Table of contents

We analyze the anisotropy signature expected if the high-energy (>10¹⁹ eV) cosmic-ray (CR) sources are extragalactic and trace the distribution of luminous matter on large scales. We investigate the dependence of the anisotropy on both the relative bias between the CR sources and the galaxy distribution and on the (unknown) intrinsic CR source density. We find that the expected anisotropy associated with the large-scale structure (LSS) should be detected once the number of CR events observed above 10¹⁹ eV is increased by a factor of ~10. This would require ~30 observation-years with existing experiments, but less than 1 year with the proposed ~5000 km² Auger detectors. We find that the recently reported concentration of the Haverah Park CR events toward the supergalactic plane is not consistent with the known LSS. If real, the Haverah Park result suggests that the CR sources are much more concentrated toward the supergalactic plane than the known LSS. Our results are not sensitive to the number density of CR sources. We show that once the number of detected events is increased by a factor of ~10, the number density would be strongly constrained by considering the probability of having repeating sources.

This study analyzes the sizes, shapes, and correlations of Lyα clouds produced by a hydrodynamic simulation of a spatially flat CDM universe with a nonzero cosmological constant (Ω₀ = 0.4, Λ₀ = 0.6, σ₈ = 0.79) over the redshift range 2 ≤ z ≤ 4. The Lyα clouds range in size from several kiloparsecs to about a hundred kiloparsecs in proper units, and they range in shape from roundish, high column density regions with N_{H I} ≥ 10¹⁵ cm^-2 to low column density sheet-like structures with N_{H I} ≤ 10¹³ cm^-2 at z = 3. The most common shape found in the simulation resembles that of a flattened cigar. The physical size of a typical cloud grows with time roughly as (1 + z)^-3/2, while its shape hardly evolves (except for the most dense regions with ρ_cut > 30). Collectively, the clouds form large networks of filaments and sheets. Our result indicates that any simple model with a population of spheres (or other shapes) of a uniform size is oversimplified; if such a model agrees with observational evidence, it is probably only by coincidence. We also illustrate why the use of pairs of quasar sight lines to set lower limits on cloud sizes is useful only when the perpendicular sight line separation is small (Δr ≤ 50 h^-1 kpc). Finally, we conjecture that high column density Lyα clouds (N_{H I} ≥ 10¹⁵ cm^-2) may be the progenitors of the lower redshift faint blue galaxies, based on consideration of their correlation, number density, and mass.

The high ionization level and nonzero metallicity (~1% Z_☉) of the intergalactic gas at redshifts z ≲ 5 implies that nonlinear structure had started to form in the universe at earlier times than we currently probe. In cold dark matter (CDM) cosmologies, the first generation of baryonic objects emerges at redshifts z ~ 10-50. Here we examine the observable consequences of the possibility that an early generation of stars reionized the universe and resulted in the observed metallicity of the Lyα forest. Forthcoming microwave anisotropy experiments will be sensitive to the damping of anisotropies caused by scattering off free electrons from the reionization epoch. For a large range of CDM models with a Scalo stellar mass function, we find that reionization occurs at a redshift z ≳ 10 and damps the amplitude of anisotropies on angular scales ≲10° by a detectable amount, ~10%-25%. However, reionization is substantially delayed if the initial stellar mass function transformed most of the baryons into low-mass stars. In this case, the mass fraction of pregalactic stars could be constrained from the statistics of microlensing events in galactic halos or along lines of sight to quasars. Deep infrared imaging with future space telescopes (such as the Space Infrared Telescope Facility or the Next Generation Space Telescope) will be able to detect bright star clusters at z ≳ 5. The cumulative bremsstrahlung emission from these star clusters yields a measurable distortion to the spectrum of the microwave background.

We have used the Very Large Array (VLA) of the National Radio Astronomy Observatory at 8.44 GHz to image a ~40 arcmin² field with an uprecedented rms sensitivity of 1.5 μJy. After correcting for the effects of discrete foreground radio sources, we examined this most sensitive microwave image of the sky for fluctuations in the cosmic microwave background radiation (CBR). At the 6'' resolution of our VLA map, ΔT/T ≈ (0.7 ± 0.8) × 10^-4, with an upper limit of 1.3 × 10^-4 at 95% confidence. At 1' resolution, we measure a fluctuation amplitude of ΔT/T = (1.2 ± 1.4) × 10^-5. We also report on our observations of the linear and circular polarization of the CBR for which we derive upper limits (at 95% confidence) of ΔT/T ≤ 1.1 × 10^-5 and ΔT/T ≤ 1.6 × 10^-5, respectively, on an angular scale of 1'.

Even the most sensitive cosmic microwave background anisotropy experiments have signal-to-noise ratios ≲5, so that an accurate determination of the properties of the cosmological signal requires a careful assessment of the experimental noise. Most of the experiments combine simultaneous multichannel observations, in which the presence of correlated noise is likely. This case is common for ground-based experiments, in which an important fraction of the noise could be atmospheric in origin. Here, the way to compute and determine the effects produced by this correlated noise is discussed; in particular, the paper considers the Tenerife experiments (three radiometers at 10, 15, and 33 GHz with two independent receivers each), showing how this effect has been taken into account properly in the more recent analysis of these data. It will be demonstrated that for each of the three radiometers of these experiments, the atmospheric noise is equivalent to a Gaussian noise common to both channels with a coherence time smaller than the binning time, the net effect being an enhancement of the error bars in the stacked scan as compared with the estimation for the case of pure uncorrelated noise. As expected from the spectral index of the atmosphere, the effect is more important at higher frequencies. The formalism is generalized and applied to the general case of simultaneous multichannel observations.

The beaming pattern of thermal annihilation radiation is broader than the beaming pattern produced by isotropic nonthermal electrons and positrons in the jets of radio-emitting active galactic nuclei that Compton scatter photons from an external isotropic radiation field. Thus, blueshifted thermal annihilation radiation can provide the dominant contribution to the high-energy radiation spectrum at observing angles θ ≳ 1/Γ, where Γ is the bulk Lorentz factor of the outflowing plasma. This effect may account for the spectral features of MeV blazars discovered with the Compton Telescope on the ComptonGammaRayObservatory. Coordinated gamma-ray observations of annihilation line radiation to infer Doppler factors and VLBI radio observations to measure transverse angular speeds of outflowing plasma blobs can be used to determine the Hubble constant.

We present a statistical comparison of three different estimates of cluster mass, namely, the dynamical masses obtained from the velocity dispersion of optical galaxies, the X-ray masses measured from the temperature of X-ray-emitting gas under the assumption of isothermal hydrostatic equilibrium, and the gravitational lensing masses derived from the strong/weak distortions of background galaxy images. Using a sample of 29 lensing clusters available in literature, we show that the dynamical masses are in agreement with the gravitational lensing masses while the X-ray method has systematically underestimated cluster masses by a factor of 2-3 as compared with the others. These results imply that galaxies indeed trace the gravitational potential of their clusters and that there is no bias between the velocities of the dark matter particles and the galaxies in clusters. The X-ray cluster mass discrepancy probably arises from the simplification in the models for the X-ray gas distribution and dynamical evolution.

We use the ROSAT All Sky Survey to compute X-ray luminosities for an optically selected sample of 32 galaxy groups. The groups were identified by an objective algorithm as three-dimensional density enhancements in the magnitude-limited Center for Astrophysics Redshift Survey. The sample of groups includes five poor Abell clusters.

We use the optical galaxy positions to limit the sky area in which we search for the faint, extended X-ray emission from the groups. Nine of the groups have significant diffuse X-ray emission, with 0.5-2.0 keV luminosities in the range 0.12-1.6 h^-2 × 10⁴³ ergs s^-1, where we use h = H₀/(50 km s^-1 Mpc^-1). Among these nine groups, only three are Abell clusters. For the remaining 23 systems we compute 3 σ upper limits.

The relationship between the X-ray luminosities and the radial velocity dispersions differs significantly from that for rich clusters. We find L ∝ σ^{1.56 ± 0.25}, whereas L ~σ⁴. This shallow slope is consistent with models in which intragroup gas concentrated around individual galaxies makes a substantial contribution to the extended X-ray emission. Previous work suggests that in this case L_X ∝ σ^1.6.

The X-ray/optical luminosity relationship for these systems is L_X ∝ L, physically consistent with the shallow L_X - σ relation. There is some correlation (Kendall's τ = 0.4, P = 0.1) between the fraction of member ellipticals and L_X.

We present new calculations of the contributions to the intergalactic radiation field from light neutrinos of the type proposed by Sciama, with rest energies near 30 eV and radiative decay lifetimes of τ = (2 ± 1) × 10²³ s. Experimental limits on the intensity of the diffuse ultraviolet background, while still controversial, can put useful lower bounds on the decay lifetime of these particles. These are centered around (2-3) × 10²³ s but range as high as (8 ± 3) × 10²³ s. The model is thus in conflict with some observations but remains marginally compatible with most of them.

A model is developed to explain the cosmological evolution of dwarf galaxies. The population of small galaxies is found to evolve rapidly for z < 1, which provides a natural explanation for the evolution observed in the galaxy luminosity function. A tail is found in the redshift distribution of the faint blue excess that can extend to a redshift of 2. The star formation history is followed in detail for these objects. Constraints on the metallicity are identified for which stars are formed with much higher efficiency in a multiphase interstellar medium than in massive galaxies. Blue dwarf galaxies at the current epoch are identified with this starburst mode.

The collapse of 1 and 2 σ perturbations of the initial density fluctuation spectrum is followed using the extended standard hierarchical clustering formalism. The collapse of these perturbations is normally associated with the formation of dwarf galaxies. These objects have shallow gravitational potential wells, and their evolution strongly depends upon the cooling time of the gas. The latter is determined by the ionization and chemical equilibrium of the gas in the presence of the intergalactic and local stellar radiation fields. The latter generally dominates and creates a feedback mechanism that regulates the evolutionary timescale. To improve upon previous models, essential new astrophysical ingredients are incorporated, such as a more detailed description of the physical processes regulating the multiphase structure of the interstellar medium in dwarf galaxies and the effects of evolution in the galaxy's metallicity on the formation of stars in molecular clouds.

It is found that for a low star formation rate of 0.1 M_☉ yr^-1, the cooling time of interstellar gas is longer than the local Hubble time until z ~ 1. At this epoch, a two-phase medium makes the dwarf interstellar medium less fragile against supernova explosions, and the volume filling factor of the hot phase (10⁷ K) becomes of order unity. The resulting X-ray luminosity is consistent with observations of nearby dwarf galaxies if exchange processes between the hot and cold phases of the interstellar medium (i.e., evaporation and ablation of clouds due the supernova blast wave) are included. The tenuous halo gas has a temperature in excess of the escape velocity and exhibits very high ionization stages.

The dark matter halo potential (M/L ~ 10²) influences the loss rate of metals and the star formation history. It is found that if the metallicity is between 0.01 and 0.1 Z_☉ then a period of rapid star formation, ~1-3 M_☉ yr^-1, can be excited for z ~ 1. The physical reason for this starburst mode in our model is that the free-fall time exceeds the ambipolar diffusion timescale for clouds. Consequently, molecular clouds are not magnetically supported and star formation can proceed much more efficiently compared with more massive and metal-rich galaxies.

Our results are consistent with the redshift distribution of dwarf galaxies observed in the Hubble Deep Field. In particular, a rapidly evolving dwarf population can account for the excess number counts of faint galaxies: the blue excess. Predictions are made for the color evolution of the old stellar population in these systems, observable with the Near-Infrared Camera and Multiobject Spectrometer (NICMOS) on the HubbleSpaceTelescope in the near future.

During some gravitational lensing events, the lens transits the disk of the star. This event causes a shift in the apparent radial velocity of the star proportional to the star's rotation speed. The magnification of such an event is different from that expected for a point source. By measuring both effects, one can determine the rotation parameter v sin i. The method is especially useful for K-giant stars because these stars have turbulent velocities that are typically large compared with their rotation speed. By making a series of radial velocity measurements, one can typically determine v sin i to the same accuracy as the individual radial velocity measurements. There are approximately 10 microlensing transit events per year that would be suitable for these measurements.

The Milky Way is often considered to be the best example of a spiral for which the dark matter not only dominates the outer kinematics but also plays a major dynamical role in the inner galaxy: the Galactic disk is therefore said to be "submaximal." This conclusion is important to the understanding of the evolution of galaxies and the viability of particular dark matter models. The Galactic evidence rests on a number of structural and kinematic measurements, many of which have recently been revised. The new constraints not only indicate that the Galaxy is a more typical member of its class (Sb to Sc spirals) than previously thought but also require a reexamination of the question of whether the Milky Way disk is maximal. By applying to the Milky Way the same definition of "maximal disk" that is applied to external galaxies, it is shown that the new observational constraints are consistent with a Galactic maximal disk of reasonable M/L. In particular, the local disk column can be substantially less than the oft-quoted required Σ_☉ ≈ 100 M_☉ pc^-2—as low as 40 M_☉ pc^-2 in the extreme case—and still be maximal, in the sense that the dark halo provides negligible rotation support in the inner Galaxy. This result has possible implications for any conclusion that rests on assumptions about the potential of the Galactic disk or dark halo and, in particular, for the interpretation of microlensing results along both LMC and bulge lines of sight.

We study the effects of thermal conduction in a hot, active corona above an accretion disk. We assume that all of the dissipative heating takes place in the corona. We find that the importance of conduction decreases with increases in the local dissipative compactness of the corona, ℓ_{diss, loc}, and increases with increasing abundance of electron-positron pairs. For ℓ_{diss, loc} < 1, a significant fraction of the energy released in the corona may be carried away by the conductive flux, leading to formation of a relatively hot transition layer below the base of the corona. Comptonization of disk radiation in such a layer may account for the presence of soft X-ray excesses in the spectra emitted by disk-corona systems.

The results of a spatial stability analysis of a two-dimensional slab jet, in which optically thin radiative cooling is dynamically important, are presented. We study both magnetized and unmagnetized jets at external Mach numbers of 5 and 20. We model the cooling rate by using two different cooling curves: one appropriate to interstellar gas, and the other to photoionized gas of reduced metallicity. Thus, our results will be applicable to both protostellar (Herbig-Haro) jets and optical jets from active galactic nuclei.

We present analytical solutions to the dispersion relations in useful limits and solve the dispersion relations numerically over a broad range of perturbation frequencies. We find that the growth rates and wavelengths of the unstable Kelvin-Helmholtz (K-H) modes are significantly different from the adiabatic limit, and that the form of the cooling function strongly affects the results. In particular, if the cooling curve is a steep function of temperature in the neighborhood of the equilibrium state, then the growth of K-H modes is reduced relative to the adiabatic jet. On the other hand, if the cooling curve is a shallow function of temperature, then the growth of K-H modes can be enhanced relative to the adiabatic jet by the increase in cooling relative to heating in overdense regions. Inclusion of a dynamically important magnetic field does not strongly modify the important differences between an adiabatic jet and a cooling jet, provided the jet is highly supermagnetosonic and not magnetic pressure-dominated. In the latter case, the unstable modes behave more like the transmagnetosonic magnetic pressure-dominated adiabatic limit. We also plot fluid displacement surfaces associated with the various waves in a cooling jet in order to predict the structures that might arise in the nonlinear regime. This analysis predicts that low-frequency surface waves and the lowest order body modes will be the most effective at producing observable features in the jet.

We use two-dimensional time-dependent hydrodynamical simulations to follow the growth of the Kelvin-Helmholtz (K-H) instability in cooling jets into the nonlinear regime. We focus primarily on asymmetric modes that give rise to transverse displacements of the jet beam. A variety of Mach numbers and two different cooling curves are studied. The growth rates of waves in the linear regime measured from the numerical simulations are in excellent agreement with the predictions of the linear stability analysis presented in the first paper in this series. In the nonlinear regime, the simulations show that asymmetric modes of the K-H instability can affect the structure and evolution of cooling jets in a number of ways. We find that jets in which the growth rate of the sinusoidal surface wave has a maximum at a so-called resonant frequency can be dominated by large-amplitude sinusoidal oscillations near this frequency. Eventually, growth of this wave can disrupt the jet. On the other hand, nonlinear body waves tend to produce low-amplitude wiggles in the shape of the jet but can result in strong shocks in the jet beam. In cooling jets, these shocks can produce dense knots and filaments of cooling gas within the jet. Ripples in the surface of the jet beam caused by both surface and body waves generate oblique shock "spurs" driven into the ambient gas. Our simulations show these shock "spurs" can accelerate ambient gas at large distances from the jet beam to low velocities, which represents a new mechanism by which low-velocity bipolar outflows may be driven by high-velocity jets. Rapid entrainment and acceleration of ambient gas may also occur if the jet is disrupted.

For parameters typical of protostellar jets, the frequency at which K-H growth is a maximum (or highest frequency to which the entire jet can respond dynamically) will be associated with perturbations with a period of ~200 yr. Higher frequency (shorter period) perturbations excite waves associated with body modes that produce internal shocks and only small-amplitude wiggles within the jet. The fact that most observed systems show no evidence for large-amplitude sinusoidal oscillation leading to disruption is indicative that the perturbation frequencies are generally large, consistent with the suggestion that protostellar jets arise from the inner regions (r < 1 AU) of accretion disks.

Near-Infrared spectroscopy combined with high spatial resolution imaging have been used in this work to probe the central 500 pc of M82. Imaging observations in the 2.36 μm CO band head are added to our previously published near-infrared hydrogen recombination line imaging, near-infrared broadband imaging, and 3.29 μm dust feature imaging observations, in order to study the nature of the starburst stellar population. A starburst model is constructed and compared with the observations of the stellarclusters in the starburst complex. Our analysis implies that the typical age for the starburst clusters is 10⁷ yr. In addition, our high spatial resolution observations indicate that there is an age dispersion within the starburst complex that is correlated with projected distance from the center of the galaxy. The inferred age dispersion is 6 × 10⁶ yr. If the starburst in M82 is propagating outward from the center, this age dispersion corresponds to a velocity of propagation, originating in the center, of ~50 km s^-1. Our quantitative analysis also reveals that a Salpeter initial mass function, extending from 0.1 to 100 M_☉, can fit the observed properties of M82 without using up more than 30% of the total dynamical mass in the starburst.

We present the results of the multiwavelength campaigns on 3C 273 in 1993-1995. During the observations in late 1993, this quasar showed an increase of its flux for energies ≥100 MeV from about 2.1 × 10^-7 photons cm^-2 s^-1 to approximately 5.6 × 10^-7 photons cm^-2 s^-1 during a radio outburst at 14.5, 22, and 37 GHz. However, no one-to-one correlation of the γ-ray radiation with any frequency could be found. The photon spectral index of the high-energy spectrum changed from Γ_γ = (3.20 ± 0.54) to Γ_γ = (2.20 ± 0.22) in the sense that the spectrum flattened when the γ-ray flux increased. Fits of the three most prominent models (synchrotron self-Comptonization, external inverse Comptonization, and the proton-initiated cascade model) for the explanation of the high γ-ray emission of active galactic nuclei were performed to the multiwavelength spectrum of 3C 273. All three models are able to represent the basic features of the multiwavelength spectrum. Although there are some differences, the data are still not decisive enough to discriminate between the models.

We present new HST optical continuum and emission line WFPC2 images and MERLIN radio observations of 3C 264 at ~01 resolution. The jet is well resolved in both the optical and radio images. In addition, we report the discovery of an apparent optical "ring" at a projected radius of ~300-400 pc. The ring is most likely the manifestation of absorption by a nearly face-on circumnuclear dust disk. We discuss the evolution of the jet properties with distance. The jet collimation, brightness, and orientation change dramatically as it crosses the outer boundary of the "ring" suggesting an interaction between the jet and dense circumnuclear gas.

We present a model for the jet propagation in which an initially relativistic jet decelerates as it crosses through a region of dense cold gas in the inner region of the galaxy. We derive the equations for brightness variations along an adiabatically expanding relativistic jet, and we model the jet brightness in 3C 264 as the combined effects of Doppler boosting, and adiabatic losses as traced through the jet velocity and width. We find that the data are consistent with a model in which the jet is initially highly relativistic (v ~ 0.98c, γ = 5) and we view it at roughly 50° inclination. We suggest that 3C 264 may serve as a laboratory for the study of relativistic entraining jets and may help us to understand the deceleration of jets, which is required in unifying schemes for FRI radio galaxies and BL Lac objects.

We model the polarization properties of line emission from an accretion disk under a range of assumptions about the source function and electron-scattering optical depth τ_es. For small values of τ_es and modest viewing angles, polarization can be in excess of the Chandrasekhar result for τ_es = ∞. The polarization vector can be either parallel or perpendicular to the projected direction of the disk axis. The polarization properties of the double-peaked Hα emission line of the broad-line radio galaxy Arp 102B observed by Antonucci, Hurt, & Agol can be understood in terms of electron scattering and line broadening within the line-emitting region if τ_es is of order unity, and if the position angle of polarization is parallel to the projected disk axis. The required small τ_es is consistent with the hypothesis that the Balmer lines in Arp 102B are produced by photoionization of the disk atmosphere.

We have mapped the [C II] 158 μm line over 85 × 65 in the Magellanic irregular galaxy IC 10, thus presenting the first complete [C II] map of an entire low-metallicity galaxy. The total luminosity in the [C II] line in IC 10 is 1.5 × 10⁶L_☉. We discuss the origin of the [C II] emission toward different regions in the galaxy. Overall, about 10% of the [C II] emission can originate in standard H I clouds (n ~ 80, T ~ 100 K), while up to about 10% of the emission can originate in ionized gas, either the low-density warm gas or the denser H II regions. For the two brightest regions, most of the [C II] emission is associated with dense photodissociation regions (PDRs). For several regions, however, the [C II] emission may not be explained by standard PDR models. For these regions, emission solely from the atomic medium can also be precluded because the cooling rate per hydrogen atom would be much greater than the heating rate provided by photoelectric UV heating. We speculate that in these regions the presence of an additional column density of H₂, 5 times that observed in H I, is required to explain the [C II] emission. The ambient UV fields present in these regions, combined with the low metallicity, create a situation where small CO cores exist surrounded by a relatively large [C II]-emitting envelope where molecular hydrogen is self-shielded. This additional molecular mass is equivalent to at least 100 times the mass in the CO core that one would derive from the CO integrated intensity alone using the standard CO-to-H₂ conversion factor. These [C II] observations may, therefore, make a more reliable inventory of the gas reservoir in dwarf irregular galaxies where use of CO alone may significantly underestimate the molecular mass.

We have observed the ¹²CO J = 2-1,¹³CO J = 2-1, and ¹²CO J = 3-2 lines in a sample of seven giant molecular clouds in the Local Group spiral galaxy M33 using the James Clerk Maxwell Telescope. The ¹²CO/¹³CO J = 2-1 line ratio is constant across the entire sample, while the observed ¹²CO J = 3-2/J = 2-1 line ratio has a weak dependence on the star formation environment of the cloud, with large changes in the line ratio seen only for clouds in the immediate vicinity of an extremely luminous H II region. A large velocity gradient analysis indicates that clouds without H II regions have temperatures of 10-20 K, clouds with H II regions have temperatures of 15-100 K, and the cloud in the giant H II region has a temperature of at least 100 K. Interestingly, the giant H II region appears capable of raising the kinetic temperature of the molecular gas only for clouds that are quite nearby (<100 pc). The continuous change of physical conditions across the observed range of star formation environments suggests that the unusual physical conditions in the cloud in the giant H II region are due to post-star formation changes in the molecular gas, rather than intrinsic properties of the gas related to the formation of the giant H II region. The results from this study of M33 suggest that similar observations of ensembles of giant molecular clouds in more distant normal spiral galaxies are likely to give meaningful measurements of the average physical conditions inside the molecular clouds. These results also imply that clouds with a factor of 3 difference in metallicity have similar density and temperature, which in turn implies that the differences in the CO-to-H₂ conversion factor seen in these clouds can be attributed to metallicity effects entirely.

We examine the consequences and implications of the possibilities that the best-fit m = 4/3 line of the silicon isotopic ratios measured in mainstream SiC grains is identical or parallel to the mean ISM evolution line of the silicon isotopes. Even though the mean ISM evolution proceeds along a line of unity slope when deviations are expressed in terms of the native representation (the mean ISM), the evolution line can become a slope 4/3 line in the solar representation, provided that the solar composition is displaced from the mean ISM evolution. During the course of this analysis, we introduce new methods for relating the solar composition to that of the mean ISM at the time of solar birth. These new developments offer a unique view on the meaning of the mainstream SiC particles and afford a new way of quantitatively answering the question whether the Sun has a special composition relative to the mean ISM at solar birth. If the correlation slope of the silicon isotopes in the mean ISM could be decisively established, then its value would quantify the difference between the solar and mean ISM silicon abundances. Our formalism details the transformations between the two representations and applies not only to ²⁹Si and ³⁰Si, but also to any two purely secondary isotopes of any element (O, Ne, Mg, and perhaps S). Both the advantages and disadvantages of this technique are critically reviewed.

Oxygen to iron abundance ratios of metal-poor stars provide information on nucleosynthesis yields from massive stars that end in Type II supernova (SN II) explosions. Using a standard model of chemical evolution of the Galaxy we have reproduced the solar neighborhood abundance data and estimated the oxygen and iron yields of genuine SN II origin. The estimated yields are compared with the theoretical yields to derive the relation between the lower and upper mass limits in each generation of stars and the initial mass function (IMF) slope. Independent of this relation, we furthermore derive the relation between the lower mass limit and the IMF slope from the stellar mass-to-light ratio in the solar neighborhood. These independent relations unambiguously determine the upper mass limit of m_u = 50 ± 10 M_☉ and the IMF slope index of 1.3-1.6 above 1 M_☉. This upper mass limit corresponds to the mass beyond which stars end as black holes without ejecting processed matter into the interstellar medium. We also find that the IMF slope index below 0.5 M_☉ cannot be much shallower than 0.8.

We have conducted a survey of HCN and HNC (two rotational transitions each) in our standard sample of 11 cirrus cores and 27 Clemens-Barvainis translucent cores whose structures and chemistry have been studied earlier in this series. Both species are seen in all 38 objects. HCNH⁺ has been searched in three objects. These results are modeled in terms of our previous hydrostatic equilibrium and n ~ r^-α structures together with other chemical and physical properties derived earlier. A detailed program has been written to handle the complex radiative transfer of the hyperfine splitting (hfs) of HCN. It is shown that serious errors are made in deriving HCN abundances by methods that ignore the hfs. Both HCN and HNC abundances are high, typically 1(-8) in most sources. The chemically important ratio HCN/HNC is found to be ~2.5 if these species are spatially centrally peaked and ~6 if not. Both species abundances increase monotonically with increasing extinction in the 1.2-2.7 mag range (edge to center), thus displaying the same characteristic transition between diffuse and dense cloud chemistry as do most other species we have studied. HCN/HNC decreases with increasing extinction to a value of 1.3 at A_v0 ~ 10, approaching the expected value of 1.0 for dense clouds. Two types of ion-molecule chemistry models have been carried out: a full model using the Standard Model rate file and comprising 409 species (by Lee and Herbst), and a simplified model comprising 21 nitrogen-bearing species for conditions relevant to translucent clouds. Good agreement between observations and chemistry models is achieved throughout the translucent extinction range. Important conclusions are that (1) neutral-neutral reactions such as N + CH₂ dominate the chemistry of HCN; (2) low ion-polar reaction rates are strongly favored over high ones; (3) the reaction C⁺ + NH₃ → H₂NC⁺ → HNC is unimportant, thus largely uncoupling the CN and NH chemistries; (4) the ratio HCN/HNC is not a particularly important diagnostic of the CN chemistry; (5) model NH₃ abundances are at least a factor 100 lower than observed in translucent clouds, even if the reaction N+H⁺₃→NH⁺₂ is permitted at Langevin rate.

We consider the stability of clouds surrounded by a hotter confining medium with respect to which they are in motion, against Kelvin-Helmholtz instabilities (KHIs). In the presence of cooling, sound waves are damped by dissipation. Whenever cooling times are shorter than sound crossing times, as they are in the normal interstellar medium, this implies that the instability generated at the interface of the two media cannot propagate far from the interface itself. To study how this influences the overall stability, first we derive an analytic dispersion relation for cooling media, separated by a shear layer. The inclusion of dissipation does not heal the instability, but it is shown that only a small volume around the interface is affected, the perturbation decaying exponentially with distance from the surface; this is confirmed by numerical simulations. Numerical simulations of spherical clouds moving in a surrounding intercloud medium by which they are pressure confined show that these clouds develop a core/halo structure, with a turbulent halo, and a core in laminar flow nearly unscathed by the KHIs. The related and previously reported "champagne effect," whereby clouds seem to explode from their top sides, is cured by the inclusion of radiative losses.

Large-scale ordered structures in the interstellar medium (ISM) are commonly explained as a manifestation of the various instabilities inherent to the motion of partially ionized, magnetized interstellar gas. We examined the process of creation of large-scale coherent eddy structures in the ISM characterized by the existence of fully developed small-scale turbulence with nonzero values of the hydrodynamic and magnetic helicities. The instability analysis shows that in such media large-scale ordered structures with characteristic scale l ~ 1-100 pc are generated. The generation coefficient of large-scale structures is determined by both the value of the hydrodynamic helicity and the combined influence of the drag force and magnetic helicity.

Magnetic fields in synchrotron sources are almost certainly inhomogeneous, mixing high-field and low-field regions. This inhomogeneity affects the evolution of a relativistic electron distribution function due to the rate of energy loss of the electrons changing as they move between the two regions. We present two models for the evolution of the distribution function, and discuss the results of these models in terms of the critical energies, or synchrotron frequencies, where the particle and photon spectra steepen. We find these critical frequencies are higher than would be the case if the electrons were confined to a homogeneous high-field region. We apply our results to the interpretation of extragalactic radio sources whose dynamical ages are known to be significantly greater than the ages inferred from their high-frequency spectral breaks.

The measures of mechanical alignment are obtained for both prolate and oblate grains whose temperatures are comparable to the grain kinetic energy divided by k, the Boltzmann constant. For such grains, the alignment of angular momentum, J, with the axis of maximal inertia, a, is only partial, which substantially alters the mechanical alignment as compared with the results obtained by Lazarian and Roberge, Hanany, & Messinger under the assumption of perfect alignment. We also describe Gold alignment when the Barnett dissipation is suppressed and derive an analytical expression that relates the measure of alignment to the parameters of grain nonsphericity and the direction of the gas-grain drift. This solution provides the lower limit for the measure of alignment, while the upper limit is given by the method derived by Lazarian. Using the results of a recent study of incomplete internal relaxation by Lazarian & Roberge, we find measures of alignment for the whole range of ratios of grain rotational energy to kT_s, where T_s is the grain temperature. To describe alignment for mildly supersonic drifts, we suggest an analytical approach that provides good correspondence with the results of direct numerical simulations by Roberge, Hanany, & Messinger. We also extend our approach to account for simultaneous action of the Gold and Davis-Greenstein mechanisms.

The fragmentation mechanism has been quite successful at providing an explanation for the formation of binary stars during the collapse phase of dense cloud cores. However, nearly all fragmentation calculations to date have ignored the effects of magnetic fields, whereas magnetic fields are generally regarded as the dominant force in molecular clouds. Here, we present the first three-dimensional, radiative hydrodynamical models of the collapse and fragmentation of dense molecular cloud cores, including the effects of magnetic fields and ambipolar diffusion. Starting from a prolate, Gaussian cloud that would collapse and fragment in the absence of magnetic fields (a thermally supercritical cloud), we introduce sufficient magnetic field support [through the magnetic field pressure, B²/8π, with B = B₀(ρ/ρ₀)^κ] to ensure a magnetically subcritical (stable) cloud. The effects of ambipolar diffusion are then simulated by reducing the magnetic pressure scaling factor (B₀) over a specified time interval (=t_AD), which leads to a magnetically supercritical cloud and collapse. The estimated timescale for ambipolar diffusion in these dense clouds is about 10 free-fall times. The numerical models show that when t_AD is as long as 10 or even 20 free-fall times, fragmentation into a binary can still occur. The main effect of the magnetic field support is to delay somewhat the formation of the binary protostar. Once the dynamic collapse phase begins, a rapidly rotating, (β_i = E_rot/| E_grav | = 0.12) cloud fragments into a binary protostar. While it remains to be seen if magnetic fields can stifle fragmentation in slowly rotating clouds, rapidly rotating, magnetically supported clouds appear to be quite capable of forming binary stars.

We report measurements of the abundance of ³He for a sample of six Galactic planetary nebulae: IC 289, NGC 3242, NGC 6543, NGC 6720, NGC 7009, and NGC 7662. Based on observations of the 8.665 GHz hyperfine transition of ³He⁺, we derive ³He/H abundances ranging from ~0.1 to 1.0 × 10^-3 by number. These abundances are more than an order of magnitude larger than those found in any H II region, the local interstellar medium, or the proto-solar system. If planetary nebulae are surrounded by large, low-density, ionized halos, modeling suggests that these abundances will decrease by a factor of about 2. Our source sample is highly biased in that we selected objects on the basis of several criteria that maximized the likelihood of ³He detections. The abundances are nonetheless consistent with the idea that ³He is produced in significant quantities by stars of 1-2 M_☉. We conclude that there is some stellar production of ³He.

We present high-resolution radio continuum, H I absorption, and H90α recombination line observations of the CTB 33 complex and nearby sources (l = 337°, b = 0°) taken with the Australia Telescope Compact Array. We estimate distances to several discrete sources on the basis of H I absorption spectra. Radio recombination lines are detected for two sources in the field. The compact source G337.0-0.1 in the CTB 33 complex, whose nature was previously in doubt, is identified as a supernova remnant with a diameter of ~5 pc. This classification is based on several contributing features: a nonthermal spectral index (-0.6), a shell structure, and the absence of detectable recombination lines. In addition, a 1720 MHz OH maser is probably associated with G337.0-0.1. We conclude that the CTB 33 region is a supernova remnant-H II complex at a distance of ~11 kpc.

In this article, I present a method for the nonparametric (model-free) estimation of intensity profiles underlying gamma-ray bursts. The algorithm, called TIPSH, is based on applying specially calibrated thresholds to the Haar wavelet coefficients of binned counts gathered from such bursts. Because wavelets are well-localized with respect to both time and scale, they are an ideal tool for working with the often sharp, abrupt nature of gamma-ray burst signals. When applied to an idealized signal in a small simulation study and a selection of actual gamma-ray bursts, the TIPSH algorithm was found to be well capable of simultaneously estimating the smooth, uniform background and the pulse-like structure of gamma-ray burst signals.

We have developed a method based on wavelet transforms (WTs) to detect sources in astronomical images obtained with photon-counting detectors, such as X-ray images. The WT is a multiscale transform that is suitable for detection and analysis of interesting image features (sources) spanning a range of sizes. This property of the WT is particularly well suited to the case in which the point-spread function is strongly varying across the image, and it is also effective in the detection of extended sources. The method allows one to measure source count rates, sizes, and ellipticity, with their errors. Care has been taken in the assessment of thresholds for detection, in the WT space, at any desired confidence level, through a detailed semianalytical study of the statistical properties of noise in wavelet-transformed images. The method includes the use of exposure maps to handle sharp background gradients produced by a nonuniform exposure across the detector, which would otherwise yield many spurious detections. The same method is applied to evaluate upper limits to the count rate of undetected objects in the field of view, allowing a sensitivity map for each observation to be constructed.

We apply to the specific case of images taken with the ROSAT PSPC detector our wavelet-based X-ray source detection algorithm presented in a companion paper. Such images are characterized by the presence of detector "ribs," strongly varying point-spread function, and vignetting, so that their analysis provides a challenge for any detection algorithm. First, we apply the algorithm to simulated images of a flat background, as seen with the PSPC, in order to calibrate the number of spurious detections as a function of significance threshold and to ascertain that the spatial distribution of spurious detections is uniform, i.e., unaffected by the ribs; this goal was achieved using the exposure map in the detection procedure. Then, we analyze simulations of PSPC images with a realistic number of point sources; the results are used to determine the efficiency of source detection and the accuracy of output quantities such as source count rate, size, and position, upon a comparison with input source data. It turns out that sources with 10 photons or less may be confidently detected near the image center in medium-length (~10⁴ s), background-limited PSPC exposures. The positions of sources detected near the image center (off-axis angles < 15') are accurate to within a few arcseconds. Output count rates and sizes are in agreement with the input quantities, within a factor of 2 in 90% of the cases. The errors on position, count rate, and size increase with off-axis angle and for detections of lower significance. We have also checked that the upper limits computed with our method are consistent with the count rates of undetected input sources. Finally, we have tested the algorithm by applying it on various actual PSPC images, among the most challenging for automated detection procedures (crowded fields, extended sources, and nonuniform diffuse emission). The performance of our method in these images is satisfactory and outperforms those of other current X-ray detection techniques, such as those employed to produce the MPE and WGA catalogs of PSPC sources, in terms of both detection reliability and efficiency. We have also investigated the theoretical limit for point-source detection, with the result that even sources with only 2-3 photons may be reliably detected using an efficient method in images with sufficiently high resolution and low background.

We describe the parallel implementation of our generalized stellar atmosphere and non-LTE (NLTE) radiative transfer computer program PHOENIX. We discuss the parallel algorithms we have developed for radiative transfer, spectral line opacity, and NLTE opacity and rate calculations. Our implementation uses a multiple instruction-multiple data design based on a relatively small number of MPI library calls. We report the results of test calculations on a number of different parallel computers and discuss the results of scalability tests.

The single assumption that neutron stars receive a kick velocity at their formation explains a large variety of observations, ranging from observed properties of individual binary radio pulsars and Be/X-ray binaries to the observed birth rates and dynamical properties of the populations of low-mass X-ray binaries and recycled radio pulsars. The rejection of this one assumption by Iben & Tutukov compels them to introduce a considerable number of ad hoc hypotheses, each explaining a only small part of the observational results. We conclude that the choice of the single powerful assumption that neutron stars receive a kick velocity at their formation is highly preferable.

A theoretical analysis of coupled mode equations describes the evolution of plasma wave turbulence in the strongly magnetized pair plasma of the pulsar polar cap. The onset of turbulence is a modulational instability of a large-amplitude electrostatic wave. Numerical solution follows the nonlinear evolution toward a strongly turbulent state characterized by the multidimensional collapse of wave packets. The turbulence generates electromagnetic modes that can escape the plasma. The modulational conversion process provides a fairly simple scenario for pulsar emission: an electrostatic plasma wave instability and the subsequent modulational mode coupling to escaping radiation.

The strong magnetic fields (B ~ 10¹²-10¹³ G) characteristic of neutron stars make all the properties of an atom strongly dependent on the transverse component K_⊥ of its generalized momentum. In particular, the photoionization process is modified substantially: (1) threshold energies are decreased as compared with those for an atom at rest, (2) cross section values are changed significantly, and (3) selection rules valid for atoms at rest are violated by the motion so that new photoionization channels become allowed. To calculate the photoionization cross sections, we employ, for the first time, exact numerical treatment of both initial and final atomic states. This enables us to take into account the quasi-bound (autoionizing) atomic states as well as coupling of different ionization channels. We extend the previous consideration, restricted to the so-called centered states corresponding to relatively small values of K_⊥, to arbitrary states of atomic motion. We fold the cross sections with the thermal distribution of atoms over K. For typical temperatures of neutron star atmospheres, the averaged cross sections differ substantially from those of atoms at rest. In particular, the photoionization edges are strongly broadened by the thermal motion of atoms; this "magnetic broadening" exceeds the usual Doppler broadening by orders of magnitude. The decentered states of the atoms give rise to the low-energy component of the photoionization cross section. This new component grows significantly with increasing temperature above 10^5.5 K and decreasing density below 1 g cm^-3, i.e., for the conditions expected in atmospheres of middle-aged neutron stars.

Observations of chromospheric activity variations for some lower main-sequence stars from the Mount Wilson Observatory's HK project reveal a cyclic behavior comparable to the sunspot cycle. Even in the relatively short interval that they have been observed, those stars show stellar cycles and other features, like grand minima. The quasi-periodic nature of such variations is not completely compatible with the standard Fourier analysis, so we applied a wavelet analysis to study the nature of regularities in the data. We computed wavelet transforms and energy spectra for the 25 yr records of surface magnetic activity in four stars: HD 3651, HD 10700, HD 10476, and HD 201091. We present a modified wavelet technique that is suitable for analysis of data with gaps and find that the common aliasing problems due to the finite length of the observations and irregularly spaced gaps between data can be reduced on both large and small scales by applying this algorithm.

We report the detection of N V 1239 Å transition region emission in HST/Goddard High Resolution Spectrograph spectra of the A7 V stars, α Aql and α Cep. Our observations provide the first direct evidence of 1-3 × 10⁵ K material in the atmospheres of normal A-type stars. For both stars, and for the mid A-type star τ³ Eri, we also report the detection of chromospheric emission in the Si III 1206 Å line. At a B-V color of 0.16 and an effective temperature of ~8200 K, τ³ Eri becomes the hottest main-sequence star known to have a chromosphere and, thus, an outer convection zone. We see no firm evidence that the Si III line surface fluxes of the A stars are any lower than those of moderately active, solar-type G and K stars. This contrasts sharply with their coronal X-ray emission, which is more than 100 times weaker than that of the later type stars. Given the strength of the N V emission observed here, it now appears unlikely that the X-ray faintness of A stars is due to their forming very cool, ≤1 MK coronae. An alternative explanation in terms of mass loss in coronal winds remains a possibility, though we conclude from moderate resolution spectra of the Si III lines that such winds, if they exist, do not penetrate into the chromospheric Si III-forming layers of the star, since the profiles of these lines are not blueshifted and may well be redshifted with respect to the star.

We present the results of a Doppler tomographic reconstruction of the UV spectra of the double-lined, O binary DH Cephei based on observations made with the InternationalUltravioletExplorer. We describe cross-correlation methods we use to obtain precise radial velocities, and we present a radial velocity curve based on combined optical and UV measurements. We also show how we use fits of the cross-correlation functions to estimate the UV flux ratio and projected rotational velocities. The individual component spectra are classified as O6 V + O7 V using UV criteria defined by Penny, Gies, & Bagnuolo. We place the individual components in the theoretical Hertzsprung-Russell diagram using the distance modulus and reddening estimated for its home cluster, NGC 7380, and we find that the stars are larger than estimates from prior studies of the "ellipsoidal" light variations. We reconsider the ellipsoidal light curve and show that there is a range in acceptable stellar radii (as a function of orbital inclination). We discuss the constraints on inclination and system masses based on cluster distance modulus, presumed synchronous rotation, and on consistency with masses derived from evolutionary tracks (which involves the issue of the temperature calibration of O stars). We find that primary and secondary masses of 39-50 M_☉ and 35-45 M_☉, respectively, satisfy all the constraints from spectroscopy, photometry, distance modulus, and single-star evolutionary tracks.

Redshifted absorption profiles that resemble the high-velocity circumstellar gas features in the spectrum of β Pictoris have been detected in IUE data for the 10 Myr old Herbig Be star, HD 100546, on 1995 March 7. In addition to Mg II, Si II, and other refractory species similar to those seen in β Pic, the HD 100546 spectra are rich in accreting gas profiles from neutral atomic gas, including C I and O I, as well as mildly refractory species such as Zn II and S II. The presence of accreting gas profiles, including neutral atomic gas, is consistent with detection of comae of star-grazing bodies potentially resembling either comets or asteroids. Overall, the IUE data for HD 100546 are consistent with the planetesimals in this system being more volatile-rich and magnesium-rich than similar bodies in the β Pic system.

High-precision radial-velocity observations of the solar-type star 16 Cygni B (HR 7504, HD 186427), taken at McDonald Observatory and at Lick Observatory, have each independently discovered periodic radial-velocity variations indicating the presence of a Jovian-mass companion to this star. The orbital fit to the combined McDonald and Lick data gives a period of 800.8 days, a velocity amplitude (K) of 43.9 m s^-1, and an eccentricity of 0.63. This is the largest eccentricity of any planetary system discovered so far. Assuming that 16 Cygni B has a mass of 1.0 M_☉, the mass function then implies a mass for the companion of 1.5/sin i Jupiter masses. While the mass of this object is well within the range expected for planets, the large orbital eccentricity cannot be explained simply by the standard model of growth of planets in a protostellar disk. It is possible that this object was formed in the normal manner with a low-eccentricity orbit and has undergone postformational orbital evolution, either through the same process that has been proposed to have formed the "massive eccentric" planets around 70 Virginis and HD 114762, or by gravitational interactions with the companion star 16 Cygni A. It is also possible that the object is an extremely low mass brown dwarf formed through fragmentation of the collapsing protostar. We explore a possible connection between stellar photospheric Li depletion, pre-mainsequence stellar rotation, the presence of a massive protoplanetary disk, and the formation of a planetary companion.

We construct a new approach to model the velocity distribution function (VDF) for the protons in stellar atmosphere expansions or planetary polar winds. The generalized Grad method of construction is used, and comparisons with the bi-Maxwellian polynomial expansion model are made in applications to the solar wind in the context of the measurements made by the Helios probes between 0.3 and 1 AU. A fitting procedure based on a sum of two Maxwellian functions is used to check the convergence property of both polynomial expansions and to calculate the predicted polynomial expansion profiles along the magnetic field orientation for typical proton VDFs in the solar wind. The generalized model is better adapted than the bi-Maxwellian polynomial expansion function to reproduce the long-tail features of a majority of the observed proton VDFs; moreover, our model does not display negative values of the VDF, contrary to the bi-Maxwellian expansion for normalized heat flux larger than unity. A 16 moment approximation, which corresponds to a third order of development, allows us to provide an associated set of generalized transport equations better closed than the equivalent system associated with a bi-Maxwellian polynomial expansion.

Thirty-seven isotopically highly anomalous presolar Al₂O₃ grains and one presolar MgAl₂O₄ grain from a separate of the Tieschitz H3.6 ordinary chondrite were identified out of 17,000 isotopically normal refractory oxide grains by an automatic ¹⁶O/¹⁸O low mass resolution ion-imaging mapping technique in the ion microprobe. Eight additional presolar Al₂O₃ grains were found by high mass resolution ion probe measurements of all three stable O isotopes in individual grains, including several that would have been missed by the ion-imaging search. Forty-five of the grains were analyzed for their ¹⁶O/¹⁷O and ¹⁶O/¹⁸O ratios. Twenty-four grains were also analyzed for Al-Mg and 17 of them have large excesses of ²⁶Mg, attributable to the radioactive decay of ²⁶Al. The highly anomalous isotopic composition of the grains is evidence for their presolar, stellar origin.

The 46 oxide grains of this study together with 42 previously identified presolar grains were divided into four groups. These groups most likely comprise grains from distinct types of stellar sources. Group1 grains have ¹⁷O excesses and moderate ¹⁸O depletions, relative to solar, and many of them exhibit ²⁶Mg excesses as well. Group2 grains have ¹⁷O excesses, large ¹⁸O depletions, and high inferred ²⁶Al/²⁷Al ratios. Group3 grains have solar or higher ¹⁶O/¹⁷O and ¹⁶O/¹⁸O ratios. Group4 grains have ¹⁷O and ¹⁸O enrichments. One Al₂O₃ grain of this study, T54, has an ¹⁶O/¹⁷O ratio of 71, lower than any previously observed, and ¹⁶O/¹⁸O much greater than the solar value.

The O-isotopic compositions of Group 1 and Group 3 grains are consistent with an origin in O-rich red giant stars, which have undergone the first dredge-up. The range of O-isotopic ratios of these groups requires multiple stellar sources of different masses and initial isotopic compositions and is well explained by a combination of Galactic chemical evolution and first dredge-up models. The inferred ²⁶Al/²⁷Al ratios of many of these grains indicate that they formed in thermally pulsing asymptotic branch (TP-AGB) stars that had undergone the third dredge-up. Group 2 grains probably formed in low-mass AGB stars as well, and their substantial ¹⁸O depletions are the likely result of "extra" mixing (cool bottom processing). The origin of the ¹⁸O enrichments in Group 4 grains is unknown, but it might be due to initial compositional differences of the stellar sources or to unusual third dredge-up in low-mass AGB stars. The highly ¹⁷O-enriched grain T54 could have formed in an AGB star undergoing hot bottom burning or in a massive star in the Of-WN phase.

O-rich circumstellar dust seems to be underrepresented in meteorites, relative to C-rich. Explanations include the possibility that most O-rich stardust grains are silicates and have been destroyed either in the laboratory or in nature and the possibility that presolar Al₂O₃ has a finer grain size distribution than SiC and graphite.

We numerically solve the relativistic Fokker-Planck equation for a beam of accelerated electrons impinging on the solar chromosphere, for several cases relevant to solar flares. We make a detailed comparison between our results and those obtained from the test-particle approach. We find that the inclusion of velocity diffusion changes significantly not only the resulting distribution function but also macroscopic quantities like the energy deposition rate and the hard X-ray emission.

We find that the beam energy is deposited in a deeper and much broader region of the atmosphere. Also, our computations predict a harder and larger hard X-ray emission. These results might be relevant to the long-standing controversy between the thermal and the nonthermal models for the X-ray production, as well as to the study of the acceleration mechanisms of electron beams.

A spectacular erupting feature with a plasmoid-like structure is observed before and during the solar flare that occurred on the limb on 1991 December 2 with the Yohkoh soft X-ray telescope. The rise of a loop structure starts ~10 min before the flare, evolving to a plasmoid-like structure in the impulsive phase of the flare. The speed of the rising loop (plasmoid) is almost constant (~96 km s^-1) throughout the observation. A clear X-shaped structure is formed underneath the rising plasmoid, and a bright soft X-ray loop is formed below the X-point. The X-shaped structure indicates a magnetic neutral point with a large-scale magnetic separatrix structure. Inverse-V-shaped high-temperature ridges are located above the soft X-ray loop and below the X-point. We interpret these as reconnected loops heated by slow shocks. A moving high-temperature (15 MK) source is found, coincident in position with the rising structure above the X-point. A hard X-ray source (33-53 keV) is located at the top of the soft X-ray flare loop. These two compact high-temperature sources located above and below the X-point would be formed by fast shocks due to the symmetric reconnection outflows both upward and downward from the X-point.

We report new high-sensitivity measurements of the energy spectra of ions from five impulsive solar flares and one gradual event observed during solar minimum by the Energetic Particles, Acceleration, Composition, and Transport (EPACT) experiment aboard the WIND spacecraft. All of the impulsive-flare events had intensities too low to be visible on previous spacecraft such as ISEE3, which observed hundreds of impulsive-flare events. Often these events cluster in or behind a coronal mass ejection (CME) where magnetic field lines provide an excellent connection to a solar active region where flares are occurring. In most cases we can see velocity dispersion as the ions of 20 keV amu^-1 to 10 MeV amu^-1 streamed out from the impulsive flare at the Sun, arriving in inverse order of their velocity. Ions from a large, magnetically well-connected gradual event, associated with a CME-driven shock, also show velocity dispersion early in the event but show identical time profiles that last for several days late in the event. These time-invariant spectra of H,⁴He, C, O, and Fe in this gradual event are well represented as power laws in energy from 20 keV amu^-1 to ~100 MeV amu^-1. In the impulsive-flare events, H,³He,⁴He, C, O, and Fe have more rounded spectra that flatten somewhat at low energies; yet the intensities continue to increase down to 20 keV amu^-1. Most of the ion energy content appears to lie below 1 MeV in the impulsive events, where it would be invisible to γ-ray line observations.

The total solar irradiance and Ca H and K fluxes (HK) for the Maunder minimum are estimated from scaling laws for solar-type stars using historical solar rotation rates and solar diameters. We found that the irradiance may be lower than modern solar minimum values by 1.23% in 1683 and by 0.37% in 1715. The estimate for 1683 is substantially lower than previously reported. Analysis of cosmogenic isotope records in ice cores and tree rings shows continuation of the Sun's magnetic cycle through the Maunder minimum; therefore, we find the HK fluxes to be 0.161 for 1683 and 0.163 for 1715, compared with the modern solar minimum flux value of ~0.164. This suggests that the Sun never reached a noncycling state.

The charge-transfer rate coefficient for the reaction Si³⁺(3s²S) + He → products is measured by means of a combined technique of laser ablation and ion storage. A cylindrical radio-frequency ion trap was used to store Si³⁺ ions produced by laser ablation of solid silicon targets. The rate coefficient of the reaction was derived from the decay rate of the ion signal. The measured rate coefficient is 6.27^{+ 0.68}_−0.52 × 10^-10 cm³ s^-1 at T_equiv = 3.9 × 10³ K. This value is about 30% higher than the Landau-Zener calculation of Butler & Dalgarno and is larger by about a factor of 3 than the recent full quantal calculation of Honvault et al.

Dissociative recombination of the polyatomic ions H₂O⁺, H₃O⁺, and CH⁺₃ with electrons has been measured at the heavy-ion storage ring ASTRID. Complete branching ratios for all the possible product channels have been determined at zero relative energy using an energy-sensitive detector masked by grids with known transmissions. In the dissociative recombination of H₃O⁺, water molecules are produced with a probability of 33%, whereas the production of atomic oxygen is negligible. Atomic carbon is, on the other hand, produced with a branching ratio of 30% in the dissociative recombination of CH⁺₃. For all three molecular ions, the three-particle breakup is a major process. Relative cross sections for dissociative recombination of H₃O⁺ and for dissociative excitation of H₃O⁺ have been measured for relative electron energies up to 40 eV. Implications for the modeling of the chemistry of interstellar molecular clouds are discussed.

The evidence for early assembly of the central parts of the giant galaxies is a challenge for the adiabatic cold dark matter (CDM) model for structure formation; it more readily fits an isocurvature version where homogeneity is broken by CDM that is the remnant of a massive scalar field frozen from quantum fluctuations during inflation. In this picture the primeval CDM mass distribution is proportional to the square of a random Gaussian process, so for a given power spectrum the prominent upward density fluctuations have contrast about three times that of a Gaussian process. This could lead to the assembly of spheroids at redshift z ~ 10 in concentrations outside of which generally smaller fluctuations become the Lyα forest at z ~ 3 and dwarf galaxies at z ~ 1, as the evidence suggests.

We present a semi-analytical model to test the hypothesis that Lyα QSO absorption lines originate in gaseous halos produced by multiple supernova explosions occurring in Population III objects in a CDM cosmological scenario hoping to assess the validity of CDM models and/or constrain their parameters. The preliminary results indicate that the range of N_{H I}, redshift distribution and metallicity of clouds are well reproduced by CDM if they are associated with galaxy halos or groups. Firm conclusions on clouds with N_{H I} ≤ 10¹⁴ cm^-2 need a more refined study.

At a distance of 20 pc from the purported supermassive black hole powering quasars, temperatures and densities are inferred from optical observations (Osterbrock) to be ~10⁴ K and ~10⁴ cm^-3. Here we present very long baseline interferometry radio observations revealing organized magnetic fields on the parsec scale in the hot plasma surrounding the quasar OQ 172 (1442+101). These magnetic fields rotate the plane of polarization of the radio emission coming from the core and inner jet of the quasar. The derived rotation measure (RM) is 40,000 rad m^-2 in the rest frame of the quasar. Only 10 mas (a projected distance of 68 pc) from the nucleus, the jet absolute values of RM fall to less than 100 rad m^-2.

We propose that the relative variability on short timescales of the multiple images of a lensed quasar, after removal of the time delay, may be caused by hot spots or other moving structures in the accretion disk crossing microlens caustics caused by stellar-mass objects in the lensing galaxy. Such variability has been reported in the two images of 0957+561. The short durations would be caused by the high rotation speed of the disk (v/c ~ 0.1), rather than by planetary mass objects in the slowly moving (v/c ~ 10^-3) lens. This interpretation could be confirmed by finding periodicity, or correlations of the spectral and flux variations caused by the Doppler effect in the disk. We also propose another signature of stationary accretion disks (with no intrinsic variability): the gradient of the magnification over the accretion disk should cause a relative color change between the images whose sign and amplitude are correlated with the time derivative of the flux difference between the images. Other color terms induced by the radial variation of disk colors are of second order in the magnification gradient. The methods proposed here can be used first to verify that accretion disks near supermassive black holes are the source of the continuum radiation from quasars, and then to study them.

Comparison of the high-resolution X-ray image of A2218 obtained with the ROSAT HRI with the optical HST image shows several interesting correlations. The X-ray emission within a 1' radius core is resolved into several components; the central dominant galaxy does not coincide with either of them or with the emission centroid. The major X-ray peak is an elongated feature that lies between the two mass concentrations known from the optical lensing analysis, and coincides with optical arcs at r ≃ 20'' from the cD galaxy. We speculate that this may be lensed X-ray emission, for example (but not necessarily) of the same object lensed in the optical. Alternatively, this feature may be a merger shock or a gas trail of an infalling subgroup. Two other X-ray enhancements are close to the two major mass concentrations. Both lensing and a merger are likely.

Previous X-ray derivations of the A2218 mass used a β-model fit to the data with angular resolution that blurred the features mentioned above into a broad constant core. As the HRI data show, such a core does not exist. Because of this, under certain assumptions and using only the improved imaging data, the hydrostatic estimate of the projected mass within the lensing radius can in principle be increased by a factor of ~1.4 (and the mass within a sphere of the same radius by a factor of 2.6) compared to previous analyses. However, for a merging cluster, the hydrostatic analysis is generally inapplicable. Most other lensing clusters are more distant than A2218, and obtaining adequate X-ray images and temperature maps of them is even more difficult. Together with the likely overestimation of mass by the lensing analysis (as in the simulations), oversimplification of the gas density and temperature models resulting from inadequate resolution may account for the lensing/X-ray mass discrepancy as suggested for A2218.

We report the identification of a cluster of galaxies around the high-redshift radio galaxy 3CR 184 at z = 0.996. The identification is supported by an excess of galaxies observed in projection in I-band images (both in ground-based and Hubble Space Telescope [HST] data), a peak in the redshift distribution comprising 11 galaxies (out of 56 with measured redshifts) in a ~2000 km s^-1 velocity interval, and the observation on HST WFPC2 frames of a gravitational arc seen projected at 42 h⁻¹₅₀ kpc away from the central radio galaxy. We thus have strong evidence for the presence of a massive cluster at z ≃ 1. The mass contained within the arc radius is in the range (1.20-2.78) × 10¹³h⁻¹₅₀M_☉ for z_arc between 1.5 and 3; the corresponding mass-to-light ratio varies from 56 h₅₀ to 140 h₅₀. The velocity dispersion deduced from the galaxy cluster redshifts is 634^{+ 206}₋₁₀₂ km s^-1, leading to a virial mass M = 6.16^{+ 3.94}_−2.40 × 10¹⁴h⁻¹₅₀M_☉ and a mass-to-light ratio of 200 h₅₀ < (M/L_B)_{400 h⁻¹50} < 500 h₅₀ within a radius of 400 h⁻¹₅₀ kpc.

Using the parameter V [ ≡ (⁶⁴)_max/(¹⁰²⁴)_max], the ratio of expected peak counts in 64 ms and 1024 ms, proposed by Che et al. recently, we find the cosmological time dilation in short gamma-ray bursts (t₉₀ < 1.5 s) directly from the Burst and Transient Source Experiment catalog. The parameter V of short bursts shows a good correlation with the brightness B when we use (⁶⁴)_max as a measure of brightness B rather than (¹⁰²⁴)_max, introduced by Lamb et al. in 1993. In a Friedmann universe with Ω = 1 and Λ = 0, the estimated redshift is about 2 and the time dilation factor (TDF) with the energy correction is about 1.8. The fact that these results generally agree with those found in long bursts supports the same origin of long and short gamma-ray bursts.

We study the correlations between the distance of a supernova (SN) from the center of the host galaxy and measurable properties such as SN spectral types and luminosity at maximum light for Type Ia supernovae (SNe Ia). The rate of SNe Ia within 1 kpc of the center of spiral galaxies is significantly lower than outside this region. The rates of Type II (SNe II) and Type Ib/c (SNe Ib/c) supernovae in the central 1 kpc region of spiral galaxies are at least as high as, and perhaps higher than, those in the outer regions. This is the first direct evidence that stellar bulges in spiral galaxies are not efficient SNe Ia producers and that novae and nova-like systems in the bulges are therefore not efficient producers of SNe Ia. The radial distribution of all types of supernovae are, within errors, the same beyond ~7 kpc. So SNe Ia are also unlikely to come from a halo population.

We also analyze a subsample of SNe Ia including the Calán/Tololo sample and 11 nearby well-observed SNe Ia for which accurate photometry is available. Supernovae at more than 7.5 kpc from the galactic center show 3-4 times smaller scatter in brightness than those closer to the center. This behavior represents an important new constraint on evolutionary models of SNe Ia and their correlation with the underlying stellar population. SNe Ia at larger galactocentric distances may define a more homogeneous group for measuring cosmological distances. The B_max, V_max, and I_max magnitudes for SNe Ia beyond 7.5 kpc from the center of the host galaxies were found to be -19.05 ± 0.20, -19.07 ± 0.18, and -18.81 ± 0.18, respectively.

We present optical images of the nucleus of the nearby radio galaxy M84 (NGC 4374 = 3C 272.1) obtained with the Wide Field/Planetary Camera 2 aboard the Hubble Space Telescope. Our three images cover the Hα + [N II] emission lines as well as the V and I continuum bands. Analysis of these images confirms that the Hα + [N II] emission in the central 5'' (410 pc) is elongated along position angle (P.A.) ≈ 72°, which is roughly parallel to two nuclear dust lanes. Our high-resolution images reveal that the Hα + [N II] emission has three components, namely, a nuclear gas disk, an "ionization cone," and outer filaments. The nuclear disk of ionized gas has diameter ≈ 1'' = 82 pc and major axis P.A. ≈ 58° ± 6°. On an angular scale of 05, the major axis of this nuclear gas disk is consistent with that of the dust. However, the minor axis of the gas disk (P.A. ≈ 148°) is tilted with respect to that of the filamentary Hα + [N II] emission at distances greater than 2'' from the nucleus; the minor axis of this larger scale gas is roughly aligned with the axis of the kiloparsec-scale radio jets (P.A. ≈ 170°). The ionization cone (whose apex is offset by ≈ 03 south of the nucleus) extends 2'' from the nucleus along the axis of the southern radio jet. This feature is similar to the ionization cones seen in some Seyfert nuclei, which are also aligned with the radio axes.

We report on the first determination of the distance to the Coma Cluster from surface brightness fluctuation (SBF) measurements obtained from Hubble Space Telescope WFPC2 observations of the bright E0 galaxy NGC 4881 in the Coma Cluster and ground-based observations of the "standard" E1 galaxy NGC 3379 in the Leo-I group. Relative distances based on the I-band fluctuation magnitude, I, are strongly dependent on the metallicity and age of the stellar population. However, the radial changes in the stellar populations of the two giant ellipticals NGC 3379 and NGC 4881 are well described by published Mg₂ gradients, and the ground-based measurements of _I at several radial points in NGC 3379 are used to calibrate _I in terms of the Mg₂ index. The distance to NGC 3379, assumed to be identical to the average SBF distance of the Leo-I group, is combined with the new SBF measurements of NGC 4881 to obtain a Coma Cluster distance of 102 ± 14 Mpc. Combining this distance with the cosmic recession velocity of the Coma Cluster (7186 ± 428 km s^-1), we find the Hubble constant to be H₀ = 71 ± 11 km s^-1 Mpc^-1.

The age of NGC 1651 is 1.6 ± 0.4 billion yr. This letter shows that accurate age determinations of young LMC clusters are achievable with aperture photometry of Wide Field Planetary Camera 2 (WFPC2) images taken in a single orbit of the HST.

We present the first numerical simulations of moderately hot, supersonic jets propagating initially along the field lines of a denser magnetized background medium with Lorentz factor W = 4.56 and evolving in a four-dimensional spacetime. Compared with previous simulations in two spatial dimensions, the resulting structure and kinematics differ noticeably: the density of the Mach disk is lower, and the head speed is smaller. This is because the impacted ambient fluid and its embedded magnetic field make efficient use of the third spatial dimension as they are deflected circularly off of the head of the jet. As a result, a significant magnetic field component normal to the jet is created near the head. If the field is strong, backflow and field reversals are strongly suppressed; upstream, the field closes back on the surface of the beam and assists the collimation of the jet. If the field is weak, backflow and field reversals are more pronounced, although still not as extended as in the corresponding plane-parallel case. In all studied cases, the high-pressure region is localized near the jet head irrespective of the presence/strength of the magnetic field, and the head decelerates efficiently by transferring momentum to the background fluid that recedes along a thin bow shock in all directions. Furthermore, two oppositely directed currents circle near the surface of the cylindrical beam, and a third current circles on the bow shock. These preliminary results underline the importance of performing fully three-dimensional simulations to investigate the morphology and propagation of relativistic extragalactic jets.

We present Compton Gamma Ray Observatory (CGRO) EGRET measurements of an outburst of greater than 100 MeV gamma-ray emission from the massive X-ray binary system Centaurus X-3 that occurred during an interval of rapid spin-down by the X-ray pulsar. For the 1994 October outburst, the phase-averaged 30 MeV- 10 GeV emission is best fitted by a power law with index α = 1.81 ± 0.37 and an integral flux above 100 MeV of (9.2 ± 2.3) × 10^-7 photons cm^-2 s^-1, corresponding to a phase-averaged luminosity in GeV gamma rays of ~5 × 10³⁶ ergs s^-1. Our phase analysis, employing contemporaneous X-ray pulse observations by CGRO BATSE, indicates modulation of the gamma-ray emission at the pulsar's spin frequency with a significance level higher than 99.5%. Straightforward interpretation of the EGRET measurements requires at least sporadic acceleration of GeV particles within the Cen X-3 binary system. The observations also suggest that Galactic X-ray binary systems may constitute a class of highly variable GeV gamma-ray sources.

We use interstellar scattering of the Vela pulsar to determine the size of its emission region. We find that radio-wave scattering in the Vela supernova remnant broadens the source by 3.3 ± 0.2 mas × 2.0 ± 0.1 mas, with the major axis at a position angle of 92° ± 10°. From the modulation of the pulsar's scintillation, we infer a size of 500 km for the pulsar's emission region, with an estimated uncertainty of about a factor of 2, including systematic errors. We suggest that radio-wave refraction within the pulsar's magnetosphere may plausibly explain this size.

We report the detection of millimeter CO emission from a dense, expanding disk of molecular gas, ≈ 20'' in diameter, at the center of the remarkable 14' × 4', bipolar, episodic jet system of the planetary nebula KjPn 8. The disk surrounds the compact photoionized core and exceeds it in mass by at least an order of magnitude. The disk axis lies at a position angle of ≈ 115° and appears to be aligned with the most prominent and youngest of the bipolar jets. The disk axis is not, however, aligned with the older, more extensive bipolar structure of the nebula at a position angle of ≈ 71°, indicating that major changes have taken place in the escape of matter from the central star system over the last ≈ 5000 yr.

Two consecutive rotational transitions of the long cyanopolyyne HC₁₁N, J = 39 → 38 and 38 → 37, have been detected in the cold dust cloud TMC-1 at the frequencies expected from recent laboratory measurements by Travers and coworkers, and at about the expected intensities. The astronomical lines have a mean radial velocity of 5.8(1) km s^-1, in good agreement with the shorter cyanopolyynes HC₇N and HC₉N observed in this very sharp line source [5.82(5) and 5.84(5) km s^-1, respectively]. The column density of HC₁₁N is calculated to be 2.8 × 10¹¹ cm^-2. The abundance of the cyanopolyynes decreases smoothly with length to HC₁₁N, the decrement from one to the next being about six for the longer carbon chains.

We report the discovery of the rotational spectrum of CH⁺ in the Infrared Space Observatory Long Wavelength Spectrometer (LWS) spectrum of the planetary nebula NGC 7027. The identification relies on a 1996 reanalysis of the LWS spectrum by Liu et al. and on new LWS data. The strong line at 179.62 μm (coinciding with the 2₁₂-1₀₁ transition of water vapor) and the lines at 119.90 and 90.03 μm (reported as unidentified by Liu et al.), whose frequencies are in the harmonic relation 2 : 3:4, are shown to arise from the J = 2-1, 3-2, and 4-3 rotational transitions of CH⁺. This identification is strengthened by the new LWS spectra of NGC 7027, which clearly show the next two rotational lines of CH⁺ at 72.140 and 60.247 μm. This is the first time that the pure rotational spectrum of CH⁺ has been observed. This discovery opens the possibility of probing the densest and warmest zones of photodissociation regions. We derive a rotational temperature for the CH⁺ lines of 150 ± 20 K and a CH⁺/CO abundance ratio of 2-6 × 10^-4.

We use Mie scattering theory to determine the expected thermal emission from dust grains in cometary comae and apply these results to mid-infrared images of comet Hyakutake (C/1996 B2) obtained preperihelion in 1996 March. Calculations were performed for dust grains in the size range from 0.1 to 10 μm for two different compositions: amorphous olivine (a silicate glass) and an organic residue mixture. The resulting emission efficiencies are complicated functions of wavelength and particle size and are significantly different for the two materials in question. The Hyakutake data set consists of three nights of high-resolution imaging (100-150 km pixel^-1 at the comet) of the inner coma at 8.7, 11.7, 12.5, and 19.7 μm. Attempts to fit the observed colors (ratios of fluxes at different wavelengths) using a single grain composition failed. However, fits to the data were achieved for all three nights using a mixture of ~1 μm olivine grains and ~7 μm organic grains. The resulting olivine mass fraction was between 8% and 16% of the total dust mass-loss rate. We also estimate the radius of the nucleus to be r = 2.1 ± 0.4 km.