Table of contents

Analytic expressions for distance-redshift relations that have been corrected for the effects of inhomogeneities in the Friedmann-Lemaître-Robertson-Walker (FLRW) mass density are given in terms of Heun functions and are used to illustrate the significance of inhomogeneities on a determination of the mass parameter Ω_m and the cosmological constant Λ. The values of these parameters inferred from a given set of observations depend on the fractional amount of matter in inhomogeneities and can significantly differ from those obtained by using the standard magnitude-redshift (m-z) result for pure dust FLRW models. As an example, a determination of Ω_m made by applying the homogeneous distance-redshift relation to SN 1997ap at z = 0.83 could be as much as 50% lower than its true value.

The photometric evolution of galaxies in a hierarchically clustering universe is investigated. The study is based on high-resolution numerical simulations that include the effects of gasdynamics, shock heating, radiative cooling, and a heuristic star formation scheme. The outcome of the simulations is convolved with photometric models, which enables us to predict the appearance of galaxies in the broadband colors U, B, V, R, I, and K. We demonstrate the effect of the mutual interplay of the hierarchical build-up of galaxies, photometric evolution, k-correction, and intervening absorption on the appearance of forming disk galaxies at redshift 1-3. We also discuss to what extent the numerical resolution of current computer simulations is sufficient to make quantitative predictions on surface density profiles and color gradients.

We have created a map of the large-scale infrared surface brightness in excess of that associated with the atomic interstellar medium, using region-by-region correlations between the far-infrared and 21 cm line surface brightness. Our study updates and extends a previous attempt with the InfraredAstronomicalSatellite and Berkeley/Parkes H I surveys; in this study we used far-infrared (60-240 μm) data from the CosmicBackgroundExplorer Diffuse Infrared Background Experiment and 21 cm data from the combined Leiden-Dwingeloo and Parkes 21 cm line surveys. Using the maps of excess infrared emission at 100, 140, and 240 μm, we created an atlas and identified the coherent structures. These infrared excess clouds can be caused both by dust that is warmer than average or by dust associated with gas other than the atomic interstellar medium. We find very few warm clouds—which are relatively bright at 60 μm—such as the H II region around the high-latitude B-type star α Vir and a new cloud of unknown origin that we name DIR 015+54. Using the ratio of 100 to 240 μm brightness, we find that infrared excess clouds are cold. The dust temperature in atomic gas is 19 ± 2 K, while the dust temperature in known high-latitude molecular clouds (all of which have infrared excess) is 15.5 ± 1 K. The dust temperature in those infrared excess clouds that are not known to be associated with molecular clouds (generally because they have never been observed) is 17 ± 2 K, suggesting they are similar to high-latitude molecular clouds. Infrared excess clouds are peaks of column density rather than dust temperature, and their excess infrared emission is likely due to dust associated with molecular gas. For a large region in Ursa Major-Ursa Minor-Camelopardalis, where the CO(1 → 0) line has been surveyed, we correlated the infrared excess CO line integral, allowing us to measure X = N(H₂)/W(CO) = (1.3 ± 0.2) × 10²⁰ cm² (K km s^-1)^-1 for high-latitude molecular clouds. Our measurement of X takes into account the low dust temperature in molecular gas; this correction amounts to a factor of 3.8 increase in the X-value that would naively be determined using only 100 μm, CO, and H I data. Our value of X is consistent with a recent γ-ray determination for the same region, while it is a factor of about 2 lower than the value determined for the inner galactic plane. The surface mass density of infrared excess clouds is 0.3 M_☉ pc^-2. The atlas of infrared excess clouds may be useful as a guide to regions of relatively high interstellar column density, which might extinct light from extragalactic objects at optical to ultraviolet wavelengths and confuse structures in the cosmic background at infrared to microwave wavelengths.

We compute the cluster autocorrelation function ξ_cc(r) of an X-ray flux-limited sample of Abell clusters (XBACs). For the total XBACs sample we find a power-law fit ξ_cc = (r/r₀)^γ with r₀ = 21.1 Mpc h^-1 and γ = -1.9, consistent with the results of R ≥ 1 Abell clusters. We also analyze ξ_cc(r) for subsamples defined by different X-ray luminosity thresholds where we find a weak tendency of larger values of r₀ with increasing X-ray luminosity, although with a low statistical significance. In the different subsamples analyzed we find 21 Mpc h^-1 < r₀ < 35 Mpc h^-1 and -1.9 < γ < -1.6. Our analysis suggests that cluster X-ray luminosities may be used for a reliable confrontation of cluster spatial distribution properties in models and observations.

We calculate the relativistic thermal bremsstrahlung Gaunt factor for the high-temperature plasma that exists in clusters of galaxies. We calculate the Gaunt factor by employing the Bethe-Heitler cross section corrected by the Elwert factor. We also calculate the Gaunt factor by using the Coulomb-distorted wave functions for nonrelativistic electrons following the method of Karzas and Latter. By comparing the Gaunt factors calculated by these two different methods, we carefully assess the accuracy of the calculation. We present the numerical results in the form of tables.

We have previously reported a significant excess of K ≳ 19 galaxies in the fields of a sample of 31 z = 1-2 quasars (Hall, Green, & Cohen). Here we examine the properties of this excess galaxy population using optical and near-IR imaging. The excess occurs on two spatial scales. One component lies at θ < 40'' from the quasars and is significant compared to the galaxy surface density at θ > 40'' in the same fields. The other component appears roughly uniform to θ ~ 100'' and is significant compared to the galaxy surface density seen in random-field surveys in the literature. The r - K color distributions of the excess galaxy populations are indistinguishable and are significantly redder than the color distribution of the field population. The excess galaxy population is consistent with being predominantly early-type galaxies at the quasar redshifts, while there is no evidence that it is associated with intervening Mg II absorption systems. The average excess within 0.5 h⁻¹₇₅ Mpc (~65'') of the quasars corresponds to Abell richness class ~0 compared to the galaxy surface density at >0.5 h⁻¹₇₅ Mpc from the quasars and to Abell richness class ~1.5 compared to that from the literature. We estimate -0.65^{+ 0.41}_−0.55 mag of evolution in M_K* to =1.67 by assuming the excess galaxies are at the quasar redshifts. We discuss the spectral energy distributions (SEDs) of galaxies in fields with data in several passbands. Most candidate quasar-associated galaxies are consistent with being 2-3 Gyr old early types at the quasar redshifts of z ~ 1.5. However, some objects have SEDs consistent with being 4-5 Gyr old at z ~ 1.5, and a number of others are consistent with ~2 Gyr old but dust-reddened galaxies at the quasar redshifts. These potentially different galaxy types suggest there may be considerable dispersion in the properties of early-type cluster galaxies at z ~ 1.5. There is also a population of galaxies whose SEDs are best modeled by background galaxies at z ≳ 2.5.

We measure quantitative structural parameters of galaxies in the Hubble deep field (HDF) on the drizzled F814W images. Our structural parameters are based on a two-component surface brightness made up of a Sérsic profile and an exponential profile. We compare our results to the visual classification of van den Bergh et al. and the C-A classification of Abraham et al. Our morphological analysis of the galaxies in the HDF indicates that the spheroidal galaxies, defined here as galaxies with a dominant bulge profile, make up for only a small fraction, namely, 8%, of the galaxy population down to m_F814W(AB) = 26.0. We show that the larger fraction of early-type systems in the van den Bergh sample is primarily due to the difference in classification of 40% of small round galaxies with half-light radii less than 031. Although these objects are visually classified as elliptical galaxies, we find that they are disk dominated with bulge fractions less than 0.5. Given the existing large data set of HDF galaxies with measured spectroscopic redshifts, we are able to determine that the majority of distant galaxies (z > 2) from this sample are disk dominated. Our analysis reveals a subset of HDF galaxies that have profiles flatter than a pure exponential profile.

We investigate the formation of disk-bulge-halo systems by including bulges in the Fall & Efstathiou theory of disk formation. This allows an investigation of bulge-dominated disk galaxies, such as S0's and disky ellipticals. These latter systems, which consist of an elliptical spheroid with an embedded disk with a scale length of typically a few hundred parsecs, seem to form a smooth sequence with spirals and S0's toward a lower disk-to-bulge ratio. The aim of this paper is to examine whether spirals, S0's, and disky ellipticals all can be incorporated in one simple galaxy-formation scenario. We investigate an inside-out formation scenario in which subsequent layers of gas cool and form stars inside a virialized dark halo. The inner, low angular momentum material is assumed to form the bulge. Stability arguments are used to suggest that this bulge formation is a self-regulating process in which the bulge grows until it is massive enough to allow the remaining gas to form a stable disk component. We assume that the baryons that build the disk do not lose their specific angular momentum, and we search for the parameters and physical processes that determine the disk-to-bulge ratio and therewith explain to a large extent the origin of the Hubble sequence. The spread in halo angular momenta coupled with a spread in the formation redshifts can explain the observed spread in disk properties and disk-to-bulge ratios from spirals to S0's. If galaxy formation is efficient, and all available baryons are transformed into the disk-bulge system, cosmologies with Ω₀ ≲ 0.3 can be excluded since stable spiral disks would not be allowed to form. If we assume, however, that the efficiency with which galaxies form depends on the formation redshift, as suggested by the small amount of scatter in the observed Tully-Fisher relation, and we assume that the probability for a certain baryon ultimately to end up in the disk or bulge is independent of its specific angular momentum, spirals are allowed to form, but only at small formation redshifts (z ≲ 1). At higher formation redshifts, stability arguments suggest the formation of systems with smaller disk-to-bulge ratios, such as S0's. Since density perturbations in clusters will generally collapse earlier than those in the field, this scenario naturally predicts a density-morphology relation, the amplitude of which depends on the baryon fraction of the universe. Disky ellipticals are too compact to be incorporated in this scenario, and thus they do not form a continuous sequence with spirals and S0's, at least not in the sense of the galaxy-formation scenario envisioned in this paper. Alternative formation scenarios for the disky ellipticals, such as gas-rich mergers or an internal mass-loss origin for the embedded disks, are much more viable.

New CO interferometer data show that the molecular gas in infrared ultraluminous galaxies is in rotating nuclear disks or rings. The CO maps yield disk radii, kinematic major axes, rotation speeds, enclosed dynamical masses, and gas masses. The CO brightness temperatures, the double-peaked CO line profiles, the limits on thermal continuum flux from dust, and the constraint that the gas mass must be less than the dynamical mass all indicate that the CO lines are subthermally excited and moderately opaque (τ = 4 to 10). We fit kinematic models in which most of the CO flux comes from a moderate-density warm intercloud medium, rather than from self-gravitating clouds. Typical ring radii are 300 to 800 pc. We derive gas masses not from a standard CO-to-mass ratio, but from a model of radiative transfer through subthermally excited CO in the molecular disks. This model yields gas masses of ~5 × 10⁹M_☉, ~5 times lower than the standard method, and a ratio M/L ≈ 0.8 M_☉ (K km s^-1 pc²)^-1. In the nuclear disks, we derive a ratio of gas to dynamical mass of M_gas/M_dyn ≈ 1/6, and a maximum ratio of gas to total mass surface density, μ/μ_tot, of 1/3. For the galaxies VII Zw 31, Arp 193, and IRAS 10565+2448, the CO position-velocity diagrams provide good evidence for rotating molecular rings with a central gap. In addition to the rotating central rings or disks, a new class of star formation region is identified, which we call an extreme starburst. These have a characteristic sizes of only 100 pc, with about 10⁹M_☉ of gas and an IR luminosity of ≈ 3 × 10¹¹L_☉ from recently formed OB stars. Four extreme starbursts are identified in the 3 closest galaxies in the sample, including Arp 220, Arp 193, and Mrk 273. These are the most prodigious star formation events in the local universe, each representing about 1000 times as many OB stars as 30 Doradus. In Mrk 231, the CO (2-1) velocity diagram along the line of nodes shows a 12 diameter inner disk and a 3'' diameter outer disk. The narrow CO line width, the single-peak line profile, the equality of the major and minor axes, and the observed velocity gradients all imply that the molecular disk is nearly face-on, yielding low optical and UV extinction to the active galactic nucleus (AGN). Such a geometry means that the molecular disk cannot be heated by the AGN; the far-infrared (FIR) luminosity of Mrk 231 is powered by a starburst, not the AGN. In Mrk 273, the CO (1-0) maps show long streamers of radius 5 kpc (7'') with velocity gradients north-south, and a nuclear disk of radius 400 pc (06) with velocity gradients east-west. The nuclear disk contains a bright CO core of radius 120 pc (02). In Arp 220, the CO and 1.3 mm continuum maps show the two "nuclei" embedded in a central ring or disk at P.A. 50° and a fainter structure extending 7'' (3 kpc) to the east, normal to the nuclear disk. Models of the CO and dust flux indicate that the two K-band sources contain high-density gas, with n(H₂) = 2 × 10⁴ cm^-3. There is no evidence that these sources really are the premerger nuclei. They are more likely to be compact extreme starburst regions, containing 10⁹M_☉ of dense molecular gas and new stars, but no old stars. Most of the HCN emission arises in the two nuclei. The luminosity-to-mass ratios for the CO sources in Arp 220 are compatible with the early phases of compact starbursts. There is a large mass of molecular gas currently forming stars with plenty of ionizing photons, and no obvious AGN. The entire bolometric luminosity of Arp 220 comes from starbursts, not an AGN. The CO maps show that the gas in ultraluminous IR galaxies is in extended disks that cannot intercept all the power of central AGNs, even if they exist. We conclude that in ultraluminous IR galaxies—even in Mrk 231, which hosts a quasar—the FIR luminosity is powered by extreme starbursts in the molecular rings or disks, not by dust-enshrouded quasars.

We report the detection of Cepheids and a new distance to the spiral galaxy NGC 2541, based on data obtained with the Wide Field Planetary Camera 2 on board the HubbleSpaceTelescope (HST). A total of 25 exposures (divided into 13 epochs) were obtained using the F555W filter (transformed to Johnson V), and nine exposures were obtained (divided into five epochs) using the F814W filter (transformed to Cousins I). Photometric reduction of the data is performed using two independent packages, DoPHOT and DAOPHOT II/ALLFRAME, which give very good agreement in the measured magnitudes. A total of 34 bona fide Cepheids, with periods ranging from 12 to over 60 days, are identified based on both sets of photometry. By fitting V and I period-luminosity relations, apparent distance moduli are derived assuming a Large Magellanic Cloud distance modulus and mean color excess of μ_LMC = 18.50 ± 0.10 mag and E(B-V) = 0.10 mag, respectively. Adopting A(V)/E(V-I) = 2.45, we obtain a true distance modulus to NGC 2541 of μ₀ = 30.47 ± 0.11 (random) ± 0.12 (systematic) mag (D = 12.4 ± 0.6 [random] ± 0.7 [systematic] Mpc), and a total (Galactic plus internal) mean color excess E(B-V) = 0.08 ± 0.05 (internal error) mag.

We report on kinematic observations of Hα emission from four late-type galaxies of Hickson Compact Group 16 (H16a, b, c, d) obtained with a scanning Fabry-Perot interferometer and samplings of 16 km s^-1 and 1''. The velocity fields show kinematic peculiarities for three of the four galaxies: H16b, H16c, and H16d. Misalignments between the kinematic and photometric axes of gas and stellar components (H16b-H16d), double gas systems (H16c) and severe warping of the kinematic major axis (H16b and H16c) were some of the peculiarities detected.

We conclude that major merger events have taken place in at least two of the galaxies of the group, H16c and H16d, based on their significant kinematic peculiarities, their double nuclei, and their high infrared luminosities. Their Hα gas content is strongly spatially concentrated; H16d contains a peculiar barlike structure confined to the inner ~1 h^-1 kpc region. These observations are in agreement with predictions of simulations, namely, that the gas flows toward the galaxy nucleus during mergers, forms bars, and fuels the central activity. Galaxy H16b, an Sb galaxy, also presents some of the kinematic evidence for past accretion events. Its gas content, however, is very sparse, limiting our ability to find other kinematic merging indicators, if they are present. We find that the merger remnants in the compact group HCG 16 have significantly smoother optical profiles than isolated mergers, i.e., they show an amorphous morphology and no signs of tidal tails. Tidal arms and tails formed during the mergers may have been stripped by the group potential, or, alternatively, they may have never been formed.

The velocity field of the galaxy H16a shows grand-design isovelocity lines with no signs of disturbances inside a radius of ~R₂₅. This result is contrary to expectations given that the galaxy has a high infrared luminosity, central activity, tidal tails at large radii, and is embedded in a common group envelope observed in H I and X-rays. The normality of the velocity field suggests that this galaxy may be a fairly recent acquisition of the compact group.

Our observations suggest that HCG 16 may be a young compact group in formation through the merging of close-by objects in a dense environment.

We present VLBI observations of the EGRET quasars 0202+149, CTA 26, and 1606+106, as well as additional analysis of VLBI observations of 1156+295 presented in a previous letter. We have produced 8 and 2 GHz VLBI images at 11 epochs, 8 epochs, and 12 epochs, spanning the years 1989-1996, of 0202+149, CTA 26, and 1606+106, respectively. The VLBI data have been taken from the Washington VLBI correlator's geodetic database. We have measured the apparent velocities of the jet components and find that CTA 26 and 1606+106 are superluminal sources, with average apparent speeds of 8.9 and 2.9 h^-1c, respectively (H₀ = 100 h km s^-1 Mpc^-1, q₀ = 0.5). The components in 0202+149 are stationary, and we identify this source as a compact F double. These sources all have apparently bent jets, and we detected nonradial motion of components in CTA 26 and 1156+295. We have not yet detected any components emerging subsequent to the γ-ray flares in CTA 26, 1156+295, and 1606+106, and we derive lower limits on the ejection times of any such components. The misalignment angle distribution of the EGRET sources is compared to the distribution for blazars as a whole, and we find that EGRET sources belong preferentially to neither the aligned nor the misaligned population. We also compare the average values for the apparent velocities and Doppler beaming factors for the EGRET and non-EGRET blazars, and find no significant differences. We thus find no indication, within the measurement errors, that EGRET blazars are any more strongly beamed than their counterparts that have not been detected in γ-rays.

We present the results of a search for CO 1 → 0 emission from NGC 147 and M32, two of the four dwarf elliptical companions of M31. Return of gas from evolved stars to the interstellar medium of these galaxies should have resulted in detections, but we find instead upper limits of 4100 and 5100 M_☉ of H₂ for NGC 147 and M32, respectively. Including and earlier H I limit, we find that the gaseous interstellar medium (ISM) of NGC 147 comprises less than 2% of what is expected. The large published H I mass limit for M32 prevents us from reaching a similarly extreme conclusion for this galaxy. These results stand in stark contrast to what is seen in NGC 185 and NGC 205, where the observed gas is approximately what is expected from stellar mass return, although some of the gas in NGC 205 probably had an external origin. There are no obvious differences in masses or luminosities that would explain the results. The proposal that differences may be related to the recent interaction histories of the galaxies with M31 is rendered moot by the lack of orbital information but does not seem to be viable for NGC 147 and NGC 185. We can offer no convincing explanation for these puzzling results, although they may point to a fundamental gap in our understanding of galaxy evolution.

We present a measurement of the deuterium to hydrogen ratio (D/H) in a metal-poor absorption system at redshift z = 2.504 toward the QSO 1009+2956. We apply the new method of Burles & Tytler to robustly determine D/H in high-resolution Lyα forest spectra and include a constraint on the neutral hydrogen column density determined from the Lyman continuum optical depth in low-resolution spectra. We introduce six separate models to measure D/H and to assess the systematic dependence on the assumed underlying parameters. We find that the deuterium absorption feature contains a small amount of contamination from unrelated H I. Including the effects of the contamination, we calculate the 67% confidence interval of D/H in this absorption system, log (D/H)=-4.40_−0.08^+0.06. This measurement agrees with the low measurement by Burles & Tytler toward Q1937-1009, and the combined value gives the best determination of primordial D/H, log (D/H)_p=-4.47_−0.035^+0.030 or D/H = 3.39 ± 0.25 × 10^-5. Predictions from standard big bang nucleosynthesis give the cosmological baryon-to-photon ratio, η = 5.1 ± 0.3 × 10^-10, and the baryon density in units of the critical density, Ω_bh² = 0.019 ± 0.001, where H₀ = 100h km s^-1 Mpc^-1. The measured value of (D/H)_p implies that the primordial abundances of both ⁴He and ⁷Li are high and consistent with some recent studies. Our two low measurements of primordial D/H also place strong constraints on inhomogeneous models of big bang nucleosynthesis.

We report observations and an analysis of the distribution of HCO⁺ (1-0) and HCO⁺ (4-3) emission in the central 1 kpc star-forming region of M82. Comparisons are made with other star formation indicators such as the millimeter continuum, the distribution of radio supernova remnants, and the molecules CO and OH. In a broad sense, the HCO⁺ is distributed in a way similar to the CO, although there are noticeable differences in detail, including an inward displacement of spiral arm emission relative to CO. A comparison of the position-velocity plots for CO, HCO⁺ (1-0), HCO⁺ (4-3), and ionized gas, with orbits expected in the presence of the nuclear bar, suggest an inward transfer of gas associated with star formation toward the nucleus.

The HCO⁺ (4-3)/(1-0) line ratios are comparatively uniform in the observed region, and according to a large velocity gradient analysis, reflect mean gas densities in the range 10⁴-10⁵ cm^-3 for kinetic temperatures in the range 20-60 K. The comparative uniformity of these conditions and the low filling factor suggest that each sampled point comprises a large number of clouds occupying a broad range of density, and possibly temperature. We briefly examine fractal-type models in the context of the HCO⁺ data as an alternative way to analyze molecular line emission in M82.

We present a study of the kinematics of the outer regions of the early-type galaxy NGC 1316, based on radial velocity measurements of 43 planetary nebulae as well as deep integrated light absorption line spectra. The smoothed velocity field of NGC 1316 indicates fast rotation at a distance of 16 kpc, possibly associated with an elongated feature orthogonal to the inner dust lanes. The mean square stellar velocity is approximately independent of radius, and the estimated total mass of the system is 2.9 × 10¹¹M_☉ within a radius of 16 kpc, implying an integrated mass-to-light ratio of M/L_B ≃ 8.

We report the results of our CO J = 4 → 3 line and rest-frame 650 μm continuum observations of the z = 0.93 hyperluminous infrared galaxy FSC 15307+3252 using the Owens Valley Millimeter Array. No line or continuum emission was detected, but the derived limits provide a useful constraint on the temperature, emissivity, and mass of the cold dust associated with FSC 15307+3252 and its molecular gas content.

The 3 σ upper limit on the velocity-integrated CO (4-3) line flux is 1.6 Jy km s^-1 (for ΔV = 300 km s^-1). This corresponds to a surprisingly small total molecular gas mass limit of 5 × 10⁹h^-2M_☉ for this galaxy with infrared luminosity L_FIR > 10¹³L_☉. Combined with existing photometry data, our 3 σ upper limit of 5.1 mJy for the 239 GHz (650 μm rest wavelength) continuum flux yields a total dust mass of 0.4-1.5 times 10⁸M_☉. The CO luminosity (thus molecular gas content) and the resulting gas-to-dust ratio are lower than the values typical for the more gas-rich infrared galaxies, but they are within the observed ranges. On the other hand, FSC 15307+3252 has a dust content and infrared luminosity 40 and 200 times larger than the infrared bright elliptical-like galaxies NGC 1275 and Cygnus A.

The far-infrared (FIR) luminosity to dust mass ratios, L_FIR/M_dust, for all three galaxies hosting a powerful active galactic nucleus (FSC 10214+4724, FSC 15307+3252, and Cygnus A) are larger than reasonably expected for a galaxy dominated by a starburst and 4 times larger than Arp 220. Therefore the bulk of the observed FIR luminosity in these galaxies is likely powered by their luminous active nuclei.

We report a multiwavelength analysis of an unusual high-energy transient: EXS 1737.9-2952. Due to the features this source exhibited in the hard X-ray domain similar to another source in the Galactic center region (1E1740.9 - 2952), and in order to study the molecular gas toward this X-ray source, we performed an observation of the EXS region in 1993 August, using the Swedish-ESO Submillimeter Telescope (SEST) located in La Silla (Chilean Andes). We observed a cloud, at the "forbidden" velocity of 135 km s^-1, using ¹²CO (1-0) transitions, giving a maximum column density of 8 × 10²¹ cm^-2. In 1994 we conducted other observations to search for higher density regions inside the cloud, using HCO+ and CS lines, but they were unsuccessful: we concluded that the cloud could be associated with the X-ray source and its mean density is of the order of ≈ 10³ cm^-3. In 1994 April, we performed a set of observations of the field containing EXS, at 20 cm and 6 cm, using the VLA in its A configuration, and found four possible radio candidates for an association with the EXS X-ray source, one of them (source 3) being extended, exhibiting a head-tail morphology, and a having a thermal spectrum with a spectral index ≈ -0.7. We reproduced our observation in 1994 November and December, using the C configuration at 6 cm, in order to investigate on possible variability and extension of these sources and found a marginal indication in the 20 cm image that source 3 may have a weak second component displaced about 15''. Nevertheless, this indication is too faint to associate this source definitely with EXS since no significant variation was detected. In addition, during the 1994 November-December observation, two more extended sources were detected but their association with EXS is unlikely.

We also analyzed the Einstein/IPC image of the 5 σ EXS error box which does not exhibit, at the time of the observation, any significant low-energy X-ray counterpart to EXS. A nearby pulsar PSR 1737-30 in the ROSAT catalog is outside this error box. Finally, IRAS maps of the EXS region do not show any IR contribution at the location of the radio sources. We conclude that (1) EXS 1737.9-2952 is a high-energy transient, (2) a persistent counterpart at other wavelengths is not demonstrated, and (3) EXS, when flaring, as well as other GC gamma-ray sources, could possibly contribute to the 511 keV bulge emission.

We have used the Long Wavelength Spectrometer aboard the InfraredSpaceObservatory in the grating mode to map the far-infrared continuum emission (45-175 μm) toward the massive giant molecular cloud core GCM 0.25+0.11 located near the Galactic center. Graybody models of the observed far-infrared spectral energy distribution indicate that the bulk of the dust in the diffuse component along the line of sight toward GCM 0.25+0.11 has a mean temperature of ~26 K and a 100 μm optical depth of ~0.17. GCM 0.25+0.11 is observed in emission at far-infrared (FIR) wavelengths (≳100 μm). However at midinfrared wavelengths (≲70 μm) the core is seen in absorption against the general Galactic center background. This indicates that GCM 0.25+0.11 is located in front of the bulk of the dust responsible for the diffuse FIR emission, most likely a few hundred parsecs from the Galactic center. By subtracting the spectrum of the diffuse component from the spectrum observed toward GCM 0.25+0.11, we have been able to extract the intrinsic spectrum of this GMC core. Graybody fits to the resulting far-infrared spectrum combined with our previous submillimeter measurements (350-800 μm) give a low temperature ~18 K for the bulk of the dust in the GCM 0.25+0.11 core. In addition, the grain emissivity is a very steep function of frequency (ν^2.8). The high grain emissivity exponent is consistent with the presence of dust grains covered with thick ice mantles. We have complemented our ISO data with CO (2-1) and HCO⁺ (3-2) observations carried out with the Caltech Submillimeter Observatory. The molecular emission shows a large velocity gradient across the southern part of the core indicative of streaming motions of the gas or of the presence of multiple, spatially overlapping velocity components. The observed gas kinematics may indicate that GCM 0.25+0.11 is in the process of being disrupted by the strong tidal forces caused by the high mass concentration in the Galactic center region. This might explain why there is no evidence for ongoing high-mass star formation associated with this core, in spite of its large molecular mass. However, the mean H₂ density of GCM 0.25+0.11 is well above the tidal stability limit for a Galactocentric distance of a few hundred parsecs implied by our observations. An alternative explanation is that we are witnessing the very early stage of a cloud-cloud collision that may result in a future star formation episode.

We have performed a detailed oxygen abundance analysis of 23 metal-poor (-3.0 < [Fe/H] < -0.3) unevolved halo stars and one giant through the OH bands in the near UV, using high-resolution echelle spectra. Oxygen is found to be overabundant with respect to iron in these stars, with the [O/Fe] ratio increasing from 0.6 to 1 between [Fe/H] = -1.5 and -3.0. The behavior of the oxygen overabundance with respect to [Fe/H] is similar to that seen in previous works based on O I IR triplet data. Contrary to the previously accepted picture, our oxygen abundances, derived from low-excitation OH lines, agree well with those derived from high-excitation lines of the triplet. For nine stars in common with Tomkin et al. we obtain a mean difference of 0.00 ± 0.11 dex with respect to the abundances determined from the triplet using the same stellar parameters and model photospheres. For four stars in our sample we have found measurements of the [O I] λ6300 line in the literature, from which we derive oxygen abundances consistent (average difference 0.09 dex) with those based on OH lines, showing that the long-standing controversy between oxygen abundances from forbidden and permitted lines in metal-poor unevolved stars can be resolved. Our new oxygen abundances show a smooth extension of the Edvardsson and coworkers [O/Fe] versus metallicity curve to much lower abundances, with a slope of -0.31 ± 0.11 (taking into account the error bars in both oxygen abundances and metallicities) in the range -3 < [Fe/H] < -1. The extrapolation of our results to very low metallicities indicates that the first Type II supernovae in the early Galaxy provided oxygen to iron ratios of [O/Fe] ≳ 1. The oxygen abundances of unevolved stars, when compared with values in the literature for giants of similar metallicity, imply that the latter may have suffered a process of oxygen depletion. As a result, unevolved metal-poor stars shall be considered better tracers of the early evolution of oxygen in the Galaxy. The higher [O/Fe] ratios we find in dwarfs has an impact on the age determination of globular clusters, suggesting that current age estimates have to be reduced by about 1-2 Gyr.

Six SX Phe variables detected in a 39 hr time series of HubbleSpaceTelescope Planetary Camera exposures in the F336W (U) filter are discussed. Two of these stars show both the fundamental and first-overtone modes and are among the first double-mode SX Phe stars reported in globular clusters. One of the double-mode stars shows evidence for a third mode near the expected period for the second overtone. Two of the other SX Phe variables in 47 Tuc are shown to oscillate simultaneously in the fourth and fifth radial overtones. The remaining two variables both have multiple oscillations excited which include nonradial modes. To support interpretation of these variables we provide evolutionary computations using current opacities and physics allowing comparison of the blue straggler stars (BSSs) and several cluster-magnitude diagram features for 47 Tuc. Linear nonadiabatic oscillation analysis is performed for a significant range of envelope and full evolutionary models in order to (1) establish the theoretical characteristics of SX Phe pulsations (e.g., location of primary driving and adiabatic nature), (2) establish the theoretical red and blue edges of the instability strip, and (3) derive the theoretical relations necessary for obtaining evolutionary and pulsational masses. Masses inferred from fundamental period-period ratio relations for the double-mode oscillators are well above the turnoff mass in 47 Tucanae and are consistent with expected masses for these blue stragglers based on position in the color-magnitude diagram (CMD) and comparison with theoretical evolutionary tracks. Combining the evolution and pulsation constraints results in mass estimates for the four double-mode BSSs in 47 Tuc of M = 1.35 ± 0.1 to 1.6 ± 0.2 M_☉.

We derive general relationships between the observed timescale of diffractive interstellar scintillations and the physical velocities of the observer, the source, and the scattering medium. Our treatment applies exclusively to saturated scintillations of point sources in the strong scattering regime. We show how scintillation observations may be combined with other observables (proper motion and dispersion measure) to yield (1) improvements in galactic models for the free-electron density and (2) estimates of the distance and transverse velocity of individual pulsars. We explicitly consider cases of current astrophysical interest, including hypervelocity pulsars too far above the Galactic plane to allow distance estimates from dispersion measures alone. We also briefly consider scintillations of extragalactic sources, including gamma-ray burst sources at great distances from the Galaxy.

We have obtained new observations of the massive molecular outflow in G192.16-3.82 in CO(J = 1-0) line and 3 mm continuum using the Owens Valley Radio Observatory millimeter-wave array. We have also imaged the outflow in the near-infrared J, H, and K bands and 2.12 μm H₂ at Lowell Observatory and in the K_s band at the Apache Point Observatory. A large-field Palomar image shows that the outflow structure may extend more than 4 pc from the young stellar object (YSO), making it one of the largest known Galactic outflows. There is approximately 80 M_☉ of molecular material in the high-velocity flow, and the mass flow rate is ~5 × 10^-4M_☉ yr^-1. The flow appears to be driven by an early-B star that is surrounded by approximately 17 M_☉ of material. A biconical infrared reflection nebula is centered near the millimeter continuum peak, and shock-excited H₂ emission is detected at the brightest peak in the K-band reflection nebula. H₂ emission is also detected just beyond the highest velocity gas in the blueshifted CO outflow. The slope of the mass spectrum is approximately -2 at velocities below 15 km s^-1 and decreases to as low as -8 at higher velocities. This is significantly steeper than in low-mass outflows, suggesting that the powering mechanism is less efficient at accelerating material in the flow. Alternatively, the outflow may have built up a substantial reservoir of low-velocity gas that steepens the mass spectrum. The observation of a wide outflow opening angle (~60°) and limb-brightened shell surrounding the high-velocity gas, together with shock-excited H₂ emission and large bow shocks that extend up to 4 pc from the YSO, are consistent with the presence of both a poorly collimated disk-wind and a jet.

We investigate the extinction produced by small carbon particles consisting of aromatic rings in configurations similar to those of polycyclic aromatic hydrocarbon (PAH) molecules, but with varying degrees of hydrogenation, ionization, and defects. Extinction produced by candidate particles is calculated using the discrete dipole array (DDA) formalism with optical constants similar to those of graphite modified to take these changes into account. Clusters of neutral molecules as well as mixed ion-neutral clusters are also investigated. We find that certain specific carbon structures such as a dehydrogenated coronene-like molecule, C₂₄, exhibit a plasmon-type resonance at the 2175 Å wavelength of the interstellar extinction bump. However, we find that similar fits can be obtained using a distribution of molecule shape and sizes containing up to several hundred C atoms. These results suggest that interstellar "graphite" may be the high-mass component of the population of PAH seen in interstellar clouds. Such particles could derive from fragments ejected from carbonaceous grains in shocks.

We present spatially resolved echelle spectroscopy, obtained at high spectral resolution, for 15 multiple-shell planetary nebulae. Most exhibit faint detached halos (IC 1295, MA 3, M 2-2, M 2-40, NGC 6804, NGC 6826, NGC 6884, NGC 6891, NGC 7662, PM 1-295, and Vy 2-3). Furthermore, we have included some with attached shells (IC 1454, K 1-20, K 3-73, and PM 1-276) to allow comparison of the kinematic properties of the two subclasses of multiple-shell planetary nebulae. In addition, some of the nebulae in our sample show a triple-shell structure, composed of the bright main nebula and a combination of two attached shells (PB 9), one attached shell and one detached halo (NGC 6826, NGC 6891, NGC 7662, and Vy 2-3), or two detached shells (NGC 6804). A new method for computing the expansion velocities of those shells that do not show line splitting has been developed. This method assumes a thick-shell model and uses the observed Hα emission brightness profile to compute the volume emissivity dependence, (r), with the distance from the center of the nebula. The expansion velocity is then worked out by modeling how much the width of a the Hα line decreases with the radius of the shell. The radial velocity, expansion velocities of each shell, and turbulence contribution to the line width are presented. The expansion velocity of the detached halos spans from 12 to 30 km s^-1. It is worth noting that the expansion velocities obtained by this method are greater than if they were computed with a thin-shell model, as has previously been done. In relation to the attached shells, their expansion velocities span from 10 to 30 km s^-1. When the expansion velocities of the outer attached shells are related to the ellipticity of the inner shells, a trend toward faster expansion of the outer than the inner shells at higher ellipticities is found. The turbulent contribution to the line width has also been established. It is smaller for halos (0 km s^-1 ≤ σ_tur ≤ 6 km s^-1) than for attached shells (0 km s^-1 ≤ σ_tur ≤ 15 km s^-1). This suggests that large-scale hydrodynamic processes are more important in attached shells than in detached halos. We have also studied the kinematics of the detached halos whose morphology is perturbed from a round shape to a dipole asymmetry, indicating its interaction with the surrounding interstellar medium. We found systematic differences between the kinematical behavior of the enhanced edge of the halo and the opposite side in these cases, thus revealing the kinematic effect of the interaction of the halos with the interstellar medium.

We present the first 22 GHz polarimetric VLBI images of low-velocity water masers in the star-forming region W51 M. The compact maser concentration found near the reference position of W51 M is identified as a protostellar cocoon with both rotational and radial motions. The inner and outer radii of this water maser cocoon are 5 and 66 AU, respectively. Adjacent to this protostellar cocoon we see a 1200 AU long linear maser structure at a position angle of 200° (the streamer), which is roughly aligned with the Galactic magnetic field projection on the sky and the polarization position angle of these masers. The streamer masers move longitudinally along this direction with a median space velocity of 25 (±8.4) km s^-1 relative to the centroid of the cocoon. In contrast to the cocoon masers, which show a mean linear polarization of only 3% (maximum 13%), the masers in the streamer exhibit higher degrees of linear polarization (mean 12%; maximum 35%). The level of distortion in the polarization directions of the streamer masers from the magnetic field direction together with the observationally estimated nonthermal velocity dispersion of the streamer spots with respect to the mean velocity of the streamer yield a preshock magnetic field strength, perpendicular to the shock velocity, of 0.9-1.2 (±0.25) mG. Inside the streamer masers we estimate the typical magnetic field strength to be 38 (±15) mG. We present statistics of the cocoon and streamer masers and discuss the origin of the maser stream, which is difficult to explain as an outflow from W51 M. Most likely the streamer is produced by shocks caused by the nearby expanding H II region, which interacts with the dense molecular core of W51 M on its western side. The proximity to the protostar suggests that these shocks have affected, or even triggered, the star formation in W51 M.

We present results from a study of starspot areas (f_S) and temperatures (T_S), primarily on active, single-lined spectroscopic binaries, determined using molecular absorption bands. Expanding upon our previous studies, we have analyzed multiorder echelle spectra of eight systems to simultaneously measure several different molecular bands and chromospheric emission lines. We determined starspot parameters by fitting the molecular bands of interest, using spectra of inactive G and K stars as proxies for the nonspotted photosphere of the active stars, and using spectra of M stars as proxies for the spots. At least two bands with different T_eff sensitivities are required. We found that fitting bands other than the TiO 7055 and 8860 Å features does not greatly extend the temperature range or sensitivity of our technique. The 8860 Å band is particularly important because of its sharply different temperature sensitivity. We did not find any substantial departures from f_S or T_S that we have measured previously based on single-order spectra. We refined our derived spot parameters using contemporaneous photometry where available. We found that using M giants as spot proxies for subgiant active stars often underestimates f_S needed to fit the photometry; this is presumably due to the increase in strength of the TiO bands with decreasing gravity. We also investigated correlations between f_S and chromospheric emission, and we developed a simple method to measure nonspot temperature (T_Q) solely from our echelle spectra.

Circularization of late-type main-sequence binaries is usually attributed to turbulent convection, while that of early-type binaries is explained by resonant excitation of g-modes. We show that the latter mechanism also operates in solar-type stars and is at least as effective as convection despite inefficient damping of g-modes in the radiative core. The maximum period at which this mechanism can circularize a binary composed of solar-type stars in 10¹⁰ yr is as low as 3 days, if the modes are damped by radiative diffusion only and g-mode resonances are fixed, or as high as 6 days if one allows for evolution of the resonances and for nonlinear damping near inner turning points. Even the larger theoretical period falls short of the observed transition period by a factor of 2.

Simultaneous X-ray/UV observations over a full day on 1996 March 14-15 have been made of the prototypical B0.5e star γ Cas using the RossiX-RayTimingExplorer satellite and the Goddard High Resolution Spectrograph (GHRS) on board the HubbleSpaceTelescope. The GHRS spectra, taken in the region of the Si IV λλ1394-1403 doublet, also permitted the construction of an extremely precise light curve from a nearby "pseudocontinuum" region. The continuum UV and X-ray light curves reveal a pair of X-ray maxima ~10 hr apart that coincide in time with UV continuum flux "dips" of ~1%. In the first paper in this series we attributed the long-term X-ray variations to magnetic activity sites on the surface of the star that undergo rotational modulation on a ~1.125 days period. In the current study we find that flux and color curves generated from a 33 hr sequence of InternationalUltravioletExplorer (IUE) echellegrams obtained in 1996 January display dip features similar to those in the GHRS data. Comparing the timings of the continuum flux dips and the Si IV line strength variations in both the GHRS and IUE data sets gives a slightly revised period of 1.123 days for both the UV and X-ray activities. This strengthens the argument that high-energy activity on γ Cas is modulated by rotation of long-lived structures close to its surface. Analysis of the pseudocontinuum light curves constructed from the GHRS and IUE light curves shows at least two surprising characteristics for the flux dips: (1) the dips last only ~0.3 cycles, which is too brief for rotation modulation of surface features, and (2) their amplitudes increase from long to short wavelengths, which attain a maximum near 1206 Å. The character of the variations of the photospheric Si IV line profiles is unexpected in that the equivalent width fluctuations do not correlate with the slow undulations of the continuum flux. Moreover, the profile variations do not show an expected blue-to-red migration of microfeatures. We show that the continuum characteristics and absence of migration of features in the Si IV lines can be explained by the presence of very cool, optically thin clouds that corotate with the star. Assuming a tilt of the rotational axis of +45° to the observer's line of sight, our model simulations of the two major dips in the UV light curves indicate that the clouds have radii of a few tenths of a stellar radius and are attached to points on the surface at low to mid-latitudes on the near hemisphere. These findings support the conclusion of the first paper in this series that γ Cas is a member of a small group of OB stars that have magnetospheres associated with X-ray activity.

Ultraviolet spectra using HubbleSpaceTelescope sampled between 1250 and 1680 Å at spectral resolution ≤0.57 Å are reported for characteristically bright regions of Jupiter's morning and afternoon northern aurora. Several observed spectra exhibit sharply enhanced resolution. We interpret this as bright auroral emission foreshortened on the morning limb with a maximum intensity at least as high as 2000 kR. We have searched for evidence that the primary precipitating particles exciting the aurora include the heavy ions known to exist in Jupiter's plasma torus and magnetosphere. We have also searched for such ambient heavy ions and neutrals at rest in the auroral ionosphere, the end products of previous precipitation, excited by the auroral cascade. We argue that primary emission would be characterized by a dramatically Doppler-broadened (~10-15 Å) and redshifted line profile resulting from the cascade process and the angle between the line of sight and the magnetic field lines in the atmosphere. In contrast, ambient emission would be distinguished by narrow emission lines. We have modeled the theoretical sulfur and oxygen line shapes for ion precipitation and conclude that electron precipitation is responsible for most of the H₂ emissions. O ions contributed <13% of the precipitating energy flux, and S ions contributed <50%. This dominance suggests that field-aligned magnetospheric currents are more important than energetization of energetic ions and subsequent scattering by plasma waves as a mechanism for generating the Jovian aurora. We set an upper limit over our spectra of 35-43 R to the emission from ambient oxygen and sulfur ions and their neutrals, except that for the S II 1256 triplet, the upper limit for the nominally brightest line, at 1260 Å, is 74 R. Hence, we find no evidence for the accumulation of sulfur in the auroral ionosphere. A single narrow emission line from an unidentified ambient specie near 1254 Å may be detected at the 4 σ level, introducing the possibility of complex auroral aeronomy. Differences were observed in the auroral spectral hydrocarbon absorption at different locations, which cannot be interpreted without ambiguity between auroral and atmospheric structural causes. We have found that the brighter emission in an auroral sector consistently shows more spectral hydrocarbon absorption than the dimmer emission. We suggest two alternative physical explanations for this phenomenon.

We present two examples of current sheets that form in a magnetic configuration when it is subjected to quasi-static motions at the footpoints. The entire system is two-dimensional. There are no preexisting X-points and the footpoint motions are continuous. The calculations are motivated by the hypothesis of Parker that quasi-static deformations of MHD equilibria are generally accompanied by the formation of current sheets. The results demonstrate that three dimensions are not a necessary condition for current sheet formation. In addition, the calculation of Van Ballegooijen is not contradicted, because the initial magnetic field in our case is not dominantly collinear. Possible applications to the solar corona are discussed.

Results from high-quality solar upper atmosphere observations in the 10⁴-10⁶ K range contradict predictions made by models that assume that a single class of structures stretching throughout the entire temperature domain, from the cold chromosphere to the hot corona, is responsible for all the radiation we see. As a result, I proposed that new types of structures—the unresolved fine structures (UFSs)—are responsible for most of the detected emission in the 3 × 10⁶ ≤ T_e ≤ 8 × 10⁶ K. In a recent paper Wikstøl, Judge, & Hansteen challenged the presence of UFSs by claiming that the interpretation of the data in terms of UFSs is not unique and is likely to be incorrect in the presence of plasmas with unresolved dynamics. Further, they claim that most or all of the evidence that was brought in support of the UFSs is amenable to a different, equally reasonable interpretation in which the transition region emission is at all times formed in the time-varying thermal interface between the corona and chromosphere. In this paper I discuss some of the assertions made by Wikstøl and collaborators. I also bring evidence from elemental abundance studies that would be difficult to support with the Wikstøl model. In contrast, the existence of UFSs is not contradicted by any of the new observations.

We examine the solar neutrino flux, as measured by the Homestake neutrino detector, to search for evidence of a dependence upon the solar latitude of the Earth-Sun line that varies from 725 south in mid-March to 725 north in mid-September. Although the flux does not obviously show any dependence on latitude, we do find evidence for a dependence of the variance of the flux upon latitude. When data from 108 runs of the Homestake experiment are divided into four quartiles, sorted according to latitude, we find that the northernmost quartile exhibits a larger variance than the other three. By applying the shuffle test, we estimate the probability that this could have occurred by chance to be in the range 1%-2%.

For more detailed information, we examine a "reconstructed flux" formed from our recent maximum likelihood spectrum analysis. This procedure indicates that the variance is largest at about 65 north. We also find that the spectrum of the variance of the reconstructed flux has a notable peak at 1 cycle y^-1 tending to confirm a latitude dependence of the variance. We also examine the 12.88 cycle yr periodicity described in our recent paper and find that the amplitude of the periodicity is greater for the northernmost quartile than for the other quartiles. We suggest that these effects may be attributed to resonant spin-flavor precession of left-hand-helicity electron neutrinos in the magnetic field of the solar radiative zone.

Ulysses magnetic field and plasma data recorded during 1992 and 1993 as the spacecraft moved through mid-heliolatitudes, have been used to identify intervals dominated by Alfvénic fluctuations in the presence of corotating interaction regions. A wavelet frame representation of Elsässer variables was used to calculate the Alfvénicity of the fluctuations as a function of time and frequency, and to describe their occurrence primarily within trailing edges of the corotating interaction regions that occurred within the interval. It was confirmed that, in general, if a dominant population of Alfvén waves was present, then these waves were outward propagating, but the power in the waves was found to be highly variable in time. A decrease in the power in the Alfvén waves was found for frequencies lower than ~1 × 10^-5 Hz, corresponding to a scale between 0.3 and 0.5 AU. A dominant population of outward-propagating Alfvén waves was typically found to occur during the trailing edge of high-speed streams, as expected from previously published studies, but they did not persist for the whole of the trailing edge. They generally occurred within the first half and ended in a sharp cutoff across all frequencies. Dominant outward-propagating Alfvénic fluctuations were also identified during some low-speed streams, but they did not persist between rotations. No latitudinal dependence of the purity of the fluctuations was found, but the fluctuations became more persistent at higher latitudes.

We determine the electron density at the temperatures of formation of O⁺⁴ and Si⁺² ions, which are about 2.5 × 10⁵ and 3.2 × 10⁴ K in ionization equilibrium, respectively. These temperatures occur in the lower transition region of the Sun's atmosphere and allow a test of the often invoked assumption of constant pressure in quiet-Sun models. The O⁺⁴ density is determined from a density-sensitive spectroscopic O V line ratio involving 2s2p³P-2p²³P transitions that fall near 760 Å. The Si⁺² density is determined from a density-sensitive Si III line ratio within the 3s3p³P-3p²³P multiplet near 1300 Å. There are few available line ratio techniques for determining the density and hence electron pressure in the quiet-Sun and coronal hole transition regions using lines emitted by the same ion, and determining these quantities is the principal motivation for this work. The spectra used in our analysis were obtained from the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) experiment on the SolarandHeliosphericObservatory (SOHO). We determine the electron density and pressure in typical quiet-Sun/coronal hole regions, and densities in active region brightenings and in an explosive event. Our O V and Si III results indicate that constant pressure is valid or nearly valid in quiet-Sun lower transition regions, although there are complications arising from the weakness of a key Si III line in the quiet-Sun disk spectra. We also discuss our results in light of other density measurements and theories regarding the structure and heating of the transition region.

The ⁷Li(n, γ)⁸Li reaction has relevance for primordial nucleosynthesis in inhomogeneous big bang models as well as in connection with the solar neutrino problem, where the mirror reaction ⁷Be(p, γ)⁸Be is responsible for the production of high-energy solar neutrinos. The cross section was measured at neutron energies of E_n = 5 meV and E_n = 54 keV. With the present results and a reanalysis of a previous measurement, the discrepancies among existing data could be resolved and the cross section can be determined with improved accuracy.

We investigate the non-Gaussian properties of cosmic string-seeded linear density perturbations with cold and hot dark matter backgrounds using high-resolution numerical simulations. We compute the one-point probability density function of the resulting density field and its skewness, kurtosis, and genus curves for different smoothing scales. A semianalytic model is then invoked to provide a physical interpretation of our results. We conclude that on scales smaller than 1.5(Ωh²)^-1 Mpc, perturbations seeded by cosmic strings are very non-Gaussian. These scales may still be in a linear or mildly nonlinear regime in an open or Λ universe with Γ=Ωh≲0.2.

In order to investigate the reality of large-scale streaming motion on scales of up to 150 Mpc, we have studied the peculiar motions of ~200 early-type galaxies in three directions of the South Equatorial Strip at distances out to ~20,000 km s^-1. The new Automatic Plate Measuring Facility South Equatorial Strip Catalog (-175<δ<+25) was used to select the sample of field galaxies in three directions: (1) 15^h10^m-16^h10^m; (2) 20^h30^m-21^h50^m; (3) 00^h10^m-01^h30^m. New R-band CCD photometry and spectroscopic data for the galaxies are used. The fundamental plane distance-indicator relation is calibrated with Coma cluster data, and a correction for inhomogeneous Malmquist bias is applied to the distance estimates. A linear bulk flow model is fitted to the peculiar velocities in the sample regions, and the results do not reflect the bulk flow observed by Lauer and Postman (LP). Accounting for the difference in geometry between the galaxy distribution in the three regions and the LP clusters confirms the disagreement; assuming a low-density CDM power spectrum, we find that the observed bulk flow of the galaxies in our sample excludes the LP bulk flow at the 99.8% confidence level.

We explore the shapes of clusters and superclusters in the IRAS 1.2 Jy redshift survey with three reconstructions spanning the range β=0.1, 0.5, and 1.0, where β=Ω^0.6/b; b is the bias factor, and Ω is the present value of the dimensionless matter density. Comparing our results with Gaussian randomized reconstructions of the IRAS catalog, we find structures having both planar and filamentary properties. For β=0.5 and 1.0, the largest structures in the survey have a distinct tendency to be filament-like, in general agreement with the results of N-body simulations.

The observed Fe II (UV+optical)/Mg II λλ2796, 2804 flux ratio from a gravitationally lensed quasar B1422+231 at z=3.62 is interpreted in terms of detailed modeling of photoionization and chemical enrichment in the broad-line region (BLR) of the host galaxy. The delayed iron enrichment by Type Ia supernovae is used as a cosmic clock. Our standard model, which matches the Fe II/Mg II ratio, requires the age of 1.5 Gyr for B1422+231 with a lower bound of 1.3 Gyr, which exceeds the expansion age of the Einstein-de Sitter Ω₀=1 universe at a redshift of 3.62 for any value of the Hubble constant in the currently accepted range, H₀=60-80 km s^-1 Mpc^-1. This problem of an age discrepancy at z=3.62 can be unraveled in a low-density Ω₀≲0.2 universe, either with or without a cosmological constant, depending on the allowable redshift range of galaxy formation. However, whether the cosmological constant is a required option in modern cosmology awaits a thorough understanding of line transfer processes in the BLRs.

We present here space-based VLBI observations with VLBI Space Observatory Program (VSOP) and a southern hemisphere ground array of the gamma-ray blazar NRAO 530 at 1.6 and 5 GHz. The brightness temperature of the core at 1.6 GHz is 5×10¹¹ K. The size is near the minimum observable value in the direction of NRAO 530 due to interstellar scattering. The 5 GHz data show a single component with a brightness temperature of ~3×10¹² K, significantly in excess of the inverse Compton limit and of the equipartition brightness temperature limit. This is strong evidence for relativistic motion in a jet requiring model-dependent Doppler boosting factors in the range 6-60. We show that a simple homogeneous sphere probably does not model the emission region accurately. We favor instead an inhomogeneous jet model with a Doppler boosting factor of 15.

It is known that the class of luminous starburst galaxies tends to have higher R=¹²CO (J=1-0)/¹³CO (J=1-0) integrated line intensity ratios (R>20) than normal spiral galaxies (R~10). Since most previous studies investigated only R, it remains uncertain whether the luminous starburst galaxies are overabundant in ¹²CO or underabundant in ¹³CO. Here we propose a new observational test to examine this problem. Our new test is to compare far-infrared luminosities [L(FIR)] with those of ¹²CO and ¹³CO [L(¹²CO) and L(¹³CO), respectively]. It is shown that there is a very tight correlation between L(¹²CO) and L(FIR), as found in many previous studies. However, we find that the ¹³CO luminosities of the high-R galaxies are lower by a factor of 3 on average than those expected from the correlation for the remaining galaxies with ordinary R values. Therefore, we conclude that the observed high R values for the luminous starburst galaxies are attributed to their lower ¹³CO line intensities.

We present images in the 450 and 850 μm continuum of the edge-on galaxy NGC 891. These measurements, carried out with the recently commissioned Submillimeter Common-User Bolometer Array, provide the deepest images yet of a nearby galaxy in the submillimeter wave band. We detect dust emission from 2/3 of the optical disk and confirm the presence of dust chimneys escaping from the main absorption layer up to z-heights of nearly 2 kpc. A comparison between the submillimeter surface brightness along the major axis with that corresponding to the IRAS 60 and 100 μm filters implies that large amounts of cold dust (~15 K) are present in the disk (in fact, an order of magnitude more grain material than the warm dust detected by IRAS). These cold grains predominate at larger galactic radii.

A scenario is proposed that explains both the observed high pulsar velocities and extragalactic gamma-ray bursts (GRBs). The model involves an ultrarelativistic jet from a supernova (SN) that produces a GRB and its afterglow, whose characteristics are similar to an isotropic fireball GRB perhaps with some differences at late times in the afterglow once some significant transverse diffusion has occurred. The timescales and many other properties of GRBs and their afterglows in this model are consistent with observations. GRBs in this model have special intrinsic properties, which can either falsify or prove this model unambiguously by observations. The most direct proof is the detection of an SN about the same time as the luminous GRB event. Most GRBs and SNe are expected occur at moderate redshift (z~1-3), if they follow the observed universal star formation history, as implied in this model. Searching for GRB/SN associations is a challenge because the majority of the SNe will be faint. Some additional, dramatic observable consequences are predicted, which can also be utilized to test the model.

Using the results of nucleosynthesis calculations for theoretical core-collapse supernova models with various progenitor masses, it is shown that the abundance patterns of C, Mg, Si, Ca, and H that are seen in extremely metal-deficient stars with [Fe/H]≲-2.5 follow those seen in the individual first-generation supernova remnants (SNRs). This suggests that most of the stars with [Fe/H]≲-2.5 were made from individual supernova (SN) events. To obtain the ratio of heavy elements to hydrogen, a formula is derived to estimate the mass of hydrogen swept up by an SNR when it occurs in the interstellar matter with the primordial abundances. We use [Mg/H] to indicate the metallicities instead of [Fe/H]. The metallicities [Mg/H] predicted from these SNRs range from ~-4 to ~-1.5, and the mass of Mg in an SN is well correlated with its progenitor mass. Thus, the observed [Mg/H] in an extremely metal-deficient star has a correspondence to the progenitor mass. A larger [Mg/H] corresponds to a larger progenitor mass. Therefore, the so-called "age-metallicity relation" does not hold for stars with [Fe/H]≲-2.5. In contrast, the [Mg/Fe] ratios in the theoretical SNRs have a different trend from those in extremely metal-deficient stars. It is also shown that from the observed trend of [Mg/Fe], one can predict the Fe yield of each SN given the correspondence of [Mg/H] to the progenitor mass. The Fe yields thus obtained are consistent with those derived from SN light-curve analyses. This indicates that there is still a problem in modeling a core-collapse supernova at the beginning of its explosion or mass cut. The abundance determination of O in extremely metal-deficient stars, which has not been done from observational analyses, is strongly desired in order to test the hypothesis that the elements in an extremely metal-deficient star come from a single SN event and to obtain reliable yields for SNe.

We investigate the pulsational properties of pre-main-sequence (PMS) stars by means of linear and nonlinear calculations. The equilibrium models were taken from models evolved from the protostellar birthline to the zero-age main sequence for masses in the range 1-4 M_☉. The nonlinear analysis allows us to define the instability strip of PMS stars in the H-R diagram. These models are used to constrain the internal structure of young stars and to test evolutionary models. We compare our results with observations of the best case of a pulsating young star, HR 5999, and we also identify possible candidates for pulsational variability among known Herbig Ae/Be stars that are located within or close to the instability strip boundaries.

New high-resolution images of HK T Tauri B, the companion to the 10⁶ yr old classical T Tauri star HK Tauri A, show it to be surrounded by an optically thick, edge-on circumstellar disk that extends to a radius of at least 50 AU. The images were taken using a modified speckle technique to achieve a linear resolution of 8 AU. The disk is strikingly coherent, showing no evidence of any strong perturbation by the primary star. The disk is illuminated by the central star, and it hides the star from direct view. The small changes in vertical thickness with wavelength require a dust + gas mass ≳10^-3M_☉. The relative position angles of the disk and the binary suggest that the disk probably does not lie in the plane of the binary orbit.

We present the first infrared interferometric observations of a young stellar object with a spatial projected resolution better than 2 AU. The observations were obtained with the Palomar Testbed Interferometer (PTI). FU Orionis exhibits a visibility of V²=0.72 ± 0.07 for a 103 ± 5 m-projected baseline at λ=2.2 μm. On the spatial scale probed by the PTI, the data are consistent with both a binary system scenario (a maximum magnitude difference of 2.7 ± 0.5 mag and the smallest separation of 0.35 ± 0.05 AU) and a standard luminous accretion disk model ( ~6 × 10⁻⁵M_☉ yr^-1), where the thermal emission dominates the stellar scattering, and are inconsistent with a single stellar photosphere.

We discuss the interpretation of the distortions to the stellar spectral lines, with particular attention to line bisectors in the presence of an orbiting planetary companion. We present a simple model whereby light reflected by the companion can cause temporal variations to the observed line profiles. These distortions have a characteristic signature that depends on the inclination angle of the system. For the known close-in extrasolar giant planets, the expected amplitude of the effect might not be far from current detection capabilities. This method could be used to detect the presence of the companion directly, yielding the orbital inclination and hence the planetary mass. Futhermore, a detection would measure a combination of the planetary radius and albedo, from which a minimum radius may be deduced.

We propose that inward, subsonic flows arise from the local dissipation of turbulent motions in molecular clouds. Such "turbulent cooling flows" may account for recent observations of spatially extended inward motions toward dense cores. These pressure-driven flows may arise from various types of turbulence and dissipation mechanisms. For the example of MHD waves and turbulence damped by ion-neutral friction, sustained cooling flow requires that the outer gas be sufficiently turbulent, that the inner gas have marginal field-neutral coupling, and that this coupling decrease sufficiently rapidly with increasing density. These conditions are most likely met at the transition between outer regions ionized primarily by UV photons and inner regions ionized primarily by cosmic rays. If so, turbulent cooling flows can help form dense cores, with speeds faster than expected for ambipolar diffusion. Such motions could reduce the time needed for dense core formation and could precede and enhance the motions of star-forming gravitational infall.

The understanding of the nature of intermediate- and high-velocity gas in the Milky Way is hampered by a paucity of distance estimates to individual clouds. A project has been started at the David Dunlap Observatory to address this lack of distance measures by observing early-type stars along the line of sight toward these clouds and searching for sodium doublet absorption at the clouds' systemic velocities. Distances to foreground stars (no absorption) and background stars (with absorption) are estimated from spectroscopic parallax, and thus the distance to the bracketed cloud is estimated. In this Letter, we present the first result from this ongoing project: a measurement of the distance to the Draco Cloud, which is the most studied of the intermediate-velocity clouds. The result presented here is the first distance bracket that tightly constrains the position of the Draco Cloud. We briefly describe our target selection and observing methodology and then demonstrate absorption at the velocity of the Draco Cloud for one star (TYC 4194 2188) and a lack of absorption for several other stars. We derive a distance bracket to the Draco Cloud of 463^{+ 192}₋₁₃₆ to 618^{+ 243}₋₁₇₄ pc.

By means of one-dimensional simulations, we study the collapse of a quiescent, nonmagnetized filamentary molecular cloud, taking into account the heating and cooling processes. At the initial state, the model cloud has the central density of n_c=10³ cm^-3 and temperature of T=15 K. We follow its contraction until the central density reaches n_c≃10¹⁰ cm^-3. The cloud contracts mainly due to the CO line cooling and cooling by gas-dust interactions. During the contraction, the cloud temperature stays nearly constant at T~10 K because the net radiative cooling rate nearly balances with the heating by gravitational compression, which is the most efficient heating source. When the central density exceeds 10⁴-10⁵ cm^-3, a shock wave is formed at r~0.05 pc. The shock wave separates the cloud into two parts, i.e., a dense spindle and a diffuse envelope. The spindle slowly contracts due to cooling by gas-dust interactions. During the contraction, the outer part of the spindle has a power-law density distribution of ρ∝r⁻², which is different from that expected by an isothermal model,ρ∝r⁻⁴. Applying linear theory, we find that the collapsing spindle is likely to fragment into dense cores by the stage that the central density reaches n_c~10⁸-10⁹ cm^-3 if the amplitude of the perturbation is greater than A≳10⁻². The masses of the dense cores depend on the initial amplitude of the perturbation. When the initial amplitude of the perturbation is small (A≲10⁻¹), the spindle fragments into cores with mass 0.1-0.5 M_☉. On the other hand, when the spindle has relatively large inhomogeneity (A~10⁻¹), it fragments into cores with mass ~10 M_☉.

We present 100 and 200 μm ISOPHOT observations of the dense core L1498. We have mapped the central core by using ΔI₂₀₀=I₂₀₀-I₁₀₀/Θ, where ΔI₂₀₀ is a measure of the emission from the cold dust and Θ=I₁₀₀/I₂₀₀ in the outer regions. The dust continuum emission provides information about the chemical depletion and the properties of cold cores where there is a lack of gas tracers. Previous observations of L1498 show that the emission from CS and CCS lies outside of the NH₃ core. The peak in ΔI₂₀₀ lies close to the previously observed NH₃ peak. A comparison with high spatial resolution observations of C¹⁸O emission shows that the ΔI₂₀₀ maximum coincides with a dip in the C¹⁸O emission at the core center. We estimate that the depletion factor for C¹⁸O in this region is at least 8 and is likely to be much larger. Such high depletion has significant implications for studies of gas-grain chemistry and protostellar cores.

We present a model that is able to shed light on the long-standing problem of the attribution of the UV interstellar extinction band at 4.6 μm. The model relies on a basic physical description of the electronic structure of carbon materials and is supported by laboratory simulations of UV processing of interstellar grains. The UV bump is attributed to a population of nano-sized, UV-processed hydrogenated amorphous carbon grains: the bump carrier carbons (BCCs). Specifically, we model the feature with a linear combination of absorption from different BCC populations present in interstellar regions sampled along a line of sight. The observed bump width variations are the result of different contributions of BCC grains along different lines of sight. The absorption from less processed particles prevails for wider bumps (denser regions), while more processed grains dominate in the case of sharper features (diffuse medium).

We examine some implications of inertial range and dissipation range correlation and spectral analyses extracted from 33 intervals of Wind magnetic field data. When field polarity and signatures of cross helicity and magnetic helicity are examined, most of the data sets suggest some role of cyclotron-resonant dissipative processes involving thermal protons. We postulate that an active spectral cascade into the dissipation range is balanced by a combination of cyclotron-resonant and noncyclotron-resonant kinetic dissipation mechanisms, of which only the former induces a magnetic helicity signature. A rate balance theory, constrained by the data, suggests that the ratio of the two mechanisms is of order unity. While highly simplified, this approach appears to account for several observed features and explains why complete cyclotron absorption, and the corresponding pure magnetic helicity signature, is usually not observed.

Using the high-resolution Mass Time-of-Flight (MTOF) spectrometer of the Charge, Element, and Isotope Analysis System (CELIAS) experiment on board the Solar and Heliospheric Observatory (SOHO), we have determined the solar wind isotope abundance ratio ¹⁴N/¹⁵N = 200±55 (1 σ error), suggesting that the relative abundance of ¹⁵N in the terrestrial atmosphere is lower than in solar matter. This result is compatible with the hypothesis that terrestrial N (¹⁴N/¹⁵N = 272) and also N found in lunar surface material are a mixture of a heavy component that is identical to solar N and an unspecified light component. The large variations of ¹⁴N/¹⁵N in solar system matter is caused by special isotope enrichment processes, as in the case of Mars, as well as by varying contributions of isotopically different components.