Table of contents

We have calculated Doppler boosting factors, D_var, for a sample of active galactic nuclei (AGNs) using total flux density variation monitoring data at 22 and 37 GHz. We argue that this method is more accurate than the other commonly used methods based on the synchrotron self-Compton X-ray flux or equipartition of energy. We compare our Doppler factors with other results and conclude that even if the average D_var for a class of sources is very similar to all others, the variability Doppler factors for individual objects are more accurate and reliable. An important application of precise Doppler factors is presented, namely, calculating the Lorentz factors, Γ, and the viewing angles, θ, of relativistic outflows inAGNs. We find that high-polarization quasars have the greatest Doppler boosting, while low-polarizationquasars and BL Lac objects are less boosted. The two groups of quasars show different characteristics because of different combinations of the Lorentz factor and viewing angle, rather than either a different Γ or θ alone.

The production rate of compact objects, i.e., neutron stars (NSs) and black holes (BHs), in active galactic nuclei (AGNs) and quasars (QSOs), where frequent supernova explosions are used to explain the high metallicity, is very high because of the interaction between the accretion disk and main-sequence stars in the nucleus of the quasar. The compact object red giant (RG) star binaries can be easily formed because of the large captured cross section of the RG stars. The (NS/BH, NS/BH) binary can be formed after the supernova explosion of the (NS/BH, RG) binary. Intense transient gamma-ray emission (gamma-ray burst) and gravitational radiation can result from the merger of these two compact objects. Collision between the helium core (Hc) of the RG and the BH may also take place and may also result in long-duration gamma-ray bursts but no gravitational waves. We estimate that the merger rate of (NS/BH, NS/BH) binaries and (Hc, BH) is proportional to the metal abundance N V/C IV and can be as high as 10^-3 [(N V/C IV)/0.01] yr^-1 per AGN/QSO.

The long-term evolution of the synchrotron emission from the parsec-scale jet in the quasar 3C 345 is analyzed on the basis of multifrequency monitoring with very long baseline interferometry (VLBI) and covering the period 1979-1994. We demonstrate that the compact radio structure of 3C 345 can be adequately represented by Gaussian model fits and that the model fits at different frequencies are sufficiently reliable for studying the spectral properties of the jet. We combine the model fits from 44 VLBI observations of 3C 345 made at eight different frequencies between 2.3 and 100 GHz. This combined database is used for deriving the basic properties of the synchrotron spectra of the VLBI core and the moving features observed in the jet. We calculate the turnover frequency, the turnover flux density, and the integrated 4-25 GHz flux and 4-25 GHz luminosity of the core and the moving features. The core has an estimated mean luminosity L_core = (7.1 ± 3.5) × 10⁴²h^-2 ergs s^-1; the estimated total luminosity of 3C 345 on parsec scales is ≈3 × 10⁴³h^-2 ergs s^-1 (about 1% of the observed luminosity of the source between the radio to infrared regimes). The luminosities of the core and most of the moving features decrease at the average rate of 1.2 × 10³⁵h^-2 (0.74 ± 0.06)^t-1979.0 ergs s^-2 (t measured in years). The derived luminosity variations require intrinsic acceleration of the moving features. The turnover frequency of one of the moving features reaches a peak during the above period. The combination of the overall spectral and kinematic changes in that feature cannot be reproduced satisfactorily by relativistic shocks, which may indicate rapid dissipation in shocks. The spectral changes in the core can be reconciled with a shock or dense plasma condensation traveling through the region where the jet becomes optically thin. We are able to describe the evolution of the core spectrum by a sequence of five flarelike events characterized by an exponential rise and decay of the particle number density of the material injected into the jet. The same model is also capable of predicting the changes in the flux density observed in the core. The flares occur approximately every 3.5-4 yr, roughly correlating with appearances of new moving features in the jet and indicating that a quasi-periodic process in the nucleus may be driving the observed emission and structural evolution of 3C 345.

We present ASCA gas temperature maps of the nearby merging galaxy clusters Cygnus A, A3667, and A2065. Cygnus A appears to have a particularly simple merger geometry that allows an estimate of the subcluster collision velocity from the observed temperature variations. We estimate it to be ~2000 km s^-1. Interestingly, this is similar to the free-fall velocity that the two Cygnus A subclusters should have achieved at the observed separation, suggesting that the merger has been effective in dissipating the kinetic energy of gas halos into thermal energy, without channeling its major fraction elsewhere (e.g., into turbulence). In A3667 we may be observing a spatial lag between the shock front seen in the X-ray image and the corresponding rise of the electron temperature. A lag of the order of hundreds of kiloparsecs is possible because of the combination of thermal conduction and a finite electron-ion equilibration time. Forthcoming better spatial resolution data will allow a direct measurement of these phenomena in the cluster gas using such lags. A2065 has gas density peaks coincident with two central galaxies. A merger with the collision velocity estimated from the temperature map should have swept away such peaks if the subcluster total mass distributions had flat cores in the centers. The fact that the peaks have survived (or quickly reemerged) suggests that the gravitational potential is also strongly peaked. Finally, the observed specific entropy variations in A3667 and Cygnus A indicate that energy injection from a single major merger may be of the order of the full thermal energy of the gas. We hope that these order-of-magnitude estimates will encourage further work on hydrodynamic simulations, as well as a more quantitative representation of the simulation results, in anticipation of the Chandra and XMM data.

We present Hubble Space Telescope UV/optical spectrophotometry and ground-based optical spectrophotometry of the brightest knots in the emission-line regions of the Seyfert galaxies NGC 5643 and NGC 5728. These objects have ionization cones, radio and soft X-ray emission, and evidence of nuclear outflows and winds, suggesting that shocks may be associated with the emission-line gas. Comparison of the UV/optical data with grids of ionization-bounded and mixed ionization- and matter-bounded photoionization models, and shock models with and without ionized precursors, are used to determine whether radiative shocks that photoionize the shock precursor material ("autoionizing shocks") are a significant excitation mechanism for the emission-line gas in these objects. Although the UV/optical spectra studied here are generally compatible with both photoionization and shock + precursor models, an unambiguous shock + precursor signature is not present. This suggests that mechanical energy input may not play the same dominant role in exciting these objects with weak radio jets as appears to be the case in strong radio jet sources such as M87.

Observations have been made, using the University of Durham Mark 6 gamma-ray telescope, of the very high energy gamma-ray emission from a number of active galactic nuclei visible from the Southern Hemisphere. Limits are presented to the very high energy gamma-ray emission from 1ES 0323+022, PKS 0829+046, 1ES 1101-232, Cen A, PKS 1514-24, RX J10578-275, and 1ES 2316-423, both for steady long-term emission and for outbursts of emission on timescales of 1 day.

We present the results of BeppoSAX observations of PKS 2155-304 during an intense gamma-ray flare. The source was in a high X-ray state. A temporal analysis of the data reveals a tendency of the amplitude of variations to increase with energy and the presence of a soft lag with a timescale of the order 10³ s. A curved continuum spectrum, with no evidence of spectral features, extends up to ~50 keV, while there is indication of a flatter component emerging at higher energies, consistent with the interpretation of the broadband spectral energy distribution (SED) due to synchrotron self-Compton emission from a single region. Notably, the fitting of the SED with such a model is consistent with an interpretation of the detected soft lag due to radiative cooling, supporting the idea that radiation losses play an important role in variability. The observed shifts of the SED peaks between the lowest and highest flux levels can be accounted for by an increase of the "break" energy in the relativistic particle spectrum. The model predicts emission at TeV energies in good agreement with the recently reported detection.

Historically, 3C 66A has been considered a relative quiescent blazar. For that reason, 3C 66A was selected as a comparison source for OJ 287 in the OJ-94 project. However, after more detailed observation it turns out that the variability of 3C 66A itself is very interesting. We have analyzed the entire project data set of 3C 66A from fall of 1993 to spring of 1998 by using structure function analysis, Deeming periodograms, Scargle periodograms, and the folded light curves. Here we present the first preliminary evidence for the 65 day period in 3C 66A observed during the bright state. Our analysis indicates that this period is slowly slowing down. We will also discuss the possible physical mechanism producing the observed periodicity.

We present VLA maps of the Seyfert 2 galaxy NGC 7319 at 3.6, 6, and 20 cm. Subarcsecond resolution is achieved at 3.6 and 6 cm. The radio emission exhibits a triple structure on a scale of ~4'' (1.7 kpc). All three components have steep spectra, consistent with synchrotron radiation. We have also analyzed an HST archival, broad-band red image, which contains structure related to the radio components. In particular, a V-shaped feature in the HST image some 37 (1.6 kpc) southwest of the nucleus is associated with highly blueshifted emission lines seen in ground-based spectra. We interpret the V-shaped feature as emission from gas compressed by the bow shock driven by the outwardly moving radio plasmoid.

We present the first observations of a Seyfert galaxy with the echelle gratings on the Space Telescope Imaging Spectrograph (STIS), which provide high-resolution (λ/Δλ ≈ 40,000) coverage of the intrinsic UV absorption lines in NGC 5548. We confirm the presence of five kinematic components of absorption in Lyα, C IV, and N V at radial velocities of -160 to -1060 km s^-1 with respect to the emission lines, and find an additional Lyα component near the systemic velocity, which probably arises in the interstellar medium of the host galaxy. Compared to Goddard High-Resolution Spectrograph spectra of the N V and C IV absorption obtained ~2 yr earlier, the kinematic components have not changed in radial velocity, but the ionic column densities for two components have decreased. We attribute these variations to changes in the total column of gas, but for one component we cannot rule out changes in the ionization of the gas. We have calculated photoionization models to match the UV column densities from each of the five components associated with the nucleus. In four of the components, the ionization parameters (U = 0.15-0.80) and effective hydrogen column densities (N_eff = 6.0 × 10¹⁸ to 2.8 × 10²⁰ cm^-2) cannot produce the O VII and O VIII absorption edges seen in the X-ray-warm absorber. The remaining component is more highly ionized (U = 2.4, N_eff = 6.5 × 10²¹ cm^-2), and our model matches the previously observed X-ray absorption columns. This component is therefore likely to be responsible for the X-ray-warm absorber. It also has the highest outflow velocity and showed the largest variations in column density.

We have obtained V and I images of the lone globular cluster that belongs to the dwarf Local Group irregular galaxy known as WLM. The color-magnitude diagram of the cluster shows that it is a normal old globular cluster with a well-defined giant branch reaching to M_V = -2.5, a horizontal branch at M_V = +0.5, and a subgiant branch extending to our photometry limit of M_V = +2.0. A best fit to theoretical isochrones indicates that this cluster has a metallicity of [Fe/H] = -1.52 ± 0.08 and an age of 14.8 ± 0.6 Gyr, thus indicating that it is similar to normal old halo globulars in our Galaxy. From the fit we also find that the distance modulus of the cluster is 24.73 ± 0.07 and the extinction is A_V = 0.07 ± 0.06, both values that agree within the errors with data obtained for the galaxy itself by others. We conclude that this normal massive cluster was able to form during the formation of WLM, despite the parent galaxy's very small intrinsic mass and size.

Synchrotron radiation from active galactic nuclei (AGNs) is often highly polarized. We present a search for linear polarization with the Very Large Array (VLA) at 4.8 and 8.4 GHz from the nearest AGN, Sagittarius A*. As a part of this study we used spectropolarimetric data that were sensitive to a rotation measure (RM) as large as 3.5 × 10⁶ rad m^-2 at 4.8 GHz and 1.5 × 10⁷ rad m^-2 at 8.4 GHz. The upper limit to the linear polarization of Sgr A* over a broad range of RM is 0.2% at both frequencies. We also present continuum observations with the VLA at 4.8 GHz that give an upper limit of 0.1% for RMs less than 10⁴ rad m^-2. We conclude that depolarization is unlikely to occur in the Galactic center scattering medium. However, it is possible for depolarization to occur in the accretion region of Sgr A* if the outer scale of turbulence is small enough. We also consider the implications of a very low intrinsic polarization for Sgr A*.

We propose a model for the origin of the isolated nonthermal filaments observed at the Galactic center based on an analogy to cometary plasma tails. We invoke the interaction between a large-scale magnetized galactic wind and embedded molecular clouds. As the advected wind magnetic field encounters a dense molecular cloud, it is impeded and drapes around the cloud, ultimately forming a current sheet in the wake. This draped field is further stretched by the wind flow into a long, thin filament the aspect ratio of which is determined by the balance between the dynamical wind and amplified magnetic field pressures. The key feature of this cometary model is that the filaments are dynamic configurations, and not static structures. As such, they are local amplifications of an otherwise weak field and not directly connected to any static global field. The derived field strengths for the wind and wake are consistent with observational estimates. Finally, the observed synchrotron emission is naturally explained by the acceleration of electrons to high energy by plasma and MHD turbulence generated in the cloud wake.

We simulate the observations of proper motion of stars very close to the Galactic center. We show that the speckle interferometry done with the Keck II telescope is accurate enough to obtain orbital parameters for stars with the period P ~ 10 yr during ~10 seasons of astrometric observations made once a year. The determination of a single orbit will give a central mass estimate with the typical uncertainty of the existing mass determinations based on velocity dispersion measurements. Much higher precision orbits will be measured in several years when the Keck Interferometer becomes operational and fainter stars are discovered even closer to Sgr A*. Astrometry alone will provide an accurate determination of M/D³, where M is the black hole mass and D is the distance to the Galactic center. If spectroscopic orbits of the stars are also measured, then both M and D will be precisely determined.

An unsolved problem in turbulent dynamo theory is the "back-reaction" problem: to what degree does the mean magnetic field suppress the turbulent dynamo coefficients that are needed to drive its growth? The answer will ultimately derive from a combination of numerical and analytical studies. Here we show that analytic approaches to the dynamo and back-reaction problems require one to separate turbulent quantities into two components: those influenced by the mean field (which are therefore anisotropic) and those independent of the mean field (and are therefore isotropic), no matter how weak the mean field is. Upon revising the standard formalism to meet this requirement, we find the following: (1) The two types of components often appear in the same equation, so that standard treatments, which do not distinguish between them, are ambiguous. (2) The usual first-order smoothing approximation that is necessary to make progress in the standard treatment is unnecessary when the distinction is made. (3) In contrast to previous suggestions, the current helicity correction to the dynamo α-coefficient is actually independent of the mean field and therefore cannot be interpreted as a quenching.

This is a preliminary report on the application of Difference Image Analysis (DIA) to Galactic bulge images. The aim of this analysis is to increase the sensitivity to the detection of gravitational microlensing. We discuss how the DIA technique simplifies the process of discovering microlensing events by detecting only objects that have variable flux. We illustrate how the DIA technique is not limited to detection of so-called "pixel lensing" events but can also be used to improve photometry for classical microlensing events by removing the effects of blending. We will present a method whereby DIA can be used to reveal the true unblended colors, positions, and light curves of microlensing events. We discuss the need for a technique to obtain the accurate microlensing timescales from blended sources and present a possible solution to this problem using the existing Hubble Space Telescope color-magnitude diagrams of the Galactic bulge and LMC. The use of such a solution with both classical and pixel microlensing searches is discussed. We show that one of the major causes of systematic noise in DIA is differential refraction. A technique for removing this systematic by effectively registering images to a common air mass is presented. Improvements to commonly used image differencing techniques are discussed.

Analysis of initial observations sky surveys has shown that the resulting photometric catalogs, combined with far-red optical data, provide an extremely effective method of finding isolated, very low-temperature objects in the general field. Follow-up observations have already identified more than 25 sources with temperatures cooler than the latest M dwarfs. A comparison with detailed model predictions (Burrows & Sharp 1999) indicates that these L dwarfs have effective temperatures between ≈2000 ± 100 K and 1500 ± 100 K, while the available trigonometric parallax data place their luminosities at between 10^-3.5 and 10. Those properties, together with the detection of lithium in one-third of the objects, are consistent with the majority having substellar masses. The mass function cannot be derived directly, since only near-infrared photometry and spectral types are available for most sources, but we can incorporate VLM/brown dwarf models in simulations of the solar neighborhood population and constrain Ψ(M) by comparing the predicted L dwarf surface densities and temperature distributions against observations from the Deep Near-Infrared Survey (DENIS) and 2 Micron All-Sky Survey (2MASS) surveys. The data, although sparse, can be represented by a power-law mass function, Ψ(M) ∝ M^-α, with 1 < α < 2. Current results favor a value nearer the lower limit. If α = 1.3, then the local space density of 0.075 > M/M_☉ > 0.01 brown dwarfs is 0.10 systems pc^-3. In that case, brown dwarfs are twice as common as main-sequence stars but contribute no more than ~15% of the total mass of the disk.

We investigate the equilibrium properties of self-gravitating magnetized clouds with polytropic equations of state with negative index n. In particular, we consider scale-free isopedic configurations that have constant dimensionless spherical mass-to-flux ratio λ_r and that may constitute "pivotal" states for subsequent dynamical collapse to form groups or clusters of stars. For given Γ = 1 + 1/n, equilibria with smaller values of λ_r are more flattened, ranging from spherical configurations with λ_r = ∞ to completely flattened states for λ_r = 1. For a given amount of support provided by the magnetic field as measured by the dimensionless parameter H₀, equilibria with smaller values of Γ are more flattened. However, logatropic (defined by Γ = 0) disks do not exist. The only possible scale-free isopedic equilibria with logatropic equation of state are spherical uniformly magnetized clouds.

We analyze the dynamics of a neutron-proton relativistic wind, paying particular attention to fireballs of cosmological gamma-ray bursts (GRBs). Specific effects of the neutron component depend on whether the final Lorentz factor of a plasma wind exceeds some critical value or not. In the first case, velocity decoupling of the neutron and proton flows takes place, giving rise to an electromagnetic cascade induced by pion production in inelastic collisions of nucleons. Otherwise, all nucleons in the wind behave as a single fluid. In both cases neutrons can strongly influence a GRB by changing the dynamics of a shock initiated by protons in the surrounding medium. Conditions for the decoupling of the neutron flow as well as observational consequences of the resulting pion-induced cascade are discussed, including preburst of high-energy photons and neutrinos and annihilation afterglow of a huge number of ejected electron-positron pairs. The critical value of the Lorentz factor is estimated to lie in the range expected for cosmological GRBs, so there possibly exist two different populations of bursts. A number of tests for decoupling of the neutron flow is suggested. The results obtained for the radiation-driven wind allow straightforward generalization for winds driven by other mechanisms, e.g., for the MHD winds.

The dynamical foundations of α disk models are described. At the heart of the viscous formalism of accretion disk models are correlations in the fluctuating components of the disk velocity, magnetic field, and gravitational potential. We relate these correlations to the large-scale mean flow dynamics used in phenomenological viscous disk models. MHD turbulence readily lends itself to the α formalism, but transport by self-gravity does not. Nonlocal transport is an intrinsic property of turbulent self-gravitating disks, which in general cannot be captured by an α model. Local energy dissipation and α-like behavior can be reestablished if the pattern speeds associated with the amplitudes of an azimuthal Fourier decomposition of the turbulence are everywhere close to the local rotation frequency. In this situation, global wave transport must be absent. Shearing box simulations, which employ boundary conditions forcing local behavior, are probably not an adequate tool for modeling the behavior of self-gravitating disks. As a matter of principle, it is possible that disks that hover near the edge of gravitational stability may behave in accord with a local α model, but global simulations performed to date suggest matters are not this simple.

We investigated the dynamical collapse of a molecular cloud core with three-dimensional numerical simulations. Our simulations show that an initially spherical core produces a bar or disk during its dynamical collapse. The disk and bar formation is due to instability. Velocity perturbations grow in proportion to ρ^1/6_c, where ρ_c denotes the central density. The growing velocity perturbations are due to two effects, rotation and shear. Rotation makes the core spin faster to produce a disk at the center. On the other hand, velocity shear elongates or shortens the core in one direction to form a bar or nonrotating disk. When the core rotates nonuniformly, the collapse produces a disk containing a bar at its center by the growth of rotation and shear. This bar is much longer than the Jeans length and is likely to be unstable against fragmentation. We expect that the bar will evolve into a binary or multiple stars. The binary or multiple stars will be surrounded by a common disk. We also demonstrate the growth of an eigenmode that leads to bar and disk formation. On the basis of our numerical simulations, we give a condition for formation of a disk and bar during the isothermal collapse phase of a molecular cloud core. If the core has an oblateness of 10% or an equivalent velocity shear at n_H2 ≃ 10⁴ cm^-3, the core produces a bar by the end of its isothermal collapse phase.

We present a CCD-based photometric survey covering 870 arcmin² in a young stellar cluster around the young multiple star σ Orionis. Our survey-limiting R, I, and Z magnitudes are 23.2, 21.8, and 21.0, respectively, the completeness being about 2.2 mag brighter. From our color-magnitude diagrams, we have selected 49 faint objects (I = 15-21 mag), which smoothly extrapolate the photometric sequence defined by more massive known members. Adopting the currently accepted age interval of 2-10 Myr for the Orion 1b association, in which σ Orionis is located, and considering recent evolutionary models, our objects may span a mass range from 0.1 down to 0.02 M_☉, well within the substellar regime. Follow-up low-resolution optical spectroscopy (635-920 nm) for eight of our candidates in the magnitude range I = 16-19.5 shows that they have spectral types M6-M8.5, consistent with what is expected for true members. Compared with their Pleiades counterparts of similar types, Hα emission is generally stronger, while Na I and K I absorption lines appear weaker, as expected for lower surface gravities and younger ages. In addition, TiO and in particular VO bands appear to be clearly enhanced in our candidate with the latest spectral type, S Ori 45 (M8.5, I = 19.5), compared to objects of similar types in older clusters and the field. We have estimated the mass of this candidate at only 0.020-0.040 M_☉; hence, it is one of the least massive brown dwarfs yet discovered. We examine the potential role of deuterium as a tracer of both substellar nature and age in very young clusters. The luminosity and mass at which the burning/preservation of deuterium takes place is a sensitive function of age and can therefore provide a determination of the age of a cluster. The σ Orionis cluster is an excellent site for determining this transition zone empirically; the most massive brown dwarfs identified are expected to have burned their deuterium content, while the lowest mass ones should have preserved it.

This completes a study of the evolution of binary systems in five open clusters of various ages. Among 21 stars observed in Praesepe, eight are found or confirmed to be spectroscopic binaries and orbital elements are derived, while one more shows long-term binary motion. Among 18 stars observed in the Coma Berenices cluster, five are found or confirmed to be spectroscopic binaries and orbital elements are derived, while a sixth has tentative elements. Among five clusters studied we searched for three expected evolutionary effects, namely an increase with age in the mass ratios, a decrease with age of the binary periods, and an increase in binary frequencies. We find that there is a progression (at the 3 σ level) from no binaries out of 10 with mass ratios greater than 0.5 in the youngest cluster (combined with the published results for NGC 6193) to 25% such stars in the intermediate-age clusters to 43% such stars in these two oldest clusters. There is no evidence for an increase in short-period binaries with age. And there is slight evidence (at the 1 σ level) for an increase with age from 15% to 28% in the fraction of large-amplitude binaries. These results are mostly consistent with the idea that most binaries are formed or modified in three-body interactions, and successive generations of formation and disruptions tend to form binaries with larger mass ratios. However, part of the initial generation of binaries is probably primordial.

We follow the chemical evolution of the Galaxy for elements from Ba to Eu, using an evolutionary model suitable for reproducing a large set of Galactic (local and nonlocal) and extragalactic constraints. Input stellar yields for neutron-rich nuclei have been separated into their s-process and r-process components. The production of s-process elements in thermally pulsing asymptotic giant branch stars of low mass proceeds from the combined operation of two neutron sources: the dominant reaction ¹³C(α, n)¹⁶O, which releases neutrons in radiative conditions during the interpulse phase, and the reaction ²²Ne(α, n)²⁵Mg, marginally activated during thermal instabilities. The resulting s-process distribution is strongly dependent on the stellar metallicity. For the standard model discussed in this paper, there is a sharp production of the Ba-peak elements around Z ≃ Z_☉/4. Concerning the r-process yields, we assume that the production of r-nuclei is a primary process occurring in stars near the lowest mass limit for Type II supernova progenitors. The r-contribution to each nucleus is computed as the difference between its solar abundance and its s-contribution, given by the Galactic chemical evolution model at the epoch of the formation of the solar system. We compare our results with spectroscopic abundances of elements from Ba to Eu at various metallicities (mainly from F and G stars), showing that the observed trends can be understood in the light of present knowledge of neutron capture nucleosynthesis. Finally, we discuss a number of emerging features that deserve further scrutiny.

We investigate the stability of a similarity solution for a gravitationally collapsing isothermal sphere by means of a normal-mode analysis. When the central density increases in proportion to ρ_c ∝ (t₀ - t)^-2, the bar mode grows in proportion to (t₀ - t)^-0.354. Here the symbol t denotes the time and the symbol t₀ denotes an epoch. When the bar mode grows, the dense part of the collapsing cloud becomes either a thin filament or a disk. Since a thin filament is likely to fragment, the bar-mode instability will probably result in multiple stars at the center of the collapsing cloud.

We summarize the general formalism describing surface flows in three-dimensional space in a form which is suitable for various astrophysical applications. We then apply the formalism to the analysis of nonradial perturbations of self-gravitating spherical fluid shells. Spherically symmetric gravitating shells (or bubbles) have been used in numerous model problems especially in general relativity and cosmology. A radially oscillating shell was recently suggested as a model for a variable cosmic object. Within Newtonian gravity we show that self-gravitating static fluid shells are unstable with respect to linear nonradial perturbations. Only shells (bubbles) with a negative mass (or with a charge the repulsion of which is compensated by a tension) are stable.

We report the results from X-ray and hard X-ray observations of the PSR B1259-63/SS 2883 system with ASCA and the Compton Gamma-Ray Observatory (CGRO), performed between 1995 February and 1996 January when the pulsar was near apastron. The system was clearly detected in each of the two ASCA observations with luminosity in the 1-10 keV band of L_X = (9 ± 3) × 10³² (d/2 kpc)² ergs s^-1, while CGRO/OSSE did not detect significant hard X-rays from the system. X-ray spectra obtained with ASCA are well fitted by a single power-law spectrum with a photon index of 1.6 ± 0.3. No pulsations were detected in either the ASCA or the OSSE data. We combine all existing X-ray and hard X-ray observations and present the orbital modulation in the luminosity and photon index for the entire orbit. The results are in agreement with predictions based on a synchrotron emission model from relativistic particles in a shocked pulsar wind interacting with the gaseous outflow from the Be star.

We study theoretically the formation of black hole (BH) X-ray binaries. Consistency of the models with the observed relative numbers of systems with low-mass (≲2 M_☉) and intermediate-mass (~2 M_☉-M_BH) donors leads to severe constraints on the evolutionary parameters of the progenitors. In particular, we find that (1) BH progenitor masses cannot exceed about 2M_BH; (2) high values of the common envelope efficiency parameter (α_CE > 1) are required, implying that energy sources other than orbital contraction must be invoked to eject the envelope; and (3) the mass-loss fraction in helium star winds is limited to ≲50%. Outside of this limited parameter space for progenitors we find that either BH X-ray binary formation cannot occur at all or donors do not have the full range of observed masses. We discuss the implications of these results for the structure of massive hydrogen-rich stars, the evolution of helium stars, and BH formation. We also consider the possible importance of asymmetric kicks.

We explore the sensitivity of the nucleosynthesis of intermediate-mass elements (28 ≤ A ≲ 80) in supernovae derived from massive stars to the nuclear reaction rates employed in the model. Two standard sources of reaction rate data are employed in pairs of calculations that are otherwise identical. Both include as a common backbone the experimental reactions rates of Caughlan & Fowler. Two stellar models are calculated for each of two masses: 15 and 25 M_☉. Each star is evolved from core hydrogen burning to a presupernova state carrying an appropriately large reaction network and then exploded using a piston near the edge of the iron core as described by Woosley & Weaver. The final stellar yields from the models calculated with the two rate sets are compared and found to differ in most cases by less than a factor of 2 over the entire range of nuclei studied. Reasons for the major discrepancies along with the physics underlying the two reaction rate sets employed are discussed in detail. The nucleosynthesis results are relatively robust and less sensitive than might be expected to uncertainties in nuclear reaction rates, though they are sensitive to the stellar model employed.

We report new statistical equilibrium calculations for Fe I and Fe II in the atmosphere of late-type stars. We used atomic models for Fe I and Fe II having, respectively, 256 and 190 levels, as well as 2117 and 3443 radiative transitions. Photoionization cross sections are from the Iron Project. These atomic models were used to investigate non-LTE (NLTE) effects in iron abundances of late-type stars with different atmospheric parameters. We found that most Fe I lines in metal-poor stars are formed in conditions far from LTE. We derived metallicity corrections of about 0.3 dex with respect to LTE values for the case of stars with [Fe/H] ~ -3.0. Fe II is found not to be affected by significant NLTE effects. The main NLTE effect invoked in the case of Fe I is overionization by ultraviolet radiation; thus classical ionization equilibrium is far from being satisfied. An important consequence is that surface gravities derived by LTE analysis are in error and should be corrected before final abundance corrections. This apparently solves the observed discrepancy between spectroscopic surface gravities derived by LTE analyses and those derived from Hipparcos parallaxes. A table of NLTE [Fe/H] and log g values for a sample of metal-poor late-type stars is given.

Almost none of the r-modes ordinarily found in rotating stars exist, if the star and its perturbations obey the same one-parameter equation of state; and rotating relativistic stars with one-parameter equations of state have no pure r-modes at all, no modes whose limit, for a star with zero angular velocity, is a perturbation with axial parity. Similarly (as we show here), rotating stars of this kind have no pure g-modes, no modes whose spherical limit is a perturbation with polar parity and vanishing perturbed pressure and density. Where have these modes gone? In spherical stars of this kind, r-modes and g-modes form a degenerate zero-frequency subspace. We find that rotation splits the degeneracy to zeroth order in the star's angular velocity Ω, and the resulting modes are generically hybrids, whose limit as Ω → 0 is a stationary current with axial and polar parts. Lindblom & Ipser have recently found these hybrid modes in an analytic study of the Maclaurin spheroids. Since the hybrid modes have a rotational restoring force, they call them "rotation modes" or "generalized r-modes." Because each mode has definite parity, its axial and polar parts have alternating values of l. We show that each mode belongs to one of two classes, axial-led or polar-led, depending on whether the spherical harmonic with lowest value of l that contributes to its velocity field is axial or polar. We numerically compute these modes for slowly rotating polytropes and for Maclaurin spheroids, using a straightforward method that appears to be novel and robust. Timescales for the gravitational-wave driven instability and for viscous damping are computed using assumptions appropriate to neutron stars. The instability to nonaxisymmetric modes is, as expected, dominated by the l = mr-modes with simplest radial dependence, the only modes which retain their axial character in isentropic models; for relativistic isentropic stars, these l = m modes must also be replaced by hybrids of the kind considered here.

Temporal and spectral properties of X-ray rapid variability of Cyg X-1 are studied through correlation analysis in the time domain on different timescales. The correlation coefficients between the total intensity in the 2-60 keV band and the hardness ratio of the 13-60 keV to the 2-6 keV band on the timescale of ~1 ms are always negative in all states. For soft states, the correlation coefficients are positive on all timescales from ~0.01 to ~100 s, which is significantly different from those for transition and low states. The temporal structures in the high-energy band are narrower than those in low-energy band in quite a few cases. The delay of high-energy photons relative to low-energy ones in the X-ray variations has also been revealed by the correlation analysis. The implications of the observed temporal and spectral characteristics for the production region and mechanism of Cyg X-1 X-ray variations are discussed.

We present an approach to the problem of particle disks in lopsided potentials—such as circumbinary dust or planetesimal disks—which focuses on planar and off-plane orbits in the restricted three-body problem. We show that several families of off-plane orbits around a circular binary are stable, and at least one of these is easily accessible to particles orbiting in the plane of the binary motion (e.g., in a circumbinary accretion or protoplanetary disk). The presence of a vertical instability in the family of simple, periodic orbits that supports such disks suggests that particles in the disk should be excited into off-plane motion. When we include a dissipational term in our equations to mimic the effects of such forces as viscosity, gas drag, or Poynting-Robertson drag, disk particles spiral slowly inward. This allows us to test the effects of in-plane and vertical resonances on particle motion and to explore the effects of nonzero binary eccentricity. In the circular case, for mass ratios 0.02 ≲ μ[= m₂/(m₁ + m₂)] ≲ 0.35, the vertical resonance located at a distance from the barycenter of just over twice the binary semimajor axis intercepts the majority of inbound particles and excites them onto the off-plane orbits. For slightly lower mass ratios (μ ≲ 0.01), the corresponding k = Ω - Ω_B : κ = -1 : 2 planar instability tends to dominate, and particles are forced onto orbits which escape from the binary before significant vertical excitation can occur; at very small mass ratios (μ ≲ 0.001), neither of these outer resonances are strong enough to intercept a significant fraction of particles. Eccentricities of e ≳ 0.01 effectively shut off this vertical excitation. But at e ~ 0.1, we find a new vertical excitation for particles on 1 : 3 planar orbits farther out, which branch from the main circumbinary orbits at the k = -1 : 3 resonance (eccentric orbits at the Keplerian 4 : 1 mean-motion resonance). We discuss applications of these results to pre-main-sequence binaries and star-planet systems with dust disks and comment on the relevance of our results to circumbinary disks dominated by collective effects such as gas pressure and self-gravity. A number of objects in the Kuiper Belt of the outer solar system may exist in highly inclined orbits as a result of the resonances discussed here.

We study a solid protoplanetary core undergoing radial migration in a protoplanetary disk. We consider cores in the mass range ~1-10 M_⊕ embedded in a gaseous protoplanetary disk at different radial locations. We suppose that the core luminosity is generated as a result of planetesimal accretion and calculate the structure of the gaseous envelope assuming hydrostatic and thermal equilibrium. This is a good approximation during the early growth of the core, while its mass is less than the critical value, M_crit, above which such static solutions can no longer be obtained and rapid gas accretion begins. The critical value corresponds to the crossover mass above which rapid gas accretion begins in time-dependent calculations. We model the structure and evolution of the protoplanetary nebula as an accretion disk with constant α. We present analytic fits for the steady state relation between the disk surface density and the mass accretion rate as a function of radius. We calculate M_crit as a function of radial location, gas accretion rate through the disk, and planetesimal accretion rate onto the core. For a fixed planetesimal accretion rate, M_crit is found to increase inward. On the other hand, it decreases with the planetesimal accretion rate and hence with the core luminosity. We consider the planetesimal accretion rate onto cores migrating inward in a characteristic time of ~10³-10⁵ yr at 1 AU, as indicated by recent theoretical calculations. We find that the accretion rate is expected to be sufficient to prevent the attainment of M_crit during the migration process if the core starts off significantly below it. Only at those small radii at which local conditions are such that dust, and accordingly planetesimals, no longer exist can M_crit be attained. At small radii, the runaway gas accretion phase may become longer than the disk lifetime if the mass of the core is too small. However, within the context of our disk models, and if it is supposed that some process halts the migration, massive cores can be built up through the merger of additional incoming cores on a timescale shorter than for in situ formation. A rapid gas accretion phase may thus begin without an earlier prolonged phase in which planetesimal accretion occurs at a reduced rate because of feeding zone depletion in the neighborhood of a fixed orbit. Accordingly, we suggest that giant planets may begin to form through the above processes early in the life of the protostellar disk at small radii, on a timescale that may be significantly shorter than that derived for in situ formation.

The identification of the emission-line feature at 17.62 Å in solar X-ray spectra is reexamined. Using a Monte Carlo technique, we compute a realistic theoretical upper limit to the observed Fe Lα photospheric fluorescent line strength caused by irradiation from an overlying corona. These calculations demonstrate that the photospheric Fe Lα characteristic line is much too weak to account for the observed 17.62 Å line flux. Instead, we identify this line with the configuration interaction 2s²2p⁴3p²P_3/2-2s2p⁶²S_1/2 transition in Fe XVIII seen in Electron Beam Ion Trap spectra and predicted in earlier theoretical work on the Fe XVIII X-ray spectrum. This line should be easily resolved and detected in stellar coronae by the spectrographs on the upcoming Chandra X-ray Observatory and X-ray Multimirror Mission.

The diffusion properties of photospheric bright points associated with magnetic elements (magnetic bright points) in the granulation network are analyzed. We find that the transport is subdiffusive for times less than 20 minutes but normal for times larger than 25 minutes. The subdiffusive transport is caused by the walkers being trapped at stagnation points in the intercellular pattern. We find that the distribution of waiting times at the trap sites obeys a truncated Lévy type (power-law) distribution. The fractal dimension of the pattern of sites available to the random walk is less than 2 for the subdiffusive range and tends to 2 in the normal diffusion range. We show how the continuous time random walk formalism can give an analytical explanation of the observations. We simulate this random walk by using a version of a phenomenological model of renewing cells introduced originally for supergranules by Simon, Title, & Weiss. We find that the traps that cause the subdiffusive transport arise when the renewed convection cell pattern is neither fixed nor totally uncorrelated from the old pattern, as required in Leighton's model, but in some intermediate state between these extremes.

Two scenarios of coronal loop heating by directly driven torsional Alfvén waves are considered. In the first scenario the driving is assumed to be harmonic. In the steady state of oscillations, wave energy dissipation mainly occurs in a narrow dissipative layer embracing an ideal resonant magnetic surface. The wave motion in the dissipative layer is characterized by very large amplitudes. It is assumed that the radiative and thermoconductive losses from loops are exactly covered by wave energy dissipation. This assumption allows expression of the maximum value of the velocity in the dissipative layer in terms of the energy losses and the loop parameters. It turns out that this maximum velocity is proportional to R^1/3, where R is the total Reynolds number accounting for both viscosity and resistivity. For typical coronal loops and R = 10⁶, the maximum velocity in the dissipative layer is between 800 and 1000 km s^-1. In the second scenario the driving is assumed to be a stationary stochastic process. Once again it is assumed that energy losses from the loop are covered by wave energy dissipation. The maximum value of the mean square velocity turns out to be proportional to R^1/6. This value is also very sensitive to the width of the driver frequency spectrum. In two considered examples, one with a narrow spectrum and another with a wide spectrum, the maximum values of the mean square velocity between 300 and 400 km s^-1 and between 150 and 200 km s^-1 were obtained, respectively, for typical coronal loops and R = 10⁶. Since the observed nonthermal velocities in coronal loops never exceed a few tens of km s^-1, these results lead to the conclusion that both scenarios do not satisfy the observational constraints.

Slow solar wind, found at low heliographic latitudes, and fast solar wind, associated with high-latitude coronal holes, are fundamentally different: slow solar wind is more variable and is biased in elements with low first ionization potentials (FIPs), whereas fast solar wind is steady and has at most limited FIP bias. It has been recently argued that these differences are consequences of a continuous reorganization of the Sun's magnetic field described by a new heliospheric magnetic field model. This continuous reorganization requires a sustained reconnection process at low latitudes between open magnetic field lines and large coronal loops. Arguably, the slow solar wind originates from material stored on large coronal loops which is released sporadically because of reconnection. The fast solar wind is released on continuously open magnetic field lines. In this paper a theory for FIP fractionation is developed which depends on the wave heating of minor ions which extends below the transition region. The wave heating may be natural on the closed field configurations of large coronal loops but not in the open configurations associated with fields emanating from coronal holes. Therefore, the theory naturally leads to a differentiation in FIP fractionation between fast and slow solar wind.

A model has been introduced for the magnetic field in the heliosphere in which the field lines execute large excursions in heliographic latitude. The excursions result from the interplay between the differential rotation of the photosphere and the nonradial expansion of the solar wind near the Sun. The model accounts for the observed ease with which low-rigidity particles propagate in latitude in the solar wind. In this paper the consequences of this model for the behavior of the coronal magnetic field are explored. It is pointed out that the model provides an explanation for the time evolution and apparent rigid rotation of polar coronal holes and the differences between fast and slow solar wind.

The contribution to p-mode line widths from the excitation of tube mode oscillations on an individual magnetic fibril is computed. An idealized model of the fibril within the photosphere is implemented, consisting of a vertical, thin magnetic flux tube embedded in a plane-parallel isentropic polytrope of index m. Bogdan et al. considered a similar model but imposed a stress-free boundary condition at the top of the photosphere, which acts to reflect any upward-propagating tube waves completely back down into the tube. The stress-free boundary condition neglects a possibly important physical process: the loss of energy to the upper solar atmosphere by the excitation of waves in the chromosphere and corona. Using simple models of the solar chromosphere and corona, we explore the consequences of applying various boundary conditions. The resultant upward energy fluxes are not large, but surprisingly the more realistic upper boundary conditions lead to a significant increase in kink mode flux out the bottom. Nevertheless, the sausage mode remains dominant in cases of interest and is essentially unaffected by the new boundary conditions. Consequently, the resultant total p-mode line width computed here can account for only a few percent of the observed line width.

We present a correlation analysis of GONG p-mode frequencies with nine solar activity indices for the period from 1995 August to 1997 August. This study includes spherical harmonic degrees in the range 2-150 and the frequency range of 1500-3500 μHz. Using three statistical tests, the measured mean frequency shifts show strong to good correlation with activity indices. A decrease of 0.06 μHz in frequency during the descending phase of solar cycle 22 and an increase of 0.04 μHz in the ascending phase of solar cycle 23 are observed. These results provide the first evidence for change in p-mode frequencies around the declining phase of cycle 22 and the beginning of new cycle 23. This analysis further confirms that the temporal behavior of the solar frequency shifts closely follow the phase of the solar activity cycle.

We run a pseudospectral magnetohydrodynamic code to simulate reconnection between two flux tubes inside a solar coronal loop. We apply a stationary velocity field at one of the footpoints consisting of two vortices in such a way as to induce the development of a current layer and force the field lines to reconnect. During the process we find a remarkable coincidence between the location of the current layer and the location of quasi-separatrix layers, which are thin magnetic volumes where the field line connectivity changes abruptly. This result lends support to a scenario in which quasi-separatrix layers are the most likely locations for impulsive energy release in the solar corona. Another important result of this simulation is the observed transient of strong magnetohydrodynamic turbulence characterized by a k^-3/2 energy spectrum. This transient reaches its peak activity in coincidence with a maximum in the energy dissipation rate, thus suggesting that the direct energy cascade associated with this turbulent transient plays a key role in enhancing energy dissipation in magnetic reconnection processes.

We study the ratio of the numbers of interplanetary to interacting protons, Γ, using a model of stochastic acceleration on open magnetic field lines in solar corona. The impact of diverging coronal magnetic field lines is incorporated into the particle transport operator and shown to be unavoidable, no matter how small the mean free path may be. We calculate the energy spectra of protons precipitating into the subcoronal regions (interacting protons) and the spectra of protons escaping into the interplanetary medium (interplanetary protons). In the case in which both injection and the magnetic field exponentially decrease with altitude, the proton spectra for interacting particles are steeper than the corresponding spectra in interplanetary space. The deduced high-energy ratio Γ varies from 1 to ≈5, being almost independent on a magnetic mirror ratio beneath the acceleration region if the latter ratio does not exceed typically ≈10. A quantitative relation between the interplanetary to interacting proton ratio and the magnetic field change from the top of the interaction region to the top of the acceleration region is established. The model results are qualitatively consistent with patterns of energetic protons, γ-rays, and neutrons produced during the approximately 40 minutes after the impulsive phase of the 1990 May 24 solar flare.

Numerical simulations of the dynamics and radiation in a solar flare loop are presented. The heating processes in the lower atmosphere include nonthermal heating by accelerated electrons and thermal soft X-ray irradiation from the flare-heated transition region and corona. Important transitions of hydrogen, helium, and singly ionized calcium and magnesium are treated in non-LTE. The principal results of the analysis are the following:

A sounding rocket observation of comet Hale-Bopp was conducted on 1997 April 6 at 3:51 UT when the comet was at heliocentric and geocentric distances of 0.92 and 1.39 AU, respectively. The instrument was a telescope and long-slit (65 × 260''), ultraviolet (1280-1880 Å) spectrograph that sampled the coma parallel to the Sun-comet line from ~10⁵ km sunward of the nucleus to ~2 × 10⁵ km tailward with ~6'' (6000 km) spatial resolution. Two spectral images were obtained with the slit offset 20'' and 40'' from the nucleus in the direction perpendicular to the long axis of the slit. Spatial profiles and production rates for C, O, CO, and S are presented. Modeling of the spatial profiles of CO and C emissions indicate that photodissociation of the detected CO can account for all of the C I emissions observed. The brightnesses of the strong bands of the CO Fourth Positive system and the S I λ1814 multiplet along near-nuclear lines of sight were diminished by self-absorption. A CO production rate of ~3 × 10³⁰ molecules s^-1 is derived.

Because of the limited size of the satellite-borne instruments, it has not been possible to observe the flux of gamma-ray bursts (GRBs) beyond GeV energy. We here show that it is possible to detect the GRB radiation of TeV energy and above by detecting the muon secondaries produced when the gamma rays shower in Earth's atmosphere. Observation is made possible by the recent commissioning of underground detectors (AMANDA, the Lake Baikal detector, and MILAGRO), which combine a low muon threshold of a few hundred GeV or less, with a large effective area of 10³ m² or more. Observations will not only provide new insights in the origin and characteristics of GRB, but they also will provide quantitative information on the diffuse infrared background.

We report on a measurement of the angular spectrum of the anisotropy of the microwave sky at 30 and 40 GHz between l = 50 and l = 200. The data, covering roughly 600 deg², support a rise in the angular spectrum to a maximum with δT_l ≈ 85 μK at l = 200. We also give a 2 σ upper limit of δT_l < 122 μK at l = 432 at 144 GHz. These results come from the first campaign of the Mobile Anisotropy Telescope on Cerro Toco, Chile. To assist in assessing the site, we present plots of the fluctuations in atmospheric emission at 30 and 144 GHz.

We report results for the angular three-point galaxy correlation function in the APM Galaxy Survey and compare them with theoretical expectations. For the first time, these measurements extend to sufficiently large scales to probe the weakly nonlinear regime. On large scales, the results are in good agreement with the predictions of nonlinear cosmological perturbation theory for a model with initially Gaussian fluctuations and linear power spectrum P(k) consistent with that inferred from the APM survey. These results reinforce the conclusion that large-scale structure is driven by nonlinear gravitational instability and that APM galaxies are relatively unbiased tracers of the mass on large scales; they also provide stringent constraints upon models with non-Gaussian initial conditions and strongly exclude the standard cold dark matter model.

The ages of two old galaxies (53W091, 53W069) at high redshifts are used to constrain the value of the cosmological constant in a flat universe (ΛCDM) and the density parameter Ω_M in Friedmann-Robertson-Walker models with no Λ term. In the case of ΛCDM models, the quoted galaxies yield two lower limits for the vacuum energy density parameter, Ω_Λ ≥ 0.42 and Ω_Λ ≥ 0.5, respectively. Although compatible with the limits from statistics of gravitational lensing and cosmic microwave background, these lower bounds are more stringent than the ones recently determined using Type Ia supernovae as standard candles. For matter-dominated universes (Ω_Λ = 0), the existence of these galaxies imply that the universe is open with the matter density parameter constrained by Ω_M ≤ 0.45 and Ω_M ≤ 0.37, respectively. In particular, these results disagree completely with the analysis of field galaxies, which gives a lower limit Ω_M ≥ 0.40.

There has been a long-standing discrepancy between the densities deduced from studies of the variability of quasar emission lines and those inferred from standard photoionization analyses. We have found that the higher metal abundances now believed to be present in most quasars lead to the higher densities being predicted by photoionization models. We explain why this is so.

Emission-line variability data on NGC 5548 argue strongly for the existence of a mass of order 7 × 10⁷M_☉ within the inner few light-days of the nucleus in the Seyfert 1 galaxy NGC 5548. The time-delayed response of the emission lines to continuum variations is used to infer the size of the line-emitting region, and these determinations are combined with measurements of the Doppler widths of the variable line components to estimate a virial mass. The data for several different emission lines spanning an order of magnitude in distance from the central source show the expected V ∝ r^-1/2 correlation and are consistent with a single value for the mass.

We discuss early results from the first N-body/hydrodynamical simulation to resolve the formation of galaxies in a volume large enough for their clustering properties to be reliably determined. The simulation follows the formation of galaxies by gas cooling within dark halos of mass a few times 10¹¹M_☉ and above, in a flat cold dark matter universe with a positive cosmological constant. Over 2200 galaxies form in our simulated volume of (100 Mpc)³. Assigning luminosities to the model galaxies using a spectral population synthesis model results in a K-band luminosity function in excellent agreement with observations. The two-point correlation function of galaxies in the simulation evolves very little since z = 3, and it has a shape close to a power law over 4 orders of magnitude in amplitude. At the present day, the galaxy correlation function in the simulation is antibiased relative to the mass on small scales and unbiased on large scales. It provides a reasonable match to observations.

The electron-scattering region (ESR) is one of the important ingredients of Seyfert nuclei because it makes it possible to observe the hidden broad-line region (HBLR) in some type 2 Seyfert nuclei (S2's). However, little is known about its physical and geometrical properties. Using the number ratio of S2's with and without HBLR, we statistically investigate where the ESR is in Seyfert nuclei. Our analysis suggests that the ESR is located at a radius between ~0.01 and ~0.1 pc from the central engine. We also briefly discuss a possible origin of the ESR.

Millimeter CO (1 → 0) interferometry and high-resolution, Hubble Space Telescope 1.1, 1.6, and 2.2 μm imaging of the radio compact galaxy PKS 1345+12 are presented. With an infrared luminosity of ~2 × 10¹²L_☉, PKS 1345+12 is a prime candidate for studying the link between the ultraluminous infrared galaxy phenomenon and radio galaxies. These new observations probe the molecular gas distribution and obscured nuclear regions of PKS 1345+12 and provide morphological support for the idea that the radio activity in powerful radio galaxies is triggered by the merger of gas-rich galaxies. Two nuclei separated by 2'' (4.0 kpc) are observed in the near-infrared; the extended southeastern nucleus has colors consistent with reddened starlight, and the compact northwestern nucleus has extremely red colors indicative of an optical quasar with a warm dust component. Further, the molecular gas, 3 mm continuum, and radio emission are coincident with the redder nucleus, confirming that the northwestern nucleus is the site of the active galactic nucleus and that the molecular gas is the likely fuel source.

At the time of its discovery, the optical and X-ray afterglow of GRB 970228 appeared to be a ringing endorsement of the previously untried relativistic fireball model of gamma-ray burst (GRB) afterglows, but now that nearly a dozen optical afterglows to GRBs have been observed, the wavering light curve and reddening spectrum of this afterglow make it perhaps the most difficult of the observed afterglows to reconcile with the fireball model. In this Letter, we argue that this afterglow's unusual temporal and spectral properties can be attributed to a supernova that overtook the light curve nearly 2 weeks after the GRB. This is the strongest case yet for a GRB/supernova connection. It strengthens the case that a supernova also dominated the late afterglow of GRB 980326 and the case that GRB 980425 is related to SN 1998bw.

GRB 980425 is distinctive in that it seems to be associated with SN 1998bw, has no X-ray afterglow, and has a rounded, structureless, single-peak light curve and a relatively soft spectrum. The supernova is itself unusual in that its expansion velocity exceeds c/6. We suggest that many of these features can be accounted for with the hypothesis that we observe the gamma-ray burst along a penumbral line of sight that contains mainly photons that have scattered off ejected baryons. The hypothesis suggests a baryon-poor jet existing within a baryon-rich outflow. The sharp distinction can be attributed to whether or not the magnetic field lines thread an event horizon. Such a configuration suggests that there will be some nonthermal acceleration of pickup ex-neutrons within the baryon-poor jet. This scenario might produce observable spallation products and neutrinos.

We offer a brief description of the bulk motion Comptonization (BMC) model for accretion onto black holes, illustrated by its application to observational data for LMC X-1 and Nova Muscae 1991. We then extract some physical parameters of these systems from observables within the context of the BMC model, drawing from results on GRO J1655-40, for which we presented extensive analysis previously. We derive estimates of the mass, (16 ± 1) M_☉ × [0.5/ cos(i)]^1/2, and mass accretion rate in the disk in Eddington units (_d ≃ 2) for LMC X-1 and (24 ± 1) M_☉ × [0.5/ cos(i)]^1/2d_5.5 and (_d ≃ 3) for Nova Muscae 1991, where cos(i) and d_5.5 are the inclination angle cosine and distance in 5.5 kpc units, respectively. Differences between these estimates and previous estimates based on dynamical studies are discussed. It is further shown that the disk inner radius increases with the high-to-low-state transition in Nova Muscae 1991. Specifically, our analysis suggests that the inner disk radius increases to r_in ≃ 17 Scwarzschild radii as the transition to the low-hard state occurs.

We have detected EUV emission from the globular cluster M15 using the Deep Survey Photometer aboard the Extreme Ultraviolet Explorer. The emission is variable at the 97% confidence level. The minimum EUV luminosity implied by our detection is ~5 × 10³⁶ ergs s^-1 for a distance and reddening appropriate to M15. We have examined a number of possible origins for this emission including post-asymptotic giant branch stars, a population of hitherto unknown, optically faint, stellar merger products or "supersoft sources," or the well-known M15 low-mass X-ray binary (LMXB) AC 211. A significant EUV flux from AC 211 is supported by the relatively strong He II λ4686 emission observed from this system. If the observed EUV flux indeed originates from AC 211, this is the first detection of an LMXB at EUV energies. Furthermore, such a luminosity is comparable to the 2-10 keV X-ray luminosity of AC 211 and may dominate the energetics of the system if absorption local to the binary is taken into account. Further observations of this and other low column LMXBs (e.g., that in NGC 1851) are required to establish the ubiquity of globular cluster/LMXB EUV emission.

A group of young, active stars in the vicinity of TW Hydrae has recently been identified as a possible physical association with a common origin. Given its proximity (~50 pc), age (~10 Myr), and abundance of binary systems, the TW Hya association is ideally suited to studies of the diversity and evolution of circumstellar disks. Here we present mid-infrared observations of 15 candidate members of the group, 11 of which have no previous flux measurements at wavelengths longer than 2 μm. We report the discovery of a possible 10 μm excess in CD -33°7795, which may be due to a circumstellar disk or a faint and as yet undetected binary companion. Of the other stars, only TW Hya, HD 98800, Hen 3-600A, and HR 4796A—all of which were detected by IRAS—show excess thermal emission. Our 10 μm flux measurements for the remaining members of the association are consistent with photospheric emission, allowing us to rule out dusty inner disks. In light of these findings, we discuss the origin and age of the TW Hya association as well as the implications for disk evolution timescales.

Since the discovery of small diamond grains in meteorites, the presence of such grains in the interstellar medium has been suspected. Here we report what we think to be the first unambiguous evidence of the presence of small crystallites of diamond in the dusty envelopes surrounding stars. Thanks to experimental results obtained in different laboratories on the diamond growth process, we identify two peculiar unidentified infrared emission bands at 3.43 and 3.53 μm as the vibrational modes of hydrogen-terminated crystalline facets of diamond. The intensities of these two strong features observed in the envelope of HD 97048 correspond to a mass of 10^-10 to 10^-9M_☉ of diamond dust at an equilibrium temperature of ~1000 K.

COMPTEL observations of the Orion/Monoceros region have shown distinct evidence for excessive 3-7 MeV emission that was attributed to nuclear de-excitation lines from accelerated ¹²C and ¹⁶O nuclei. Unfortunately, we must conclude now that this appears to be a spurious result. This conclusion follows from a better understanding of the instrumental background, from a better exposure of the region, and from an improved analysis method. We show here how the impact of each of these gradually reduces the signal to a less than 3 σ result. The prime underlying cause seems to be ²⁴Na activation in and around the upper COMPTEL detectors. Combining all available data, we now set a 2 σ flux upper limit on the 3-7 MeV emission of Orion of 3 × 10^-5 γ cm^-2 s^-1, to be compared with the previously derived flux of ~10^-4 γ cm^-2 s^-1.

The successful modeling of the dynamics of H_2v bright points in the nonmagnetic chromosphere by Carlsson & Stein gave as a by-product a part-time chromosphere lacking the persistent outward temperature increase of time-average empirical models, which is needed to explain observations of UV emission lines and continua. We discuss the failure of the dynamical model to account for most of the observed chromospheric emission, arguing that their model uses only about 1% of the acoustic energy supplied to the medium. Chromospheric heating requires an additional source of energy in the form of acoustic waves of short period (P < 2 minutes), which form shocks and produce the persistent outward temperature increase that can account for the UV emission lines and continua.

Electron densities in the lower solar atmosphere (photosphere and chromosphere) are derived using a number of different atmospheric models which are constrained by observed spectral lines. Comparing these atmospheric densities to coronal electron densities derived from polarization brightness measurements in the region from about 1.1 to several solar radii, it is shown that there is a discrepancy between the two sets of densities. The atmospheric electron densities are in agreement with a density of maximum 10⁷ cm^-3 at 1.1 R_☉. The polarized brightness densities given in the literature are typically 5 × 10⁷ cm^-3 or higher. It is shown that this discrepancy might be due to an overestimation of the coronal electron densities below 1.5-2 R_☉.

We present an H latitudinal profile of Saturn, obtained in 1998 October using the CSHELL spectrometer on the NASA Infrared Telescope Facility. The profile, measured at 3.953 μm, shows that the majority of the emission is concentrated in the auroral ovals, making Saturn similar to Jupiter and different from Uranus. The spatial resolution is sufficient to resolve the southern auroral oval, currently fully displayed around the south pole, into two peaks separated by 12. At the time of the observations reported here, the emission flux in the H line is 8.3 (±1.7) × 10^-18 W m^-2 for the intensity integrated over a 10 swath along the southern aurora and 5.8 (±1.3) × 10^-18 W m^-2 for the northern aurora. There may also be some mid- to low-latitude emission, similar to that on Jupiter. We suggest that planetwide H emission from Saturn is between 1.2 and 3.6 × 10¹¹ W.

We report on the first astronomical observations with a photon-counting pixel detector that provides arrival time (δt = 100 ns) and energy (δE_γ ≤ 0.15 eV) resolved measurements from the near-IR through the near UV. Our test observations were performed by coupling this transition edge sensor device to a 0.6 m telescope; we have obtained the first simultaneous optical near-IR phase-resolved spectra of the Crab pulsar. A varying infrared turnover gives evidence of self-absorption in the pulsar plasma. The potential of such detectors in imaging arrays from a space platform is briefly described.