Table of contents

Most work on foreground removal has treated the case where the frequency dependence of all components is perfectly known and independent of position. In contrast, real-world foregrounds are generally not perfectly correlated between frequencies, with the spectral index varying slightly with position and (in the case of some radio sources) with time. A method incorporating this complication is presented and illustrated with an application to the upcoming satellite missions MAP and Planck. We find that even spectral index variations as small as Δα ~ 0.1 can have a substantial impact on how channels should be combined and on attainable accuracy.

We study the generalized Kompaneets equation (the kinetic equation for the photon distribution function taking into account Compton scattering by electrons) using a relativistically covariant formalism. Using the generalized Kompaneets equation, we derive an analytic expression for the Sunyaev-Zeldovich effect, which takes into account up to O(θ⁵_e) terms, where θ_e = k_BT_e/mc² is the relativistic expansion parameter and T_e is the electron temperature. We carefully study the applicable region of the obtained analytic expression by comparing it with the result of the direct numerical integration. We conclude that the present analytic expression can be reliably applied to the calculation of the Sunyaev-Zeldovich effect for k_BT_e ≤ 15 keV, which is the highest electron temperature in the clusters of galaxies presently known. Therefore, the present analytic expression can be applied to all known clusters of galaxies.

In order to study the size and shape of the absorbers that lie in front of the QSOs, in particular the Lyα forest, we present an analysis of 785 absorption lines in the spectra of five QSOs in close groupings: a pair (LB 9605: 1517+2357 at z = 1.834 and LB 9612: 1517+2356 at z = 1.903, with a separation of 102'' between them) and a triplet (KP 76: 1623+2651A at z = 2.467, KP 77: 1623+2653 at z = 2.526, and KP 78: 1623+2651B at z = 2.605, with separations of 127'', 147'', and 177'' between pairs 76:78, 76:77, and 77:78, respectively). Both of these QSO groups have been observed before, but these data represent a drastic increase in signal-to-noise ratio and/or wavelength coverage over earlier data and provide a qualitatively different view of the nature of the absorbers. The pair samples a scale critical in determining the size upper bound of Lyα absorbers, with significant leverage in redshift compared to previous studies. In the case of the triplet, this represents the spatially densest sample of Lyα forest absorbers ever studied and an almost ideally suited probe of the shape of absorbers. We observe a significant number of Lyα lines in common between the triplet sight lines, for lines stronger than rest equivalent width W₀ > 0.4 Å (and no detected metal lines) and velocity differences up to 200 km s^-1, corresponding to a two-point correlation function ξ=1.88^{+ 0.78}_−0.50 on scales of 0.5-0.8 h^-1 Mpc with ⟨z⟩ = 2.14 and inconsistent at the 99.999% level with the absence of any clustering. These data also show that a significant fraction of the W₀ > 0.4 Å Lyα forest absorbers spans all three sight lines to the KP triplet, indicating that the strong-lined absorbers are consistent with nearly round shapes, chosen from a range of possible cylinders of different elongations. This may be inconsistent with results from hydrodynamic/gravitational simulations of H I in the early Universe, indicating that the theoretical counterparts of Lyα forest clouds are long and filamentary. Furthermore, there is a probable correlation of W₀ with Δv suggestive of the clouds being flattened and expanding with the Hubble flow in their long dimension, as would be indicative of sheets or filaments. This is supported by the uniformity of line strengths between the three sight lines for W₀ > 0.4 Å. We conclude, tentatively, that the W₀ > 0.4 Å Lyα forest objects are sheetlike. In contrast, the weaker lines, 0.2 Å > W₀ > 0.4 Å, show no evidence of spanning the sight lines of these groups but have sizes significantly larger than the luminous portions of galaxies and C IV absorbers as revealed by closer separation QSO pairs. When the LB sight-line pair is included with other pairs at different redshifts and sight-line separations, one finds no strong evidence for evolution of Lyα absorber size with redshift. We also show that there is no evidence of large-scale structure in the Lyα forest consistent with ionization of H II by foreground QSOs as seen in the spectrum of background QSOs (the "foreground proximity effect"). Finally, we see a marginal detection of the sight-line two-point cross-correlation function for C IV lines ξ=2.05^{+ 1.82}_−1.21 over scales of 0.5-1 h^-1 Mpc. This is significantly weaker than ξ measured by autocorrelation along single sight lines for 200 km s^-1 < Δv < 600 km s^-1, suggesting that most of the latter signal may be due to the internal motions within absorbers that are smaller than 0.5 h^-1 Mpc.

We use the rotation curves of a sample of dark matter-dominated dwarf and low surface brightness (LSB) late-type galaxies to study their radial mass distributions. We find that the shapes of the rotation curves are remarkably similar for all (both dwarf and LSB) galaxies in the sample, suggesting a self-similar density distribution of their dark matter (DM) halos. This shape can be reproduced well by a density profile with a shallow central cusp [ρ(r) ∝ 1/r^γ, γ ≈ 0.2-0.4] corresponding to a steeply rising velocity curve [v(r) ∝ r^g, g ≈ 0.9-0.8]. We further show that the observed shapes of the rotation curves are well matched by the average density profiles of dark matter halos formed in very high resolution simulations of the standard cold dark matter model (CDM), the low-density CDM model with cosmological constant (ΛCDM), and the cold + hot dark matter model with two types of neutrino (CHDM). This is surprising in light of several previous studies, which suggested that the structure of simulated dark matter halos is inconsistent with the dynamics of dwarf galaxies. We discuss possible explanations for this discrepancy and show that it is most likely caused by the systematic differences at small radii between the analytic model proposed by Navarro, Frenk, & White, with γ_NFW = 1, and the actual central density profiles of the dark matter halos. We also show that the mass distributions in the hierarchically formed halos are onaverage consistent with the shape of rotation curves of dark matter-dominated galaxies. However, the scatter of the individual profiles around the average is substantial. Finally, we show that the dark matter halos in our hierarchical simulations and the real galaxies in our sample exhibit a similar decrease in their characteristic densities with increasing characteristic radial scales and show increase in their maximum rotation velocities with increase in the radii at which their maximum velocities occur.

To elucidate the permeation of cosmic ultraviolet (UV) background radiation into a pregalactic cloud and the subsequent ionization, the frequency-dependent radiative transfer equation is solved, coupled with the ionization process, for a spherical top-hat cloud composed of pure hydrogen. The calculations properly involve scattering processes of ionizing photons that originate from radiative recombination. As a result, it is shown that the self-shielding, although it is often disregarded in cosmological hydrodynamic simulations, could start to emerge shortly after the maximum expansion stages of density fluctuations. Quantitatively, the self-shielding is prominent above a critical number density of hydrogen, which is given by n_crit = 1.4 × 10^-2 cm^-3 (M/10⁸M)^−1/5I^3/5₂₁ for 10⁴ K gas, where M is the cloud mass and the UV background intensity is assumed to be I_ν = 10^-21I₂₁(ν/ν_L)^-1 ergs cm^-2 s^-1 sr^-1 Hz^-1, with ν_L being the Lyman limit frequency. The weak dependence of n_crit upon the mass is worth noting. The corresponding critical optical depth (τ_crit) turns out to be independent of either M or I₂₁, which is τ_crit = 2.4 for 10⁴ K gas. The present analysis reveals that the Strömgren approximation leads to overestimation of the photoionization effects. Also, the self-shielded neutral core is no longer sharply separated from surrounding ionized regions; a low but noticeable degree of ionization is caused by high-energy photons even in the self-shielded core. The present results may be substantial when one considers the biasing by photoionization against low-mass galaxy formation.

We describe a new approach for the determination of cosmological parameters using gravitational lensing systems with multiple arcs. We exploit the fact that a given cluster can produce multiple arcs from sources over a broad range in redshift. The coupling between the critical radius of a single arc and the projected mass density of the lensing cluster can be avoided by considering the relative positions of two or more arcs. Cosmological sensitivity appears through the angular size-redshift relation. We present a simple analytic argument for this approach using an axisymmetric, power-law cluster potential. In this case, the relative positions of the arcs can be shown to depend only on the ratios of the angular size distances of the source and the distances between the lens and source. Provided that the astrometric precision approaches ~001 (e.g., via the HubbleSpaceTelescope) and the redshifts of the arcs are known, we show that the system can, in principle, provide cosmological information through the angular size-redshift relation. Next we consider simulated data constructed using a more general form for the potential, realistic sources and an assumed cosmology. We present a method for simultaneously inverting the lens and extracting the cosmological parameters. The input data required are the image and measured redshifts for the arcs. The technique relies on the conservation of surface brightness in gravitationally lensed systems. We find that for a simple lens model, our approach can recover the cosmological parameters assumed in the construction of the simulated images.

We use Bayesian methods to analyze the distribution of gamma-ray burst intensities reported in the Third BATSE Catalog (3B catalog) of gamma-ray bursts, presuming the distribution of burst sources ("bursters") is isotropic. We study both phenomenological and cosmological source distribution models, using Bayes's theorem both to infer unknown parameters in the models and to compare rival models. We analyze the distribution of the time-averaged peak photon number flux, Φ, measured on both 64 ms and 1024 ms timescales, performing the analysis of data based on each timescale independently. Several of our findings differ from those of previous analyses that modeled burst detection less completely. In particular, we find that the width of the intrinsic luminosity function for bursters is unconstrained, and the luminosity function of the actually observed bursts can be extremely broad, in contrast to the findings of all previous studies. Useful constraints probably require observation of bursts significantly fainter than those visible to BATSE. We also find that the 3B peak flux data do not usefully constrain the redshifts of burst sources; useful constraints require the analysis of data beyond that in the 3B catalog (such as burst time histories) or data from brighter bursts than have been seen by BATSE (such as those observed by the PioneerVenusOrbiter). In addition, we find that an accurate understanding of the peak flux distributions reported in the 3B almost certainly requires consideration of data on the temporal and spectral properties of bursts beyond that reported in the 3B catalog and more sophisticated modeling than has so far been attempted.

We first analyze purely phenomenological power-law and broken power-law models for the distribution of observed peak fluxes. We find that the 64 ms data are adequately fitted by a single power law, but that the 1024 ms data significantly favor models with a sharp, steep break near the highest observed fluxes. At fluxes below the break, the distribution of 1024 ms fluxes is flatter than that of 64 ms fluxes. Neither data set is consistent with the power-law distribution expected from a homogeneous, Euclidean distribution of sources. Next we analyze three simple cosmological models for burst sources: standard candles with constant burst rate per comoving volume, a distribution of standard-candle sources with comoving burst rate proportional to a power law in (1 + z), and a bounded power-law burster luminosity function with constant comoving burst rate but variable power-law index and luminosity bounds. We find that the 3B data can usefully constrain the luminosity of a standard-candle cosmological population of bursts if there is no density evolution. But the 3B data allow strong density evolution and arbitrarily broad luminosity functions; consequently, they do not usefully constrain the redshifts or luminosities of cosmological burst sources. We elucidate the properties of the models responsible for these results.

For sufficiently flexible models, the inferred values for parameters describing the shapes of the distributions of 64 ms and 1024 ms peak fluxes formally differ at the 68%-95% level. Because the measurements on these two timescales are not independent, it is difficult to ascertain the true significance of this discrepancy; since many bursts are common to both data sets, it is likely its significance is larger than these formal values indicate. In addition, the inferred amplitude (in bursts per year) of the distribution of 64 ms peak fluxes is about twice that of 1024 ms peak fluxes. These results strongly suggest that a complete understanding of the measured peak flux distributions requires simultaneous modeling and analysis of temporal properties of bursts. We study models that attempt to reconcile the two data sets by accounting for "peak dilution," the underestimation of the peak intensity that results from using data accumulated over a timescale exceeding the peak duration. A phenomenological model strongly correlating peak duration with peak flux is moderately successful at reconciling the data. A model that correlates peak duration with peak flux due to cosmological time dilation and relativistic beaming is less successful, but remains of interest in that it is a simple physical model illustrating how one can jointly model and analyze temporal and spectral properties of bursts with peak flux data. A more rigorous accounting for the differences between the 64 ms and 1024 ms data requires analysis of temporal and spectral information about bursts beyond that available in the 3B catalog.

We use Bayesian methods to study anisotropic models for the distribution of gamma-ray burst intensities and directions reported in the Third BATSE Catalog (3B catalog) of gamma-ray bursts. We analyze data obtained using both the 64 ms and 1024 ms measuring timescales. We study both purely local models, in which burst sources ("bursters") are presumed to be distributed in extended halos about the Galaxy and M31, and mixed models consisting of a cosmological population of standard-candle bursters and a local population distributed throughout a standard Bahcall-Soneira dark matter halo with a 2 kpc core. In a companion paper we study isotropic models, including a variety of cosmological models, using the same methodology adopted here, allowing us to rigorously and quantitatively compare isotropic and anisotropic models. We find that the purely local models we have studied can account for the 3B data as successfully as cosmological models, provided one considers halos with core sizes significantly larger than those used to model the distribution of dark matter. A preference for cosmological over local models, or vice versa, must therefore be justified by means of information other than the distribution of burst directions and intensities. We infer core sizes for the halo distribution that are smaller than one might expect based on popular semiquantitative arguments that consider the superposed dipole moments of shells centered on the Galactic center and show why such arguments can lead to unwarranted conclusions. We also find that the 3B data do not constrain the width of power-law luminosity functions for burst sources. This disagrees with the findings of previous studies; we elucidate the qualitative reasons for the lack of a constraint, and discuss why our results differ from those of earlier studies. Our analysis of mixed models finds two families of models that can successfully account for the data: models with up to 20% of observed bursts in a bright local population visible to ~50 kpc and models with up to 50% of observed bursts in a dim local population visible only nearby (to less than a disk scale height). These models fit as well or better than purely cosmological models. They indicate that a surprisingly large local, anisotropic component could be present whose size is comparable to the sizes of hypothetical classes of bursts inferred from analyses of temporal and spectral characteristics. Finally, as in our study of isotropic models, we find substantial systematic differences between results based on 64 ms and 1024 ms data, which indicates that a thorough understanding of the distribution of burst intensities and directions is likely to require detailed analysis of temporal properties.

Simulations of large-scale structure formation predict the formation of sheet and filamentary structures, which are often invoked as the origin of the Lyα forest. In their simplest description, these sheets and filaments require a differential distribution of observed line-of-sight velocity widths (b), which will decrease as power laws at velocities well above the observed peak in this distribution; for filaments, the differential distribution is dN/db ∝ b^-3, while for sheets it is dN/db ∝ b^-2. These functional dependences on b arise a priori because of the geometry of these absorbing structures—assuming random orientations relative to the line of sight—and are otherwise unrelated to the physical state in the absorbing structure. We find the distribution at b > 35 km s^-1 in three previously published data sets to be steeper than dN/dB ∝ b^-3 (99.99% confidence). This implies that evidence of the finite length of these kinds of absorbing structures is present in the b-distribution data.

Using the spectrophotometry of a large sample of galaxies in 19 Abell clusters, we have selected 42 candidate active galactic nuclei (AGNs) using the criteria used by Dressler and coworkers in their analysis of the statistics of 22 AGNs in 14 rich cluster fields, which are based on the equivalent width of [O II] 3727 Å, Hβ, and [O III] 5007 Å emission. We have then discriminated AGNs from H II region-like galaxies (hereafter H II galaxies) in the manner developed by Veilleux & Osterbrock using the additional information provided by Hα and [N II] 6583 Å or Hα and [S II] 6716 + 6731 Å emission, in order to test the reliability of the selection criteria used by Dressler and coworkers. We find that before we discriminate AGNs from H II galaxies, our sample is very similar to that of Dressler and coworkers and it leads to similar conclusions. However, we find that their method inevitably mixes H II galaxies with AGNs, even for the most luminous objects in our sample. We estimate a contamination of at least 38% at a formal 90% confidence level. Since the study of Dressler and coworkers, other authors have attempted to quantify the relative fraction of cluster-to-field AGNs and have reached similar conclusions, but they have used criteria similar to Dressler and coworkers to select AGNs (or have used the [O III] 5007 Å/Hβ flux ratio test that also mixes H II galaxies with AGNs). Our sample of true AGNs remains too small to reach statistically meaningful conclusions, therefore a new study with a more time-consuming method that includes the other lines will be required to quantify the true relative fraction of cluster-to-field AGNs.

Most clusters and groups of galaxies contain a giant elliptical galaxy in their centers that far outshines and outweighs normal ellipticals. The origin of these brightest cluster galaxies is intimately related to the collapse and formation of the cluster. Using an N-body simulation of a cluster of galaxies in a hierarchical cosmological model, we show that galaxy merging naturally produces a massive central galaxy with surface brightness and velocity dispersion profiles similar to those of observed BCGs. To enhance the resolution of the simulation, 100 dark halos at z = 2 are replaced with self-consistent disk + bulge + halo galaxy models following a Tully-Fisher relation using 100,000 particles for the 20 largest galaxies and 10,000 particles for the remaining ones. This technique allows us to analyze the stellar and dark-matter components independently. The central galaxy forms through the merger of several massive galaxies along a filament early in the cluster's history. Galactic cannibalism of smaller galaxies through dynamical friction over a Hubble time only accounts for a small fraction of the accreted mass. The galaxy is a flattened, triaxial object whose long axis aligns with the primordial filament and the long axis of the cluster galaxy distribution, agreeing with observed trends for galaxy cluster alignment.

The drag force on a satellite of mass M moving with speed V in the gravitational field of a spherically symmetric background of stars is computed. During the encounter, the stars are subject to a time-dependent force that alters their equilibrium. The resulting distortion in the stellar density field acts back to produce a force F_Δ that decelerates the satellite. This force is computed using a perturbative technique known as linearresponsetheory. In this paper, we extend the formalism of linear response to derive the correct expression for the back-reaction force F_Δ that applies when the stellar system is described by an equilibrium one-particle distribution function. F_Δ is expressed in terms of a suitable correlation function that couples the satellite dynamics to the unperturbed dynamics of the stars. At time t, the force depends upon the whole history of the composite system. In the formalism, we account for the shift of the stellar center of mass resulting from linear momentum conservation. The self-gravity of the response is neglected since it contributes to a higher order in the perturbation. Linear response theory applies also to the case of a satellite orbiting outside the spherical galaxy. The case of a satellite moving on a straight line, at high speed relative to the stellar dispersion velocity, is explored. We find that the satellite during its passage raises (1) global tides in the stellar distribution and (2) a wake, i.e., an overdense region behind its trail. If the satellite motion is external to the galaxy, it suffers a dissipative force that is not exclusively acting along V but acquires a component along R, the position vector relative to the center of the spherical galaxy. We derive the analytical expression of the force in the impulse approximation. In penetrating short-lived encounters, the satellite moves across the stellar distribution and the transient wake excited in the density field is responsible for most of the deceleration. We find that dynamical friction arises from a memory effect involving only those stars perturbed along the path. The force can be written in terms of an effective Coulomb logarithm that now depends upon time. The value of ln Λ is computed for two simple equilibrium density distributions; it is shown that the drag increases as the satellite approaches the denser regions of the stellar distribution and attains a maximum after pericentric passage. When the satellite crosses the edge of the galaxy, the force does not vanish since the galaxy keeps memory of the perturbation induced and declines on a time comparable to the dynamical time of the stellar system. In the case of a homogeneous cloud, we compute the total energy loss. In evaluating the contribution resulting from friction, we derive self-consistently the maximum impact parameter, which is found to be equal to the length traveled by the satellite within the system. Tides excited by the satellite in the galaxy reduce the value of the energy loss by friction; in close encounters, this value is decreased by a factor of ~1.5.

We study the dynamical evolution of a satellite (of mass M) orbiting around a companion spherical galaxy. The satellite is subject to a back-reaction force F_Δ resulting from the density fluctuations excited in the primary stellar system. We evaluate this force using the linear response theory developed by Colpi & Pallavicini in 1998. F_Δ is computed in the reference frame comoving with the primary galaxy and is expanded in multipoles. To lowest order, the force depends on a time integral involving a dynamical four-point correlation function of the unperturbed background. The equilibrium stellar system (of mass Nm) is described in terms of a Gaussian one-particle distribution function. To capture the relevant features of the physical process determining the evolution of the detached binary, we introduce in the Hamiltonian the harmonic potential as interaction potential among stars. The evolution of the composite system is derived solving for a set of ordinary differential equations; the dynamics of the satellite and of the stars is computed self-consistently. We determine the conditions for tidal capture of a satellite from an asymptotic free state and give an estimate of the maximum kinetic energy above which encounters do not end in a merger as a function of the mass ratio M/Nm. We find that capture always leads to final coalescence. If the binary forms as a bound pair, stability against orbital decay is lost if the pericentric distance is smaller than a critical value. This instability is interpreted in terms of a near-resonance condition and establishes when the orbital Keplerian frequency becomes comparable to the internal frequency ω of the stellar system. We show that before coalescence, eccentric orbits become progressively less eccentric; the circularization is explored as a function of mass ratio. The timescale of binary coalescence τ_b is a sensitive function of the eccentricity e for a fixed semimajor axis a and M/Nm ratio: the mismatch between τ_b at e ~ 0 and τ_b at e ~ 1 can be very large, typically τ_b(e ≲ 1) ~ 6ω^-1, and the time ratio τ_b(0)/τ_b(1) ≳ 5 (for M/Nm = 0.05). In addition, we find that τ_b obeys a scaling relation with M/Nm for circular orbits: τ_b ∝ (M/Nm)^-α, with α ~ 0.4. In grazing encounters, τ_b is nearly independent of mass.

We use a Monte Carlo technique and assume spatial distributions of dust and supernova (SN) progenitors in a simple model of a characteristic SN-producing disk galaxy to explore the effects of extinction on the radial distributions of SN properties in their parent galaxies. The model extinction distributions and projected radial number distributions are presented for various SN types. Even though the model has no core-collapse SNe within 3 kpc of the center, a considerable fraction of the core-collapse SNe are projected into the inner regions of inclined parent galaxies owing to their small vertical scale height. The model predicts that because of extinction, SNe that are projected into the central regions should, on average, appear dimmer and have a much larger magnitude scatter than those in the outer regions. In particular, the model predicts a strong deficit of bright core-collapse events inside a projected radius of a few kiloparsec. Such a deficit is found to be present in the observations. It appears to be a natural consequence of the characteristic spatial distributions of dust and core-collapse SNe in galaxies, and it leads us to offer an alternative to the conventional interpretation of the Shaw effect.

Centimeter to submillimeter total flux and polarization monitoring data are used to investigate the nature of a prominent flare in the quasar 3C 273 during 1995/6. After removal of the quiescent level, the resulting "flare spectra" are well fitted by a simple homogeneous synchrotron source model, which in turn allows the movement of the self-absorption turnover to be tracked during the flare. Both the flare amplitude/time delay relationship and the overall spectral evolution are qualitatively consistent with existing models. The early evolution of the spectrum is best determined and is shown to be in excellent agreement with the Compton stage of the Marscher & Gear shock model. However, the polarization behavior during the flare is different at millimeter and centimeter wavelengths and the observations are difficult to reconcile with a simple transverse shock. They are, however, consistent with a conical shock for which the observed polarization properties vary with distance along the jet. Such variations may be caused, for example, by a change in cone angle owing to disruption caused by the growing component of the magnetic field parallel to the jet axis or by a moderate change in viewing angle.

A new cosmological scenario for the origin of gamma-ray bursts (GRBs) is proposed. In our scenario, a highly evolved central core in the dense galactic nucleus is formed, containing a subsystem of compact stellar remnants (CSRs), such as neutron stars and black holes. Those subsystems result from the dynamical evolution of dense central stellar clusters in the galactic nuclei through merging of stars, thereby forming (as has been realized by many authors) the short-lived massive stars and then CSRs. We estimate the rate of random CSR collisions in the evolved galactic nuclei by taking into account, in a procedure similar to that of Quinlan & Shapiro, the dissipative encounters of CSRs, mainly due to radiative losses of gravitational waves, which result in the formation of intermediate short-lived binaries, with further coalescence of the companions to produce GRBs. We also consider how the possible presence of a central supermassive black hole, formed in a highly evolved galactic nucleus, influences the CSR binary formation. This scenario does not postulate ad hoc a required number of tight binary neutron stars in the galaxies. Instead, it gives, for the most realistic parameters of the evolved nuclei, the expected rate of GRBs consistent with the observed one, thereby explaining the GRB appearance as a natural part of the dynamical evolution of galactic nuclei. In addition, this scenario provides an opportunity for a cosmological GRB recurrence, previously considered to be a distinctive feature of GRBs of a local origin only. We also discuss some other observational tests of the proposed scenario.

We present HST/WFPC2 images, in narrowband filters containing the [O III] λ5007 and Hα + [N II] emission lines and their adjacent continua, of a sample of seven Seyfert 2 galaxies selected because they possess either extended emission-line regions in ground-based observations or a hidden broad-line region in polarized light. Six of the galaxies have also been observed with the VLA in order to obtain radio maps of better quality and angular resolution than those in the literature. We find detailed correspondences between features in the radio and emission-line images, clearly indicating strong interactions between the radio jets and the interstellar medium. Such interactions play a major role in determining the morphology of the NLR, as the radio jets sweep up and compress ambient gas, producing ordered structures with enhanced surface brightness in line emission. In at least three galaxies, namely, Mrk 573, ESO 428-G14, and Mrk 34 (and perhaps also NGC 7212), off-nuclear radio lobes coincide with regions of low gaseous excitation (as measured by the [O III]/(Hα + [N II]) ratio). In Mrk 573 and NGC 4388, there is a clear trend for low-brightness ionized gas to be of higher excitation. These results may be understood if radio lobes and regions of high emission-line surface brightness are associated with high gas densities, reducing the ionization parameter. [O III]/(Hα + [N II]) excitation maps reveal bipolar structures that can be interpreted as either the ionization cones expected in the unified scheme or widening, self-excited gaseous outflows. Only NGC 4388 and Mrk 573 show a clearly defined, straight-edged ionization cone.

We have determined detailed radio spectra for 26 compact sources in the starburst nucleus of M82, between 74 and 1.3 cm. Seventeen show low-frequency turnovers. One other has a thermal emission spectrum, and we identify it as an H II region. The low-frequency turnovers result from absorption by interstellar thermal gas in M82. New information on the active galactic nucleus candidate 44.01+595 shows it to have a nonthermal falling power-law spectrum at the highest frequencies and that it is strongly absorbed below 2 GHz. We derive large magnetic fields in the supernova remnants of order (1-2)(1 + k)^2/7ϕ^-2/7 mG; hence, large pressures in the sources suggest that the brightest ones are either expanding or are strongly confined by a dense interstellar medium. From the largest source in our sample we derive a supernova rate of 0.016 yr^-1.

We present high dynamic range (20,000:1 at 1414 MHz) VLBA observations of NGC 1275 (3C 84) at 1414, 612, and 330 MHz. With these observations, we have discovered a previously undetected counterjet component located ~80 mas (20 h^-1 pc) to the north of the compact core. This counterjet component, like those closer in to the nucleus, appears to be free-free absorbed. We have also discovered the first radio halo around an active galactic nucleus on small (<1 kpc) scales. This millihalo cannot be the result of scattering and is most likely produced by synchrotron radiation from relativistic particles that have diffused out from the parsec-scale jets over the lifetime of 3C 84.

In this paper we report the detection of a massive counterrotating molecular gas disk in the early-type spiral NGC 3626, observed in the 1-0 and 2-1 lines of ¹²CO, mapped with the 30 m telescope and Bure interferometer.¹²CO emission is concentrated in a compact nuclear disk of average radius r ~ 12'' (1.2 kpc). In the outer disk, from r = 20'' to r = 100'' (2-10 kpc),¹²CO is not detected, and the neutral gas content is largely dominated by H I. The observed ¹²CO velocity field pattern corresponds to a gaseous disk with a sense of rotation opposite to that of the stars. Counterrotation is shared by molecular and ionized gas in the center. There is no strong evidence of ¹²CO emission from gas in direct rotation. The estimated molecular mass in the ¹²CO nuclear disk is M(H₂) ~ 0.3 × 10⁹M_☉, 3 times lower than the mass of the H I disk. Within the nuclear disk,¹²CO is distributed in a central source of ~1'' (100 pc) radius, where the derived ¹²CO rotation curve reaches ~240 km s^-1, surrounded by a pseudoring of average radius ~6'' (600 pc) characterized by strong noncircular motions. The dynamics of molecular gas, characterized by a regular counterrotating pattern and streaming motions typical of a steady density-wave driven flow and normal ¹²CO line widths, preclude the occurrence of violent large-scale shocks or of a nonequilibrium dynamical state for the gas. The available optical (Hα, N [II] and S [II]) and radio continuum data (at 12.6 cm and 21 cm) indicate that no violent burst of star formation is associated with the nuclear molecular gas. This is confirmed by the lack of IRAS flux record for the nucleus of NGC 3626. The present ¹²CO observations suggest that we are probably seeing a late stage of a merger happened in NGC 3626.

Observational evidence for shock-induced star formation is found in the northeast radio lobe of the nearby radio galaxy Centaurus A (NGC 5128). A gas cloud, recently detected in H I, is impacted by the adjacent radio jet to the extent that cloud collapse is triggered and loose chains of blue supergiant stars are formed. Diffuse clouds and filaments of ionized gas have been observed near the interface of the H I cloud and the radio jet. These show velocities that cover a range of more than 550 km s^-1. Line intensities in their spectra are characteristic of a shock-related origin with strong [N II] and [S II] relative to Hα. The [O III]/Hα line ratio indicates a large range in excitation that is not correlated with velocity. Distinct from this component is a group of four apparently normal H II regions that are excited by embedded young stars and whose velocities are very close to that of the H I cloud. Star formation will continue for as long as the gas cloud remains close to the radio jet. The loose chains of blue stars in the area are resolved only because NGC 5128 is so close. The reported faint blue extensions and plumes in more distant analogs probably have similar origins.

We have investigated the variability of the binary X-ray pulsar SMC X-1 in data from several X-ray observatories. We confirm the ~60 day cyclic variation of the X-ray flux in the long-term monitoring data from the RXTE and CGRO observatories. X-ray light curves and spectra from the ROSAT, Ginga, and ASCA observatories show that the uneclipsed flux varies by as much as a factor of 20 between a high-flux state when 0.71 s pulses are present and a low-flux state when pulses are absent. In contrast, during eclipses when the X-rays consist of radiation scattered from circumsource matter, the fluxes and spectra in the high and low states are approximately the same. These observations prove that the low state of SMC X-1 is not caused by a reduction in the intrinsic luminosity of the source or a spectral redistribution thereof, but rather by a quasi-periodic blockage of the line of sight, most likely by a precessing tilted accretion disk. In each of two observations in the midst of low states a brief increase in the X-ray flux and reappearance of 0.71 s pulses occurred near orbital phase 0.2. A similar brief flux increase near orbital phase 0.2 was observed during a low-state observation that did not have sufficient time resolution to detect 0.71 s pulses. These brief increases result from an opening of the line of sight to the pulsar that may be caused by wobble in the precessing accretion disk. The records of spin-up of the neutron star and decay of the binary orbit are extended during 1991-1996 by pulse-timing analysis of ROSAT, ASCA, and RXTE PCA data. The pulse profiles in various energy ranges from 0.1 to greater than 21 keV are well represented as a combination of a pencil beam and a fan beam. Finally, there is a marked difference between the power spectra of random fluctuations in the high-state data from the RXTE PCA below and above 3.4 keV. In the higher energy range the spectrum has a sharp break at 3.3 Hz, with fitted power-law indices of 0.45 and 1.76 below and above the break. No break is evident in the power spectrum below 3.4 keV, and the fitted power-law index is 0.51. In both spectra there is a positive deviation from the fitted power law around 0.06 Hz that may be quasi-periodic oscillation.

Panoramic images of ¹²CO J = 1-0 and thermal dust emissions from the W3-W4-W5 region of the outer Galaxy are presented. These data and recently published H I 21 cm line emission images provide a ~1' resolution perspective to the dynamics and thermal energy content of the interstellar gas and dust components contained within a 9° arc of the Perseus spiral arm. We tabulate the molecular properties of 1560 clouds identified as closed surfaces within the l-b-v CO data cube at a threshold of 0.9 K T^*_R. Relative surface densities of the molecular (28:1) and atomic (2.5:1) gas components determined within the arm and interarm velocity intervals demonstrate that the gas component that enters the spiral arm is predominantly atomic. Molecular clouds must necessarily condense from the compressed atomic material that enters the spiral arm and are likely short lived within the interarm regions. From the distribution of centroid velocities of clouds, we determine a random cloud-to-cloud velocity dispersion of 4 km s^-1 over the width of the spiral arm but find no clear evidence within the molecular gas for streaming motions induced by the spiral potential. The far-infrared images are analyzed with the CO J = 1-0 and H I 21 cm line emission. The enhanced UV radiation field from members of the Cas OB6 association and embedded newborn stars provide a significant source of heating to the extended dust component within the Perseus arm relative to the quiescent cirrus regions. Much of the measured far-infrared flux (69% at 60 μm and 47% at 100 μm) originates from regions associated with star formation rather than the extended, infrared cirrus component.

Measurements of the cosmic-ray hydrogen and helium spectra at energies from 20 to 800 TeV are presented. The experiments were performed on a series of twelve balloon flights, including several long duration Australia to South America and Antarctic circumpolar flights. No clear evidence is seen for a spectral break. Both the hydrogen and the helium spectra are consistent with power laws over the entire energy range, with integral spectral indices 1.80 ± 0.04 and 1.68^{+ 0.04}_−0.06 for the protons and helium, respectively. The results are fully consistent with expectations based on supernova shock acceleration coupled with a "leaky box" model of propagation through the Galaxy.

We report the measurement of polycyclic aromatic hydrocarbons (PAHs) in individual circumstellar graphite grains extracted from two primitive meteorites, Murchison and Acfer 094. The ¹²C/¹³C isotope ratios of the grains in this study range from 2.4 to 1700. Roughly 70% of the grains have an appreciable concentration of PAHs (500-5000 parts per million [ppm]). Independent molecule-specific isotopic analyses show that most of the PAHs appear isotopically normal, but in several cases correlated isotopic anomalies are observed between one or more molecules and their parent grains. These correlations are most evident for ¹³C-depleted grains. Possible origins of the PAHs in the graphite grains are discussed.

This paper discusses star formation in the 40 C¹⁸O molecular cloud cores in Taurus by studying relations between the cores and young stellar objects (YSOs). These C¹⁸O cores constitute a complete sample of dense gas in Taurus as described in detail by Onishi and coworkers. The YSOs treated in the present work fall into the following four categories: (1) starless H¹³CO⁺ compact cores, (2) cold IRAS sources, (3) warm IRAS sources, and (4) T Tauri stars without far-infrared emission. Starless H¹³CO⁺ compact cores are assumed to be protostellar condensations that are in a phase just prior to star formation, although they are not yet considered to be YSOs in the usual sense. In the present study, we shall call the objects 1 and 2 coldobjects and objects 3 and 4 warmobjects. They constitute the most complete sample of YSOs, including compact dense molecular condensations, at present.

It is found that C¹⁸O cores with coldobjects have clearly higher molecular column densities than the rest. These C¹⁸O cores also tend to be massive and large in size. If we assume that the coldobjects represent the protostellar condensations or protostars in the main accretion phase, the results imply that the actual process of star formation, i.e., collapse and significant mass accretion, takes place when a column density of the molecular cloud core becomes greater than a certain value. This threshold value for star formation is ~8.0 × 10²¹ cm^-2 in average column density. On the other hand, the C¹⁸O cores without coldobjects are characterized by a typical column density of 5.5 × 10²¹ cm^-2, significantly smaller than the threshold value. The fact that all the C¹⁸O cores whose average column density is higher than 8.0 × 10²¹ cm^-2 are associated with coldobjects suggests that star formation takes place immediately and without exception when the column density satisfies the above value. Such C¹⁸O cores with coldobjects typically have more than one YSO. The number of YSOs in a C¹⁸O core increases with core mass. The C¹⁸O core mass divided by the number of YSOs does not depend strongly on the C¹⁸O core mass and is nearly a constant mass of 11 M_☉, i.e., the number of YSOs within a C¹⁸O core is proportional to the core mass. This suggests that formation of a protostellar condensation requires ~11 M_☉ molecular gas of n(H)₂ ~ 10⁴ cm^-3 indicating a rather uniform star formation efficiency (M_star/M_gas) of ~6% (the mass of a YSO is taken to be 0.7 M_☉) at a density of ~10⁴ cm^-3. The mean projected separation of the coldobjects in a C¹⁸O core is estimated to be ~0.3 pc. This separation is roughly in a range explicable by existing theoretical studies of gravitational instability, while the core morphology is not yet exactly taken into account in these studies. The C¹⁸O data suggest that the amount of the surrounding gas decreases on a timescale of, ~5 × 10⁵ yr or larger. Dissipation of surrounding gas occurs not only in the vicinity of a star but also in the scale of C¹⁸O cores, ≳0.1 pc. This dissipation cannot be explained simply by an outflow or mass accretion onto a central star.

Aperture synthesis observations of HCO⁺J = 1-0,¹³CO 1-0, and C¹⁸O 1-0 obtained with the Owens Valley Millimeter Array are used to probe the small-scale (5'' ≈ 700 AU) structure of the molecular envelopes of a well-defined sample of nine embedded low-mass young stellar objects in Taurus. The interferometer results can be understood in terms of: (1) a core of radius ≲1000 AU surrounding the central star, possibly flattened and rotating; (2) condensations scattered throughout the envelope that may be left over from the inhomogeneous structure of the original cloud core or that may have grown during collapse; and (3) material within the outflow or along the walls of the outflow cavity. Masses of the central cores are 0.001-0.1 M_☉, and agree well with dust continuum measurements. Averaged over the central 20'' (3000 AU) region, an HCO⁺ abundance of 4 × 10^-8 is inferred, with a spread of a factor of 3 between the different sources. Reanalysis of previously presented single-dish data yields an HCO⁺ abundance of (5.0 ± 1.7) × 10^-9, which may indicate an average increase by a factor of a few on the smaller scales sampled by the interferometer. Part of this apparent abundance variation could be explained by contributions from extended cloud emission to the single-dish C¹⁸O lines, and uncertainties in the assumed excitation temperatures and opacities. The properties of the molecular envelopes and outflows are further investigated through single-dish observations of ¹²CO J = 6-5, 4-3, and 3-2,¹³CO 6-5 and 3-2, and C¹⁸O 3-2 and 2-1, obtained with the James Clerk Maxwell and IRAM 30 m telescopes, along with the Caltech Submillimeter Observatory. Ratios of the mid-J CO lines are used to estimate the excitation temperature, with values of 25-80 K derived for the gas near line center. The outflow wings show a similar range, although T_ex is enhanced by a factor of 2-3 in at least two sources. In contrast to the well-studied L1551 IRS 5 outflow, which extends over 10' (0.4 pc), seven of the remaining eight sources are found to drive ¹²CO 3-2 outflows over ≤1' (0.04 pc); only L1527 IRS has a well-developed outflow of some 3' (0.12 pc). Estimates are obtained for the outflow kinetic luminosity, L_kin, and the flow momentum rate, F_CO, applying corrections for line opacity and source inclination. The flow force F_CO correlates with the envelope mass and with the 2.7 mm flux of the circumstellar disk. Only a weak correlation is seen with L_bol, while none is found with the relative age of the object as measured by ∫ T_mb(HCO⁺ 3-2)dV/L_bol. These trends support the hypothesis that outflows are driven by accretion through a disk, with a global mass infall rate determined by the mass and density of the envelope. The association of compact HCO⁺ emission with the walls of the outflow cavities indicates that outflows in turn influence the appearance of the envelopes. It is not yet clear, however, whether they are actively involved in sweeping up envelope material, or merely provide a low-opacity pathway for heating radiation to reach into the envelope.

We report multi-epoch interferometric radio observations of the IRAS source 18162-2048 that drives the thermal radio jet in the HH 80-81/GGD 27 complex. Our main goal was to follow the proper motion and flux density decay of the two inner young jet condensations N4 and S4, discovered by Martí et al., on their way out from the driving source. The tangential velocity estimated for the condensations amounts to ~60 mas yr^-1, equivalent to ~500 km s^-1 at the distance of the complex. The brightness decay can be fitted by a power law of the time elapsed since ejection, yielding power law exponents of about -2 and -3 for the northern and southern condensations, respectively. We also discuss our observations in the context of simple biconical jet models, suggesting that both condensations are consistent with relatively weak density enhancements in the otherwise steady jet flow. Their fade in brightness seems to be in agreement with the density decay expected in a freely expanding jet.

We present a series of two-dimensional hydrodynamic simulations of massive disks around protostars. We simulate the same physical problem using both a Piecewise Parabolic Method (PPM) code and a Smoothed Particle Hydrodynamic (SPH) code and analyze their differences. The disks studied here range in mass from 0.05M_* to 1.0M_* and in initial minimum Toomre Q value from 1.1 to 3.0. We adopt simple power laws for the initial density and temperature in the disk with an isothermal (γ = 1) equation of state. The disks are locally isothermal. We allow the central star to move freely in response to growing perturbations. The simulations using each code are compared to discover differences due to error in the methods used. For this problem, the strengths of the codes overlap only in a limited fashion, but similarities exist in their predictions, including spiral arm pattern speeds and morphological features. Our results represent limiting cases (i.e., systems evolved isothermally) rather than true physical systems. Disks become active from the inner regions outward. From the earliest times, their evolution is a strongly dynamic process rather than a smooth progression toward eventual nonlinear behavior. Processes that occur in both the extreme inner and outer radial regions affect the growth of instabilities over the entire disk. Effects important for the global morphology of the system can originate at quite small distances from the star. We calculate approximate growth rates for the spiral patterns; the one-armed (m = 1) spiral arm is not the fastest growing pattern of most disks. Nonetheless, it plays a significant role because of factors that can excite it more quickly than other patterns. A marked change in the character of spiral structure occurs with varying disk mass. Low-mass disks form filamentary spiral structures with many arms while high-mass disks form grand design spiral structures with few arms. In our SPH simulations, disks with initial minimum Q = 1.5 or lower break up into protobinary or protoplanetary clumps. However, these simulations cannot follow the physics important for the flow and must be terminated before the system has completely evolved. At their termination, PPM simulations with similar initial conditions show uneven mass distributions within spiral arms, suggesting that clumping behavior might result if they were carried further. Simulations of tori, for which SPH and PPM are directly comparable, do show clumping in both codes. Concerns that the pointlike nature of SPH exaggerates clumping, that our representation of the gravitational potential in PPM is too coarse, and that our physics assumptions are too simple suggest caution in interpretation of the clumping in both the disk and torus simulations.

Observational constraints on the primordial lithium abundance are important for the evaluation of the baryonic density of the universe. Its precise determination, however, suffers from uncertainties arising from the possible depletion of this element inside stars. Here we present and discuss new results for the lithium abundances in Population II stars obtained with the most recent stellar models including the best available physics. We show that it is possible to account for the general behavior of lithium observed in Population II stars without any free parameters. Macroscopic motions inside the stars are needed, but it is not necessary to specify their exact nature in order to interpret the observational data. This fact allows us to derive a parameter-free value for the primordial lithium abundance: log Li_p = 2.35 ± 0.10 in the log H = 12 scale.

We discuss the spin evolution of pulsars in the case where a superfluid component of the star is coupled to the observable crust on long, spin-down timescales. The momentum transfer from the superfluid interior results in an apparent decay of the external torque and, after a dramatic increase, in an asymptotic decrease of the generic value of the braking index, e.g., n = 3, to values n ~ 2.5 if the magnetic field of the star does not decay over its lifetime. In the case where an exponential decay of the magnetic field toward a residual value occurs, the star undergoes a spin-up phase after which it could emerge in the millisecond sector of the P- diagram.

The nonlinear growth of a nonaxisymmetric instability (the Papaloizou-Pringle instability) is followed numerically in thin Keplerian disks with the use of a time-dependent two-dimensional polytropic hybrid Fourier-Chebyshev spectral method of collocation. The nonaxisymmetric instability (a corotation resonance) develops in the inner disk when the inner boundary is rigid (corresponding here to the surface of an accreting compact star). All the modes of the instability have high Q-values and a period of rotation on the order of the Keplerian period at the inner edge of the disk. The high-order modes have growth rates larger than the low-order modes. When the viscosity is large, the higher modes are the first to be damped and saturate at moderate values: the energy is contained in the low-order modes, which dominate the flow. When the viscosity is low, the high-order modes dominate the flow, while the low-order modes do not grow at all: the energy is contained in the higher modes. When the order m and the amplitude a of the unstable mode are high enough (in the present calculations m ≳ 15 and a ≳ 0.3 for α = 0.001), the flow undergoes a subcritical transition to turbulence. The turbulence is confined in the inner region of the disk, inside the "resonant cavity," where it sustains itself because of the overreflection of waves (i.e., like the nonaxisymmetric instability itself). Some of the low-order modes are dominant during transient phases of the turbulent flow. The turbulence obtained in this work cannot account for angular momentum transport in the disk. However, the instability provides a new robust mechanism to explain the appearance of short-period oscillations (dwarf nova oscillations and quasi-periodic oscillations) observed in the inner disk of cataclysmic variables and other related systems.

We describe radial velocity observations of a large sample of apparently single white dwarfs (WDs), obtained in a long-term effort to discover close double-degenerate (DD) pairs, which might comprise viable Type Ia supernova (SN Ia) progenitors. We augment the WD sample with a previously observed sample of apparently single subdwarf B (sdB) stars, which are believed to evolve directly to the WD cooling sequence after the cessation of core helium burning. We have identified 18 new radial velocity variables, including five confirmed sdB + WD short-period pairs. Our observations are in general agreement with the predictions of the theory of binary star evolution. We describe a numerical method to evaluate the detection efficiency of the survey and estimate the number of binary systems not detected because of the effects of varying orbital inclination, orbital phase at the epoch of the first observation, and the actual temporal sampling of each object in the sample. Follow-up observations are in progress to solve for the orbital parameters of the candidate velocity variables.

We present and discuss 25 spectra obtained in 1996 November, covering all phases of the CAL 87 binary system. These spectra are superior both in signal-to-noise ratio and wavelength coverage to previously published data, so additional spectral features can be measured. Photometry obtained on the same nights is used to confirm the ephemeris and for comparison with light curves from previous years. Analysis of the color variation through the orbital cycle has been carried out using archival MACHO data. When a barely resolved red field star is accounted for, there is no (V-R) color variation, even through eclipse. There have been substantial changes in the depth of minimum light since 1988; it has decreased more than 0.5 mag in the last several years. The spectral features and radial velocities are also found to vary not only through the 0.44 day orbit but also over timescales of a year or more. Possible interpretations of these long-term changes are discussed. The 1996 spectra contain phase-modulated Balmer absorption lines not previously seen, apparently arising in gas flowing from the region of the compact star. The changes in emission-line strengths with orbital phase indicate that there are azimuthal variations in the accretion disk structures. Radial velocities of several lines give different amplitudes and phasing, making determination of the stellar masses difficult. All solutions for the stellar masses indicate that the companion star is considerably less massive than the degenerate star. The Balmer absorption-line velocities correspond to masses of ~1.4 M_☉ for the degenerate star and ~0.4 M_☉ for the mass donor. However, the strong He II emission lines indicate a much more massive accreting star, with M_X > 4 M_☉.

We have detected circumstellar absorption lines of the ¹²CN and ¹³CN violet and red system in the spectrum of the post-AGB star HD 56126. From a synthetic spectrum analysis, we derive a Doppler broadening parameter of b = 0.51 ± 0.04 km s^-1,¹²CN/¹³CN = 38 ± 2, and a lower limit of 2000 on ¹²CN/¹⁴CN and ¹²C¹⁴N/¹²C¹⁵N. A simple chemical model has been computed of the circumstellar shell surrounding HD 56126 that takes into account the gas-phase ion-molecule reaction between CN and C⁺. From this we infer that this reaction leads to isotopic fractionation of CN. Taking into account the isotopic exchange reaction and the observed ¹²CN/¹³CN, we find ¹²C/¹³C ~ 67 (for T_kin = 25 K). Our analysis suggests that ¹²CN has a somewhat higher rotational temperature than ¹³CN: T_rot = 11.5 ± 0.6 and 8.0 ± 0.6 K, respectively. We identify possible causes for this difference in excitation temperature, among which is the N'' dependence of the isotopic exchange reaction.

We use recent data obtained by three (OSSE, BATSE, and COMPTEL) of four instruments on board the ComptonGammaRayObservatory (CGRO) to construct a model of Cyg X-1 that describes its emission in a broad energy range, from soft X-rays to MeV γ-rays, self-consistently. The γ-ray emission is interpreted to be the result of Comptonization, bremsstrahlung, and positron annihilation in a hot, optically thin, and spatially extended region surrounding the whole accretion disk. For the X-ray emission, a standard corona-disk model is applied. We show that the Cyg X-1 spectrum accumulated by the CGRO instruments during a ~4 year time period between 1991 and 1995, as well as the HEAO3 γ₁ and γ₂ spectra, can be well represented by our model. The derived parameters match the observational results obtained from X-ray measurements.

Preliminary trigonometric parallaxes and BVI photometry are presented for two dwarf carbon stars, LP 765-18 (=LHS 1075) and LP 328-57 (=CLS 96). The data are combined with the literature values for a third dwarf carbon star, G77-61 (=LHS 1555). All three stars have very similar luminosities (9.6 < M_V < 10.0) and very similar broadband colors across the entire visual-to-near-IR (BVIJHK) wavelength range. Their visual (BVI) colors differ from all known red dwarfs, subdwarfs, and white dwarfs. In the M_V versus V-I color-magnitude diagram, they are approximately 2 mag subluminous compared to normal disk dwarfs with solar-like metallicities, occupying a region also populated by O-rich subdwarfs with -1.5 < [M/H] < -1.0. The kinematics indicate that they are members of the Galactic spheroid population. The subluminosity of all three stars is due to an as yet unknown combination of (undoubtedly low) metallicity, possibly enhanced helium abundance, and unusual line blanketing in the bandpasses considered. The properties of the stars are compared with models for the production of dwarf carbon stars.

Scorpius X-1 is the brightest extrasolar point source of X-rays and may serve as a prototype for low-mass X-ray binaries as a class. It has been suggested that the UV and optical emission arise as a result of reprocessing of X-rays and that a likely site for such reprocessing is an accretion disk around the X-ray source. If UV and optical emission are enhanced by the reprocessing of X-rays, the X-ray variability may be manifest in UV emission. We test this by using high temporal resolution UV data obtained simultaneously with high temporal resolution X-ray data collected by the Goddard High Resolution Spectrograph (GHRS) on the HubbleSpaceTelescope and by the X-RayTimingExplorer. We analyze the variability behavior of the UV spectrum and of the X-rays, and we also measure the properties of the emission-line profiles as viewed at high resolution (resolving power ≃ 25,000) with the echelle gratings. The variability behavior does not provide direct support for the reprocessing scenario, although the correlated variability between UV and X-rays does not conflict with this hypothesis. Furthermore, the emission-line profiles do not fit with simple models for disk emission lines.

We develop and test a numerical code that provides a self-consistent deconvolution of energy-dependent hard X-ray (HXR) time profiles I(ε, t) into two HXR-producing electron components, i.e., directly precipitating and trap-precipitating electrons. These two HXR components can be physically distinguished because their energy-dependent time delays have an opposite sign. The deconvolution is based on the following model assumptions: (1) nonthermal electrons are injected from the acceleration site into coronal flare loops by an injection function f(E, α, t) that consists of synchronized pulses in energy E and pitch angle α, (2) electrons with initially small pitch angles (α ≤ α₀) precipitate directly to the HXR emission site, (3) electrons with initially large pitch angles (α ≥ α₀) are temporarily trapped and precipitate after the collisional deflection time, and (4) nonthermal electrons lose their energy by Coulomb collisions and emit thick-target HXR bremsstrahlung in a high-density (fully collisional) site (near or inside the chromosphere). The numerical deconvolution provides a self-consistent determination of three physical parameters: (1) the electron time-of-flight distance l^TOF between the acceleration/injection site and the HXR emission site, (2) the electron density n_e in the trap region, and (3) the fraction of HXR-emitting electrons that precipitate directly, q^prec, which relates to the loss cone angle by q^prec(α₀) = (1 - cos α₀) for isotropic pitch angle distributions. This yields the magnetic mirror ratio R = B^loss/B^inj = 1/sin² (α₀) between the injection and loss cone site. With this method, we measure for the first time magnetic field ratios in coronal loops by means of HXR data. Based on this ratio, together with the knowledge of the photospheric field at the footpoint, a direct measurement of the magnetic field in the coronal acceleration region can be obtained. We simulate energy-dependent HXR data I(ε, t) with typical solar flare parameters (l^TOF = 15,000 km, n_e = 10¹¹ cm^-3, q^prec = 0.5) and test the accuracy of the inversion code. We perform the inversion in 30 different simulations over the entire physically plausible parameter space and demonstrate that a satisfactory inversion of all three physical parameters l^TOF, n_e, and q^prec is achieved in a density range of n_e = 10¹⁰-10¹² cm^-3 for precipitation ratios of q^prec = 0.1-0.9 and for signal-to-noise ratios of ≳100 (requiring HXR count rates of ≳10⁴ counts s^-1). Applications of this inversion method to solar flare observations in hard X-rays (CGRO/BATSE, Yohkoh/Hard X-Ray Telescope) and microwaves (Nobeyama) will be presented in subsequent papers.

Based on the deconvolution method developed in the first paper of this series, we present here the data analysis of 20-200 keV hard X-ray (HXR) data from the Burst and Transient Source Experiment (BATSE) on board the ComptonGammaRayObservatory (CGRO) recorded during 103 solar flares in 1991-1995. These are all of the flares simultaneously observed by CGRO with high time resolution (64 ms) and by Yohkoh in flare mode. The deconvolution method takes the measured HXR count rates as function of energy and time, I(ε, t), and computes the following self-consistently: the electron injection function n(E, t), the directly precipitating electron flux n^prec(E, t), the trapped-precipitating flux n^trap(E, t), the fraction of directly precipitating electrons (q^prec), the electron time-of-flight distance (l^TOF), and the electron density at the loss cone site of the trap (n_e). We find that the electron time-of-flight distances (l^TOF = 20.0 ± 7.3 Mm) inferred with the deconvolution method are fully consistent with those obtained earlier using a Fourier filter method. The trap electron densities (n_e = 10^10.96±0.57 cm^-3) obtained from deconvolving the e-folding decay times of HXR pulses (according to the trap model of Melrose & Brown) are found to be statistically a factor of 1.5 lower than those inferred from cross-correlation delays. The fraction q^prec of directly precipitating electrons, measured for the first time here, is found to have a mean (and standard deviation) of q^prec = 0.42 ± 0.16. Based on this precipitation fraction, we infer loss cone angles of α₀ ≈ 20°-70° and magnetic mirror ratios of R = B^loss/B^inj ≈ 1.2 - 3 (with a median value of R_median = 1.6) between the loss cone site and injection/acceleration site, assuming an isotropic pitch angle distribution at the injection site. The TOF distances and mirror ratios constrain magnetic scale heights in flare loops to λ_B = 10-150 Mm. The fact that this two-component model (with free-streaming and trapped electrons) satisfactorily fits the energy-dependent time delays in virtually all flares provides strong evidence that electron time-of-flight differences and collisional scattering times dominate the observed HXR timing, while the injection of electrons appears to be synchronized (independent of energy) and does not reveal the timing of the acceleration process.

Numerical simulations of rising magnetic flux tubes in the solar convection zone have contributed significantly to our understanding of the basic properties of sunspot groups. They have provided an important clue to the operation of the solar dynamo by predicting strong (super-equipartition) magnetic fields near the bottom of the convection zone. We have investigated to what extent the simulation results (obtained on the basis of the thin flux tube approximation) depend on the assumptions made about the initial state of a magnetic flux tube at the start of the simulation. Two initial conditions used in the literature have been considered in detail: mechanical equilibrium (MEQ) and temperature balance (TBL). It turns out that the requirement of super-equipartition field strength is a robust feature of the simulations, largely independent of the choice of initial conditions: emergence of active regions at low latitudes and the correct dependence of their tilt angle (with respect to the east-west direction) as a function of heliographic latitude require an initial magnetic field strength on the order of 10⁵ G. Other properties of rising flux tubes, such as the asymmetries of shape and field strength between the leading and following wings (with respect to the direction of rotation) of a rising loop, or the anchoring of part of the flux tube in the overshoot region, depend on the initial condition. Observed asymmetries in the magnetic flux distribution and of proper motions in emerging active regions favor MEQ over TBL as the proper initial condition. MEQ should also be preferred for other theoretical reasons: it allows for fewer free parameters, it requires no fine tuning for the tube geometry and background stratification in the overshoot region, and it can be easily made compatible with an encompassing model of the generation, storage, and eruption of the magnetic flux. We have also studied whether an external upflow (convective updraft) can trigger the eruption of an otherwise stably stored flux tube in the overshoot region. We find that a significant deformation and destabilization of a flux tube with equipartition field strength requires coherent upflow velocities of 20-50 m s^-1 in the overshoot layer, which is an order of magnitude larger than current estimates for such velocities.

The Big Bear Solar Observatory (BBSO) has a long tradition of flare observations. In this paper, we would like to direct the reader's attention to observations of a small δ spot that produced a moderate flare activity characterized by 18 C-class and 2 M-class flares. Active region NOAA 8076 (BBSO 3877) was one of the first active regions in the new solar cycle 23. We present for the first time high spatial resolution white-light observations obtained on 1997 August 31 with the speckle masking technique to study mechanisms that trigger flares. Almost diffraction-limited speckle reconstructions revealed the complex and highly dynamical behavior of a small emerging δ configuration in the central part of NOAA 8076. We found strong shear flows and indications of strong transverse fields in the small δ spot. The flare-producing mechanism for this small activity complex was very similar to that of the outstanding flare-producing region NOAA 5395 of 1989 March however, on a completely opposite spatial scale. As an important by-product, the speckle-interferometric techniques provided information about the seeing quality at a site. We used the spectral ratio technique to estimate the Fried parameter r₀. We measured a maximum Fried parameter of r=10.3 cm and an average Fried parameter of r₀ = 9.0 ± 0.7 cm in which the standard deviation reflects the temporal variations of the seeing, indicating good seeing conditions during our observations.

We study the spatial distribution of dark matter halos in the universe in terms of their number density contrast, related to the underlying dark matter fluctuation via a nonlocal and nonlinear bias random field. The description of the matter dynamics is simplified by adopting the "truncated" Zeldovich approximation to obtain both analytical results and simulated maps. The halo number density field in our maps and its probability distribution reproduce with excellent accuracy those of halos in a high-resolution N-body simulation with the same initial conditions. Our nonlinear and nonlocal bias prescription matches the N-body halo distribution better than any Eulerian linear and local bias.

We propose that the gravitational collapse of supermassive objects (M≳10⁴M_☉), either as relativistic star clusters or as single supermassive stars (which may result from stellar mergers in dense star clusters), could be a cosmological source of γ-ray bursts. These events could provide the seeds of the supermassive black holes observed at the center of many galaxies. Collapsing supermassive objects will release a fraction of their huge gravitational binding energy as thermal neutrino pairs. We show that the accompanying neutrino/antineutrino annihilation-induced heating could drive electron/positron "fireball" formation, relativistic expansion, and associated γ-ray emission. The major advantage of this model is its energetics: supermassive object collapses are far more energetic than solar mass-scale compact object mergers; therefore, the conversion of gravitational energy to fireball kinetic energy in the supermassive object scenario need not be highly efficient, nor is it necessary to invoke directional beaming. The major weakness of this model is difficulty in avoiding a baryon-loading problem for one dimensional collapse scenarios.

We present a model for gamma-ray bursts (GRBs) in which a stellar mass black hole acquires a massive accretion disk by merging with the helium core of its red giant companion. The black hole enters the helium core after it, or its neutron star progenitor, first experiences a common envelope phase that carries it inward through the hydrogen envelope. Accretion of the last several solar masses of helium occurs on a timescale of roughly a minute and provides a neutrino luminosity of approximately 10⁵¹-10⁵² ergs s⁻¹. Neutrino annihilation, 0.01%-0.1% efficient, along the rotational axis then gives a baryon-loaded fireball of electron-positron pairs and radiation (about 10⁵⁰ ergs total) whose beaming and relativistic interaction with the circumstellar material makes the GRB (see, e.g., Rees & Mészáros). The useful energy can be greatly increased if energy can be extracted from the rotational energy of the black hole by magnetic interaction with the disk. Such events should occur at a rate comparable to that of merging neutron stars and black hole neutron star pairs and may be responsible for long complex GRBs but not short hard ones.

Gamma-ray bursts (GRBs) and following afterglows are considered to be produced by dissipation of kinetic energy of a relativistic fireball, and the radiation process is widely believed to be synchrotron radiation or inverse Compton scattering of electrons. We argue that the transfer of kinetic energy of ejecta into electrons may be an inefficient process and hence the total energy released by a GRB event is much larger than that emitted in soft gamma rays by a factor of ~(m_p/m_e). We show that, in this case, very strong emission of TeV gamma rays is possible due to synchrotron radiation of protons accelerated up to ~10²¹ eV, which are trapped in the magnetic field of afterglow shocks and radiate their energy on an observational timescale of about a few days. This suggests the possibility that GRBs are most energetic in the TeV range, and such TeV gamma rays may be detectable from GRBs even at cosmological distances, i.e., z~1, by currently working ground-based telescopes. Furthermore, this model naturally gives a quantitative explanation for the famous long-duration GeV photons detected from GRB 940217. If TeV gamma-ray emission that is much more energetic than GRB photons is detected, it provides a strong evidence for acceleration of protons up to ~10²¹ eV.

NGC 1705 is one of the optically brightest and best studied dwarf galaxies. It appears to be in the late stage of a major starburst and contains a young super-star cluster. Type II supernovae are therefore likely to have been a major event in the recent evolution of this galaxy and are likely to have produced a superbubble whose effects on the low-density ambient interstellar medium can be ideally studied. ROSAT PSPC observations of this galaxy reveal two striking blobs of X-ray emission embedded in Hα loops that can be interpreted as both sides of the upper plumes of the same superbubble. These sources are a surprise. They are much softer than those observed from other starburst dwarf galaxies and are so soft that they should have been blocked if the observed Galactic H I column density were uniformly distributed across NGC 1705 or if the sources were embedded in the H I disk of NGC 1705. In addition, the total X-ray luminosity in the ROSAT energy band of 1.2×10³⁸ ergs s^-1 is low in comparison with similar objects. We discuss possible models for the two X-ray peaks in NGC 1705 and find that the sources most likely originate from the relatively cool gas of one single superbubble in NGC 1705. The implications of the exceptional softness of these sources are addressed in terms of the intrinsic properties of NGC 1705 and the nature of the foreground Galactic absorption.

We present results of high-resolution VLBA observations at 1.4 GHz of the nuclear region of the nearby elliptical galaxy NGC 3894. An angular resolution of 9 mas (1.4 h^-1 pc) was obtained, with a velocity resolution of 7 km s^-1 and an rms noise of 1.5 mJy beam^-1 per channel. The 21 cm atomic hydrogen line is seen in absorption slightly redshifted with respect to the systemic velocity toward the core, jet, and counterjet of this source. There are four components in the gas that are spatially and/or spectrally distinct, two of which appear to be part of a single larger structure, possibly a circumnuclear torus.

This Letter investigates the hypothesis that the lensing objects toward the Large Magellanic Cloud are brown dwarfs by analyzing the effects of velocity anisotropy on the inferred microlensing masses. To reduce the masses, the transverse velocity of the lenses with respect to the microlensing tube must be minimized. In the outer halo, radial anisotropy is best for doing this; closer to the solar circle, azimuthal anisotropy is best. By using a constraint on the total kinetic energy of the tracer population from the Jeans equations, the microlensing mass is minimized over orientations of the velocity dispersion tensor. This minimum mass is ≳0.1 M_☉, which lies above the hydrogen-burning limit. This demonstrates explicitly that populations of brown dwarfs with smoothly decreasing densities and dynamically mixed velocity distributions cannot be responsible for the microlensing events. Brown dwarfs are no white knights! There is one caveat. If there are demons sitting on the microlensing tube, they can drop brown dwarfs so as to reproduce the microlensing data set exactly. Such a distribution is not smooth and does not give well-mixed velocities in phase space. It is a permissible solution only if the outer halo is dynamically young and lumpy. In such a case, theorists cannot rule out brown dwarfs. Only exorcists can!

Microlensing is increasingly gaining recognition as a powerful method for the detection and characterization of extrasolar planetary systems. Naively, one might expect that the probability of detecting the influence of more than one planet on any single microlensing light curve would be small. Recently, however, Griest & Safizadeh have shown that, for a subset of events, those with minimum-impact parameter u_min ≲ 0.1 (high-magnification events), the detection probability is nearly 100% for Jovian-mass planets with projected separations in the range 0.6-1.6 of the primary Einstein ring radius R_E and remains substantial outside this zone. In this Letter, we point out that this result implies that, regardless of orientation, all Jovian-mass planets with separations near 0.6-1.6R_E dramatically affect the central region of the magnification pattern and thus have a significant probability of being detected (or ruled out) in high-magnification events. The joint probability, averaged over all inclinations and phases, of two planets having projected separations within 0.6-1.6R_E is substantial: 1%-15% for two planets with the intrinsic separations of Jupiter and Saturn orbiting around 0.3-1.0 M parent stars. We illustrate by example the complicated magnification patterns and light curves that can result when two planets are present, and we discuss the possible implications of our result on detection efficiencies and the ability to discriminate between multiple and single planets in high-magnification events.

The old open cluster NGC 6791 is believed to be more metal-rich than any other, yet several hot blue horizontal-branch (BHB) stars are probable members. We have performed an abundance analysis of the coolest BHB candidate, 2-17, whose proper motion and radial velocity both support cluster membership. Its luminosity and its low rotational velocity, vsini=16 ± 1 km s^-1, suggest that it is a BHB star rather than a very luminous blue straggler. We find an effective temperature T_eff=7300 ± 50 K, gravity logg=3.6 ± 0.3 dex, and an iron abundance more than twice solar, [Fe/H]=+0.4 ± 0.1 dex. This result is free from the serious line blending and continuum suppression that hamper spectroscopic analyses of metal-rich giants. The Ca abundance is in the solar proportion with respect to iron, ruling out A-star peculiarities in 2-17. The relative abundances of C, O, and Al are nearly solar; mixing effects are probably small but not ruled out. The light elements Mg and Si are enhanced, N and Na especially so, as seen in metal-rich galaxies. NGC 6791 thus provides an excellent template for the study of the stellar content of metal-rich galactic systems. Both its abundances and its continued existence suggest that metal-rich systems might generally form in locally deep potential wells. Further study of its hot population should clarify the mechanisms responsible for producing hot stars in a metal-rich environment and thus assist the interpretation of the integrated light and the ultraviolet upturn in elliptical galaxies.

OSSE observed the transient black hole candidate GRO J0422+32 (XN Persei 92) between 1992 August 11 and 1992 September 17. High time resolution data were obtained in several energy bands over the ≃35-600 keV range with a timing resolution of 8 ms. Power spectra at energies below 175 keV show substantial low-frequency red noise with a shoulder at a few times 10⁻² Hz, peaked noise with characteristic frequency near 0.2 Hz, and a second shoulder at a few Hz. The frequencies of the shoulders and the peak are independent of energy and source intensity. The complex cross spectrum indicates that photons in the 75-175 keV band lag photons in the 35-60 keV band by a time roughly proportional to the inverse of the Fourier frequency. The maximum lag observed is ≃300 ms. The power and lag spectra are consistent with the production of the γ-rays through thermal Comptonization in an extended hot corona with a power-law density profile.

We present results from 16 snapshots of Aquila X-1 with the Rossi X-Ray Timing Explorer during the rising phase of its recent outburst. The observations were carried out at a typical rate of once or twice per day. The source shows interesting spectral evolution during this period. Phenomenologically, it bears remarkable similarities to "atoll" sources. Shortly after the onset of the outburst, the source is seen to be in an "island" state, but with little X-ray variability. It then appears to have made a rapid spectral transition (on a timescale of less than half a day) to another island state, where it evolves slightly and stays for 4 days. In this state, the observed X-ray flux becomes increasingly variable as the source brightens. Quasi-periodic oscillation (QPO) in the X-ray intensity is detected in the frequency range 670-870 Hz. The QPO frequency increases with the X-ray flux while its fractional rms decreases. The QPO becomes undetectable following a transition to a "banana" state, where the source continues its evolution by moving up and down the banana branch in the color-color diagram as the flux (presumably the mass accretion rate) fluctuates around the peak of the outburst. Throughout the entire period, the power-density spectrum is dominated by very low frequency noises. Little power can be seen above ~1 Hz, which is different from typical atoll sources. In the banana state, the overall X-ray variability remains low (with a fractional rms ~3%-4%) but roughly constant. The observed X-ray spectrum is soft, with few photons from above ~25 keV, implying the thermal origin of the emission. The evolution of both spectral and temporal X-ray properties is discussed in the context of disk-instability models.

The numerical hydrodynamic modeling of beat Cepheid behavior has been a long-standing quest in which purely radiative models have failed miserably. We find that beat pulsations occur naturally when turbulent convection is accounted for in our hydrodynamics codes. The development of a relaxation code and of a Floquet stability analysis greatly facilitates the search for and analysis of beat Cepheid models. The conditions for the occurrence of beat behavior can be understood easily and at a fundamental level with the help of amplitude equations. Here a discriminant arises whose sign decides whether single-mode or double-mode pulsations can occur in a model, and this depends only on the values of the nonlinear coupling coefficients between the fundamental and the first overtone modes. For radiative models is always found to be negative, but with sufficiently strong turbulent convection its sign reverses.

An unusual mass-loss event observed in the B2e star μ Centauri during the course of 5 days in 1994 April is described and discussed within the framework of contemporary ideas on the Be phenomenon. The onset of the activity occurred in less than 1 day and was characterized by variable emission in He I λ6678 that displayed a distinctive character. Unlike the transient microemission in He I that frequently occurs in μ Cen and other Be stars, the emission-line variations seen in this event took place more slowly in three discrete velocity intervals. On two occasions, violet (v)- and red (r)-shifted emission components declined on a timescale of ≲2 hr, while the emission at/near the line center increased. The short timescale and observed velocity behavior suggest that the site of the activity was near the photosphere. The possible importance of nonradial pulsations and magnetic fields in precipitating the event is discussed. A scenario is suggested to explain the observations in which material originating from an active site on the photosphere is injected into a slab. Layers in the active region become visible in He I λ6678 as the prevailing density builds to values favorable for the production of this emission line. It is estimated that the slab covered ~30% of the star. A 22% increase in the Hα emission strength by the final day of the observations indicates that the activity did indeed add material to the circumstellar disk.

Hubble Space Telescope (HST) images of HK Tauri reveal that the companion star in this 24 (340 AU) pre-main-sequence binary system is an entirely nebulous object at visual wavelengths. HK Tau/c appears as two elongated reflection nebulosities separated by a dark lane. Near-infrared adaptive optics observations made at the Canada-France-Hawaii Telescope show a similar morphology and no directly visible star at λ≤2.2 μm. HK Tau/c is strikingly similar to scattered light models of an optically thick circumstellar disk seen close to edge-on and to the HST images of HH 30. HK Tau/c is therefore the first disk to be clearly resolved around an individual star in a young binary system. The disk properties have been constrained by fitting model reflection nebulae to the HST images. The disk has a radius of 105 AU, an inclination of about 5°, a scale height of 3.8 AU at r=50 AU, and is flared. The absence of a point source in the near-IR requires A_V>50 mag toward the unseen central star. The thickness of the dark lane establishes a disk mass near 10^-4M_☉ (~0.1 M_Jupiter) of dust and gas, if the dust grains have interstellar properties and remain fully mixed vertically. With the observed disk radius equal to one-third of the projected separation of the binary, there is a strong possibility that tidal truncation of the circumsecondary disk has occurred in this system.

Several of the young stellar objects observed with the Hubble Space Telescope in the Orion Nebula near θ¹C Ori show disklike structures with sizes r~100 AU, similar to our own planetary system. These disklike shapes appear as dark silhouettes in [O III], [S II], [N II], Hα, and the continuum but are seen in emission in the [O I] λ6300 line. We propose in this Letter that the [O I] emission is emerging from a H/H₂ photodissociation front that lies close to the disk surface. The H/H₂ front lies inside a photodissociation region between the disk surface and an ionization front that typically has a standoff distance of several disk radii from the disk surface. OH is produced efficiently at the warm H/H₂ front by the endothermic chemical reaction O+H₂→OH+H. However, OH is also efficiently destroyed by photodissociation caused by FUV photons. Approximately 50% of the photodissociated OH produces electronically excited atomic oxygen in the upper level of the 6300 Å transition, which radiatively decays as intense [O I] λ6300 emission. Essentially, broadband FUV photons are absorbed by OH and efficiently converted to λ6300 line photons. The theoretically predicted [O I] λ6300 emission agrees well with that observed in 182-413 (HST -10), the best-studied object with a clearly resolved disk. The H/H₂ photodissociation front is close to the disk surface of 182-413, and the [O I] line, which peaks at the photodissociation front, thus traces the disk surface. The [O I] emission provides additional evidence in a number of proplyds for the existence of an extended PDR between the disk surface and the ionization front, and the penetration of OH-dissociating FUV photons from the ionization front to the disk surface.

We present 1.3 and 3.3 mm polarization maps of Orion-KL obtained with the BIMA array at approximately 4'' resolution. Thermal emission from magnetically aligned dust grains produces the polarization. Along the Orion "ridge" the polarization position angle varies smoothly from about 10° to 40°, in agreement with previous lower resolution maps. In a small region south of the Orion "hot core," however, the position angle changes by 90°. This abrupt change in polarization direction is not necessarily the signpost of a twisted magnetic field. Rather, in this localized region, processes other than the usual Davis-Greenstein mechanism might align the dust grains with their long axes parallel with the field, orthogonal to their normal orientation.

We have detected H I 21 cm line absorption by the warm neutral medium (WNM) using the Westerbork synthesis radio telescope. The absorption was detected toward Cygnus A at LSR velocities of -40 and -70 km s⁻¹. These two velocity ranges were previously identified as being relatively free of cold absorbing clouds. The measured optical depth for the WNM along the line of sight to Cygnus A is (8.9 ± 1.9) × 10⁻⁴ at -70 km s⁻¹ and (8.5 ± 2.0) × 10⁻⁴ at -40 km s⁻¹, with corresponding spin temperatures of 6000 ± 1700 and 4800 ± 1600 K, respectively. The volume filling factor for the WNM appears to be fairly high (f ≈ 0.4).

Bright extreme-UV sunspot plumes have been observed in eight out of 11 different sunspot regions with the Coronal Diagnostic Spectrometer on Solar and Heliospheric Observatory. From wavelength shifts, we derive the line-of-sight velocity relative to the average velocity in the rastered area, 120'' × 120''. In sunspot plumes, we find that the motion is directed away from the observer and increases with increasing line formation temperature, reaches a maximum between 15 and 41 km s^-1 close to log logT ≈ 5.5, then decreases abruptly. The flow field in the corona is not well correlated with the flow in the transition region, and we discuss briefly the implication of this finding.

We present a new, pencil-beam survey of the Kuiper Belt taken with the Keck 10 m telescope. The cumulative surface density of Kuiper Belt Objects measured to apparent red magnitude m_R=26.1 is 31^{+ 12}₋₁₄ deg^-2, while to m_R=26.6 it is 40±33 deg^-2. These numbers are compatible with an extrapolation of the luminosity function deduced earlier from measurements in the 20≤m_R≤25 range, and they confirm a Kuiper Belt differential size distribution index q~4.