Table of contents

We report the detection of Cepheid variable stars in the barred spiral galaxy NGC 1365, located in the Fornax cluster, using the Hubble Space Telescope (HST) Wide Field and Planetary Camera 2 (WFPC2). Twelve V (F555W) and four I (F814W) epochs of observation were obtained. The two photometry packages ALLFRAME and DoPHOT were separately used to obtain profile-fitting photometry of all the stars in the HST field. The search for Cepheid variable stars resulted in a sample of 52 variables, with periods between 14 and 60 days, common to both data sets. ALLFRAME photometry and light curves of the Cepheids are presented. A subset of 34 Cepheids were selected on the basis of period, light curve shape, similar ALLFRAME and DoPHOT periods, color, and relative crowding, to fit the Cepheid period-luminosity relations in V and I for both ALLFRAME and DoPHOT. The measured distance modulus to NGC 1365 from the ALLFRAME photometry is 31.31±0.20 (random)±0.18 (systematic) mag, corresponding to a distance of 18.3±1.7 (random)±1.6 (systematic) Mpc. The reddening is measured to be E(V-I)=0.16±0.08 mag. These values are in excellent agreement with those obtained using the DoPHOT photometry, namely a distance modulus of 31.26±0.10 mag and a reddening of 0.15±0.10 mag (internal errors only).

Using the Hubble Space Telescope, 37 long-period Cepheid variables have been discovered in the Fornax Cluster spiral galaxy NGC 1365. The resulting V and I period-luminosity relations yield a true distance modulus of μ₀=31.35±0.07 mag, which corresponds to a distance of 18.6±0.6 Mpc. This measurement provides several routes for estimating the Hubble constant. (1) Assuming this distance for the Fornax Cluster as a whole yields a local Hubble constant of 70±18 (random) ±7 (systematic) km s⁻¹ Mpc⁻¹. (2) Nine Cepheid-based distances to groups of galaxies out to and including the Fornax and Virgo Clusters yield H₀=73±16 (random) ±7 (systematic) km s⁻¹ Mpc⁻¹. (3) Recalibrating the I-band Tully-Fisher relation using NGC 1365 and six nearby spiral galaxies, and applying it to 15 galaxy clusters out to 100 Mpc, give H₀=76±3 (random) ±8 (systematic) km s⁻¹ Mpc⁻¹. (4) Using a broad-based set of differential cluster distance moduli ranging from Fornax to Abell 2147 gives H₀=72±3 (random) ±6 (systematic) km s⁻¹ Mpc⁻¹. Finally, (5) assuming the NGC 1365 distance for the two additional Type Ia supernovae in Fornax and adding them to the SN Ia calibration (correcting for light-curve shape) gives H₀=67±6 (random) ±7 (systematic) km s⁻¹ Mpc⁻¹ out to a distance in excess of 500 Mpc. All five of these H₀ determinations agree to within their statistical errors. The resulting estimate of the Hubble constant, combining all of these determinations, is H₀=72±5 (random) ±7 (systematic) km s⁻¹ Mpc⁻¹. An extensive tabulation of identified systematic and statistical errors, and their propagation, is given.

Abell 754, a cluster undergoing merging, was observed in hard X-rays with the Rossi X-Ray Timing Explorer (RXTE) in order to constrain its hottest temperature component and search for evidence of nonthermal emission. Simultaneous modeling of RXTE data and those taken with previous missions yields an average intracluster temperature of ~9 keV in the 1-50 keV energy band. A multitemperature component model derived from numerical simulations of the evolution of a cluster undergoing a merger produces similar quality of fit, indicating that the emission measure from the very hot gas component is sufficiently small that it renders the two models indistinguishable. No significant nonthermal emission was detected. However, our observations set an upper limit of 7.1×10⁻¹⁴ ergs cm⁻² s⁻¹ keV⁻¹ (90% confidence limit) to the nonthermal emission flux at 20 keV. Combining this result with the radio synchrotron emission flux, we find a lower limit of 0.2 μG for the intracluster magnetic field. We discuss the implications of our results for the theories of magnetic field amplifications in cluster mergers.

The virialized regions of galaxies and clusters contain significant amounts of substructure; clusters have hundreds to thousands of galaxies, and satellite systems and globular clusters orbit the halos of individual galaxies. These orbits can decay owing to dynamical friction. Depending on their orbits and their masses, the substructures either merge, are disrupted, or survive to the present day. We examine the distributions of eccentricities of orbits within mass distributions similar to those we see for galaxies and clusters. A comprehensive understanding of these orbital properties is essential to calculate the rates of physical processes relevant to the formation and evolution of galaxies and clusters. We derive the orbital eccentricity distributions for a number of spherical potentials. These distributions depend strongly on the velocity anisotropy, but only slightly on the shape of the potential. The eccentricity distributions in the case of an isotropic distribution function are strongly skewed toward high eccentricities, with a median value of typically ~0.6, corresponding to an apocenter-to-pericenter ratio of 4.0. We also present high-resolution N-body simulations of the orbital decay of satellite systems on eccentric orbits in an isothermal halo. The dynamical friction timescales are found to decrease with increasing orbital eccentricity because of the dominating deceleration at the orbit's pericenter. The orbital eccentricity stays remarkably constant throughout the decay; although the eccentricity decreases near pericenter, it increases again near apocenter, such that there is no net circularization. We briefly discuss several applications for our derived distributions of orbital eccentricities and the resulting decay rates from dynamical friction. We compare the theoretical eccentricity distributions to those of globular clusters and galactic satellites for which all six phase-space coordinates (and therewith their orbits) have been determined. We find that the globular clusters are consistent with a close-to-isotropic velocity distribution, and they show large orbital eccentricities because of this (not in spite of this, as has been previously asserted). In addition, we find that the limited data on the Galactic system of satellites appears to be different and warrants further investigation as a clue to the formation and evolution of our Milky Way and its halo substructure.

We develop the "simulated extinction method" to measure average foreground Galactic extinction from field galaxy number counts and colors. The method comprises simulating extinction in suitable reference fields by changing the isophotal detection limit. This procedure takes into account selection effects; in particular, the change in the isophotal detection limit (and hence in the isophotal magnitude completeness limit) with extinction, and the galaxy color-magnitude relation. We present a first application of the method to the Hubble Space Telescope WFPC2 images of the gamma-ray burster GRB 970228. Four different WFPC2 high-latitude fields, including the Hubble Deep Field, are used as reference to measure the average extinction toward the gamma-ray burst (GRB) in the F606W passband. From the counts, we derive an average extinction of A_V=0.5 mag, but the dispersion of 0.4 mag between the estimates from the different reference fields is significantly larger than can be accounted for by Poisson plus clustering uncertainties. Although the counts differ, the average ⟨F606W-F814W⟩ colors of the field galaxies agree well. The extinction implied by the average color difference between the GRB field and the reference galaxies is A_V=0.6 mag, with a dispersion in the estimated extinction from the four reference fields of only 0.1 mag. All of our estimates are in good agreement with the value of 0.81±0.27 mag obtained by Burstein & Heiles, and with the extinction of 0.78±0.12 measured by Schlegel et al. from maps of dust IR emission. However, the discrepancy between the widely varying counts and the very stable colors in these high-latitude fields is worth investigating.

Surface brightness fluctuations (SBF) have been detected for three elliptical galaxies—NGC 3379 in the Leo group, NGC 4406 in the Virgo cluster, and NGC 4373 in the Hydra-Centaurus supercluster—using marginally sampled, deep images taken with the Planetary Camera of the WFPC2 instrument on the Hubble Space Telescope (HST). The power spectrum of the fluctuations image is well fitted by an empirical model of the point-spread function constructed using point sources identified in the field. Comparison with high-quality ground-based observations of all three galaxies show excellent agreement in the measurement of the distance modulus over a substantial range in distance. This demonstrates the capability of the Planetary Camera of WFPC2 to measure distances using the SBF technique despite the marginal sampling and small spatial coverage of the images. The residual variance due to unresolved sources in all three galaxies is only 2%-5% of the detected fluctuations signal, which confirms the advantage of HST imaging in minimizing the uncertainty of this SBF correction. Extensive consistency checks, including an independent SBF analysis using an alternate software package, suggest that our internal uncertainties are <0.02 mag. The fluctuations magnitude for NGC 4373 is I_F814W=31.31±0.05 mag, corresponding to a distance modulus of (m-M)₀=32.99±0.11. This implies a peculiar velocity for this galaxy of 415±330 km s⁻¹, which is smaller than derived from the D_n-σ relation.

We present models for the interstellar medium in disk galaxies. In particular, we investigate whether the ISM in low surface brightness galaxies can support a significant fraction of molecular gas given their low metallicity and surface density. It is found that the abundance and line brightness of CO in LSB galaxies is small and typically below current observational limits. Still, depending on physical details of the ISM, the fraction of gas in the form of molecular hydrogen can be significant in the inner few kiloparsecs of a low surface brightness galaxy. This molecular gas would be at temperatures of ~30-50 K, rather higher than in high surface brightness galaxies. These results may help explain the star-forming properties and inferred evolutionary history of LSB galaxies.

Deep Hα images of a sample of nearby late-type spiral galaxies have been analyzed to characterize the morphology and energetic significance of the "diffuse ionized medium" (DIM). We find that the DIM properties can be reasonably unified as a function of relative surface brightness (Σ/, where is the mean Hα surface brightness within regions lying above a fixed, very faint isophotal level). We measured the images down to this common isophotal limit and constructed a fundamental dimensionless surface brightness distribution function that describes the dependence of the area (normalized to total area) occupied by gas with a given relative surface brightness (Σ/). This function determines both the flux and area contribution by the DIM to global values. The function is found to be almost the same at high surface brightness (Σ/≳1) and less similar for Σ/≲1. We show the universal distribution function at high surface brightness can be understood as a consequence of the general properties of H II regions, including their Hα luminosity function and exponential radial brightness profiles. We suggest that relative surface brightness (rather than an absolute value) is a more physically meaningful criterion to discriminate the DIM from H II regions. The use of the dimensionless distribution function to quantify the DIM is consistent with the fundamentally morphological definition of the DIM as being "diffuse." The difference in the distribution function from galaxy to galaxy at low surface brightness quantifies the different prominence of the DIM in the galaxies. This variation is found to be consistent with results from other complementary ways of determining the DIM's global importance. The variation of the DIM among the galaxies that is indicated by the distribution function is small enough to guarantee that the fractional contribution of the DIM to the global Hα luminosity in the galaxies is fairly constant, as has been observed. The continuous transition from H II regions to the DIM in the distribution function suggests that the ionizing energy for the DIM mainly comes from H II regions, consistent with the "leaky H II regions model."

In the course of our studies of the gaseous halo surrounding the Milky Way, we have recently identified several high-velocity clouds (HVCs; V_LSR<-100 km s^-1) in the directions of Mrk 509 and PKS 2155-304 that have unusual ionization properties. The clouds exhibit strong C IV absorption with little or no detectable low ion (C II, Si II) absorption or H I 21 cm emission down to very sensitive levels. As the closest known analog to the outer diffuse halos of damped Lyα absorbers and the low H I column density metal-line absorption systems seen in the spectra of high-redshift quasars, these "C IV HVCs" present unique opportunities for relating the conditions within the Milky Way halo and nearby intergalactic gas to the properties of galactic halos at higher redshift. In this paper we present new Goddard High Resolution Spectrograph (GHRS) intermediate-resolution measurements of the absorption lines within these C IV HVCs and study the ionization properties of the gas in detail. The present data represent the most complete set of measurements available for studying the ionization conditions within HVCs. The C IV HVCs have ionization properties consistent with photoionization by extragalactic background radiation, although some contribution by collisional ionization within a hot plasma cannot be ruled out. The clouds are probably low density (n_H~10⁻⁴ cm^-3), large (greater than several kiloparsecs), and mostly ionized (n_HI/n_H~10⁻³) regions located well beyond the neutral gas layer of the Galaxy. The presence of weak H I HVCs detected through their 21 cm emission near both sight lines indicates that the C IV HVCs trace the extended, ionized, low-density regions of the H I HVCs. Several lines of evidence, including very low thermal pressures (P/k ~2 cm^-3 K), favor a location for the C IV HVCs in the Local Group or very distant Galactic halo. If the clouds are intergalactic in nature, their metallicities could be [Z/H]~-1 or lower, but higher metallicities [Z/H]>-1 are favored if the clouds are located in the distant Galactic halo since the cloud sizes scale inversely with metallicity. We provide a summary of the HVCs detected in absorption at intermediate resolution with the GHRS and the IUE satellite and find that C IV HVCs are detected along three of 10 extragalactic sight lines down to a level of logN(C IV)≈13.3 (3σ).

We have reanalyzed the Large Magellanic Cloud's (LMC) ultraviolet (UV) extinction using data from the IUE final archive. Our new analysis takes advantage of the improved signal-to-noise ratio of the IUE NEWSIPS reduction, the exclusion of stars with very low reddening, the careful selection of well-matched comparison stars, and an analysis of the effects of Galactic foreground dust. Differences between the average extinction curves of the 30 Dor region and the rest of the LMC are reduced compared with previous studies. We find that there is a group of stars with very weak 2175 Å bumps that lie in or near the region occupied by the supergiant shell, LMC 2, on the southeast side of 30 Dor. The average extinction curves inside and outside LMC 2 show a very significant difference in 2175 Å bump strength, but their far-UV extinctions are similar. While it is unclear whether or not the extinction outside the LMC 2 region can be fitted with the relation of Cardelli, Clayton, & Mathis (CCM), sight lines near LMC 2 cannot be fitted with CCM because of their weak 2175 Å bumps. While the extinction properties seen in the LMC lie within the range of properties seen in the Galaxy, the correlations of UV extinction properties with environment seen in the Galaxy do not appear to hold in the LMC.

We present ASCA observations of the radio-selected BL Lacertae objects 1749+096 (z=0.32) and 2200+420 (BL Lac, z=0.069) performed in 1995 September and November, respectively. The ASCA spectra of both sources can be described as a first approximation by a power law with photon index Γ~2. This is flatter than for most X-ray-selected BL Lacs observed with ASCA, in agreement with the predictions of current blazar unification models. While 1749+096 exhibits tentative evidence for spectral flattening at low energies, a concave continuum is detected for 2200+420: the steep low-energy component is consistent with the high-energy tail of the synchrotron emission responsible for the longer wavelengths, while the harder tail at higher energies is the onset of the Compton component. The two BL Lacs were observed with ground-based telescopes from radio to TeV energies contemporaneously with ASCA. The spectral energy distributions are consistent with synchrotron self-Compton emission from a single homogeneous region shortward of the IR/optical wavelengths, with a second component in the radio domain related to a more extended emission region. For 2200+420, comparing the 1995 November state with the optical/GeV flare of 1997 July, we find that models requiring inverse Compton scattering of external photons provide a viable mechanism for the production of the highest (GeV) energies during the flare. In particular, an increase of the external radiation density and of the power injected in the jet can reproduce the flat γ-ray continuum observed in 1997 July. A directly testable prediction of this model is that the line luminosity in 2200+420 should vary shortly after (~1 month) a nonthermal synchrotron flare.

We present H I observations and analyses of the kinematics of 24 satellite-primary galaxy pairs with projected separations between 4.9 and 240 kpc. The satellites have masses of less than 3% of their primary spirals. Two estimates for the masses of the primaries are available, one from their rotation curves and one from the orbital properties of the satellites. Defining χ as the ratio of these two mass estimates, it is a measure of the presence, or absence, of a significant halo. The χ-distribution for these 24 pairs is presented and the selection effects are discussed. Moreover, we show that the χ-distribution of more numerous pairs, with projected separations of less than 200 kpc, identified by Zaritsky et al., after adopting selection criteria quite different from ours, is similar to our χ-distribution. We show that the observational biases have a negligible effect; the biased and unbiased distributions are essentially identical. In order to understand this distribution, N-body calculations were executed to simulate the dynamical behavior of relatively low mass satellites orbiting primary disk galaxies with and without extended halos. The models and the real galaxies were "observed" in the same fashion. In addition, we made a partially analytical analysis of the behavior of orbits in a logarithmic potential. We find that a "generic" model, characterized by a single disk/halo combination, cannot reproduce the observed P(χ) distribution. However, a simple two-component population of galaxies, composed of not more than 60% with halos and 40% without halos, is successful, if galaxies have dimensions of order 200 kpc. If galaxies are considerably larger with sizes extending to 400 kpc or more, the constraints become more onerous. No generic model can describe the full range of the observed P(χ), particularly if the distribution for r_p<200 kpc is compared with that for r_p>200 kpc. Regardless of the mix of orbital eccentricities, neither pure halo, nor canonical (disk and halo masses are comparable within the disk radius) models will work. A multicomponent approximation to reality can be constructed for which the canonical model must be mixed with a small fraction of systems essentially devoid of a massive dark halo. Only by including these complexities can the full range of P(χ) be modeled with any degree of success over all radial extents. We show that dynamical friction cannot be ignored in these explorations and that the average mass of a galaxy is in the range of (1-5)×10¹²M_☉, with a mass-to-luminosity ratio of at most a few hundred. This is insufficient to close the universe.

We derive abundances and central star parameters for 15 planetary nebulae (PNe) in M31: 12 in the bulge and three in a disk field 14 kpc from the nucleus. No single abundance value characterizes the bulge stars: although the median abundances of the sample are similar to those seen for PNe in the LMC, the distribution of abundances is several times broader, spanning over one decade. None of the PNe in our sample approach the super-metal-rich ([Fe/H]~0.25) expectations for the bulge of M31, although a few PNe in the sample of Stasińska, Richer, & Mc Call come close. This [O/H] versus [Fe/H] discrepancy is likely due to a combination of factors, including an inability of metal-rich stars to produce bright PNe, a luminosity selection effect, and an abundance gradient in the bulge of M31. We show that PNe that are near the bright limit of the [O III] λ5007 planetary nebula luminosity function (PNLF) span nearly a decade in oxygen abundance and, thus, support the use of the PNLF for deriving distances to galaxies (in the work by Jacobi and collaborators) with differing metallicities. We also identify a correlation between central star mass and PN dust formation that partially alleviates any dependence of the PNLF maximum magnitude on population age. In addition, we identify a spatially compact group of five PNe having unusually high O/H; this subgroup may arise from a recent merger, but velocity information is needed to assess the true nature of the objects.

We analyze the dependence of circumstellar extinction on core mass for the brightest planetary nebulae (PNs) in the Magellanic Clouds and M31. We show that in all three galaxies, a statistically significant correlation exists between the two quantities, such that high-core mass objects have greater extinction. We model this behavior and show that the relation is a simple consequence of the greater mass loss and faster evolution times of high-mass stars. The relation is important because it provides a natural explanation for the invariance of the [O III] λ5007 planetary nebula luminosity function (PNLF) with population age: bright Population I PNs are extinguished below the cutoff of the PNLF. It also explains the counter-intuitive observation that intrinsically luminous Population I PNs often appear fainter than PNs from older, low-mass progenitors.

G359.87+0.18 is an enigmatic object located 15' from Sgr A*. It has been variously classified as an extragalactic source, a Galactic jet source, and a young supernova remnant. We present new observations of G359.87+0.18 between 0.33 and 15 GHz and an H I absorption spectrum and use these to argue that this source is an FR II radio galaxy. We are able to place a crude limit on its redshift of z≳0.1. The source has a spectral index of α<-1 (S∝ν^α), suggestive of a radio galaxy with a redshift z≳2. The scattering diameters of Sgr A* and several nearby OH masers (≈1'' at 1 GHz) indicate that a region of enhanced scattering is along the line of sight to the Galactic center. If the region covers the Galactic center uniformly, the implied diameter for a background source is at least 600'' at 0.33 GHz, which is in contrast with the observed 20'' diameter of G359.87+0.18. Using the scattering diameter of a nearby OH maser OH 359.762+0.120 and the widths of two nearby nonthermal threads, G0.08+0.15 and G359.79+0.17, we show that a uniform scattering region should cover G359.87+0.18. We therefore conclude that the Galactic center scattering region is inhomogeneous on a scale of 5' (≈10 pc at a distance of 8.5 kpc). This scale is comparable with the size scale of molecular clouds in the Galactic center. The close agreement between these two length scales is an indication that the scattering region is linked intimately to the Galactic center molecular clouds.

A large number of microlensing events have been observed in the direction of the Galactic bulge, with a measured optical depth in the range 2-3×10⁻⁶. It has been shown that most of these events are due to bulge stars being lensed by other bulge stars or by foreground disk stars. Among the stars observed in the bulge fields there should also be disk stars located behind the bulge; here we consider their effect on the microlensing rates. The optical depth of background disk stars is much higher than that of typical bulge stars, reaching 10^-5 at 6 kpc behind the bulge. Thus, although background disk stars are a very small fraction of the stars in Baade's window, we find that ~5%-10% of the optical depth should be due to disk stars more than 3 kpc behind the bulge. This fraction is sensitive to the luminosity function of disk stars at large scale height, to the magnitude cutoff of the survey, and to the amplification bias effect causing large numbers of "blended" events. We consider also the effect of a warp and flare in the disk at large distances behind the bulge; this could increase the optical depth from the background disk to ~20% of the total. Events on background disk stars should on average be longer than other events and could be distinguished as well by measuring the proper motion or distance of the stars that have been microlensed. The number of these events could be an interesting probe to the structure and stellar population of the far side of the Galactic disk.

We present measurements of the 0.05-10 MeV gamma-ray spectra as a function of longitude from the inner Galactic ridge using the Oriented Scintillation Spectrometer Experiment (OSSE) on the Compton Gamma Ray Observatory. The differential continuum emission relative to intensities at ±10° Galactic latitude appears to be a composite of at least three independent components: a soft low-energy component with a broad longitude distribution and with spectra well approximated by an exponentially absorbed power law; a hard component with a similarly broad longitude distribution modeled by a power law from ~200 keV to 10 MeV with photon index ~-1.75; and strong positron annihilation line and continuum contributions observed toward the center with intensities that decrease rapidly with longitude distance from the center. Although OSSE cannot distinguish between a simple one-component latitude distribution and a more complicated one with, for example, broad and narrow latitude components, an "effective" 5°-6° FWHM Gaussian latitude width gives a spectrum and intensity for the power-law component that agrees with extrapolations of measurements at higher energies using a cosmic-ray interaction model. However, the latitude distribution of the emission is not well measured. Near the Galactic center, bright variable sources contribute significantly to the low-energy spectrum. When account is taken of these variable-source contributions, both the soft low-energy and hard power-law components show a consistent longitude distribution that follows the Galactic matter distribution as evidenced by the Galactic CO distribution. These results, in conjunction with previous measurements, provide new information for determining the Galactic cosmic-ray electron spectrum at lower energies.

In this paper we study the very early phases of the evolution of our Galaxy by means of a chemical evolution model that reproduces most of the observational constraints in the solar vicinity and in the disk. We have restricted our analysis to the solar neighborhood, and we present the predicted abundances of several elements (C, N, O, Mg, Si, S, Ca, and Fe) over a more extended range of metallicities, [Fe/H]=-4.0 to [Fe/H]=0.0, than previous models. We adopt the most recent yield calculations for massive stars taken from two different works, and compare the results with a very large sample of data, one of the largest ever used for this purpose. We have obtained this data set by selecting the most recent and higher quality abundance data from a number of sources and renormalizing them to the same solar abundances. These data have been analyzed with a new and powerful statistical method that allows us to quantify the observational spread in measured elemental abundances and to obtain a more meaningful comparison with the predictions from our chemical evolution model. Our analysis shows that the "plateau" observed for the [α/Fe] ratios at low metallicities (-3.0<[Fe/H]<-1.0) is not perfectly constant, but shows a slope, especially for oxygen. This slope is very well reproduced by our model with both sets of yields. This is not surprising, since realistic chemical evolution models, taking stellar lifetimes into account in detail, never predicted a completely flat plateau. This is due either to the fact that massive stars of different mass produce a slightly different O/Fe ratio or to the often forgotten fact that supernovae of type Ia, originating from white dwarfs, already start appearing at a Galactic age of 30 Myr and reach their maximum at 1 Gyr. For lower metallicities (-4.0<[Fe/H]<-3.0), the two sets of adopted yields differ, especially for iron. In this range, the "plateau" is almost constant, since at such low metallicities there is almost no contribution from type Ia supernovae. However, there are not enough data in this domain to significantly test this point. Finally, we show the evolution with redshift of the [O/Fe] ratio for different cosmologies and conclude that a sharp rise of this ratio should be observed at high redshift, irrespective of the adopted yields. The same behavior is expected for the [O/Zn] ratio, which should be easier to compare with the abundances observed in high-redshift damped Lyα systems, since these elements are not likely to be affected by dust. Future measurements of either [α/Fe] or [α/Zn] ratios in very metal poor stars will be useful to infer the nature and the age of high-redshift objects.

Performing one-dimensional hydrodynamical calculations coupled with nonequilibrium processes for hydrogen molecule formation, we pursue the thermal and dynamical evolution of filamentary primordial gas clouds and attempt to make an estimate on the mass of Population III stars. The cloud evolution is computed from the central proton density n_c~10²-10⁴ cm^-3 up to ~10¹³ cm^-3. It is found that, almost independent of initial conditions, a filamentary cloud continues to collapse nearly isothermally owing to H₂ cooling until the cloud becomes optically thick against the H₂ lines (n_c~10¹⁰-10¹¹ cm^-3). During the collapse the cloud structure separates into two parts, i.e., a denser spindle and a diffuse envelope. The spindle contracts quasi-statically, and thus the line mass of the spindle keeps a characteristic value determined solely by the temperature (~800 K), which is ~1×10³M_☉ pc^-1 during the contraction from n_c~10⁵ cm^-3 to 10¹³ cm^-3. Applying a linear theory, we find that the spindle is unstable against fragmentation during the collapse. The wavelength of the fastest growing perturbation (λ_m) lessens as the collapse proceeds. Consequently, successive fragmentation could occur. When the central density exceeds n_c~10¹⁰-10¹¹ cm^-3, the successive fragmentation may cease, since the cloud becomes opaque against the H₂ lines and the collapse decelerates appreciably. Resultantly, the minimum value of λ_m is estimated to be ~2×10⁻³ pc. The mass of the first star is then expected to be typically ~3 M_☉, which may grow up to ~16 M_☉ by accreting the diffuse envelope. Thus, the first-generation stars are anticipated to be massive but not supermassive.

We report the detection of a peculiar molecular cloud, CO 0.02-0.02, lying about 5' Galactic east from the center of the Galaxy. ¹²CO images taken with Nobeyama Radio Observatory (NRO) 45 m telescope showed that it is relatively compact (~3×4 pc²) as well as having a very large velocity width (ΔV≥100 km s^-1). The cloud has a virial mass about 1 order of magnitude larger than the LTE mass, 9×10⁴M_☉, indicating the cloud is apparently gravitationally unbound.

New observations with the James Clerk Maxwell Telescope 15 m and the NRO 45 m telescopes show that CO 0.02-0.02 is very bright in the CO (J=3-2) and in the HCN and HCO⁺ (J=1-0) lines. It appears that the environment may have an unusually high density and temperature, which may be related to the very broad CO line width.

We propose that CO 0.02-0.02 may have been accelerated, heated, and compressed in a series of supernovae shocks that have occurred within the last (3-5)×10⁴ yr.

We propose a test that can in principle detect any systematic errors in the Hipparcos parallaxes toward the Hyades cluster at the level of 0.3 mas. We show that the statistical parallax algorithm subsumes the classical moving cluster methods and provides more precise estimates of the distance and the first two moments of the velocity distribution of the Hyades cluster, namely, its bulk space velocity and the velocity dispersion tensor. To test the Hipparcos parallaxes, we first rescale the bulk velocity determined from statistical parallax to force agreement with the distance scale determined from Hipparcos parallaxes. We then predict the parallaxes of Hyades cluster members using this common cluster space velocity and their Hipparcos proper motions. We show that the parallaxes determined in this manner (π_pm) are consistent at the 1 σ level with the parallaxes (π_orb) of three Hyades spectroscopic binary systems with orbital solutions. We find that ⟨π_pm-π_orb⟩=0.52±0.47 mas, where the error is dominated by the errors in the orbital parallaxes. A reduction in these errors would allow a test of the systematic errors in the Hipparcos parallaxes at the 0.3 mas level. If the Hyades distance scale is fixed by Hipparcos parallaxes, then its bulk velocity in equatorial coordinates is (V_x,V_y,V_z)=(-5.70±0.20, 45.62±0.11, 5.65±0.08) kms⁻¹, its velocity dispersion is 320±39 m s⁻¹, and the distance modulus to the centroid of our sample of 43 cluster members is 3.34±0.02 mag.

We report observations of the J=(1-0) C¹⁸O molecular emission line toward the L977 molecular cloud. To study the correlation between C¹⁸O emission and dust extinction we constructed a Gaussian smoothed map of the infrared extinction measured by Alves et al. at the same angular resolution (50'') as our molecular-line observations. This enabled a direct comparison of C¹⁸O integrated intensities and column densities with dust extinction over a relatively large range of cloud depth (2<A_V<30 mag) at 240 positions inside L977. We find a good linear correlation between these two column density tracers for cloud depths corresponding to A_V≲10 mag. For cloud depths above this threshold there is a notable break in the linear correlation. Although either optically thick C¹⁸O emission or extremely low (T_ex<5 K) excitation temperatures at high extinctions could produce this departure from linearity, CO depletion in the denser, coldest regions of L977 may be the most likely cause of the break in the observed correlation. We directly derive the C¹⁸O abundance in this cloud over a broad range of cloud depths and find it to be virtually the same as that derived for IC 5146 from the data of Lada et al. In regions of very high extinction (A_V>10 mag), such as dense cores, our results suggest that C¹⁸O would be a very poor tracer of mass. Consequently, using C¹⁸O as a column density tracer in molecular clouds can lead to a 10% to 30% underestimation of overall cloud mass. We estimate the minimum total column density required to shield C¹⁸O from the interstellar radiation field to be 1.6 ± 0.5 mag of visual extinction.

We have carried out Very Large Array (VLA) Zeeman observations of absorption lines of H I and OH toward the molecular cloud associated with the NGC 2024 (Orion B) H II region. The synthesized beam diameters are 68'' × 52'', p.a.=38°, and 81'' × 65'', p.a.=-6°, for OH and H I, respectively. The absorption lines could be mapped over the NGC 2024 continuum source, which has an extent (at the 1 Jy beam⁻¹ level) of Δα≈7' by Δδ≈5'. The maps of the magnetic field, together with comparisons with additional data from the published literature, lead to the following conclusions: (1) The magnetic field comes from a line subcomponent at v_LSR≈10.2 km s^-1, which corresponds in velocity and in spatial morphology with the northern dense molecular ridge in NGC 2024. (2) B_los varies from 0 to the northeast of the northern molecular ridge to almost 100 μG to the southwest. The variation in B_los may be due to the field being mainly in the plane of the sky to the northeast but having a significant line-of-sight component to the southwest. (3) Velocities in the cloud are supersonic but approximately equal to the Alfvén velocity, which is consistent with motions being dominated by magnetohydrodynamical waves rather than thermal motion or hydrodynamical turbulence. (4) The mass-to-magnetic flux ratio is supercritical, which suggests that the static magnetic field does not support the cloud against collapse. Simple virial estimates of the relative importance of gravitational, kinetic, and magnetic energies show that the ratio of kinetic/gravitational energy is about 0.5, while the magnetic/gravitational energy ratio is less than 0.1. At face value, these results imply that the cloud is supported mainly by nonthermal motions rather than by the static magnetic field. However, since we only measure directly the line-of-sight component of B, this result is not conclusive.

We examine the idea that diffuse H I and giant molecular clouds and their substructure form as density fluctuations induced by large-scale interstellar turbulence. We do this by closely investigating the topology of the velocity, density, and magnetic fields within and at the boundaries of the clouds emerging in high-resolution two-dimensional simulations of the interstellar medium (ISM) including self-gravity, magnetic fields, parameterized heating and cooling, and a simple model for star formation. We find that the velocity field is continuous across cloud boundaries for a hierarchy of clouds of progressively smaller sizes. Cloud boundaries defined by a density-threshold criterion are found to be quite arbitrary, with no correspondence to any actual physical boundary, such as a density discontinuity. Abrupt velocity jumps are coincident with the density maxima, which indicates that the clouds are formed by colliding gas streams. This conclusion is also supported by the fact that the volume and surface kinetic terms in the Eulerian virial theorem for a cloud ensemble are comparable in general and by the topology of the magnetic field, which exhibits bends and reversals where the gas streams collide. However, no unique trend of alignment between density and magnetic features is observed. Both sub- and super-Alfvénic motions are observed within the clouds. In light of these results, we argue that thermal pressure equilibrium is irrelevant for cloud confinement in a turbulent medium, since inertial motions can still distort or disrupt a cloud, unless it is strongly gravitationally bound. Turbulent pressure confinement appears self-defeating because turbulence contains large-scale motions that necessarily distort Lagrangian cloud boundaries or equivalently cause flux through Eulerian boundaries. We then discuss the compatibility of the present scenario with observational data. We find that density-weighted velocity histograms are consistent with observational line profiles of comparable spatial and velocity resolution, exhibiting similar FWHMs and similar multicomponent structure. An analysis of the regions contributing to each velocity interval indicates that the histogram "features" do not come from isolated "clumps" but rather from extended regions throughout a cloud, which often have very different total velocity vectors. Finally, we argue that the scenario presented here may also be applicable to small scales with larger densities (molecular clouds and their substructure, up to at least n~10³-10⁵ cm^-3) and conjecture that quasi-hydrostatic configurations cannot be produced from turbulent fluctuations unless the thermodynamic behavior of the flow becomes nearly adiabatic. We demonstrate, using appropriate cooling rates, that this will not occur except for very small compressions (≲10^-2 pc) or until protostellar densities are reached for collapse.

We have carried out VLA Zeeman observations of H I absorption lines toward the H II region in the M17 giant molecular cloud complex. The resulting maps have 60'' × 45'' spatial resolution and 0.64 km s^-1 velocity separation. The H I absorption lines toward M17 show between 5 and 8 distinct velocity components, which vary spatially in a complex manner across the source. We explore possible physical connections between these components and the M17 region based on calculations of H I column densities, line-of-sight magnetic field strengths, as well as comparisons with a wide array of previous optical, infrared, and radio observations. In particular, an H I component at the same velocity as the southwestern molecular cloud (M17 SW; ~20 km s^-1) seems to originate from the edge-on interface between the H II region and M17 SW in unshocked photodissociation region (PDR) gas. We have detected a steep enhancement in the 20 km s^-1 H I column density and line-of-sight magnetic field strengths (B_los) toward this boundary. A lower limit for the peak 20 km s^-1 H I column density is N_HI/T_s≥5.6 × 10¹⁹ cm^-2 K^-1, whereas the peak B_los is ~-450 μG. In addition, blended components at velocities of 11-17 km s^-1 appear to originate from shocked gas in the PDR between the H II region and an extension of M17 SW, which partially obscures the southern bar of the H II region. The peak N_HI/T_s and B_los for this component are ≥4.4 × 10¹⁹ cm⁻² K^-1 and ~+550 μG, respectively. Comparison of the peak magnetic fields detected toward M17 with virial equilibrium calculations suggest that ≈½ of M17 SW's total support comes from its static magnetic energy, while the other half of its support is supplied by the turbulent kinetic energy (including MHD waves).

Observed variations in the slope of the stellar initial mass function (IMF) are shown to be consistent with a previously introduced model in which the protostellar gas is randomly sampled from clouds with a self-similar hierarchical structure. Root mean square variations in the IMF slope around the Salpeter value are ±0.4 when only 100 stars are observed, and ±0.1 when 1000 stars are observed. Similar variations should be present in other stochastic models as well. The hierarchical sampling model reproduces the tendency for massive stars to form closer to the center of a cloud at a time somewhat later than the formation time of the lower mass stars. The systematic variation in birth position results from the tendency for the trunk and larger branches of the hierarchical tree of cloud structure to lie closer to the cloud center, while the variations in birth order result from the relative infrequency of stars with larger masses. The hierarchical cloud sampling model has now reproduced most of the reliably observed features of the cluster IMF. The power-law part of the IMF comes from cloud hierarchical structure that is sampled during various star formation processes with a relative rate proportional to the square root of the local density. These processes include turbulence compression, magnetic diffusion, gravitational collapse, and clump or wavepacket coalescence, all of which have about this rate dependence. The low-mass flattening comes from the inability of gas to form stars below the thermal Jeans mass at typical temperatures and pressures. The thermal Jeans mass is the only relevant scale in the problem. Considerations of heating and cooling processes indicate why the thermal Jeans mass should be nearly constant in normal environments and why this mass might increase in starburst regions. In particular, the relative abundance of high-mass stars should increase where the average density of the interstellar medium is very large; accompanying this increase should be an increase in the average total efficiency of star formation. Alternative models in which the rate of star formation is independent of density and the local efficiency decreases systematically with increasing stellar mass can also reproduce the IMF, but this is an adjustable result and not a fundamental property of hierarchical cloud structure, as is the preferred model. The steep IMF in the extreme field is not explained by the model, but other origins are suggested, including one in which massive stars in low-pressure environments halt star formation in their clouds. In this case, the slope of the extreme field IMF is independent of the slope of each component cluster IMF and is given by (γ-1)/α for a cloud mass function slope, -γ~-2, and a power-law relation, M_L∝M^α_c, between the largest star in a low-pressure cloud, M_L, and the cloud mass, M_c. A value of α~1/4 is required to explain the extreme field IMF as a superposition of individual cluster IMFs; cloud destruction by ionizing has this property. We note that the similarity between cluster IMFs and the average IMF from global studies of galaxies implies that most stars form in clusters and that massive stars do not generally halt star formation in the same cloud.

We present a model of X-ray emission from rotation-powered pulsars, which in general consist of one nonthermal component, two hard thermal components, and one soft thermal component. The nonthermal X-rays come from synchrotron radiation of e ± pairs created in the strong magnetic field near the neutron star surface by curvature photons emitted by charged particles on their way from the outer gap to the neutron star surface. The first hard thermal X-ray component results from polar-cap heating by the return current in the polar gap. The second hard thermal X-ray component results from polar-cap heating by the return particles from the outer gap. Because of cyclotron resonance scattering, most of the hard thermal X-rays will be effectively reflected back to the stellar surface and eventually reemitted as soft thermal X-rays. However, some of the hard thermal X-rays can still escape along the open magnetic field lines, where the e⁺/e⁻ pair density is low. Furthermore, the characteristic blackbody temperatures of the two hard X-ray components emitted from the polar-cap area inside the polar gap and the polar-cap area defined by the footprints of the outer-gap magnetic field lines are strongly affected by the surface magnetic field, which can be much larger than the dipolar field. In fact, the strong surface magnetic field can explain why the effective blackbody radiation area is nearly 2 orders of magnitude larger than that deduced from the dipolar field for young pulsars (2 orders of magnitude less for old pulsars). Our model indicates how several possible X-ray components may be observed, depending on the magnetic inclination angle and viewing angle. Using the expected X-ray luminosity and spectra, we explain the observed X-ray spectra from pulsars such as Geminga, PSR B1055-52, PSR B0656+14, and PSR B1929+10.

We describe the far-UV (1140-1740 Å) spectrum of the hydrogen-deficient R Coronae Borealis (RCB) star RY Sgr, obtained near maximum light (pulsational phase ~0.1) by the Space Telescope Imaging Spectrograph (STIS) on Hubble Space Telescope. The far-UV spectrum shows a photospheric continuum rising steeply toward longer wavelengths and two prominent emission features at the shorter wavelengths: C II λ1335 and Cl I λ1351 (the latter is radiatively fluoresced by the 10 times stronger C II multiplet). We also find evidence for CO A-X 4th-positive system absorption band heads and possible weak CO fluorescent emissions pumped by C II λ1335, but the inferred column densities are low (~few times 10¹⁶ cm^-2), consistent with formation in a warm (~5000 K) atmospheric layer. The detection of CO molecules, if confirmed, would be significant, because they are thought to play a key role in the dust ejection episodes of RCB stars through the initiation of "molecular cooling catastrophes."

We have determined the visual orbit for the spectroscopic binary ι Pegasi with interferometric visibility data obtained by the Palomar Testbed Interferometer in 1997. ι Peg is a double-lined binary system whose minimum masses and spectral typing suggests the possibility of eclipses. Our orbital and component diameter determinations do not favor the eclipse hypothesis: the limb-to-limb separation of the two components is 0.151±0.069 mas at conjunction. Our conclusion that the ι Peg system does not eclipse is supported by high-precision photometric observations. The physical parameters implied by our visual orbit and the spectroscopic orbit of Fekel & Tomkin are in good agreement with those inferred by other means. In particular, the orbital parallax of the system is determined to be 86.9±1.0 mas, and masses of the two components are determined to be 1.326±0.016 and 0.819±0.009 M, respectively.

Recent work has shown that the spinning magnetic fields of neutron stars (NSs) can cause soft/hard spectral state switches. The spectral state switches of black hole candidates (BHCs) could be produced in the same way. Spin frequencies and magnetic field strengths are estimated for them. Spins below 100 Hz and fields above 10¹⁰ G can account for their spectral state switches and their quiescent luminosities. It also appears that large flickering amplitudes and ultrasoft spectral peaks would be expected from radiating surfaces of massive NSs. Since BHCs share most of their spectral properties with NSs and there is as yet no proof of event horizons, the possibility that they might simply be massive NSs must be considered seriously. This opens an avenue for proof by negation but requires the use of a spacetime metric that has no event horizon. The Yilmaz exponential metric used here is shown to have an innermost marginally stable orbit with radius, binding energy, and Keplerian frequency that are within a few percent of the same quantities for the Schwarzschild metric. A maximum NS mass of ~10 M_☉ is found for the Yilmaz metric. Since the two metrics essentially differ only by the presence/absence of a surface for the BHCs it should at last be possible to prove or disprove the existence of event horizons.

A stellar model of mass 11 M_☉ and Population I composition is evolved from the hydrogen-burning main sequence through the core carbon-burning phase. In contrast with 9, 10, and 10.5 M_☉ models studied in earlier papers of this series, carbon burning is ignited at the center of the 11 M_☉ model. Like the 10.5 M_☉ model, the 11 M_☉ model experiences a dredge-out episode at the end of the carbon-burning phase. At the beginning of this episode, a semiconvective zone forms at the base of the hydrogen-rich envelope and carries hydrogen inward in mass toward the outer edge of a fully convective zone that is sustained by helium burning at its base. Hydrogen diffuses into the helium-rich convective zone untila hydrogen shell flash occurs. Helium burning dies out and the outer edge of the convective layer,sustained by fluxes due to hydrogen burning, extends outward in mass through hydrogen-rich material, mixing freshly synthesized nuclei outward. Then, hydrogen burning dies out and the outer edge of the convective shell, now sustained primarily by fluxes due to the release of gravothermal energy, moves outward until it reaches the inner edge of the convective envelope. Freshly synthesized material is then convected to the surface. Mixing during the final phase of homogenization in the convective envelope is maintained by fluxes due to the release of gravothermal energy. At the end of the dredge-out phase, the surface nitrogen abundance has decreased and the C/N ratio has changed from less than unity to larger than unity, showing that mixing has extended into regions where helium burning has manufactured substantial quantities of ¹²C and destroyed ¹⁴N. Prior to the dredge-out phase, neon burning is narrowly averted, and, after the dredge-out phase, neutrino losses due to electron capture and decay reactions between A=25 and A=23 isotopes in and above convective Urca shells cool the inner portions of the electron-degenerate oxygen-neon (ONe) core. Ultimately, the model becomes a thermally pulsing super-asymptotic giant branch (TPSAGB) star with an ONe core of mass ~1.368 M_☉. Hydrogen and helium burning over a period of ~1.4×10⁴ yr of TPSAGB evolution add a carbon-oxygen layer of mass ~0.014 M_☉ to the electron-degenerate core. Then, electron captures on products of carbon burning lead to the collapse of the core into a neutron star and expulsion of the envelope in a weak Type II supernova explosion. The ratio of helium to hydrogen in the ejecta is approximately twice solar.

The pair of drifting subpulses of the radio pulsar PSR B0031-07 appear to be well separated from each other on average, overlapping at less than 1/30 of the peak energy. Strictly, the integrated profile of the pair (after removing the drift) does not show more than two subpulses; however, this result is likely to be modified with the availability of more data. On the other hand, rare instances of three simultaneous subpulses in a single period have been noticed. This is consistent with the Ruderman & Sutherland model of the drifting phenomenon in which several sparks, uniformly spaced on a circle, rotate around the polar cap. The third subpulse appears to show up strongly when subpulses belonging to the primary drift bands weaken. Although this conclusion is based mainly on two reliable examples, it suggests an anticorrelation of the subpulse energies. The normalized correlation coefficient between the energies of the two subpulses in a period is -0.16 ± 0.02, after accounting for a known selection effect. Simulations suggest that this result could not be an artifact. This suggests the idea of competing drifting subpulses which, if true, will have important ramifications for the growth and breakdown of the particle acceleration zones in pulsars.

We present hard X-ray and optical observations of the eclipsing AM Her system V2301 Oph. The X-ray data were obtained using the PCA detector of the Rossi X-Ray Timing Explorer satellite during 1997 May, and the optical data were obtained using the 1 and 1.5 m telescopes of the Cerro Tololo Inter-American Observatory during 1996 May and 1997 June. V2301 Oph was bright in both the optical and hard X-rays during our observations. This, when coupled with its eclipsing nature, makes V2301 Oph an ideal testbed for theories of the large-scale topology of AM Her flows and the radiative shocks in AM Her systems. The X-ray emission from V2301 Oph was modulated strongly on the orbital period. During the bright orbital phases, the X-ray flux was F_x≈3.6 × 10⁻¹¹ ergs cm^-2 s^-1 over the energy range E=2-10 keV. The X-ray emission did not go to zero during the faint orbital phases; it was ~10% of the bright phase level. The X-ray spectrum could be fitted by (1) optically thin thermal bremsstrahlung (temperature kT_x≈9-19 keV) models with an absorption line at 5.1-5.2 keV or an emission line at ~7 keV, and (2) power-law continuum (index ≈2) models with an absorption line at 5.1-5.2 keV or an emission line at ~7 keV. The absorption columns were large for all fits, n_H~(3-10) × 10²² cm^-2. The n_H are model dependent, but their large sizes are secure because they are set by the rollover in the X-ray spectrum at 3-4 keV. The hardness of the X-ray spectrum was roughly constant during the bright orbital phases. During the faint orbital phases, the X-ray spectral properties were not well determined, but it did appear that the spectrum hardened. There were total eclipses in both the X-ray and optical light curves. The X-ray light curves and eclipses were consistent with a dominant hot spot and a secondary hot spot. The dominant hot spot was not a point source; it had to cover about 50° in longitude on the surface of the white dwarf. We argue that the X-ray light curve and eclipse shape also suggest that the accretion occurs in a sheetlike geometry rather than in a columnar geometry. The optical light curves and eclipses were consistent with emission from the white dwarf photosphere, an extended emission region that sat above the surface of the white dwarf, and the X-ray-heated face of the companion star.

A numerical method of mode analysis of rapidly rotating relativistic stellar models by the Cowling approximation is applied to rotating neutron stars with realistic equations of state. For selected equations of state, eigenvalues and eigenfunctions of f-modes are numerically solved for stellar models from nonrotating to maximally rotating states.

Neutral points of the lower order f-modes are determined as a function of the stellar rotational frequency. As in the polytropic case, we find that the bar mode can have neutral points for models with relatively strong gravity. The rotational frequency at the neutral point increases as the gravitational mass of the model becomes larger.

As astrophysical applications of our analysis, we discuss the timescales of gravitational radiation-induced instability and the possibility of the resonant excitation of f-modes during inspiraling motion of compact binary systems.

We present Extreme Ultraviolet Explorer (EUVE) spectroscopy and photometry of the nearby F8 V star HD 35850 (HR 1817). The EUVE short-wavelength 75-175 Å and medium-wavelength 160-365 Å spectra reveal 28 emission lines from Fe IX and Fe XV to Fe XXIV. The Fe XXI λλ102, 129 ratio yields an upper limit for the coronal electron density, logn_e<11.6 cm⁻³. The EUVE short-wavelength spectrum shows a small but clearly detectable continuum. The 75-150 Å line-to-continuum ratio indicates approximately solar Fe abundances, with 0.8<Z<1.6 (90% confidence interval). Upper limits have been derived for a dozen high-emissivity Fe X through Fe XIV lines. The resulting emission measure distribution is characterized by two broad temperature components at logT of 6.8 and 7.4. Over the course of the 1 week observation, large-amplitude, long-duration flares were not seen in the EUVE Deep Survey light curve, although the light curve does show signs of persistent, low-level flaring and possible rotational modulation. The EUVE spectra have been compared with nonsimultaneous ASCA SIS spectra of HD 35850 obtained in 1995. The SPEX DEM analysis of the SIS spectrum indicates the same temperature distribution as the EUVE DEM analysis. However, the SIS spectra suggest subsolar abundances, 0.34<Z<0.81. Although some of the discrepancy may be the result of incomplete X-ray line lists, we cannot explain the disagreement between the EUVE line-to-continuum ratio and the ASCA-derived Fe abundance. The X-ray surface flux on HD 35850 is comparable to that of cooler dwarfs of comparable age and rotation like EK Draconis (G0 V) and AB Doradus (K1 V). Given its youth (t≈100 Myr), its rapid rotation (vsini≈50 km s⁻¹), and its high X-ray activity (L_X≈1.5×10³⁰ ergs s⁻¹), HD 35850 may represent an activity extremum for single, main-sequence F-type stars. The variability and emission measure distribution can be reconstructed using the continuous flaring model of Güdel provided that the flare distribution has a power-law index α≈1.8. Similar results obtained for other young solar analogs suggest that continuous flaring is a viable coronal heating mechanism on rapidly rotating, late-type, main-sequence stars.

Parameters describing quasi-steady reconnecting current sheets in the plasma of the solar photosphere and chromosphere are computed using the VAL-C atmospheric model. In particular, the inflow speed for the Sweet-Parker magnetic reconnection is found for a sheet whose width is determined by the density scale height. The resulting speed of several tens of meters per second corresponds closely to the speeds implied by observations of canceling magnetic features on the Sun. This and other arguments support photospheric magnetic reconnection as the cancellation mechanism. The reconnection process should be most efficient around the temperature minimum region about 600 km above the lower photospheric boundary.

The properties of the evolution of solar granulation have been studied using an 80 minute time series of high spatial resolution white-light images obtained with the Swedish Vacuum Solar Telescope at the Observatorio del Roque de los Muchachos, La Palma. An automatic tracking algorithm has been developed to follow the evolution of individual granules, and a sample of 2643 granules has been analyzed. To check the reliability of this automatic procedure, we have manually tracked a sample of 481 solar granules and compared the results of both procedures. An exponential law gives a good fit to the distribution of granular lifetimes, T. Our estimated mean lifetime is about 6 minutes, which is at the lower limit of the ample range of values reported in the literature. We note a linear increase in the time-averaged granular sizes and intensities with the lifetime. T=12 minutes marks a sizeable change in the slopes of these linear trends. Regarding the location of granules with respect to the meso- and supergranular flow field, we find only a small excess of long-lived granules in the upflows. Fragmentation, merging, and emergence from (or dissolution into) the background are the birth and death mechanisms detected, resulting in nine granular families from the combination of these six possibilities. A comparative study of these families leads to the following conclusions: (1) fragmentation is the most frequent birth mechanism, while merging is the most frequent death mechanism; (2) spontaneous emergence from the background occurs very rarely, but dissolution into the background is much more frequent; and (3) different granular mean lifetimes are determined for each of these families; the granules that are born and die by fragmentation have the longest mean lifetime (9.23 minutes). From a comparison of the evolution of granules belonging to the most populated families, two critical values appear for the initial area in a granular evolution: 0.8 Mm² (d_g=139) and 1.3 Mm² (d_g=177). These values mark limits characterizing the birth mechanism of a granule, and predict its evolution to some extent. The findings of the present work complement the earlier results presented in this series of papers and reinforce with new inputs, as far as the evolutionary aspects are concerned, the conclusion stated there that granules can be classified into two populations with different underlying physics. The boundary between these two classes could be established at the scale of d_g=14.

It is well known that estimating the mean pairwise velocity of galaxies v₁₂ from the redshift space galaxy correlation function is difficult because this method is highly sensitive to the assumed model of the pairwise velocity dispersion. Here we propose an alternative method to estimate v₁₂ directly from peculiar velocity samples, which contain redshift-independent distances as well as galaxy redshifts. In contrast to other dynamical measures which determine β≡Ω^0.6σ₈, this method can provide an estimate of Ω^0.6σ²₈ for a range of σ₈, where Ω is the cosmological density parameter, while σ₈ is the standard normalization for the power spectrum of density fluctuations. We demonstrate how to measure this quantity from realistic catalogs.

The quasar in the Hubble Deep Field South (HDF-S), J2233-606 (z_em=2.23), has been observed exhaustively by ground-based telescopes and by the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope at low, medium, and high resolution in the spectral interval from 1120 to 10000 Å. The combined data give continuous coverage of the Lyα forest from redshift 0.9 to 2.24. This very large baseline represents a unique opportunity to study in detail the distribution of clouds associated with emitting structures in the field of the quasar and in nearby fields already observed as part of the HDF-S campaign. Here we report on the main properties obtained from the large spectroscopic data set that is available for the Lyα clouds in the intermediate-redshift range of 1.20-2.20, where our present knowledge has been complicated by the difficulty in producing good data. The number density is shown to be higher than what is expected by extrapolating the results from both lower and higher redshifts: 63±8 lines with logN_{H I}≥14.0 are found (including metal systems) at ⟨z⟩=1.7, compared with the ~40 lines predicted by extrapolating from previous studies. The redshift distribution of the Lyα clouds shows a region spanning z≃1.383-1.460 (comoving size of 94 h⁻¹₆₅ Mpc, Ω₀=1) with a low density of absorption lines; we detect five lines in this region, compared with the 16 expected from an average density along the line of sight. The two-point correlation function shows a positive signal up to scales of about 3 h⁻¹₆₅ Mpc and an amplitude that is larger for larger H I column densities. The average Doppler parameter is about 27 km s⁻¹, which is comparable to the mean value found at z>3, thus casting doubts on the temperature evolution of the Lyα clouds.

We report the detection by the Gamma-Ray Burst Monitor on board BeppoSAX of the strongest and longest outburst ever detected from SGR 1900+14. Oscillations are detectable with a period of ~5.16 s for the entire duration of the event (~300 s). The temporal analysis reveals also a remarkable periodic substructure: after about 35 s from the event onset, each 5.16 s pulse shows a pattern of four subpulses and a dip, each separated by ~1 s. Significant spectral variation is detected during the event and for each individual oscillation. The first and most intense part of the outburst is quite hard and similar to what was previously detected from the "March 5 event." A hard nonthermal spectral component persists for ~200 s. SGR 1900+14 was proposed to be a strongly magnetized neutron star (B≳10¹⁴ G) undergoing violent instabilities by internal magnetic/crustal stresses. However, the onset of an apparent 1 s periodicity within the 5.16 s pulsations and the observed spectral properties show a complex behavior that is not satisfactorily modeled yet.

This Letter presents the color distributions of the globular cluster (GC) systems of 12 Virgo elliptical galaxies, measured using data from the Hubble Space Telescope. Bright galaxies with large numbers of detected GCs show two distinct cluster populations with mean V-I colors near 1.01 and 1.26. The GC population of M86 is a clear exception; its color distribution shows a single, sharp peak near V-I=1.03. The absence of the red population in this galaxy, and the consistency of the peak colors in the others, may be indications of the origins of the two populations found in most bright elliptical galaxies.

We have obtained high-resolution UV images with the Hubble Space Telescope/Wide-Field Planetary Camera 2 of the central region of the dwarf elliptical galaxy NGC 205. Our images reveal that many of the hot UV stars previously detected and studied from the ground are actually multiple systems, open clusters, and star associations. We have performed photometry of two such clusters, and we find that our data are consistent with stellar ages of 50 and 100 Myr, respectively. From the number of massive stars in NGC 205, we estimate that the star formation episode in this galaxy has turned ~1000 M_☉ of gas into stars over the last 100 Myr.

A prediction for the energy shift of the synchrotron spectrum of flat-spectrum radio quasars (FSRQs) during high-energy flares is presented. If the γ-ray emission of FSRQs is produced by Comptonization of external radiation, then the peak of the synchrotron spectrum is predicted to move to lower energies in the flare state. This is opposite to the well-known broadband spectral behavior of high-frequency peaked BL Lac objects in which the external radiation field is believed to be weak and synchrotron self-Compton scattering might be the dominant γ-ray radiation mechanism. The synchrotron peak shift, if observed in FSRQs, can thus be used as a diagnostic to determine the dominant radiation mechanism in these objects. I suggest a few FSRQs as promising candidates to test the prediction of the external Comptonization model.

We have obtained high-resolution Ca II and Na I absorption-line spectra toward 68 OB stars in the direction of the Vela supernova remnant. The stars lie at distances of 190-2800 pc as determined by Hipparcos and spectroscopic parallax estimations. The presence of high-velocity absorption attributable to the remnant along some of the sight lines constrains the remnant distance to 250±30 pc. This distance is consistent with several recent investigations that suggest that the canonical remnant distance of 500 pc is too large.

We report the first detection outside of the solar system of the lowest pure rotational J=1→0 transition of the HD molecule at 112 μm. The detection was made toward the Orion Bar using the Fabry-Pérot interferometer of the Long Wavelength Spectrometer (LWS) on board the Infrared Space Observatory. The line appears in emission with an integrated flux of (0.93±0.17)×10⁻¹⁹ W cm⁻² in the LWS beam, implying a beam-averaged column density in the v=0, J=1 state of (1.2±0.2)×10¹⁷ cm⁻². Assuming LTE excitation, the total HD column density is (2.9±0.8)×10¹⁷ cm⁻² for temperatures between 85 and 300 K. Combined with the total, warm H₂ column density of ~(1.5-3.0)×10²² cm⁻² derived from either the H₂ pure rotational lines, the C¹⁸O observations, or the dust continuum emission, the implied HD abundance, HD/H₂, ranges from 0.7×10⁻⁵ to 2.6×10⁻⁵, with a preferred value of (2.0±0.6)×10⁻⁵. The corresponding deuterium abundance of [D]/[H] = (1.0±0.3)×10⁻⁵ is compared with recent values derived from ultraviolet absorption-line observations of atomic H I and D I in interstellar clouds in the solar neighborhood and in Orion.

We report observations of dramatic morphological changes in the extremely young Herbig-Haro object ejected by the pre-main-sequence binary XZ Tauri. Hubble Space Telescope images taken in 1995 showed a filled bubble of emission nebulosity extending 4'' north-northeast of the system in the direction of a previously known jet. New images obtained in 1998 show that the bubble has undergone a remarkable transition into a limb-brightened structure as it continues its motion away from the binary. The new images suggest that we may be witnessing the initial formation of the postshock cooling zone in a newly emerging Herbig-Haro bow shock system, probably accompanied by significant temporal evolution in the nebula's emission line spectrum.

We consider the possibility that a substantial fraction of the magnetic field in the quiet photosphere is generated locally by dynamo action associated with the granular and supergranular flows. The argument is based on recent advances in the theory of fast dynamos and is supported by large-scale numerical simulations that show that thermally driven turbulent convection can indeed be an efficient source of small-scale, highly intermittent magnetic fields. Some aspects of the resulting magnetic field, such as its strength and degree of intermittency, are discussed.