperatures within the active region. Using this method, peak temperatures of 2.1 × 10⁶ K were identified. This is in good agreement with electron temperatures derived using EIT filter ratios and the two-temperature model of Zhang et al. The high-temperature emission is confined to the active region core, while emission from cooler (1-1.6) × 10⁶ K lines originates in a system of loops visible in EIT 171 and 195 Å images. Finally, the three-dimensional geometry of the active region is investigated using potential field extrapolations from a Kitt Peak magnetogram. The combination of EUV and magnetic field extrapolations extends the "core-halo" picture of active region structure to one in which the core is composed of a number of compact coronal loops that confine the hot, dense, high-pressure core plasma while the halo emission emerges from a system of cooler and more extended loops.

An important objective of the solar physics community is the unambiguous determination of the morphology of the fine structures of the solar upper atmosphere in quiet-Sun and coronal hole regions and the relationship of the cold chromosphere to the hot corona. Recently the Solar Ultraviolet Measurements of Emitted Radiation spectrometer on board the Solar and Heliospheric Observatory succeeded in obtaining observations that can be used to achieve this goal. In this paper we study the spatial relationship between previously unresolved fine structures and the chromospheric emissions that underlie them. The main result is that looplike structures seen in transition region lines with length scales of 10''-20'' straddle the chromospheric network and have no chromospheric counterpart near their apparent footpoints.

We apply empirical mode decomposition (EMD) and Hilbert analysis to time series of rotation residuals at all latitudes and at all depths in the convection zone derived from 49 Global Oscillation Network Group data sets covering the period 1995 May 7 to 2000 May 15. Hilbert analysis combined with EMD is a tool to analyze nonlinear and nonstationary signals and is used to localize events in time-frequency space. We calculate Hilbert power spectra, power as a function of time and frequency, for each time series in order to determine whether the rotation rate in the convection zone shows any other systematic temporal variation besides the so-called torsional oscillation pattern in the upper convection zone and the periodicity of 1.3 yr near the base of the convection zone. In other regions of the convection zone, the temporal variations of the rotation residuals are compatible with a noise signal except near about 0.86 R_☉ in radius, where we find indications of a long-term period of about 6 yr. However, it is uncertain whether this signal is of solar origin, since the available data set is too short to rule out the possibility of an artifact. In addition, we calculate the amount of power contained in the torsional oscillation signal as a function of time, latitude, and radius to study the variation of the torsional oscillation pattern. The depth to which the pattern extends apparently changes with time. For example, at midlatitudes the pattern extends to deeper layers with increasing time. The degree of stationarity doubles from the surface to about 0.92 R_☉ in radius, which indicates that the torsional oscillation pattern disappears with increasing depth in agreement with previous results.

We present X-ray observations of the afterglow of GRB 000926, performed around and after the break observed in the optical light curve 2 days after the burst. The steep X-ray light curve observed around the break confirms the presence of this feature in X-rays. However, the spectral and temporal properties are not consistent with a standard jet scenario based on synchrotron emission, requiring a more complicated model. We find that X-ray and optical data are compatible with a moderately collimated fireball (with opening angle θ ≈ 25°) expanding in a dense medium (n ≈ 4 × 10⁴ cm^-3). This produces two breaks in the light curve. The first, at t ≈ 2 days, is due to jet behavior. The second, around 5 days, is attributed to the transition of the fireball to a nonrelativistic expansion. This transition predicts a flattening of the light curve, which explains the late X-ray measurement in excess above the extrapolation of the simple jet scenario, and is also consistent with optical data.

We model the X-rays reprocessed by an accretion disk in a fiducial low-mass X-ray binary system with a neutron star primary. An atmosphere, or the intermediate region between the optically thick disk and a Compton temperature corona, is photoionized by the neutron star continuum. X-ray lines from the recombination of electrons with ions dominate the atmosphere emission and should be observable with the Chandra and XMM-Newton high-resolution spectrometers. The self-consistent disk geometry agrees well with optical observations of these systems, with the atmosphere shielding the companion from the neutron star. At a critical depth range, the disk gas has one thermally unstable and two stable solutions. A clear difference between the model spectra exists between evaporating and condensing disk atmospheres. This difference should be observable in high-inclination X-ray binaries, or whenever the central continuum is blocked by absorbing material and the extended disk emission is not.

We evaluate the co-orbital corotation torque on a planet on a fixed circular orbit embedded in a viscous protoplanetary disk for the case of a steady flow in the planet frame. This torque can be evaluated just from the flow properties at the separatrix between the librating (horseshoe) and circulating streamlines. A stationary solution is searched for the flow in the librating region. When used to evaluate the torque exerted by the circulating material of the outer and inner disk on the trapped material of the librating region, this solution leads to an expression of the co-orbital corotation torque in agreement with previous estimates. An analytical expression is given for the corotation torque as a function of viscosity. Last, we show that additional terms in the torque expression can play a crucial role. In particular, they introduce a coupling with the disk density profile perturbation (the "dip" that surrounds the planet) and add to the corotation torque a small, positive fraction of the one-sided Lindblad torque. As a consequence, the migration could well be directed outward in very thin disks (aspect ratio smaller than a few percent). This two-dimensional analysis is especially relevant for mildly embedded protoplanets (sub-Saturn-sized objects).

We have developed a simulation code with the techniques that enhance both spatial and time resolution of the particle-mesh (PM) method, for which the spatial resolution is restricted by the spacing of structured mesh. The adaptive-mesh refinement (AMR) technique subdivides the cells that satisfy the refinement criterion recursively. The hierarchical meshes are maintained by the special data structure and are modified in accordance with the change of particle distribution. In general, as the resolution of the simulation increases, its time step must be shortened and more computational time is required to complete the simulation. Since the AMR enhances the spatial resolution locally, we reduce the time step locally also, instead of shortening it globally. For this purpose, we used a technique of hierarchical time steps (HTS), which changes the time step, from particle to particle, depending on the size of the cell in which particles reside. Some test calculations show that our implementation of AMR and HTS is successful. We have performed cosmological simulation runs based on our code and found that many of halo objects have density profiles that are well fitted to the universal profile proposed in 1996 by Navarro, Frenk, & White over the entire range of their radius.

We present a new constraint on the biased galaxy formation picture. Gravitational instability theory predicts that the two-point mass density correlation function, ξ(r), has an inflection point at the separation r = r₀, corresponding to the boundary between the linear and nonlinear regime of clustering, ξ ≃ 1. We show how this feature can be used to constrain the biasing parameter b² ≡ ξ_g(r)/ξ(r) on scales r ≃ r₀, where ξ_g is the galaxy-galaxy correlation function, which is allowed to differ from ξ. We apply our method to real data: the ξ_g(r), estimated from the Automatic Plate Measuring (APM) galaxy survey. Our results suggest that the APM galaxies trace the mass at separations r ≳ 5 h^-1 Mpc, where h is the Hubble constant in units of 100 km s^-1 Mpc^-1. The present results agree with earlier studies, based on comparing higher order correlations in the APM with weakly nonlinear perturbation theory. Both approaches constrain the b factor to be within 20% of unity. If the existence of the feature that we identified in the APM ξ_g(r)—the inflection point near ξ_g = 1—is confirmed by more accurate surveys, we may have discovered gravity's smoking gun: the long-awaited "shoulder" in ξ, predicted by Gott and Rees 25 years ago.

We present results from stacking analyses, using the 1 Ms Chandra Deep Field-North data, that constrain the X-ray emission of Lyman break galaxies at z ≈ 2-4. Stacking the counts from 24 individually undetected Lyman break galaxies located within the Hubble Deep Field-North, we have obtained average detections of these objects in the resulting 0.5-8.0 and 0.5-2.0 keV images; these images have effective exposure times of 22.4 Ms (260 days). Monte Carlo testing empirically shows the detections to be highly significant. The average rest-frame 2-8 keV luminosity of a Lyman break galaxy is derived to be ≈3.2 × 10⁴¹ ergs s^-1, comparable to that of the most X-ray luminous starbursts in the local universe. The observed ratio of X-ray to B-band luminosity is somewhat, but not greatly, higher than that seen from local starbursts. The X-ray emission probably arises from a combination of high-mass X-ray binaries, "super-Eddington" X-ray sources, and low-luminosity active galactic nuclei.

We have measured the correlation between the lensing signal induced by (dark) matter and the number counts of galaxies on scales ranging from 0.15 to 3.0 h Mpc (which correspond to aperture radii of 1'-15'). This provides a direct probe of the scale dependence of the ratio of the classical bias parameter b and the galaxy-mass correlation coefficient r. The results presented here are based on 16 deg² of R_C-band data taken with the Canada-France-Hawaii Telescope as part of the Red-Sequence Cluster Survey. We used a sample of lens galaxies with 19.5 < R_C < 21 and a sample of source galaxies with 21.5 < R_C < 24. The results are consistent with a scale-independent value of b/r, which provides valuable constraints on models of galaxy formation on scales that can only be probed through weak lensing. For the currently favored cosmology (Ω_m = 0.3, Ω_Λ = 0.7), we find that b/r = 1.05, similar to what is found on larger scales (~10 h Mpc) from local dynamical estimates. These results support the hypothesis that light traces mass on scales ranging from 0.15 out to ~10 h Mpc. The accuracy of the measurement will improve significantly in the coming years, enabling us to measure both b and r separately as a function of scale.

We present a Chandra observation of Abell 2052, a cooling flow cluster with a central cD galaxy that hosts the complex radio source 3C 317. The data reveal "holes" in the X-ray emission that are coincident with the radio lobes. The holes are surrounded by bright "shells" of X-ray emission. The data are consistent with the radio source displacing and compressing, and at the same time being confined by, the X-ray gas. The compression of the X-ray shells appears to have been relatively gentle and, at most, slightly transonic. The pressure in the X-ray gas (the shells and surrounding cooler gas) is approximately an order of magnitude higher than the minimum pressure derived for the radio source, suggesting that an additional source of pressure is needed to support the radio plasma. The compression of the X-ray shells has speeded up the cooling of the shells, and optical emission-line filaments are found coincident with the brightest regions of the shells.

In this Letter, analytical arguments are presented for the existence of a synchrotron X-ray jet on large scales in most radio-loud active galactic nuclei, based on the knowledge of the nature and physics of blazars. In blue blazars and blue blazar-like radio galaxies, the large-scale X-ray jet may become faint along the jet, while in most red blazars and red blazar-like radio galaxies, the X-ray jet is bright on 10 kpc scales whether the jet is highly relativistic on large scales or not. In extreme red blazars in which the jet is still highly relativistic on large scales and the synchrotron peak of the inner jet lies in the infrared bands, the X-ray jet may become fainter along the jet from 10 to 100 kpc scales, while the optical and IR jet becomes brighter. The predictions can be tested with the ongoing observations of the Chandra X-Ray Observatory.

This Letter proposes that the "profile crisis" in cold dark matter halo models is partly due to a weakly dissipative dark baryonic component. This component is built by rescaling the rotation curves of faint galaxies onto those of bright systems, and we argue that the collapse factor of the baryonic disk relative to the halo must be a constant. If this component is made of molecular gas in a frozen form, a large hole is expected in the central region that is due to UV heating by the stellar component. This hole, combined with a weakly peaked distribution of halo particles, may mimic an isothermal halo core that is wrongly attributed to the particles alone. Halos made of cold particles with or without a finite-scattering cross section are considered in this context. For faint objects, a concordance model is achieved with roughly half the mass in baryons, a mass concentration c_M ≈ 6.2 ± 0.5, and a inner profile ρ ∝ r^-0.5±0.3 at r = 0 for the halos.

Radio observations of starburst regions in galaxies have revealed groups of compact nonthermal sources that we interpret as radiative supernova remnants expanding in the interclump medium of molecular clouds. Because of the high pressure in starburst regions, the interclump medium may have a density ~10³ H atoms cm^-3 in a starburst nucleus like M82 and ≳10⁴ H atoms cm^-3 in an ultraluminous galaxy like Arp 220. In M82, our model can account for the sizes, the slow evolution, the high radio luminosities, and the low X-ray luminosities of the sources. We predict expansion velocities ~500 km s^-1, which is slower than the one case measured by VLBI techniques. Although we predict the remnants to be radiative, the expected radiation is difficult to detect because it is at infrared wavelengths and the starburst is itself very luminous; one detection possibility is broad [O I] 63 μm line emission at the positions of the radio remnants. The more luminous and compact remnants in Arp 220 can be accounted for by the higher molecular cloud density. In our model, the observed remnants lose most of the supernova energy to radiation. Other explosions in a lower density medium may directly heat a hot, low density interstellar component, leading to the superwinds that are associated with starburst regions.

The global star formation rate (SFR) density is estimated from the star formation histories (SFHs) of Local Group galaxies. This is found to be broadly consistent with estimates of the global SFH from existing redshift surveys for two favored cosmologies. It also provides additional evidence for a relatively constant global SFR density at high redshift (z > 1).

We report a detection of far-ultraviolet absorption from the supernova remnant SNR 0057-7226 in the Small Magellanic Cloud (SMC). The absorption is seen in the Far Ultraviolet Spectroscopic Explorer (FUSE) spectrum of the luminous blue variable/Wolf-Rayet star HD 5980. Absorption from O VI λ1032 and C III λ977 is seen at a velocity of 300 km s^-1 with respect to the Galactic absorption lines, 170 km s^-1 with respect to the SMC absorption. The O VI λ1038 line is contaminated by H₂ absorption but is present. These lines are not seen in the FUSE spectrum of Sk 80, only ~1' (~17 pc) away from HD 5980. No blueshifted O VI λ1032 absorption from the SNR is seen in the FUSE spectrum. The O VI λ1032 line in the SNR is well described by a Gaussian with FWHM = 75 km s^-1. We find log N(O ) = 14.33-14.43, which is roughly 50% of the rest of the O VI column in the SMC (excluding the SNR) and greater than the O VI column in the Milky Way halo along this sight line. The N(C IV)/N(O VI) ratio for the SNR absorption is in the range of 0.12-0.17, similar to the value seen in the Milky Way disk and lower than the halo value, supporting models in which SNRs produce the highly ionized gas close to the plane of the Galaxy, while other mechanisms occur in the halo. The N(C IV)/N(O VI) ratio is also lower than the SMC ratio along this sight line, suggesting that other mechanisms contribute to the creation of the global hot ionized medium in the SMC. The O VI, C IV, and Si IV apparent column density profiles suggest the presence of a multiphase shell followed by a region of higher temperature gas.

The recent discovery of a population of high proper-motion white dwarfs by Oppenheimer and coworkers has caused a lot of speculation as to the origin of these stars. I show that the age distribution of the white dwarfs offers a kind of sanity check in these discussions. In particular, the majority of the identified population appears to have a similar age distribution to those in the standard, thin-disk white dwarf population. This is not what is expected for either the halo or thick disk, which are thought to be old populations. A subset of the Oppenheimer "halo" sample does indeed possess an age distribution consistent with a halo origin, but the density is smaller and consistent with the results of Gould, Flynn, & Bahcall for a high-end mass function slope of -0.9.

We present the first spectroscopic orbit for the massive X-ray binary LS 5039, which we find to be a short-period (P = 4.117 ± 0.011 days) and highly eccentric (e = 0.41 ± 0.05) system. The low-mass function for the orbit appears to be most consistent with a neutron star companion, although a black hole remains a possibility if the system has a low inclination. The spectrum of the O7 V optical star appears to be normal for its type (suggesting that there is little flux in the red from an accretion disk) except that the C IV λλ5801, 5812 lines are very weak, perhaps indicating the presence of CNO-processed gas in the O star. There is no evidence of Hα emission, so the system is probably not currently undergoing Roche lobe overflow. The projected rotational velocity, V sin i = 131 ± 6 km s^-1, suggests that the optical star is rotating faster than synchronously with the orbit. The peculiar component of the systemic radial velocity is -17 ± 3 km s^-1, so the system is not a runaway star (at least not in this dimension).

The soft gamma repeater SGR 1900+14 became active on 2001 April 18 after about 2 years of quiescence; it had remained at a very low state of activity since the fall of 1998, when it exhibited extraordinary flaring. We have observed the source in the gamma-rays and X-rays with Ulysses and Chandra and in the radio with MERLIN. We report here the confirmation of a two-component X-ray spectrum (power law + blackbody), indicating emission from the neutron star surface. We have determined that there is a dust halo, due to scattering in the interstellar medium, surrounding the source that extends up to ≳100'' from the center of SGR 1900+14.

We report the results of deep infrared observations of brown dwarf candidates in the Trapezium Cluster in Orion. Analysis of the JHK color-color diagram indicates that a large fraction (~65% ± 15%) of the observed sources exhibit infrared-excess emission. This suggests the extreme youth of these objects and, in turn, provides strong independent confirmation of the existence of a large population of substellar objects in the cluster. Moreover, this suggests that the majority of these substellar objects are presently surrounded by circumstellar disks similar to the situation for the stellar population of the cluster. This evidence for a high initial disk frequency (>50%) around cluster members of all masses, combined with the smooth continuity of the cluster's initial mass function across the hydrogen-burning limit, suggests that a single physical mechanism is likely responsible for producing the entire cluster mass spectrum down to near the deuterium-burning limit. The results may also indicate that even substellar objects are capable of forming with systems of planetary companions.

The relationship between the magnetic field and the circular polarization of astrophysical maser radiation due to the Zeeman effect under idealized conditions is investigated when the Zeeman splitting is much smaller than the spectral line breadth and when radiative saturation is significant. The description of the circular polarization as well as inferences about the magnetic field from the observations are clearest when the rate for stimulated emission is much less than the Zeeman splitting. The calculations here are performed in this regime, which is relevant for some (if not most) observations of astrophysical masers. We demonstrate that the Stokes V parameter is proportional to the Zeeman splitting and that the fractional linear polarization is independent of the Zeeman splitting when the ratio of the Zeeman splitting to the spectral line breadth is small—less than about 0.1. In contrast to its behavior for ordinary spectral lines, the circular polarization for masers that are at least partially saturated does not decrease with increasing angle between the magnetic field and the line of sight until they are nearly perpendicular.

The energy spectra of Fe in the very large solar energetic particle (SEP) event of 2000 July 14 are strikingly different from those of lighter species. We show that this difference can be explained by shock acceleration from a two-component source population, comprising solar wind suprathermals and a small (~5%) admixture of remnant flare particles, as previously proposed to explain enhanced ³He/⁴He in some gradual SEP events. Flare remnants can also account for several previously unexplained features of high-energy solar heavy ions as well as important aspects of SEP event-to-event variability. These results offer a new perspective on the enduring controversy over the relative roles of flares and coronal mass ejections (CMEs) in producing SEPs. Flare activity clearly makes a unique and critical contribution to the source population. But the predominate accelerator in large gradual SEP events is the CME-driven shock, and many spectral, compositional, and charge state characteristics of high-energy heavy ions can be understood without invoking other acceleration mechanisms.

We observed the fast coronal mass ejection (CME) of 1998 April 20 with the radioheliograph at Nançay, France, between 164 and 432 MHz. Spectroscopic data were obtained between 40 and 800 MHz by the spectrometer at Tremsdorf, Germany, and between 20 kHz and 14 MHz with the WAVES instrument on board the Wind spacecraft. Energetic particle data were obtained from the Wind 3D Plasma and Energetic Particle experiment. The CME was observed in white light by the Large-Angle Spectrometric COronagraph experiment on board the Solar and Heliospheric Observatory spacecraft. For the first time, the expanding CME loops are imaged directly at radio wavelengths. We show that the radio-emitting CME loops are the result of nonthermal synchrotron emission from electrons with energies of ~0.5-5 MeV interacting with magnetic fields of ~0.1 to a few gauss. They appear nearly simultaneously with the onset of an associated type II radio burst, shock-accelerated type III radio bursts, and the initiation of a solar energetic particle event. We suggest possible sources of the energetic electrons responsible for this "radio CME" and point out diagnostic uses for synchrotron emission from CME loops.

Adaptive correction on large ground-based telescopes is enabling a variety of novel studies that would be impossible at the limits of spatial and spectral resolution imposed by the Earth's turbulent atmosphere. Even relatively high-order systems, however, do not yield a perfect correction and as a result are compromised by a low-intensity halo of remnant light that is removed from the core of the point-spread function (PSF) and dispersed in the focal plane. Worse still, this halo is neither constant in time nor uniform in position but is concentrated in transient spots that move about as mutually coherent patches of phase over the telescope aperture happen to combine constructively in the image plane. These "speckles" in the PSF set limits on ground-based searches for faint companions to bright stars. We describe here simple properties of the physics of speckle formation that will affect the statistics of residual speckles in a fundamental way: at high correction, the secondary maxima of the static PSF will coherently amplify or "pin" the time-varying speckles, influencing their effective characteristic lifetimes and dramatically changing their spatial distribution. Furthermore, as a result of speckle pinning, temporal variations in the noncommon path errors that occur in practical adaptive optics systems will cause an additional gradual drift in the spatial distribution of speckles, as Airy rings shift. Speckle pinning may be exploited to suppress speckle noise by tailoring the PSF with static aberrations artificially injected via the deformable mirror so as to clear a region of the PSF of Airy rings and hence of pinned speckles, a technique that we term "speckle sweeping."