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In the standard formulation of cosmological action, the final-time boundary conditions are taken to be the three position coordinates of each galaxy at z = 0. It can be shown that a partial canonical transformation of the action recasts the problem in a coordinate system in which the natural boundary conditions at z = 0 are the angular positions and redshifts relative to a reference galaxy that may itself be in motion. Successful reconstruction of particle orbits back in time, as well as the correct prediction of cosmological parameters, are demonstrated for small systems of particles through conventional N-body simulations.

We use the radio-infrared (IR) flux correlation between star-forming galaxies in the local universe to examine the connection between their cumulative contributions to the cosmic IR and radio backgrounds. The general expression relating the intensities of the two backgrounds is complicated and depends on details of the evolution of the galaxies' IR luminosity function with redshift. However, in the specific case in which the radio-IR flux correlation is linear, the relation between the intensity of the IR background and the brightness temperature of the radio background reduces to a simple analytical expression that at 178 MHz is I_CIB (nW m^-2 sr^-1) = 2.7T_crb (K), where the numerical coefficient was calculated for a radio spectral index of 0.7. This relation is insensitive to the star-formation history of the galaxies that produce the cosmic IR background (CIB). We use the observed CIB intensity to constrain the cosmic star formation history and the relation between the CIB and the cosmic radio background (CRB) to constrain the relative contribution of star-forming galaxies to the CRB. Current limits on the CIB intensity predict a 178 MHz brightness temperature of ~18 ± 9 K, about half of the 37 ± 8 K inferred for an isotropic radio component. This suggests that star-forming galaxies and active galactic nuclei (AGNs) contribute about equally to the CRB intensity at that frequency.

Use of the Sunyaev-Zeldovich effect as a precise cosmological probe necessitates a realistic assessment of all possible contributions to Comptonization of the cosmic microwave background in clusters of galaxies. We have calculated the additional intensity change due to various possible populations of energetic electrons that have been proposed in order to account for measurements of intracluster radio, nonthermal X-ray, and (possibly also) EUV emission. Our properly normalized estimates of (the highly model-dependent value of) the predicted intensity change due to these electrons are well below ~6% and ~35% of the usual Sunyaev-Zeldovich effect as a result of electrons in the hot gas in Coma and A2199, respectively. These levels constitute high upper limits since they are based on energetic electron populations whose energy densities are comparable to those of the thermal gas. The main impact of nonthermal Comptonization is a shift of the crossover frequency (where the thermal effect vanishes) to higher values. Such a shift would have important consequences for our ability to measure cluster peculiar velocities from the kinematic component of the Sunyaev-Zeldovich effect.

We develop a semianalytic model of hierarchical galaxy formation with an improved treatment of the evolution of galaxies inside dark matter halos. We take into account not only dynamical friction processes building up the central dominant galaxy but also binary aggregations of satellite galaxies inside a common halo. These deplete small to intermediate mass objects, affecting the slope of the luminosity function at its faint end, with significant observable consequences. We model the effect of two-body aggregations using the kinetic Smoluchowski equation. This flattens the mass function by an amount that depends on the histories of the host halos as they grow by hierarchical clustering. The description of gas cooling, star formation and evolution, and supernova feedback follows the standard prescriptions widely used in semianalytic modeling. We find that binary aggregations are effective in depleting the number of small/intermediate mass galaxies over the redshift range 1 < z < 3, thus flattening the slope of the luminosity function at the faint end. At z ≈ 0 the flattening occurs for -20 < M_B < -18, but an upturn is obtained at the very faint end for M_B > -16. We compare our predicted luminosity functions with those obtained from deep multicolor surveys in the Hubble Deep Field-North, Hubble Deep Field-South, and New Technology Telescope Deep Field in the rest-frame B and UV bands for the redshift ranges 0 < z < 1 and 2.5 < z < 3.5, respectively. The comparison shows that the discrepancy of the predictions of other semianalytic models with the observations is considerably reduced at z > 1 and even more at z ≈ 3 by the effect of the binary aggregations. The predictions from our dynamical model are discussed and compared with the effects of complementary processes (additional starburst recipes, alternative sources of feedback, different mass distribution of the dark matter halos) that may conspire in affecting the shape of the luminosity function.

In cold dark matter (CDM) cosmogonies, low-mass objects play an important role in the evolution of the universe. Not only are they the first luminous objects to shed light in a previously dark universe, but if their formation is not inhibited by their own feedback, they dominate the galaxy mass function until redshift z ~ 5. In this paper we present and discuss the implementation of a three-dimensional cosmological code that includes most of the needed physics to simulate the formation and evolution of the first galaxies with a self-consistent treatment of radiative feedback. The simulation includes continuum radiative transfer using the optically thin variable Eddington tensor (OTVET) approximation and line radiative transfer in the H₂ Lyman-Werner bands of the background UV radiation. We include detailed chemistry for H₂ formation/destruction, molecular and atomic cooling/heating processes, ionization by secondary electrons, and heating by Lyα resonant scattering. We find that the first galaxies ("small-halo galaxies") are characterized by bursting star formation, self-regulated by a feedback process that acts on cosmological scales. The mass in stars produced by these objects can exceed the mass in stars produced by normal galaxies; therefore, their impact on cosmic evolution cannot be neglected. The main focus of this paper is on the methodology of the simulations, and we only briefly introduce some of the results. An extensive discussion of the results and the nature of the feedback mechanism are the focus of a companion paper.

We use three-dimensional cosmological simulations with radiative transfer to study the formation and evolution of the first galaxies in a ΛCDM cosmology. The simulations include continuum radiative transfer using the optically thin variable Eddington tensor (OTVET) approximation and line radiative transfer in the H₂ Lyman-Werner bands of the UV background radiation. Chemical and thermal processes are treated in detail, particularly the ones relevant for H₂ formation and destruction. We find that the first luminous objects ("small-halo objects") are characterized by bursting star formation (SF) that is self-regulated by a feedback process acting on cosmological instead of galactic scales. The global SF history is regulated by the mean number of ionizing photons that escape from each source, _UV⟨f_esc⟩. It is almost independent of the assumed SF efficiency parameter, _*, and the intensity of the dissociating background. The main feedback process that regulates the SF is the reformation of H₂ in front of H II regions and inside relic H II regions. The H II regions remain confined inside filaments, maximizing the production of H₂ in overdense regions through cyclic destruction/reformation of H₂. If _UV⟨f_esc⟩ > 10^-7/_*, the SF is self-regulated, photoevaporation of small-halo objects dominates the metal pollution of the low-density intergalactic medium, and the mass of produced metals depends only on ⟨f_esc⟩. If _UV⟨f_esc⟩ ≲ 10^-7/_*, positive feedback dominates, and small-halo objects constitute the bulk of the mass in stars and metals until at least redshift z ~ 10. Small-halo objects cannot reionize the universe because the feedback mechanism confines the H II regions inside the large-scale structure filaments. In contrast to massive objects ("large halos"), which can reionize voids, small-halo objects partially ionize only the dense filaments while leaving the voids mostly neutral.

A method is developed to evaluate the magnifications of the images of galaxies with lensing potentials stratified on similar concentric ellipses. In a quadruplet system, there are two even-parity images and two odd-parity images, together with a demagnified and usually missing central image. A simple contour integral is provided which enables the sums of the magnifications of the even-parity or the odd-parity images or the central image to be separately calculated without explicit solution of the lens equation. We find that the sums for pairs of images generally vary considerably with the position of the source, while the signed sums of the two pairs can be remarkably uniform inside the tangential caustic in the absence of naked cusps. For a family of models in which the lensing potential is a power law of the elliptic radius, ψ ∝ (a² + x² + y²q^-2)^β/2, the number of visible images is found as a function of flattening q, external shear γ, and core radius a. The magnification of the central image depends on the size of the core and the slope β of the gravitational potential. It grows strongly with the source offset if β > 1, but weakly if β < 1. For typical source and lens redshifts, the missing central image leads to strong constraints; the slope β must be ≲1 and the core radius a must be ≲300 pc. The mass distribution in the lensing galaxy population must be nearly cusped, and the cusp must be isothermal or stronger. This is in good accord with the cuspy cores seen in high-resolution photometry of nearby, massive, early-type galaxies, which typically have β ≈ 0.7 (or surface density falling as distance^-1.3) outside a break radius of a few hundred parsecs. Cuspy cores by themselves can provide the explanation of the missing central images. Dark matter at large radii may alter the slope of the projected density; however, provided the slope remains isothermal or steeper and the break radius remains small, then the central image remains unobservable. The sensitivity of the radio maps must be increased fifty-fold to find the central images in abundance.

We combine our recent measurements of the velocity dispersion and the surface brightness profile of the lens galaxy D in the system MG 2016+112 (z = 1.004) with constraints from gravitational lensing to study its internal mass distribution. We find the following: (1) dark matter accounts for more than 50% of the total mass within the Einstein radius (99% confidence limit [CL]), whereas ~75% is the more likely contribution. In particular, we can exclude at the 8 σ level that mass follows light inside the Einstein radius with a constant mass-to-light ratio (M/L). (2) The total mass distribution inside the Einstein radius is well described by a density profile ∝ r, with an effective slope γ' = 2.0 ± 0.1 ± 0.1, including random and systematic uncertainties. (3) The offset of galaxy D from the local fundamental plane independently constrains the stellar M/L and matches the range derived from our models, leading to a more stringent lower limit of more than 60% on the fraction of dark matter within the Einstein radius (99% CL). Under the assumption of adiabatic contraction, we show that the inner slope of the dark matter halo before the baryons collapsed to form the lens galaxy is γ_i < 1.4 (68% CL), only marginally consistent with the highest resolution cold dark matter simulations that indicate γ_i ~ 1.5. This might indicate either that adiabatic contraction is a poor description of early-type galaxy formation or that additional processes play a role as well. Indeed, the apparently isothermal density distribution inside the Einstein radius is not a natural outcome of adiabatic contraction models, where it appears to be a mere coincidence. By contrast, we argue that isothermality might be the result of a stronger coupling between luminous and dark matter, possibly the result of (incomplete) violent relaxation processes during the formation of the innermost regions of the galaxy. Hence, we conclude that galaxy D appears already relaxed ~8 Gyr ago. We briefly discuss the importance of our results for lens statistics and the determination of the Hubble constant from gravitational lens time delays.

Optical and near-IR Hubble Space Telescope and Gemini North adaptive optics images, further improved through deconvolution, are used to explore the gravitationally lensed radio source PKS 1830-211. The line of sight to the quasar at z = 2.507 appears to be very busy, with the presence, within 05 from the source, of (1) a possible galactic main-sequence star, (2) a faint red lensing galaxy visible only in H band, and (3) a new object whose colors and morphology match those of an almost face-on spiral. The V-I color and faint I magnitude of the latter suggest that it is associated with the molecular absorber seen toward PKS 1830-211, at z = 0.89 rather than with the z = 0.19 H I absorber previously reported in the spectrum of PKS 1830-211. While this discovery might ease the interpretation of the observed absorption lines, it also complicates the modeling of the lensing potential well, hence decreasing the interest in using this system as a means to measure H₀ through the time delay between the lensed images. This is the first case of a quasar lensed by an almost face-on spiral galaxy.

We present new Hubble Space Telescope images of the gravitational lens PKS 1830-211, which allow us to characterize the lens galaxy and update the determination of the Hubble constant (H₀) from this system. The I-band image shows that the lens galaxy is a face-on spiral galaxy with clearly delineated spiral arms. The southwestern image of the background quasar passes through one of the spiral arms, explaining the previous detections of large quantities of molecular gas and dust in front of this image. The lens galaxy photometry is consistent with the Tully-Fisher relation, suggesting the lens galaxy is a typical spiral galaxy for its redshift. The lens galaxy position, which was the main source of uncertainty in previous attempts to determine H₀, is now known precisely. Given the current time delay measurement and assuming the lens galaxy has an isothermal mass distribution, we compute H₀ = 44 ± 9 km s^-1 Mpc^-1 for an Ω_m = 0.3 flat cosmological model. We describe some possible systematic errors and how to reduce them. We also discuss the possibility raised by Courbin et al. (2002), that what we have identified as a single lens galaxy is actually a foreground star and two separate galaxies.

We predict the redshift distribution of gamma-ray bursts (GRBs), assuming that they trace the cosmic star formation history. We find that a fraction ≳50% of all GRBs on the sky originate at a redshift z ≳ 5, even though the fraction of the total stellar mass formed by z ~ 5 is only ~15%. These two fractions are significantly different, because they involve different cosmological factors when integrating the star formation rate over redshift. Hence, deep observations of transient events, such as GRB afterglows or supernovae, provide an ideal strategy for probing the high-redshift universe. We caution, however, that existing or planned flux-limited instruments are likely to detect somewhat smaller fractions of high-redshift bursts. For example, we estimate that the fraction of all bursts with redshifts z ≳ 5 is ~10% in the case of the BATSE instrument and ~25% in the case of Swift. We also show that the intrinsic distribution of GRB durations is bimodal but significantly narrower and shifted toward shorter durations than the observed distribution.

We extended the disk corona model to the inner region of galactic nuclei by including different temperatures in ions and electrons as well as Compton cooling. We found that the mass evaporation rate, and hence the fraction of accretion energy released in the corona, depend strongly on the rate of incoming mass flow from the outer edge of the disk, a larger rate leading to more Compton cooling, less efficient evaporation, and a weaker corona. We also found a strong dependence on the viscosity, with higher viscosity leading to an enhanced mass flow in the corona and therefore more evaporation of gas from the disk below. If we take accretion rates in units of the Eddington rate, our results become independent of the mass of the central black hole. The model predicts weaker contributions to the hard X-rays for objects with higher accretion rate like narrow-line Seyfert 1 galaxies, in agreement with observations. For luminous active galactic nuclei, strong Compton cooling in the innermost corona is so efficient that a large amount of additional heating is required to maintain the corona above the thin disk.

We present new hard X-ray spectra of three radio-loud active galactic nuclei (AGNs) of moderately high X-ray luminosity (L_X ≈ 10⁴⁵ ergs s^-1; PKS 2349-01, 3C 323.1, and 4C 74.26) obtained with ASCA and BeppoSAX. The X-ray continua are described in all three cases with a power-law model with photon indices Γ ≈ 1.85, modified at low energies by absorption in excess of the galactic value, which appears to be due to neutral gas. At higher energies, an Fe Kα emission line is detected in PKS 2349-01 and 4C 74.26 and is tentatively detected in 3C 323.1. The equivalent widths of the lines are consistent, albeit within large uncertainties, with the values for radio-quiet AGNs of comparable X-ray luminosity. The Fe Kα line is unresolved in 4C 74.26. In the case of PKS 2349-01, however, the inferred properties of the line depend on the model adopted for the continuum: if a simple power-law model is used, the line is resolved at more than 99% confidence with a full width at half-maximum corresponding to approximately 50,000 km s^-1 and a rest-frame equivalent width of 230 ± 120 eV; however, if a Compton "reflection" model is used, the line is found to be a factor of 2 weaker for an assumed full width at half-maximum of 50,000 km s^-1. In 4C 74.26, a strong Compton "reflection" component is detected. Its strength suggests that the scattering medium subtends a solid angle of 2π to the illuminating source. Overall, the spectral indices of these radio-loud quasars are remarkably similar to those of their radio-quiet counterparts. On the other hand, if the absorber is indeed neutral, as our results suggest, this would be consistent with the typical properties of radio-loud AGNs.

We report the BeppoSAX observations of six flat-spectrum radio quasars. Three of them have a clear detection up to 100 keV with the PDS instrument. For four objects the X-ray spectrum is satisfactorily fitted by a power-law continuum with Galactic absorption. QSO 2251+158 shows the presence of absorption higher than the Galactic value, while the spectrum of the source QSO 0208-512 shows a complex structure, with evidence of absorption at low energy. We construct the spectral energy distributions adding historical data to the broadband X-ray spectra obtained with BeppoSAX and reproduce them with a one-zone synchrotron-inverse Compton model (including both synchrotron self-Compton and external Compton). The implications are briefly discussed.

We present observations of CO (1-0) and CO (2-1) emission from the z = 4.12 quasi-stellar object (QSO) PSS J2322+1944 using the Very Large Array. The CO emission is extended on a spatial scale of 2''. This extension could reflect the double nature of the QSO, as seen in the optical, or could be diffuse emission with a (redshift-corrected) mean brightness temperature of 2.8 K for the CO (2-1) line. We find that CO excitation conditions are lower than in two other IR-luminous z > 4 QSOs, suggesting the presence of a significant contribution from cooler, lower density molecular gas [n(H₂) ~ 5 × 10³ cm^-3], although such a conclusion is complicated by the possibility of differential gravitational magnification.

We use Hubble Space Telescope broadband images and VLA and VLBI continuum data to study the three-dimensional orientation of jets relative to nuclear dust disks in 20 radio galaxies. The comparison between the position angles of the jets and those of the dust disk major axes shows a wide distribution, suggesting that they are not aligned preferentially perpendicular to each other. We use a statistical technique to determine the three-dimensional distribution of angles between jets and dust disk rotation axes. This analysis shows that the observations are consistent with jets homogeneously distributed over a large region, extending over polar caps of 55°-77° but seeming to avoid lying close to the plane of the dust disks. We argue that the lack of close alignment between jets and dust disks axes is not likely to be caused by feeding the nucleus with gas from mergers originated from random directions. We suggest that the misalignment can be due to a warping mechanism in the accretion disk, such as self-irradiation instability or the Bardeen-Petterson effect, or that the gravitational potential in the inner regions of the galaxy is misaligned with respect to that of the dust disk.

Through observations and modeling, we demonstrate how the recently discovered large-scale bar in NGC 5248 generates spiral structure that extends from 10 kpc down to 100 pc, fuels star formation on progressively smaller scales, and drives disk evolution. Deep inside the bar, two massive molecular spirals cover nearly 180° in azimuth, show streaming motions of 20-40 km s^-1, and feed a starburst ring of super-star clusters at 375 pc. They also connect to two narrow K-band spirals that delineate the UV bright star clusters in the ring. The data suggest that the K-band spirals are young, and the starburst has been triggered by a bar-driven spiral density wave (SDW). The latter may even have propagated closer to the center where a second Hα ring and a dust spiral are found. The molecular and Hubble Space Telescope data support a scenario where stellar winds and supernovae efficiently clear out gas from dense star-forming regions on timescales less than a few Myr. We have investigated the properties of massive CO spirals within the framework of bar-driven SDWs, incorporating the effect of gas self-gravity. We find good agreement between the model predictions and the observed morphology, kinematics, and pitch angle of the spirals. This combination of observations and modeling provides the best evidence to date for a strong dynamical coupling between the nuclear region and the surrounding disk. It also confirms that a low central mass concentration, which may be common in late-type galaxies, is particularly favorable to the propagation of a bar-driven gaseous SDW deep into the central region of the galaxy, whereas a large central mass concentration favors other processes, such as the formation and decoupling of nuclear bars.

Hubble Space Telescope Space Telescope Imaging Spectrograph slitless spectroscopy of LMC planetary nebulae (PNs) is the ideal tool to study their morphology and their ionization structures at once. We present the results from a group of 29 PNs that have been spatially resolved, for the first time, in all the major optical lines. Images in the light of Hα, [N II], and [O III] are presented, together with line intensities, measured from the extracted one- and two-dimensional spectra. A study on the surface brightness in the different optical lines, the electron densities, the ionized masses, the excitation classes, and the extinction follows, illustrating an ideal consistence with the previous results found by us on LMC PNs. In particular, we find the surface brightness decline with the photometric radius to be the same in most emission lines. We find that asymmetric PNs form a well-defined cooling sequence in the excitation-surface brightness plane, confirming their different origin and larger progenitor mass.

We construct a simplified pressure equilibrium model for the confinement of high-velocity clouds (HVCs) in the Galactic halo of the Milky Way. The ambient pressure is obtained from a model for the distribution of the coronal gas density. This is assumed as an isothermal plasma in hydrostatic equilibrium with the gravitational field of the Galaxy. The cloud internal pressure, for either subsonic or supersonic HVCs, is expressed in terms of its observed parameters and as a function of distance from the Sun. The distances to three HVCs in complex M, observationally determined by Danly and coworkers in 1993, are used to set the values of the two free parameters of the model, namely, the thermal pressure of the coronal gas in the solar vicinity (~4 × 10^-13 dyn cm^-2) and its temperature T_g ~ 10⁶ K. This model can be more rigorously tested when a set of cloud distances, velocities, velocity dispersion, column densities, and angular sizes becomes available.

We present three sets of ROSAT Position Sensitive Proportional Counter and four sets of ASCA observations of the supernova remnant (SNR) W28. The overall shape of X-ray emission in W28 is elliptical, dominated by a centrally concentrated interior emission, sharply peaked at the center. There are also partial northeastern and southwestern shells, and both the central and shell X-ray emission is highly patchy. The ASCA spectra reveal emission lines of Ne, Mg, Si, and Fe Kα and continuum extending at least up to 7 keV, showing that X-ray emission in W28 is mostly of thermal origin with a hot thermal component. We found that spectral variations are present in W28. The southwestern shell can be fitted well by a plane-shock model with a temperature of 1.5 keV and an ionization timescale of 1.5 × 10¹¹ cm^-3 s. The long ionization timescale combined with a low estimated electron density of ~0.2 cm^-3 imply an SNR age of several times 10⁴ yr. The low density in the southwest is consistent with the shock breakout away from molecular clouds in the north and northeast. The northeastern shell, with a lower temperature of 0.56 keV and a longer ionization timescale of 1.7 × 10¹³ cm^-3 s, spatially coincides with the radiative shell delineated by radio and optical filaments, but a relatively high temperature and a low density of X-ray-emitting gas in the northeastern shell indicate that we are not observing gas cooling from high temperatures. Unlike for the southwestern and northeastern shells, the central emission cannot be fitted well by a single-temperature model, but two components with temperatures of 0.6 and 1.8 keV are required. The long ionization timescales imply that the gas is close to the ionization equilibrium. The low-temperature component is similar to those seen in other mixed-morphology SNRs. The X-ray luminosity of W28 is ~6 × 10³⁴ ergs s^-1, and the estimated X-ray mass is only ~20-25 M_☉. A comparison of W28 with other typical mixed-morphology SNRs reveals significant differences in its X-ray properties; W28 has a higher temperature and noticeable spectral variations. W28 belongs to a class of SNRs considered by Chevalier, with a radiative shell interacting with clumpy molecular clouds. X-ray emission at its center is a "fossil" radiation from gas that was shocked early in the evolution of the remnant, and its centrally peaked morphology could have been caused by processes such as evaporation, electron thermal conduction, and mixing induced by various hydrodynamical instabilities, but W28 poses a challenge for existing models of X-ray emission because the evaporation model of White & Long is in conflict with observations, while the presence of temperature variations seems inconsistent with SNR models with efficient thermal conduction.

In the standard model of low-frequency synchrotron radiation propagation through the Galaxy, the absorbing warm ionized medium (WIM) is considered to be a thick slab of thermal electrons of uniform density. When the calculated polar radio spectrum is compared with the observed Galactic background radio spectrum, it is found that this model requires a much higher electron density n_e or much lower temperature T_e than permitted by current observations. A more realistic plane-parallel model, in which electron density, temperature, and cosmic-ray electron emissivity have smooth distributions with height z above the Galactic plane, is also found to suffer from the same setbacks as the standard model. However, a plane-parallel model in which the absorbing WIM has a clumpy distribution with clump densities of ~0.2 cm^-3 and filling factor of 0.08-0.15 agrees with both the low-frequency radio synchrotron spectrum and the observational parameters ⟨n_e(z = 0)⟩ ≈ 0.025 cm^-3, T_e ≈ 7000 K, and DM ≈ 23 pc cm^-3. The clumpy WIM model also supports the idea of a local interstellar cloud (LIC), which is required to provide adequate absorption below ~0.5 MHz. This LIC appears to become optically thick only below ~0.1 MHz where future radio measurements may be used to determine the emissivity spectrum and, therefore, the local interstellar cosmic-ray electron spectrum at energies of ~40 MeV.

We describe a Faraday rotation structure in the interstellar medium detected through polarimetric imaging at 1420 MHz from the Canadian Galactic Plane Survey (CGPS). The structure, at l = 918,b = -25, has an extent of ~2°, within which polarization angle varies smoothly over a range of ~100°. Polarized intensity also varies smoothly, showing a central peak within an outer shell. This region is in sharp contrast to its surroundings, where low-level chaotic polarization structure occurs on arcminute scales. The Faraday rotation structure has no counterpart in radio total intensity and is unrelated to known objects along the line of sight, which include a Lynds Bright Nebula, LBN 416, and the star cluster M39 (NGC 7092). It is interpreted as a smooth enhancement of electron density. The absence of a counterpart, in either optical emission or total intensity, establishes a lower limit to its distance. An upper limit is determined by the strong beam depolarization in this direction. At a probable distance of 350 ± 50 pc, the size of the object is 10 pc, the enhancement of electron density is 1.7 cm^-3, and the mass of ionized gas is 23 M_☉. It has a very smooth internal magnetic field of strength 3 μG, slightly enhanced above the ambient field. G91.8-2.5 is the second such object to be discovered in the CGPS, and it seems likely that such structures are common in the magneto-ionic medium.

We analyze H₂ and CO absorption along the line of sight toward HD 37903 over the 1045-1060 Å and 1086-1102 Å wavelength regions, which were observed by the Berkeley Extreme and Far-Ultraviolet Spectrometer on the Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometer (ORFEUS) telescope. HD 37903 is a bright UV-emitting star embedded in the L1630 molecular cloud, creating the reflection nebula NGC 2023. Using the theory of formation and dissociation of molecular hydrogen in the far-UV spectral range, we derive the physical conditions of the foreground gas toward HD 37903, such as the density n, temperature T, and the UV intensity I_UV, by analyzing the H₂ lines from the J'' = 0-5 rotational levels. In addition, we identify the CO absorption band at 1088 Å and calculate the abundance ratio [CO]/[H₂] directly, which is found to be 1.3 × 10^-7. The higher rotational levels of H₂ are excited in the outer boundary of a molecular cloud where UV pumping dominates, while the lower J levels of H₂ and other molecules, such as CO, prevail in the neutral core of the cloud. In this regard, our analysis indicates that the observed gas extends from the neutral region to the photodissociation region of NGC 2023 at a distance of 0.2 pc from HD 37903. These results are similar to those found in previous studies of the photodissociation region of NGC 2023 and chemical analysis of the foreground gas toward HD 37903. The total amount of molecular hydrogen, including the gas behind HD 37903, is estimated from the total visual extinction of the dark cloud and our H₂ column density of the foreground gas. By measuring the CO and ¹³CO integrated intensities via radio observations, we find the conversion factor N_tot(H₂)/W(CO) to be 1.5 × 10²⁰ cm^-2 (K km s^-1)^-1, which is in accord with canonical conversion factors.

The reflection nebula NGC 2023 was observed by a rocket-borne long-slit imaging spectrograph in the 900-1400 Å bandpass on 2000 February 11. A spectrum of the star, as well as that of the nebular scattered light, was recorded. Through the use of a Monte Carlo modeling process, the scattering properties of the dust were derived. The albedo is low, 0.2-0.4, and decreasing toward shorter wavelengths, while the phase function asymmetry parameter is consistent with highly forward-scattering grains, g ~ 0.85. The decrease in albedo, while the optical depth increases to shorter wavelengths, implies that the far-UV rise in the extinction curve is due to an increase in absorption efficiency.

We have observed the H¹³CN (J = 1-0) and HC¹⁵N (J = 1-0) lines simultaneously toward three dense cores in the Taurus molecular cloud complex and have found a significant source-to-source variation of the column density ratio, N(H¹³CN)/N(HC¹⁵N). The lowest ratio of 2.6-3.1 is observed toward L1521E, while the highest ratio of 8-11 is observed toward L1498. We have also observed the ¹²C¹⁸O (J = 1-0, 2-1) and ¹³C¹⁸O (J = 1-0) lines, and the ¹²C/¹³C ratio has been derived to be 58.8 ± 3.7 and ≥117 toward L1521E and L1498, respectively. These results imply that a substantial isotope fractionation effect may exist in the formation processes of these molecules, or elemental abundances are not uniform among the observed sources. If the source-to-source variation of the (¹⁴N/¹⁵N)/(¹²C/¹³C) ratio in HCN and the ¹²C/¹³C ratio in CO originates from the elemental abundance variation, the ¹⁴N/¹⁵N ratio is calculated to be 151 ± 16 and ≥813 for L1521E and L1498, respectively. These ratios significantly deviate from the values obtained toward Galactic disk sources.

A 28 g sample of the Qingzhen enstatite (EH3) chondrite was subjected to chemical and physical separation procedures to yield several grain-size residues. Ion mapping of isotopes of Si, O, and C in the ion microprobe of two size fractions (QZR4: 0.4-0.8 μm; QZR5: 0.8-2 μm) identified 55 ³⁰Si-depleted candidates out of 37,917 Si-rich grains and six ¹⁸O-depleted grains out of 54,410 oxides. Subsequent isotopic analyses of C, N, and Si of 48 grains of the ³⁰Si-depleted candidates and additional randomly selected SiC and Si₃N₄ grains confirmed 36 of X-type SiC, nine of X-type Si₃N₄, and one of A+B-type SiC. The isotopic compositions of most X grains overlap those of previously measured X grains from the Murchison carbonaceous chondrite, but ~25% show more pronounced ²⁹Si deficits, suggestive of multiple stellar origins of X grains. Presolar Si₃N₄ grains have isotopic compositions similar to those of X SiC grains, except that their C isotopic ratios are close to solar. The relative abundances of various presolar grain types in Qingzhen are different from those in Murchison, suggestive of heterogeneity and/or size sorting in the primitive solar nebula.

We report an attempt to construct unified cloudy models of M, L, and T dwarfs. For this purpose, we first discuss opacities as well as thermochemical properties of the cool and dense matter. Below about 2000 K, refractory material condenses, and dust will play a major role as a source of opacity. Then a major problem in modeling the photospheres of very cool dwarfs is how to treat dust and, especially, how dust could be sustained in the static photosphere for a long time. Under the high density of the photospheres of cool dwarfs, dust forms easily at the condensation temperature, T_cond, but the dust will soon grow larger than its critical radius r_cr (at which the Gibbs free energy of condensation attains the maximum) at the critical temperature T_cr. Such large dust grains with r_gr ≳ r_cr will soon segregate from the gas and precipitate below the photosphere. For this reason, dust exists effectively only in the limited region of T_cr ≲ T ≲ T_cond in the photosphere, and this means that a dust cloud is formed deep in the photosphere rather than in the cooler surface region. With this simple model of the dust cloud, we show that the nongray model photosphere in radiative-convective equilibrium can be extended to T_eff values as low as 800 K. Since T_cond ≈ 2000 K for the first condensates such as corundum and iron, the dust cloud is rather warm and necessarily located deeper in the photosphere (τ > 1) for the cooler objects (note that T ≈ T_eff at τ ≈ 1). This explains why dust apparently shows little observable effect in T dwarfs. For warmer objects, the dust cloud, which is always formed at the same temperature range of T_cr ≲ T ≲ T_cond, can be located nearer the surface (τ < 1), and, for this reason, L dwarfs appear to be dusty. We show that the recently proposed spectral classification of L and T dwarfs can consistently be interpreted by a single grid of our unified cloudy models with the thin dust cloud deep in the photosphere.

We present the results of a deep search for the molecular hydrogen shock fronts associated with young stellar outflows in the giant molecular cloud and massive star-forming region W51. A total of 14 outflows were identified by comparing images in the H and K bands and in a narrowband filter centered on the H₂ 1-0 S(1) line at 2.122 μm. A few of the newly discovered outflows were subsequently imaged at higher spatial resolution in the S(1) filter; one outflow was also imaged in the 1.644 μm emission line of [Fe II]. For two of the outflows, high-resolution echelle spectroscopy in the H₂ 1-0 S(1) line was obtained using NIRSPEC at Keck. For one outflow additional high-resolution spectra were obtained in the [Fe II] line and in Brγ. The largest and best-studied outflow shock front shows a remarkably broad [Fe II] line, an unusual high-velocity component in Brγ, and comparably narrow line widths in the H₂ 1-0 S(1) line. A scenario involving high-velocity shocks and UV excitation of preshock material is used to explain these spectra.

The collapse of rotating magnetized molecular cloud cores is studied with axisymmetric magnetohydrodynamic (MHD) simulations. Because of the change of the equation of state of the interstellar gas, molecular cloud cores experience several phases during the collapse. In the earliest isothermal runaway collapse (n ≲ 10¹⁰ H₂ cm^-3), a pseudodisk is formed, and it continues to contract until an opaque core is formed at the center. In this disk, a number of MHD fast and slow shock pairs appear whose wave fronts are parallel to the disk. We assume that the interstellar gas obeys a polytropic equation of state with the exponent of Γ > 1 above the critical density at which the core becomes optically thick against the thermal radiation from dusts n_cr ~ 10¹⁰ cm^-3. After the equation of state becomes hard, an adiabatic quasi-static core forms at the center (the first core), which is separated from the isothermal contracting pseudodisk by the accretion shock front facing radially outward. By the effect of the magnetic tension, the angular momentum is transferred from the disk midplane to the surface. The gas with an excess angular momentum near the surface is finally ejected, which explains the molecular bipolar outflow. Two types of outflows are found. When the poloidal magnetic field is strong (its energy is comparable to the thermal one), a U-shaped outflow is formed, in which gas is mainly outflowing through a region whose shape looks like a capital letter U at a finite distance from the rotation axis. The gas is accelerated by the centrifugal force and the magnetic pressure gradient of the toroidal component. The other is a turbulent outflow in which magnetic field lines and velocity fields seem to be randomly oriented. In this case, globally the gas moves out almost perpendicularly from the disk, and the outflow looks like a capital letter I. In this case, although the gas is launched by the centrifugal force, the magnetic force working along the poloidal field lines plays an important role in expanding the outflow. The continuous mass accretion leads to a quasi-static contraction of the first core. A second collapse due to the dissociation of H₂ occurs in it. Finally, another less massive quasi-static core is formed by atomic hydrogen (the second core). At the same time, it is found that another outflow is ejected around the second atomic core, which seems to correspond to the optical jets or the fast neutral winds.

We have carried out 21 cm radio continuum, H76α radio recombination line, and various (¹²CO, ¹³CO, CS, and C³⁴S) molecular line observations of the W31 complex. Our radio continuum data show that W31 is composed of two extended H II regions, G10.2-0.3 and G10.3-0.1, each of which comprises an ultracompact H II region, two or more compact components, and a diffuse envelope. The W31 cloud appears as an incomplete shell on the whole and consists of southern spherical and northern flat components, which are associated with G10.2-0.3 and G10.3-0.1, respectively. For an assumed distance of 6 kpc, the molecular cloud has a size of 48 pc and a mass of 6.2 × 10⁵M_☉. The IR luminosity-to-mass ratio and the star formation efficiency are derived to be 9 L_☉/M_☉ and 3%, respectively. These estimates are greater than average values of the inner Galactic plane. We detect two large (16 and 11 pc) and massive (2.1 × 10⁵ and 8.2 × 10⁴M_☉) CS-emitting regions in the northern and southern cloud components. The large amount (48% in mass and 16% in area) of dense gas may suggest that the W31 cloud has the ability to form rich stellar clusters and that star formation has only recently begun. The extended envelopes of both G10.2-0.3 and G10.3-0.1 are likely to be results of the champagne flows, based on the distributions of ionized and molecular gas and the velocity gradient of H76α line emission. According to the champagne model, the dynamical ages of the two H II regions would be (4-12) × 10⁵ yr. We find strong evidence of bipolar molecular outflows associated with the two ultracompact H II regions. In the vicinity of the ultracompact and compact H II regions in G10.3-0.1, the ¹²CO J = 2-1/J = 1-0 intensity ratio is high (1.4), and a small but prominent molecular gas hollow exists. Together, these observations strongly indicate that the H II regions and their ionizing stars are interacting with the molecular cloud. Therefore, it is most likely that recently formed massive stars are actively disrupting their parental molecular cloud in the W31 complex.

Seven Class 0 sources mapped with SCUBA at 850 and 450 μm are modeled using a one-dimensional radiative transfer code. The modeling takes into account heating from an internal protostar, heating from the interstellar radiation field (ISRF), realistic beam effects, and chopping to model the normalized intensity profile and spectral energy distribution. Power-law density models, n(r) ∝ r^-p, fit all of the sources; best-fit values are mostly p = 1.8 ± 0.1, but two sources with aspherical emission contours have lower values (p ~ 1.1). Including all sources, ⟨p⟩ = 1.63 ± 0.33. Based on studies of the sensitivity of the best-fit p to variations in other input parameters, uncertainties in p for an envelope model are Δp = ±0.2. If an unresolved source (e.g., a disk) contributes 70% of the flux at the peak, p is lowered in this extreme case and Δp = . The models allow a determination of the internal luminosity (⟨L_int⟩ = 4.0 L_☉) of the central protostar as well as a characteristic dust temperature for mass determination (⟨T_iso⟩ = 13.8 ± 2.4 K). We find that heating from the ISRF strongly affects the shape of the dust temperature profile and the normalized intensity profile, but it does not contribute strongly to the overall bolometric luminosity of Class 0 sources. There is little evidence for variation in the dust opacity as a function of distance from the central source. The data are well fitted by dust opacities for coagulated dust grains with ice mantles (Ossenkopf & Henning). The density profile from an inside-out collapse model (Shu) does not fit the data well, unless the infall radius is set so small as to make the density nearly a power law.

NGC 1333, a highly active star formation region within the Perseus molecular cloud complex, has been observed with the ACIS-I detector on board the Chandra X-Ray Observatory. In our image with a sensitivity limit of ~10²⁸ ergs s^-1, we detect 127 X-ray sources, most with subarcsecond positional accuracy. While 32 of these sources appear to be foreground stars and extragalactic background, 95 X-ray sources are identified with known cluster members. The X-ray luminosity function of the discovered young stellar object (YSO) population spans a range of log L_X ≃ 28.0-31.5 ergs s^-1 in the 0.5-8 keV band, and absorption ranges from log N_H ≃ 20 to 23 cm^-2. Most of the sources have plasma energies between 0.6 and 3 keV, but a few sources show higher energies up to ~7 keV. Comparison with K-band source counts indicates that we detect all of the known cluster members with K < 12 and about half of the members with K > 12. K ≃ 11, the peak of the K-band luminosity function, corresponds to 0.2-0.4 M_☉ stars for a cluster age of ~1 Myr. We detect seven of the 20 known YSOs in NGC 1333 producing jets or molecular outflows as well as one deeply embedded object without outflows. No evident difference in X-ray emission of young stars with and without outflows is found. Based on the complete subsample of T Tauri stars, we also find no difference in X-ray properties and X-ray production mechanism of stars with and without K-band excess disks. Several other results are obtained. We suggest that the X-ray emission from two late B stars that illuminate the reflection nebula originates from unresolved late-type companions. Two T Tauri stars are discovered in the ACIS images as previously unknown components of visual binaries. A good correlation L_X ∝ J is seen, which confirms the well-known relation L_X ∝ L_bol found in many star-forming regions. Based on spectral analysis for the X-ray counterpart of SVS 16, we establish that the column density N_H is much lower than that expected from near-IR photometry so that its X-ray luminosity, log L_X ≃ 30.6 ergs s^-1, is not unusually high.

It has been proposed recently that Galactic microquasars may be prodigious emitters of TeV neutrinos that can be detected by upcoming km² neutrino telescopes. In this paper we consider a sample of identified microquasars and microquasar candidates for which available data enable rough determination of the jet parameters. By employing the parameters inferred from radio observations of various jet ejection events, we determine the neutrino fluxes that should have been produced during these events by photopion production in the jet. Despite the large uncertainties in our analysis, we demonstrate that in several of the sources considered the neutrino flux at Earth, produced in events similar to those observed, would exceed the detection threshold of a km² neutrino detector. The class of microquasars may contain also sources with bulk Lorentz factors larger than those characteristic of the sample considered here, directed along our line of sight. Such sources, which may be very difficult to resolve at radio wavelengths and hence may be difficult to identify as microquasar candidates, may emit neutrinos with fluxes significantly larger than those typically obtained in the present analysis. These sources may eventually be identified through their neutrino and gamma-ray emission.

We present seven eclipse timings of the low-mass X-ray binary (LMXB) EXO 0748-676 obtained with the Unconventional Stellar Aspect (USA) Experiment during 1999-2000, as well as 122 eclipse timings obtained with the Rossi X-Ray Timing Explorer (RXTE) during 1996-2000. According to our analysis, the mean orbital period has increased by ~8 ms between the pre-RXTE era (1985-1990) and the RXTE/USA era (1996-2000). This corresponds to an orbital period derivative of P_orb/_orb ~ 2 × 10⁷ yr. However, neither a constant orbital period derivative nor any other simple ephemeris provides an acceptable fit to the data; individual timings of eclipse centers have residuals of up to 15 or more seconds away from our derived smooth ephemerides. When we consider all published eclipse timing data, including those presented here, a model that includes observational measurement error, cumulative period jitter, and underlying period evolution is found to be consistent with the timing data. We discuss several physical mechanisms for LMXB orbital evolution in an effort to account for the change in orbital period and the observed intrinsic jitter in the mideclipse times.

We study the pulse morphologies and pulse amplitudes of thermally emitting neutron stars with ultrastrong magnetic fields. The beaming of the radiation emerging from a magnetar was recently shown to be predominantly nonradial, with a small pencil and a broad fan component. We show that the combination of this radiation pattern with the effects of strong lensing in the gravitational field of the neutron star yields pulse profiles that show a qualitatively different behavior compared to that of the radially peaked beaming patterns explored previously. Specifically, we find that (i) the pulse profiles of magnetars with a single hot emission region on their surface exhibit 1-2 peaks, whereas those with an antipodal emission geometry have 1-4 peaks, depending on the neutron star compactness, the observer's viewing angle, and the size of the hot regions; (ii) the energy dependence of the beaming pattern may give rise to weakly or strongly energy-dependent pulse profiles and may introduce phase lags between different energy bands; (iii) the nonradial beaming pattern can give rise to high pulsed fractions even for very relativistic neutron stars; (iv) the pulsed fraction may not vary monotonically with neutron star compactness; (v) the pulsed fraction does not decrease monotonically with the size of the emitting region; (vi) the pulsed fraction from a neutron star with a single hot pole has, in general, a very weak energy dependence, in contrast to the case of an antipodal geometry. Comparison of these results to the observed properties of anomalous X-ray pulsars strongly suggests that they are neutron stars with a single hot region of ultrastrong magnetic field.

The interaction of high-velocity neutron stars with the interstellar medium produces bow shock nebulae, in which the relativistic neutron star wind is confined by ram pressure. We present multiwavelength observations of the Guitar Nebula, including narrowband Hα imaging with Hubble Space Telescope (HST) WFPC2, which resolves the head of the bow shock. The HST observations are used to fit for the inclination of the pulsar velocity vector to the line of sight and to determine the combination of spin-down energy loss, velocity, and ambient density that sets the scale of the bow shock. We find that the velocity vector is most likely in the plane of the sky. We use the Guitar Nebula and other observed neutron star bow shocks to test scaling laws for their size and Hα emission, discuss their prevalence, and present criteria for their detectability in targeted searches. The set of Hα bow shocks shows remarkable consistency, in spite of the expected variation in ambient densities and orientations. Together, they support the assumption that a pulsar's spin-down energy losses are carried away by a relativistic wind that is indistinguishable from being isotropic. Comparison of Hα bow shocks with X-ray and nonthermal radio-synchrotron bow shocks produced by neutron stars indicates that the overall shape and scaling is consistent with the same physics. It also appears that nonthermal radio emission and Hα emission are mutually exclusive in the known objects and perhaps in all objects.

We present new Hubble Space Telescope Space Telescope Imaging Spectrograph observations of the short-period dwarf novae LL And and EF Peg during deep quiescence. We fit stellar models to the UV spectra and use optical and IR observations to determine the physical parameters of the white dwarfs in the systems, the distances to the binaries, and the properties of the secondary stars. Both white dwarfs are relatively cool, having T_eff near 15,000 K, and consistent with a mass of 0.6 M_☉. The white dwarf in LL And appears to be of solar abundance or slightly lower, while that in EF Peg is near 0.1-0.3 times solar. LL And is found to be 760 pc away, while EF Peg is closer, at 380 pc. EF Peg appears to have an ~M5 V secondary star, consistent with that expected for its orbital period, while the secondary object in LL And remains a mystery.

The binary star LS I +61°303 is remarkable for its periodic radio outbursts every 26.5 days. We recently discovered a ~4.4 yr periodic modulation of the phase and amplitude of these outbursts using the Gregory-Loredo Bayesian algorithm for detecting periodic signals of unknown shape. In this paper we obtain improved estimates of both the orbital period P₁ and the modulation period P₂ based on a larger data set and examine the behavior of the spectral index between 2.2 and 8.3 GHz versus the modulation period. The new estimates are P₁ = 26.4960 ± 0.0028 and P₂ = 1667 ± 8 days, and our best estimate of the radio phase of periastron is ~0.4. Analysis of the spectral index data indicates that the optical depth in the synchrotron emission region is always ≪1 at 8.3 GHz and can reach values of ~2.7 at 2.2 GHz. A test of the precessing Be star model of Lipunov & Nazin indicates that it is unlikely to be the correct mechanism to explain the 1667 day periodic modulation.

The classical B0.5e star γ Cassiopeia is known to be a unique X-ray source by virtue of its moderate L_X (10³³ ergs s^-1), hard X-ray spectrum, and light curve punctuated by ubiquitous flares and slow undulations. The peculiarities of this star have led to a controversy concerning the origin of these emissions: whether they are from wind infall onto a putative degenerate companion, as in the case of normal Be/X-ray binaries, or from the Be star itself. Recently, much progress has been made to resolve this question: (1) the discovery that γ Cas is a moderately eccentric binary system (P = 203.59 days) with unknown secondary type, (2) the addition of RXTE observations at six epochs in 2000, adding to three others in 1996-1998, and (3) the collation of robotic telescope (Automated Photometric Telescope) B- and V-band photometric observations over four seasons that show a 3%, cyclical flux variation with cycle lengths of 55-93 days. We find that X-ray fluxes at all nine epochs show random variations with orbital phase, thereby contradicting the binary accretion model, which predicts a substantial modulation. However, these fluxes correlate well with the cyclical optical variations. In particular, the six flux measurements in 2000, which vary by a factor of 3, closely track the interpolated optical variations between the 2000 and 2001 observing seasons. The energy associated with the optical variations greatly exceeds the energy in the X-rays, so that the optical variability cannot simply be due to reprocessing of X-ray flux. However, the strong correlation between the two suggests that they are driven by a common mechanism. We propose that this mechanism is a cyclical magnetic dynamo excited by a Balbus-Hawley instability located within the inner part of the circumstellar disk. According to our model, variations in the field strength directly produce the changes in the magnetically related X-ray activity. Turbulence associated with the dynamo results in changes to the density (and therefore the emission measure) distribution within the disk and creates the observed optical variations.

The variability of the Hg II λ3984 line in the primary of the binary star α And was discovered through the examination of high-dispersion spectra with signal-to-noise ratios greater than 500. This first definitively identified spectrum variation in any mercury-manganese star is not due to the orbital motion of the companion. Rather, the variation is produced by the combination of the 2.38236 day period of rotation of the primary that we determined and a nonuniform surface distribution of mercury that is concentrated in its equatorial region. If the surface mercury distribution exhibits long-term stability, then it is likely that a weak magnetic field operates in its atmosphere, but if changes are observed in the line profile over a period of a few years, then these would constitute direct evidence for diffusion.

We apply a model of dynamo-driven mass loss, magnetic braking, and tidal friction to the evolution of stars with cool convective envelopes; in particular, we apply it to binary stars where the combination of magnetic braking and tidal friction can cause angular momentum loss from the orbit. For the present we consider the simplification that only one component of a binary is subject to these nonconservative effects, but we emphasize the need in some circumstances to permit such effects in both components.The model is applied to examples of (1) the Sun, (2) BY Dra binaries, (3) Am binaries, (4) RS CVn binaries, (5) Algols, and (6) post-Algols. A number of problems regarding some of these systems appear to find a natural explanation in our model. There are indications from other systems that some coefficients in our model may vary by a factor of 2 or so from system to system; this may be a result of the chaotic nature of dynamo activity.

Infrared OH lines at 1.5-1.7 μm in the H band were obtained with the NIRSPEC high-resolution spectrograph at the 10 m Keck Telescope for a sample of seven metal-poor stars. Detailed analyses have been carried out, based on optical high-resolution data obtained with the Fiber-fed Extended Range Optical Spectrograph at ESO. Stellar parameters were derived by adopting infrared flux method effective temperatures, trigonometric and/or evolutionary gravities, and metallicities from Fe II lines. We obtain that the sample stars with metallicities [Fe/H] < -2.2 show a mean oxygen abundance [O/Fe] ≈ 0.54 for a solar oxygen abundance of (O) = 8.87, or [O/Fe] ≈ 0.64 if (O) = 8.77 is assumed.

I report the results of a survey for low-mass (0.030 M_☉ ≳ M ≳ 0.013 M_☉) brown dwarfs in the direction of the TW Hya association using the Two Micron All Sky Survey (2MASS). Two late M dwarfs show signs of low surface gravity and are strong candidates to be young, very low mass (M ≈ 0.025 M_☉) brown dwarfs related to the TW Hya association. The object 2MASSW J1207334-393254 is particularly notable for its strong Hα emission. The number of detected brown dwarfs is consistent with the substellar mass function in richer star formation environments. Newly identified late M and L dwarfs in the field are also discussed. Unusual objects include an L dwarf with strong Hα emission, a possible wide M8-M9 triple system, and a possible L dwarf companion to an LHS star.

Transit photometry is a promising method for discovering extrasolar planets as small as Earth from space-based photometers, and several near-term photometric missions are on the drawing board. In particular, NASA's recently selected Kepler mission is devoted primarily to detecting extrasolar planets. The success of these efforts depends in part on the ability to detect transit signatures against the inherent photometric variability of the target stars. While other noise sources such as shot noise and CCD noise are under the control of the instrument designers, this one is not. The photometric variability of solar-like stars presents a fundamental lower limit to the minimum detectable planet radius for a given star and number of observed transits. In this paper we examine the capability of such missions using bolometric data for the only star for which sufficient photometric precision exists to address this question: the Sun. The results indicate that solar-like variability does not prevent the detection of Earth-sized planets even for stars rotating significantly faster than the Sun. Four transits are detectable for m_v = 12 stars with rotation periods as short as ~21 days, while six transits allow detection for stellar rotation periods as short as ~16 days. Indeed, the limits posed by solar-like variability allow for the detection of planets significantly smaller than Earth orbiting Sun-like stars. Planets as small as 0.6 Earth radii exhibiting at least six transits can be detected orbiting bright (m_v = 10) solar analogs.

We investigate the statistical properties of Ellerman bombs in the dynamic emerging flux region NOAA Active Region 8844, underneath an expanding arch filament system. High-resolution chromospheric Hα filtergrams (spatial resolution 08), as well as photospheric vector magnetograms (spatial resolution 05) and Dopplergrams, have been acquired by the balloon-borne Flare Genesis Experiment. Hα observations reveal the first "seeing-free" data set on Ellerman bombs and one of the largest samples of these events. We find that Ellerman bombs occur and recur in preferential locations in the low chromosphere, either above or in the absence of photospheric neutral magnetic lines. Ellerman bombs are associated with photospheric downflows, and their loci follow the transverse mass flows on the photosphere. They are small-scale events, with typical size 18 × 11 , but this size depends on the instrumental resolution. A large number of Ellerman bombs are probably undetected, owing to limited spatial resolution. Ellerman bombs occur in clusters that exhibit fractal properties. The fractal dimension, with an average value ~1.4, does not change significantly in the course of time. Typical parameters of Ellerman bombs are interrelated and obey power-law distribution functions, as in the case of flaring and subflaring activity. We find that Ellerman bombs may occur on separatrix, or quasi-separatrix, layers, in the low chromosphere. A plausible triggering mechanism of Ellerman bombs is stochastic magnetic reconnection caused by the turbulent evolution of the low-lying magnetic fields and the continuous reshaping of separatrix layers. The total energies of Ellerman bombs are estimated in the range (10²⁷, 10²⁸) ergs, the temperature enhancement in the radiating volume is ~2 × 10³ K, and the timescale of radiative cooling is short, of the order of a few seconds. The distribution function of the energies of Ellerman bombs exhibits a power-law shape with an index ~-2.1. This suggests that Ellerman bombs may contribute significantly to the heating of the low chromosphere in emerging flux regions.

We propose the use of principal component analysis (PCA) to invert spectropolarimetric data from prominences. Observation of the full Stokes profiles in prominences is very important for a deeper understanding of magnetic-field topology in these solar structures, and for the testing of theoretical models. The line formation problem, however, is complicated by the special conditions of prominences: anisotropy of light, low magnetic intensities, temperature and density ranges, etc. We created a code to solve this problem in the limit of optically thin plasma and of a collisionless regime, and use it in combination with PCA techniques to invert synthetic data. The results show that inversion is feasible.

During the 1996-1997 activity minimum, the Large Angle and Spectrometric Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO) recorded numerous jetlike ejections above the Sun's polar regions. In a previous study, we showed that these white-light ejections were the outward extensions of extreme-ultraviolet (EUV) jets, which in turn originated from flaring bright points inside the polar coronal holes. Here we investigate a number of jetlike events observed with LASCO during the current sunspot maximum. To identify the solar surface counterparts of these events, we again use Fe XII λ195 images obtained by the EUV Imaging Telescope on SOHO. The white-light jets in our sample have angular widths of ~3°-7° and velocities typically of order 600 km s^-1; they tend to be brighter and wider than the polar jets observed near sunspot minimum and are distributed over a much greater range of latitudes. Many of the ejections are recurrent in nature and originate from active regions located inside or near the boundaries of nonpolar coronal holes. We deduce that the jet-producing regions consist of systems of closed magnetic loops partially surrounded by open fields; perturbations in the closed fields caused them to reconnect with the overlying open flux, releasing the trapped energy in the form of jetlike ejections. In some events, the core of the active region erupts, producing fast, collimated ejections with widths of up to ~15°.

The purpose of this paper is (1) to confirm and establish the working of a dual-etalon Fabry-Pérot imaging spectroscopy system at the National Solar Observatory/Sacramento Peak Dunn Solar Telescope and (2) to use this system to extend previous work by many authors and understand the structure and dynamics of sunspots. A detailed investigation of the thermal and velocity structure in an isolated sunspot, using the Fe I 5576 Å spectral line, is presented. The concept of flowless maps is incorporated, to separate velocity and intensity effects. The resulting intensities are used to generate thermal maps of the sunspot along the height of formation of a spectral line, followed by a thermal span map. The thermal span in penumbral regions is in the range of 1350-1580 K. It is a factor of 2 smaller in the umbra. Using spectral line bisectors, we extend the concept of a velocity span to a sunspot, following Gray. The velocity span is used to study the velocity gradients across a sunspot. The velocity span maximizes in the middle of the sunspot penumbra and falls off on either side. The Doppler-neutralized mean bisectors from the disk-side and limb-side penumbra show more sharply inclined gradients, when compared with the C-shaped photospheric bisectors. The mean umbral bisectors show sharp, <-shaped profiles. In most of the penumbra, the individual bisectors are sharply inclined, with a shape of "/" or "\," indicative of a highly suppressed convective flow. The intensity and velocity data show that a new family of penumbral filaments rises in the middle penumbra. Bisector intensity-velocity relationships display opposite gradients in the inner and outer penumbra, showing the rising and falling parts of curved penumbral flux tubes. Some clustering of the bisector intensity-velocity relationship is perhaps due to the fluted nature of flux tubes.

The heating of the lower solar corona is examined using numerical simulations and theoretical models of magnetohydrodynamic turbulence in open magnetic regions. A turbulent energy cascade to small length scales perpendicular to the mean magnetic field can be sustained by driving with low-frequency Alfvén waves reflected from mean density and magnetic field gradients. This mechanism deposits energy efficiently in the lower corona, and we show that the spatial distribution of the heating is determined by the mean density through the Alfvén speed profile. This provides a robust heating mechanism which can explain observed high coronal temperatures and accounts for the significant heating (per unit volume) distribution below 2 solar radii needed in models of the origin of the solar wind. The obtained heating per unit mass, on the other hand, is much more extended, indicating that the heating on a per-particle basis persists throughout all the lower coronal region considered here.

We derive the minimum energy state resulting from complete magnetic reconnection in a translationally or axisymmetric MHD system, in the limit of a low plasma beta and high magnetic Reynolds number—conditions appropriate to the solar corona. The results are necessary for determining the amount of energy that can be liberated by reconnection and, hence, are important for understanding coronal heating and other forms of solar activity. The key difference between our approach and previous work is that because of line tying at the high-beta photosphere, reconnection is limited to occur only at magnetic null points initially present in the system. We find that under these circumstances the minimum energy state is not the usual linear force-free field but a state in which the nonpotential component of the field is distributed uniformly on equal flux surfaces. We discuss the implications of our results for the Sun's corona and for laboratory plasmas.

Strong gravitational lensing has traditionally been one of the few phenomena said to oppose a large cosmological constant; many analyses of lens statistics have given upper limits on Ω_Λ that are marginally inconsistent with the concordance cosmology. Those conclusions were based on models in which the predicted number counts of galaxies at moderate redshifts (z ~ 0.5-1) increased significantly with Ω_Λ. I argue that the models should now be calibrated by counts of distant galaxies. When this is done, lens statistics lose most of their sensitivity to the cosmological constant.

The diffuse extragalactic gamma-ray background (EGRB) above 100 MeV encodes unique information about high-energy processes in the universe. Numerous sources for the EGRB have been proposed, but the two systems that are certain to make some contribution are active galaxies (blazars) as well as normal galaxies. In this Letter we evaluate the contribution to the background from both sources. The active galaxy contribution arises from unresolved blazars. We compute this contribution using the Stecker-Salamon model. For normal galaxies, the emission is due to cosmic-ray interactions with diffuse gas. Our key assumption here is that the cosmic-ray flux in a galaxy is proportional to the supernova rate and thus the massive star formation rate, quantified observationally by the cosmic star formation rate (CSFR). In addition, the existence of stars today requires a considerably higher interstellar medium mass in the past. Using the CSFR to compute both these effects, we find that normal galaxies are responsible for a significant portion (~) of the EGRB near 1 GeV but make a smaller contribution at other energies. Finally, we present a "minimal" two-component model that includes contributions from both normal galaxies and blazars. We show that the spectrum of the diffuse radiation is a key constraint on this model: while neither the blazar spectra nor the galactic spectra are separately optimal fits to the observed spectrum, the combined emission provides an excellent fit. We close by noting key observational tests of this two-component model, which can be probed by future gamma-ray observatories, such as the Gamma-ray Large Area Space Telescope.

The detection of spectral variability of the γ-ray blazar Mrk 421 at TeV energies is reported. Observations with the Whipple Observatory 10 m γ-ray telescope taken in 2000/2001 revealed exceptionally strong and long-lasting flaring activity. Flaring levels of 0.4-13 times that of the Crab Nebula flux provided sufficient statistics for a detailed study of the energy spectrum between 380 GeV and 8.2 TeV as a function of the flux level. These spectra are well described by a power law with an exponential cutoff: dN/dE ∝ E^-αe m^-2 s^-1 TeV^-1. There is no evidence for variation in the cutoff energy with flux, and all spectra are consistent with an average value for the cutoff energy of 4.3 TeV. The spectral index varies between 1.89 ± 0.04_stat ± 0.05_syst in a high flux state and 2.72 ± 0.11_stat ± 0.05_syst in a low state. The correlation between spectral index and flux is tight when averaging over the total 2000/2001 data set. Spectral measurements of Mrk 421 from previous years (1995/1996 and 1999) by the Whipple collaboration are consistent with this flux-spectral index correlation, which suggests that this may be a constant or a long-term property of the source. If a similar flux-spectral index correlation were found for other γ-ray blazars, this universal property could help disentangle the intrinsic emission mechanism from external absorption effects.

SAX J1808.4-3658 is a unique source, being the first low-mass X-ray binary showing coherent pulsations at a spin period comparable to that of millisecond radio pulsars. Here we present an XMM-Newton observation of SAX J1808.4-3658 in quiescence, the first that assessed its quiescent luminosity and spectrum with a good signal-to-noise ratio. XMM-Newton did not reveal other sources in the vicinity of SAX J1808.4-3658, likely indicating that the source was also detected by previous BeppoSAX and ASCA observations, even with large positional and flux uncertainties. We derive a 0.5-10 keV unabsorbed luminosity of L_X = 5 × 10³¹ ergs s^-1, a relatively low value compared with other neutron star soft X-ray transient sources. At variance with other soft X-ray transients, the quiescent spectrum of SAX J1808.4-3658 was dominated by a hard (Γ ~ 1.5) power law with only a minor contribution (≲10%) from a soft blackbody component. If the power law originates in the shock between the wind of a turned-on radio pulsar and matter outflowing from the companion, then a spin-down to an X-ray luminosity conversion efficiency of η ~ 10^-3 is derived; this is in line with the value estimated from the eclipsing radio pulsar PSR J1740-5340. Within the deep crustal heating model, the faintness of the blackbody-like component indicates that SAX J1808.4-3658 likely hosts a massive neutron star (M ≳ 1.7 M_☉).

We report the discovery by the Rossi X-Ray Timing Explorer Proportional Counter Array of a second transient accreting millisecond pulsar, XTE J1751-305, during regular monitoring observations of the Galactic bulge region. The pulsar has a spin frequency of 435 Hz, making it one of the fastest pulsars. The pulsations contain the signature of orbital Doppler modulation, which implies an orbital period of 42 minutes, the shortest orbital period of any known radio or X-ray millisecond pulsar. The mass function, f_X = (1.278 ± 0.003) × 10^-6M_☉, yields a minimum mass for the companion of between 0.013 and 0.017 M_☉, depending on the mass of the neutron star. No eclipses were detected. A previous X-ray outburst in 1998 June was discovered in archival All-Sky Monitor data. Assuming mass transfer in this binary system is driven by gravitational radiation, we constrain the orbital inclination to be in the range 30°-85° and the companion mass to be 0.013-0.035 M_☉. The companion is most likely a heated helium dwarf. We also present results from the Chandra High Resolution Camera-S observations, which provide the best-known position of XTE J1751-305.

We analyze measurements of bipolar, Debye-scale electrostatic structures and turbulence measured in the transition region of the Earth's collisionless bow shock. In this region, the solar wind electron population is slowed and heated, and we show that this turbulence correlates well in amplitude with the measured electron temperature change. The observed bipolar structures are highly oblate and longitudinally polarized and may instantaneously carry up to 10% of the plasma energy ψ ≡ eϕ/k_bT_e ≈ 0.1 before dissipating. The relationship between ψ and the field-aligned scale size Δ_∥ of the Gaussian potential suggests that the bipolar structures are BGK trapped particle equilibria or electron hole modes. We suggest a generation scenario and a potential role in dissipation.

We have developed a model for molecular hydrogen formation under astrophysically relevant conditions. This model takes fully into account the presence of both physisorbed and chemisorbed sites on the surface, allows quantum mechanical diffusion as well as thermal hopping for absorbed H atoms, and has been benchmarked versus recent laboratory experiments on H₂ formation on silicate surfaces. The results show that H₂ formation on grain surfaces is efficient in the interstellar medium up to some 300 K. At low temperatures (≤100 K), H₂ formation is governed by the reaction of a physisorbed H with a chemisorbed H. At higher temperatures, H₂ formation proceeds through a reaction between two chemisorbed H atoms. We present simple analytical expressions for H₂ formation that can be adopted to a wide variety of surfaces once their surface characteristics have been determined experimentally.

We show that in a system of two planets initially in nearly circular orbits, an impulse perturbation that imparts a finite eccentricity to one planet's orbit causes the other planet's orbit to become eccentric as well and also naturally results in a libration of their relative apsidal longitudes for a wide range of initial conditions. We suggest that such a mechanism may explain orbital eccentricities and apsidal resonance in some exoplanetary systems. The eccentricity impulse could be caused by the ejection of a planet from these systems or by torques from a primordial gas disk. The amplitude of secular variations provides an observational constraint on the dynamical history of such systems.

We examine the unique abundance variations of Fe/O and He/H in solar energetic particles from a W09 event of 2001 April 10 that have leaked through the flank of an interplanetary shock launched from W04 on April 9. Shock waves from both events reached the Wind spacecraft on April 11. During the second event, both Fe/O and He/H began at low values and rose to maxima near the time of passage of the shock waves, indicating greater scattering for the species with the highest rigidity at a given velocity. Strong modulation of Fe/O suggests preferential scattering and trapping of Fe by the wave spectrum near and behind the intermediate shock. A significant factor may be the residual proton-generated waves from the very hard proton spectrum accelerated by the early shock wave prior to the onset of the second event. Thus, ion abundances from the later event probe the residual wave spectrum at the earlier shock.

Corbard & Thompson analyzed quantitatively the strong radial differential rotation that exists in a thin layer near the solar surface. We investigate the role of this radial shear in driving a flux transport dynamo operating with such a rotation profile. We show that despite being strong, near-surface radial shear effectively contributes only ~1 kG (~30% of the total) to the toroidal fields produced there unless an abnormally high, surface α-effect is included. While 3 kG spot formation from ~1-2 kG toroidal fields by convective collapse cannot be ruled out, the evolutionary pattern of these model fields indicates that the polarities of spots formed from the near-surface toroidal field would violate the observed polarity relationship with polar fields. This supports previous results that large-scale solar dynamos generate intense toroidal fields in the tachocline, from which buoyant magnetic loops rise to the photosphere to produce spots. Polar fields generated in flux transport models are commonly much higher than observed. We show here that by adding enhanced diffusion in the supergranulation layer (originally proposed by Leighton), near-surface toroidal fields undergo large diffusive decay preventing spot formation from them, as well as reducing polar fields closer to the observed values. However, the weaker polar fields lead to the regeneration of a toroidal field of less than ~10 kG at the convection zone base, too weak to produce spots that emerge in low latitudes, unless an additional poloidal field is produced at the tachocline. This is achieved by a tachocline α-effect, previously shown to be necessary for coupling the north and south hemispheres to ensure toroidal and poloidal fields that are antisymmetric about the equator.

Time-distance helioseismology analysis of Dopplergrams provides maps of torsional oscillations and meridional flows. Meridional flow maps show a time-varying component that has a banded structure that matches the torsional oscillations with an equatorward migration over the solar cycle. The time-varying component of meridional flow consists of a flow diverging from the dominant latitude of magnetic activity. These maps are compared with other torsional oscillation maps and with magnetic flux maps, showing a strong correlation with active latitudes. These results demonstrate a strong link between the time-varying component of the meridional flow and the torsional oscillations.