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New near-infrared galaxy counts in the J and K bands are presented over a total area of 0.70 and 0.97 deg², respectively. The limiting magnitudes of the deepest regions are 19.5 in J and 18.0 in K. At J > 16 and K > 15, our J- and K-band number counts agree well with existing surveys, provided that all data are corrected to a common magnitude scale. There are real differences from field to field, and the European Large-Area ISO Survey (ELAIS) N1 and N2 fields show an overdensity of J < 16, K < 15 galaxies. The slopes of log N(m)/dm are ~0.40-0.45 at 15 < K < 18 and 16 < J < 19.5. Our counts favor galaxy models with a high normalization of the local luminosity function and without strong evolution.

While most of the microwave background (CMB) fluctuations on angular scales greater than a few arcminutes were generated at z > 800, the low-redshift universe does distort the microwave background. Since the Sloan Digital Sky Survey (SDSS) traces the structures in the low-redshift universe, we can gain additional insights into the physics of the low-redshift universe by cross-correlating microwave background maps with template maps produced from the SDSS. We present a formalism for cross-correlating data from the Microwave Anisotropy Probe (MAP) with the Sloan Survey for the thermal Sunyaev-Zeldovich (SZ) effect, the Integrated Sachs-Wolfe (ISW) effect, and weak lensing. This formalism is used to compute the signal-to-noise ratio for cross-correlating these effects with various classes of tracer objects from the SDSS. The anticipated samples of SDSS quasars and galaxies with photometrically determined redshifts are found to be good tracers for cross-correlating with the CMB. We find that the SZ-galaxy cross-correlation would give good constraints on pressure fluctuations in supercluster-scale gas. Cross-correlating weakly lensed quasars with maps of the convergence of the CMB is found to give strong constraints on Ω₀ as well as the equation of state, w. We find that the ISW cross-correlation gives a poor signal-to-noise ratio using these techniques.

We define a 90% complete, volume-limited sample of 31 z < 0.1 X-ray clusters and present a systematic analysis of public ROSAT PSPC data on 22 of these objects. Our efforts are undertaken in support of the Penn/OVRO Sunyaev-Zeldovich Effect (SZE) survey, and to this end we present predictions for the inverse Compton optical depth toward all 22 of these clusters. We have performed detailed Monte Carlo simulations in order to understand the effects of the cluster profile uncertainties on the SZE predictions given the OVRO 5.5 m telescope beam and switching patterns. We also present a similar analysis for the upcoming ACBAR experiment. For most of the clusters in the sample, we find less than a 5% uncertainty in the SZE predictions due to an imperfect knowledge of the profile. A comparison of different cooling-flow modeling strategies shows that our results are robust in this respect. The profile uncertainties are then one of the least significant components of our error budget for SZE-based distance measurements. The density models that result from this analysis also yield baryonic masses and, under the assumption of hydrostatic equilibrium, total masses and baryon mass fractions. Our Monte Carlo profile analysis indicates that the baryon masses within 1 h Mpc for these clusters are accurate to better than ~5% and unaffected by realistic PSPC systematics. In the sample as a whole, we find a mean gas mass fraction of (7.02 ± 0.28)h × 10^-2 internal to R₅₀₀ ~ 1 h Mpc. This is in agreement with previous X-ray cluster analyses, which indicate an overabundance of baryons relative to the prediction of big bang nucleosynthesis for an Ω_M = 1 universe. Our analysis of the X-ray spectra confirms a previous claim of an excess absorbing column density toward A478, but we do not find evidence for anomalous column densities in the other 21 clusters. We also find some indications of an excess of soft counts in the ROSAT PSPC data. A measurement of H₀ using these models and OVRO SZE determinations will be presented in a second paper.

A sample of 35 Type Ia supernovae (SNe Ia) with good to excellent photometry in B and V, minimum internal absorption, and 1200 < v ≲ 30,000 km s^-1 is compiled from the literature. As far as their spectra are known, they are all Branch-normal. For 29 of the SNe Ia, peak magnitudes in I are also known. The SNe Ia have uniform colors at maximum, i.e., = -0.012 mag (σ = 0.051) and = -0.276 mag (σ = 0.078). In the Hubble diagram, they define a Hubble line with a scatter of σ_M = 0.21-0.16 mag, decreasing with wavelength. The scatter is further reduced if the SNe Ia are corrected for differences in decline rate, Δm₁₅, or color (B-V). A combined correction reduces the scatter to σ ≲ 0.13 mag. After the correction, no significant dependence remains on Hubble type or Galactocentric distance. The Hubble line suggests some curvature that can be differently interpreted. A consistent solution is obtained for a cosmological model with Ω_M = 0.3, Ω_Λ = 0.7, which is also indicated by much more distant SNe Ia. Absolute magnitudes are available for eight equally blue (Branch-normal) SNe Ia in spirals whose Cepheid distances are known. If their well-defined mean values of M_B, M_V, and M_I are used to fit the Hubble line to the above sample of SNe Ia, one obtains H₀ = 58.3 km s^-1 Mpc^-1, or, after adjusting all SNe Ia to the average values of Δm₁₅ and (B-V), H₀ = 60.9 km s^-1 Mpc^-1. Various systematic errors are discussed whose elimination tends to decrease H₀. The value finally adopted at the 90% level, including random and systematic errors, is H₀ = 58.5 ± 6.3 km s^-1 Mpc^-1. Several higher values of H₀ from SNe Ia, as suggested in the literature, are found to depend on large corrections for variations of the light-curve parameter and/or on an unwarranted reduction of the Cepheid distances of the calibrating SNe Ia.

We monitored 23 quasars and Seyfert 1 galaxies on timescales of minutes, hours, days, weeks, 3 weeks, and 3 months. Observations were made in broad continuum bands of blue, yellow, and far-red light. We attained typical 1 σ relative flux uncertainties of 2% through differential photometry of field stars in the same CCD frames as the active galactic nuclei (AGNs).

In 77 intranight comparisons (23 in blue light, 2 in yellow, and 52 in red) on 20 different AGNs, we found no evidence for significant intranight optical variations. An upper limit for the amplitude of variability for this sample of AGNs on timescales of 1 hr or less was derived to be 0.03 mag. The shortest timescales for which variations were detected, with confidence greater than 99%, were 25 hr in Mrk 79 and 27 hr in NGC 4151. Variations on even shorter timescales might have been missed, as we observed few AGNs over time intervals of less than 1 day.

We found evidence for variability on timescales of 1-10 days in 11% of the AGNs in both the blue and yellow filters, and 26% of the AGNs in the red continuum. In fact, five of the six lowest luminosity AGNs were significantly variable in the red on a timescale of days.

We found optical variability to be more common on a month-to-month timescale; detectable in 60% of the AGNs in the blue filter and 40% in the red filter. Averaging over all the variable objects in the three wave bands, the rms variation was 0.12 mag over 25 days and 0.21 mag over 75 days. We confirmed the overall trend for average variability amplitude to increase with timescale (from 1 to 100 days) by computing autocorrelation and structure functions. The average power density spectra of AGN fluctuations had a logarithmic slope ~ -1. ± 0.5. The exceptions are the five low-luminosity Seyferts which varied rapidly in the red, displaying relatively flat power-density spectra of variability in that wave band.

The fastest significant variations detected, on a timescale of days, are consistent with the dynamical (orbital) timescale of a black-hole accretion disk. Larger amplitude variations are observed on a timescale of 1 month, by which time the amplitude of the luminosity autocorrelation function has typically dropped to 0.5. This variability timescale is consistent with the predicted thermal timescale of accretion disks.

The largest peak-to-peak changes observed were 0.3 mag in the red (NGC 4151) and 0.5 mag in the blue (MCG 8-11-11)—15 and 30 times our measurement uncertainties. When large variations were seen in one filter, they were usually detected, over the same interval and in the same sense, in other filters (12 out of 14 times). On timescales of months, the amplitude of blue variations is generally larger than that of the red. In many objects the amplitude difference is so large that the intrinsic spectrum of the active nucleus must have been bluer when brighter.

The most rapid variations tend to occur in the least luminous AGNs. Luminosity was inversely correlated with the variability χ² in the red filter (r = -0.47, significant at the 98% probability level). Nonetheless, the luminosity dependence of variability is not strong. In fact, the amplitudes of variability that we measure on timescales of months extrapolate very well from those observed in quasar variability monitoring in the blue over timescales of years (although they are somewhat smaller than the variations measured in the ultraviolet). Those AGNs with the strongest Fe II emission lines were also marginally less likely to show strong variability (r = -0.39). There was little correlation between the incidence of variability and any other AGN property. These results suggest that the time variability of the nuclear continuum may be essentially similar in all Seyfert 1 galaxies, and that many apparent differences in their light curves are due to differences in sampling.

We have examined the histograms of flux increases and decreases. This reveals, for example, whether or not the light curves would be statistically equivalent with their flux scales inverted. Averaged over all variable objects, the light curves have a nearly symmetric distribution of points above and below the mean flux level. Physically this indicates that the variations are not predominantly due to "flares and outbursts," on the one hand, nor to "eclipses and dropouts," on the other. The light curves are not a sum of impulsive "shots" superposed on a quiescent brightness level.

We also plotted the flux-change histograms as a function of time interval. In general, the symmetry of flux increases and decreases does not depend on the observation interval, so the light curves would be statistically equivalent with their time axes reversed. A possible exception is made by the low-luminosity Seyferts with fast variations in the red. There is a marginal (3 σ) indication that their red luminosity rises more rapidly than it decays.

The high-redshift radio galaxy 4C 41.17 has been shown in earlier work to consist of a powerful radio source in which there is strong evidence for jet-induced star formation along the radio axis. We argue that nuclear photoionization is not responsible for the excitation of the emission line clouds, and we construct a jet-cloud interaction model to explain the major features revealed by the detailed radio, optical, and spectroscopic data of 4C 41.17. The interaction of a high-powered (~10⁴⁶ ergs s^-1) jet with a dense cloud in the halo of 4C 41.17 produces shock-excited emission-line nebulosity through ~1000 km s^-1 shocks and induces star formation. The C IV luminosity emanating from the shock implies that the preshock density in the line-emitting cloud is high enough (hydrogen density ~ 1-10 cm^-3) that shock-initiated star formation could proceed on a timescale (~a few × 10⁶ yr) well within the estimated dynamical age (~3 × 10⁷ yr) of the radio source. Broad (FWHM ≈ 1100-1400 km s^-1) emission lines are attributed to the disturbance of the gas cloud by a partial bow shock, and narrow emission lines (FWHM ≈ 500-650 km s^-1; in particular, C IV λλ1548, 1550) arise in precursor emission in relatively low-metallicity gas or in shocked line emission in the lateral regions of the bow shock.

The implied baryonic mass ~8 × 10¹⁰M_☉ of the cloud is high and implies that Milky Way size condensations existed in the environments of forming radio galaxies at a redshift of 3.8. Our interpretation of the data provides a physical basis for the alignment of the radio, emission-line, and UV continuum images in some of the highest redshift radio galaxies, and the analysis presented here may form a basis for the calculation of densities and cloud masses in other high-redshift radio galaxies.

If gamma-ray bursts (GRBs) occur at high redshifts, then their bright afterglow emission can be used to probe the ionization and metal enrichment histories of the intervening intergalactic medium during the epoch of reionization. In contrast to other sources, such as galaxies or quasars, which fade rapidly with increasing redshift, the observed infrared flux from a GRB afterglow at a fixed observed age is only a weak function of its redshift. This results from a combination of the spectral slope of GRB afterglows and the time stretching of their evolution in the observer's frame. Assuming that the GRB rate is proportional to the star formation rate and that the characteristic energy output of GRBs is ~10⁵² ergs, we predict that there are always ~15 GRBs from redshifts z ≳ 5 across the sky that are brighter than ~100 nJy at an observed wavelength of ~2 μm. The infrared spectrum of these sources could be taken with the future Next Generation Space Telescope as a follow-up to early X-ray localization with the Swift satellite.

We discuss observations of the prompt X- and γ-ray emission and X-ray afterglow from GRB 981226. This event has the weakest gamma-ray peak flux detected with the BeppoSAX Gamma-Ray Burst Monitor. It shows an isolated X-ray precursor and the highest X-ray to gamma-ray fluence ratio measured thus far with the BeppoSAX Wide Field Cameras. The event was followed up with the BeppoSAX Narrow Field Instruments, and the X-ray afterglow was detected up to 10 keV. The afterglow flux is observed to rise from a level below the sensitivity of the MECS/LECS telescopes up to a peak flux of (5 ± 1) × 10^-13 ergs cm^-2 s^-1 in the 2-10 keV energy band. This rise is followed by a decline according to a power law with an index of 1.31. We discuss these results in the light of the current GRB models.

We demonstrate that the radiation emitted by ultrarelativistic electrons in highly nonuniform, small-scale magnetic fields is different from synchrotron radiation if the electron's transverse deflections in these fields are much smaller than the beaming angle. A quantitative analytical theory of this radiation, which we refer to as jitter radiation, is developed. It is shown that the emergent spectrum is determined by statistical properties of the magnetic field. The jitter radiation theory is then applied to internal shocks of γ-ray bursts (GRBs). The model of a magnetic field in GRBs proposed by Medvedev & Loeb in 1999 is used. The spectral power distribution of radiation produced by the power-law-distributed electrons with a low-energy cutoff is well described by a sharply broken power law: P(ω) ∝ ω¹ for ω ≲ ω_jm and P(ω) ∝ ω^-(p-1)/2 for ω ≳ ω_jm, where p is the electron power-law index and ω_jm is the jitter break frequency, which is independent of the field strength but depends on the electron density in the ejecta, ω_jm ∝ n^1/2, as well as on the shock energetics and kinematics. The total emitted power of jitter radiation is, however, equal to that of synchrotron radiation.

Since large-scale fields may also be present in the ejecta, we construct a two-component, jitter + synchrotron spectral model of the prompt γ-ray emission. Quite surprisingly, this model seems to be readily capable of explaining several properties of time-resolved spectra of some GRBs, such as (1) the violation of the constraint on the low-energy spectral index called the synchrotron "line of death," (2) the sharp spectral break at the peak frequency, inconsistent with the broad synchrotron bump, (3) the evidence for two spectral subcomponents, and (4) possible existence of emission features called "GRB lines." We believe these facts strongly support both the existence of small-scale magnetic fields and the proposed radiation mechanism from GRB shocks. As an example, we use the composite model to analyze GRB 910503, which has two spectral peaks. At last, we emphasize that accurate GRB spectra may allow precise determination of fireball properties as early as several minutes after the explosion.

This paper presents four-color narrowband photometry of clusters A115 (z = 0.191) and A2283 (z = 0.182) in order to follow the star formation history of various galaxy types. Although located at similar redshifts, the two clusters display very different fractions of blue galaxies (i.e., the Butcher-Oemler effect; f_B = 0.13 for A115, f_B = 0.30 for A2283). A system of photometric classification is applied to the cluster members that divides the cluster population into four classes based on their recent levels of star formation. It is shown that the blue population of each cluster is primarily composed of normal star-forming (star formation rate < 1 M_☉ yr^-1) galaxies at the high-luminosity end but with an increasing contribution from a dwarf starburst population below M₅₅₀₀ = -20. This dwarf starburst population appears to be the same population of low-mass galaxies identified in recent Hubble Space Telescope imaging, possible progenitors to present-day cluster dwarf ellipticals, irregulars, and blue compact dwarfs. Deviations in the color-magnitude relationship for the red galaxies in each cluster suggest that a population of blue S0s is evolving into present-day S0 colors at this epoch. The radial distribution of the blue population supports the prediction of galaxy harassment mechanisms for tidally induced star formation operating on an infalling set of gas-rich galaxies.

We present new redshift measurements for 55 galaxies in the vicinity of the rich galaxy cluster Abell 665. When combined with results from the literature, we have good velocity measurements for a sample of 77 confirmed cluster members from which we derive the cluster's redshift z = 0.1829 ± 0.0005 and line-of-sight velocity dispersion σ = 1390 km s^-1. Our analysis of the kinematical and spatial data for the subset of galaxies located within the central 750 kpc reveals only subtle evidence for substructure and non-Gaussianity in the velocity distribution. We find that the brightest cluster member is not moving significantly relative to the other galaxies near the center of the cluster. On the other hand, our deep ROSAT high-resolution image of A665 shows strong evidence for isophotal twisting and centroid variation, thereby confirming previous suggestions of significant substructure in the hot X-ray-emitting intracluster gas. In light of this evident substructure, we have compared the optical velocity data with N-body simulations of head-on cluster mergers. We find that a merger of two similar mass subclusters (mass ratios of 1 : 1 or 1 : 2) seen close to the time of core-crossing produces velocity distributions that are consistent with that observed.

We present two-dimensional spectroscopy of the 125 × 125 pc² circumnuclear region of M31, obtained using two optical fiber systems, INTEGRAL and 2D-FIS, installed at the 4.2 m William Herschel Telescope at the Roque de los Muchachos Observatory on the island of La Palma, Spain. We present continuum and emission-line maps, emission-line quotients, and stellar and ionized gas velocity fields obtained under subarcsecond seeing conditions. We have discovered Hβ, [O III] λλ4959, 5007, Hα, and [N II] λλ6548, 6584 emission in the inner 40 × 40 pc² circumnuclear region. The intensity maps obtained from the emission lines show several isolated systems of ionized gas (clouds) a few parsecs in size. The kinematics of the clouds is decoupled from the stellar rotation, and strong radial motions are likely to be present. We have also discussed the ionization mechanisms present in the clouds. Although the emission-line quotients are compatible with ionization produced by post-asymptotic giant branch stars, the estimated ionization fluxes exceed what is expected according to the distribution of planetary nebulae in the bulge of M31.

Comparison of the line quotients and spectra found in M31 with those found in the extended narrow-line region of the Seyfert galaxy NGC 4151 indicates that we could be in presence of ultralow-intensity nuclear activity. These results would be compatible with the existence of a "dead quasar" in the nucleus of M31 and underline the hybrid character of the LINER phenomenon.

We demonstrate that the Biermann battery mechanism for the creation of large-scale magnetic fields can arise in a simple model protogalaxy. Analytic calculations and numerical simulations follow explicitly the generation of vorticity (and hence magnetic field) at the outward-moving shock that develops as the protogalactic perturbation collapses. Shear angular momentum then distorts this field into a dipole-like configuration. The magnitude of the field created in the fully formed disk galaxy is estimated to be 10^-17 G, approximately what is needed as a seed for the galactic dynamo.

We present the first detection of HCN J = 4-3 in the starburst galaxy M82 and a fully sampled grid of the distribution of its emission in the nuclear region. The angular resolution is 14'' and the velocity resolution 6.3 km s^-1. The distribution of HCN emission is compared with that of HCO⁺J = 4-3 made with the same resolution, and the conclusion is that the HCO⁺/HCN brightness ratio is higher at J = 4-3 than at lower transitions, with a median ratio of 3.5. There is evidence for a variation in this ratio across the nuclear region, with higher values toward the edge of the nuclear disk. This variation is consistent with a variation in gas density, with higher densities associated with regions closer to the nucleus of M82. However, some of this variation could also be produced by a variation in the HCO⁺/HCN abundance ratio. A large-scale velocity gradient (LVG) analysis of three transitions in each molecule centered at the nucleus shows that the line ratios within each species and between the two species are consistent with a common set of physical conditions for the emitting regions, namely, a kinetic temperature of 50 K, gas density 10⁵ cm^-3, and gas column density 10²² cm^-2. Although it has been suggested that HCO⁺ may be subject to mechanisms of excitation other than collisions with H₂, such as electron collisions, our analysis shows that there is no reason to invoke any such mechanisms to account for the line ratios present. The HCO⁺/HCN intensity ratio is known to vary significantly from one galaxy to another, with M82 at the high end of the spectrum of variation. This effect is briefly discussed in the context of a bistable chemical equilibrium model in which the HCO⁺ abundance is a sensitive function of the cosmic-ray ionization rate and gas density in the cloud cores. Within the framework of this picture, a high cosmic-ray flux could be inversely related to HCO⁺ abundance, contrary to what is generally assumed.

We analyze the gas kinematics and star formation properties of the nearby RSab galaxy NGC 4736 using interferometric and single-dish CO(1-0) data and previously published Hα and H I data. The CO morphology is dominated by a central molecular bar and tightly wound spiral arms associated with a bright ring of star formation. Strong H I emission is also found in the ring, but H I is absent from the central regions. Comparison of the H I and Hα distributions suggests that H I in the ring is primarily dissociated H₂. Modeling of the CO kinematics reveals gas motion in elliptical orbits around the central bar, and we argue that the ring represents both the outer Lindblad resonance of the bar and the inner Lindblad resonance of a larger oval distortion. The H I kinematics show evidence for axisymmetric inflow toward the ring and are inconsistent with streaming in aligned elliptical orbits, but the highly supersonic (~40 km s^-1) inflow velocities required, corresponding to mass inflow rates of ~2 M_☉ yr^-1, suggest that more sophisticated models (e.g., gas orbiting in precessed elliptical orbits) should be considered. The radial CO and Hα profiles are poorly correlated in the vicinity of the nuclear bar but show a better correlation (in rough agreement with the Schmidt law) at the ring. Even along the ring, however, the azimuthal correspondence between CO and Hα is poor, suggesting that massive stars form more efficiently at some (perhaps resonant) locations than at others. These results indicate that the star formation rate per unit gas mass exhibits strong spatial variations and is not solely a function of the available gas supply. The localization of star formation to the ring is broadly consistent with gravitational instability theory, although the instability parameter Q ~ 3 on average in the ring, only falling below 1 in localized regions. Large-scale dynamical effects, by concentrating gas at resonances and influencing the star formation rate, appear to play a key role in this galaxy's evolution.

Based on our high-resolution two-dimensional hydrodynamical simulations, we propose that large cavities may be formed by the nonlinear development of the combined thermal and gravitational instabilities, without need for stellar energy injection in a galaxy modeling the Large Magellanic Cloud (LMC). Our numerical model of star formation allows us to follow the evolution of the blast waves due to supernovae in the inhomogenous, multiphase, and turbulent-like media self-consistently. Formation of kiloparsec-scale inhomogeneity, such as cavities as seen in the observed H I map of the LMC, is suppressed by frequent supernovae (the average supernova rate for the whole disk is ~0.001 yr^-1). However, the supernova explosions are necessary for the hot component (T_g > 10⁶-10⁷ K). Position-velocity maps show that kiloparsec-scale shells/arcs formed through nonlinear evolution in a model without stellar energy feedback have kinematics similar to explosive phenomena, such as supernovae. We also find that dense clumps and filamentary structure are formed as a natural consequence of the nonlinear evolution of the multiphase interstellar medium (ISM). Although the ISM on a small scale looks turbulent-like and transient, the global structure of the ISM is quasi-stable. In the quasi-stable phase, the volume filling factor of the hot, warm, and cold components are ~0.2, ~0.6, and ~0.2, respectively. We compare observations of H I and molecular gas of the LMC with the numerically obtained H I and CO brightness temperature distributions. The morphology and statistical properties of the numerical H I and CO maps are discussed. We find that the cloud mass spectra of our models represent a power-law shape, but their slopes change between models with and without the stellar energy injection. We also find that the slope depends on the threshold brightness temperature of CO.

A new investigation of the supernova remnant (SNR) N157B was carried out with the Australia Telescope Compact Array. Radio continuum images of the entire 30 Doradus region have been made at 3.5 and 6 cm wavelengths with a resolution of ~2''. These data allow a high-resolution study of the spectral index distribution and polarization properties of both N157B and the nearby 30 Doradus nebula (the latter will be reported in a subsequent paper). N157B is an extended Crab-type SNR which may be beginning the transition to a composite remnant. There is little apparent fine structure, and the brightest radio region is several parsecs from the probable position of the X-ray pulsar. The SNR has a radio spectral index of -0.19 and is significantly polarized at 3.5 cm but not at longer wavelengths.

The pressures of giant H II regions in six dwarf irregular galaxies are found to be a factor of ~10 larger than the average pressures of the corresponding galaxy disks, obtained from the stellar and gaseous column densities. This is unlike the situation for spiral galaxies, where these two pressures are approximately equal. Either the H II regions in these dwarfs are all so young that they are still expanding, or there is an unexpected source of disk self-gravity that increases the background pressure. We consider first whether any additional self-gravity might come from disk dark matter that either is cold H₂ gas in diffuse or self-gravitating clouds with weak CO emission, or is the same material as the halo dark matter inferred from rotation curves. The H₂ solution is possible because cold molecular clouds would be virtually invisible in existing surveys if they were also CO-weak from the low metal abundances in these galaxies. Cosmological dark matter might be possible too because of the relatively large volume fraction occupied by the disk within the overall galaxy potential. There is a problem with both of these solutions, however: the vertical scale heights inferred for irregular galaxies are consistent with the luminous matter alone. The amount of disk dark matter that is required to explain the high H II region pressures would give gas and stellar scale heights that are too small. The anomalous pressures in star-forming regions are more likely the result of local peaks in the gravitational field that come from large gas concentrations. These peaks also explain the anomalously low average column density thresholds for star formation that were found earlier for irregular galaxies, and they permit the existence of a cool H I phase as the first step toward dense molecular cores. The evidence for concentrations of H I in regions of star formation is summarized; the peak column densities are shown to be consistent with local pressure equilibrium for the H II regions. Strongly self-gravitating star-forming regions should also limit the dispersal of metals into the intergalactic medium. The third possibility is that all of the visible H II regions in these dwarf galaxies are strongly overpressured and still expanding. The mean time to pressure equilibrium is ~15 times their current age, which implies that the observed population is only 7% of the total if they live that long; the rest are presumably too faint to see. The expansion model also implies that the volume-filling factor can reach ~100 times the current factor, in which case faint and aging H II regions should merge and occupy nearly the entire dwarf galaxy volume. This would explain the origin of the giant H I shells seen in these galaxies as the result of old, expanded H II regions that were formerly driven by OB associations. The exciting clusters would now be so old and dispersed that they would not be recognized easily. The shells are still round because of a lack of shear.

A sample of 4208 objects with magnitude 15 < g* < 22 and colors of main-sequence A stars have been selected from 370 deg² of Sloan Digital Sky Survey (SDSS) commissioning observations. The data is from two long, narrow stripes, each with an opening angle of greater than 60°, at Galactic latitudes 36^° < |b| < 63^° on the celestial equator. Relative photometric calibrations good to 2% and consistent absolute photometry allows this uniform sample to be treated statistically over the large area. An examination of the sample's distribution shows that these stars trace considerable substructure in the halo. Large overdensities of A-colored stars in the north at (l, b, R) = (350^°, 50°, 46 kpc) and in the south at (157, -58, 33 kpc) and extending over tens of degrees are present in the halo of the Milky Way. Ivezic et al. have detected the northern structure from a sample of RR Lyrae stars in the SDSS.

Using photometry to separate the stars by surface gravity, both structures are shown to contain a sequence of low surface gravity stars consistent with identification as a blue horizontal branch (BHB). Both structures also contain a population of high surface gravity stars 2 mag fainter than the BHB stars, consistent with their identification as blue stragglers (BSs). The majority of the high surface gravity stars in the Galactic halo may be BS stars like these. A population of F stars associated with the A star excess in the southern structure is detected (the F stars in the northern structure at 46 kpc would be too faint for the SDSS to detect). From the numbers of detected BHB stars, lower limits to the implied mass of the structures are 6 × 10⁶M_☉ and 2 × 10⁶M_☉, although one does not yet know the full spatial extent of the structures. The fact that two such large clumps have been detected in a survey of only 1% of the sky indicates that such structures are not uncommon in the halo.

Simple spheroidal parameters are fit to a complete sample of the remaining unclumped BHB stars and yield (at r < 40 kpc) a fit to a halo distribution with flattening (c/a = 0.65 ± 0.2) and a density falloff exponent of α = -3.2 ± 0.3.

The Trifid Nebula (M20) is a well-known prominent optical H II region trisected by obscuring dust lanes. Radio continuum VLA observations of this nebula show free-free emission at λ = 3.6 and 6 cm from three stellar sources lying close to the O7 V star at the center of the nebula. We argue that neutral material associated with these stars is photoionized externally by the UV radiation from the hot central star. We also report the discovery of a barrel-shaped supernova remnant, SNR G7.06-0.12, at the northwest rim of the nebula, and two shell-like features, G6.67-0.42 and G6.83-0.21, adjacent to W28 and M20. We discuss the nature of these features and their possible relationship to the pulsar PSR 1801-2306 and W28 OH (1720 MHz) masers.

We reveal cold Galactic clouds of neutral hydrogen in unprecedented detail. Our 21 cm synthesis maps, taken from the Canadian Galactic Plane Survey, show a numerous and diverse population of H I self-absorption (HISA) features in gas outside the solar circle. These objects vary in size, shape, and contrast against the background H I. All display a high level of angular and velocity structure, and most would appear significantly diluted, if not invisible, in lower resolution H I surveys. A number of Perseus arm features remain unresolved by the 1' beam of our survey, with apparent diameters less than 0.6 pc at 2 kpc distance. The majority of HISA features we detect have no obvious ¹²CO emission counterparts. This suggests that either HISA is not found predominantly in molecular clouds, as has often been presumed in the past, or that CO is not a good tracer of H₂. Some HISA lacking CO shows far-infrared dust emission, though whether this arises from shielded molecular gas or from diffuse atomic clouds is not clear. Constraining the gas properties of HISA remains a difficult problem, but we introduce a new method that aids this process. Our approach relates a number of physical parameters via gas law and line integral relationships and should prove powerful if the input variables are sufficiently well known. We explore the current allowed parameter ranges for three sample features of very different appearance. We find spin temperatures ≲50 K and densities ≳10² cm^-3.

The existence of small-scale variations in the interstellar medium, between sight lines separated by 10-100 AU, is well established from pulsar, VLBI, MERLIN, and VLA observations. The angular resolutions are in the range 10-100 mas. During an outburst of the microquasar GRS 1915+105, the proper motion of the approaching component is ~250 AU (23 mas) day^-1. Using the VLA to observe the H I line at 21 cm, we have measured the time-variable H I absorption along the changing line of sight during such an outburst. We detect opacity changes of Δτ ~ 0.67 ± 0.16 in gas at v_LSR of 55 km s^-1, near the velocity of the tangent point at 65 km s^-1, over ~6 days, for lines of sight ~1000 AU apart. Another detection at v_LSR of 5 km s^-1 has Δτ ~ 0.24 ± 0.06, on lines of sight ~40 AU (or 440 AU) apart, depending on the near (or far) distance to the absorbing gas. From the several days of spectra, 3 σ upper limits are derived: Δτ ≤ 0.5 for gas at 55 km s^-1, on scales ~150-900 AU; and Δτ ≤ 0.25 at 5 km s^-1, on scales ~25-150 AU (gas at 1 kpc) or ~275-1650 AU (gas at 11 kpc). We thus probe the H I structure in distant Galactic material, using a Galactic background source with rapid structural changes. Our approach complements those using extragalactic sources and nearby pulsars. Our results are consistent with extrapolation of the power-law index of 0.375 for the structure function of H I opacity (equivalent to an index of 2.75 for the power spectrum of H I), as measured by others from 4 pc to 0.02 pc toward Cassiopeia A.

We discuss recent experimental results for ion/molecule reactions of ionized and multiply-ionized fullerenes, and of derivatized fullerene ions, with molecules relevant to the chemistry of interstellar clouds and circumstellar envelopes These reactions were studied using a selected-ion flow tube (SIFT) at 294 ± 2 K in helium at a pressure of 0.35 ± 0.01 torr. The present study supplements an earlier discussion on aspects of interstellar fullerene ion chemistry explored by the same technique. Several implications are apparent for the chemical processing of fullerenes in various astrophysical environments. Triply charged fullerene ions, such as C₆₀³⁺, may be formed under conditions prevailing within dense IS clouds, but their abundance will be very low owing to the large number of loss processes identified for such species. Derivatization of fullerene ions under interstellar or circumstellar conditions is less probable for larger fullerenes than for fullerenes smaller than C₆₀. Hydrogenation may severely impede the efficiency of fullerene ion association with polar molecules and small unsaturated molecules, but should not substantially affect the efficiency of addition of radicals or PAHs under these conditions.

We discuss prospects for neutralization of ionized fullerene adducts. Four classes of adduct ions are described, differing in their structure and expected neutralization tendencies. Adducts of fullerene ions with interstellar isonitriles, with radicals, and with linear polycyclic aromatic hydrocarbons (PAHs; class 1) are most likely to form derivatized fullerenes on neutralization, while fullerene ion adducts of nitriles, most hydrocarbons (class 3), and nonlinear PAHs (class 4) are most likely to yield the bare fullerene cage upon neutralization. Adducts of ammonia (class 2) appear to have an intermediate probability of surviving neutralization with the functionalizing group(s) intact.

The Orion Bar is an ideal astrophysical laboratory for studying photodissociation regions because of its nearly edge-on orientation in the observer's line of sight. High angular resolution (~9'') maps of the Orion Bar in the J = 1-0 emission lines of HCO⁺ and HCN have been made by combining single-dish millimeter observations with interferometric data. This mapping technique provides both large-scale structural information and high resolution. The new maps show that HCO⁺ and HCN have globally similar spatial distributions in the Orion Bar. Both molecular species show the same clumpy NE to SW bar seen by previous observers in molecular line emission from the Orion Bar. However, our maps show HCN emission to be more confined to the bar structure and to clump cores than HCO⁺ emission. We do a crosscut comparison of our full-synthesis maps with previously published observations of the Orion Bar in: (1) the rotational transitions of ¹²CO J = 1-0, ¹³CO J = 1-0, CN N = 3-2, and CS J = 7-6; (2) the UV-pumped rovibrational transition of H₂ at 2.122 μm; (3) 3.3 μm emission attributed to the aromatic C–H bond stretching of polycyclic aromatic hydrocarbons (PAH); and (4) the atomic fine-structure transitions of C I (609 μm), O I (63 μm), and C II (158 μm). The crosscuts show the same chemical stratification seen by previous observers as expected from an edge-on photodissociation region. In addition, we see that the HCN peak profile is relatively narrow and symmetrical compared to the broader asymmetrical HCO⁺ peak. We argue that this difference in peak shape supports a previously published suggestion that HCO⁺ production is enhanced in warm gas at the surface of the photodissociation region. We explain these observations using a nonhomogeneous photodissociation region model to which we have added nitrogen chemistry and the thermal chemical effects of polycyclic aromatic hydrocarbons. Instead of using a homogeneous model, we follow more recent models employing two components because the clumpiness seen in all the recent observations suggests at least two density components in the Orion Bar. From our model calculations, we have found that a ridge of dense clumps (3 × 10⁶ cm^-3) embedded in a lower density interclump medium (5 × 10⁴ cm^-3) explains our observations very well. Although some of the observations (e.g., emissions from H₂, CO, O I, C I and C II) arise from the interclump medium, we show that HCN and HCO⁺J = 1-0 emission must come from a ridge of dense clumps near the ionization front. This result agrees with the findings of previous observers, who have suggested the presence of dense clumps in the Orion Bar.

This paper explores the effects of strong magnetic fields on the Compton scattering of relativistic electrons. Recent studies of upscattering and energy loss by relativistic electrons that have used the nonrelativistic, magnetic Thomson cross section for resonant scattering or the Klein-Nishina cross section for nonresonant scattering do not account for the relativistic quantum effects of strong fields (>4 × 10¹² G). We have derived a simplified expression for the exact QED scattering cross section for the broadly applicable case in which relativistic electrons move along the magnetic field. To facilitate applications to astrophysical models, we have also developed compact approximate expressions for both the differential and total polarization-dependent cross sections, with the latter representing well the exact total QED cross section even at the high fields believed to be present in environments near the stellar surfaces of soft gamma repeaters and anomalous X-ray pulsars. We find that strong magnetic fields significantly lower the Compton scattering cross section below and at the resonance when the incident photon energy exceeds m_ec² in the electron rest frame. The cross section is strongly dependent on the polarization of the final scattered photon. Below the cyclotron fundamental, mostly photons of perpendicular polarization are produced in scatterings, a situation that also arises above this resonance for subcritical fields. However, an interesting discovery is that for supercritical fields, a preponderance of photons of parallel polarization results from scatterings above the cyclotron fundamental. This characteristic is both a relativistic and a magnetic effect not present in the Thomson or Klein-Nishina limits.

The contribution of the source cosmic rays (SCRs), confined in supernova remnants, to the diffuse high-energy γ-ray emission above 1 GeV from the Galactic disk is studied. Gamma-rays produced by the SCRs have a much harder spectrum compared with those generated by the Galactic cosmic rays that occupy a much larger residence volume uniformly. SCRs contribute less than 10% at GeV energies and become dominant at γ-ray energies above 100 GeV. The contributions from π⁰-decay and inverse Compton γ-rays have comparable magnitude and spectral shape, whereas the bremsstrahlung component is negligible. At TeV energies the contribution from SCRs increases the expected diffuse γ-ray flux by almost an order of magnitude. It is shown that for the inner Galaxy the discrepancy between the observed diffuse intensity and previous model predictions at energies above a few GeV can be attributed to the SCR contribution.

The nucleosynthesis of Be and B by spallation processes provides unique insight into the origin of cosmic rays. Namely, different spallation schemes predict sharply different trends for the growth of LiBeB abundances with respect to oxygen. "Primary" mechanisms predict BeB ∝ O and are well motivated by the data if O/Fe is constant at low metallicity. In contrast, "secondary" mechanisms predict BeB ∝ O² and are consistent with the data if O/Fe increases toward low metallicity as some recent data suggest. Clearly, any primary mechanism, if operative, will dominate early in the history of the Galaxy. In this paper, we fit the BeB data to a two-component scheme which includes both primary and secondary trends. In this way, the data can be used to probe the period in which primary mechanisms are effective. We analyze the data using consistent stellar atmospheric parameters based on Balmer line data and the continuum infrared flux. Results depend sensitively on Population II O abundances (and O/Fe trends), which have recently seen renewed interest. We explore the implications of these results phenomenologically, using a systematic and consistent compilation and fitting of BeBOFe data. Two-component Be-O fits indicate that primary and secondary components contribute equally at [O/H]_eq = -1.8 for Balmer line data; and [O/H]_eq = -1.4 to -1.8 for IRFM. We apply these constraints to recent models for LiBeB origin. The Balmer line data do not show any evidence for primary production. On the other hand, the IRFM data do indicate a preference for a two-component model, such as a combination of standard GCR and metal-enriched particles accelerated in superbubbles. These conclusions rely on a detailed understanding of the abundance data including systematic effects which may alter the derived O-Fe and BeB-Fe relations.

Self-similar solutions provide good descriptions for the gravitational collapse of spherical clouds or stars when the gas obeys a polytropic equation of state, p = Kρ^γ (with γ ≤ 4/3, and γ = 1 corresponds to isothermal gas). We study the behaviors of nonradial (nonspherical) perturbations in the similarity solutions of Larson, Penston, and Yahil, which describe the evolution of the collapsing cloud prior to core formation. Our global stability analysis reveals the existence of unstable bar modes (l = 2) when γ ≤ 1.09. In particular, for the collapse of isothermal spheres, which applies to the early stages of star formation, the l = 2 density perturbation relative to the background, δρ(,t)/ρ(r,t), increases as (t₀ - t)^-0.352 ∝ ρ_c(t)^0.176, where t₀ denotes the epoch of core formation, and ρ_c(t) is the cloud central density. Thus, the isothermal cloud tends to evolve into an ellipsoidal shape (prolate bar or oblate disk, depending on initial conditions) as the collapse proceeds. This shape deformation may facilitate fragmentation of the cloud. In the context of Type II supernovae, core collapse is described by the γ ≃ 1.3 equation of state, and our analysis indicates that there is no growing mode (with density perturbation) in the collapsing core before the protoneutron star forms, although nonradial perturbations can grow during the subsequent accretion of the outer core and envelope onto the neutron star. We also carry out a global stability analysis for the self-similar expansion-wave solution found by Shu, which describes the postcollapse accretion ("inside-out" collapse) of isothermal gas onto a protostar. We show that this solution is unstable to perturbations of all l's, although the growth rates are unknown.

We investigate the stability of a gravitationally collapsing iron core against nonspherical perturbation. The gravitationally collapsing iron core is approximated by a similarity solution for a dynamically collapsing polytropic gas sphere. We find that the similarity solution is unstable against nonspherical perturbations. The perturbation grows in proportion to (t - t₀)^-σ while the central density increases in proportion to (t - t₀)^-2. The growth rate is σ = + ℓ(γ - 4/3), where γ and ℓ denote the polytropic index and the parameter ℓ of the spherical harmonics, Y(θ,φ), respectively. The growing perturbation is dominated by vortex motion. Thus, it excites global convection during the collapse and may contribute to material mixing in a Type II supernova.

We present a new parallel supercomputer implementation of the Monte Carlo method for simulating the dynamical evolution of globular star clusters. Our method is based on a modified version of Hénon's Monte Carlo algorithm for solving the Fokker-Planck equation. Our code allows us to follow the evolution of a cluster containing up to 5 × 10⁵ stars to core collapse in ≲40 hours of computing time. In this paper we present the results of test calculations for clusters with equal-mass stars, starting from both Plummer and King model initial conditions. We consider isolated as well as tidally truncated clusters. Our results are compared to those obtained from approximate, self-similar analytic solutions, from direct numerical integrations of the Fokker-Planck equation, and from direct N-body integrations performed on a GRAPE-4 special-purpose computer with N = 16384. In all cases we find excellent agreement with other methods, establishing our new code as a robust tool for the numerical study of globular cluster dynamics using a realistic number of stars.

To explore the amount of secondary irradiation and the long-term effects of a superoutburst (SOB) on tremendous outburst amplitude dwarf novae (TOADs), we obtained spectra of EG Cnc for 3-17 months past its SOB, of SW UMa, WX Cet, and USNO 1425.09823278 at 2 months past their SOBs and HV Vir and LL And during their quiescent states at 3 and 5 yr past SOB. The quiescent spectra of EG Cnc, HV Vir, LL And, and USNO 1425 show emission cores surrounded by broad absorption lines from the white dwarf, consistent with very low mass accretion onto low-temperature white dwarfs. SW UMa and WX Cet likely have higher accretion rates and more extensive disks. SW UMa exhibits unusual disk structure, with three zones of emission that persist for 3 days, while EG Cnc and USNO 1425 have a strong orbital modulation of the Balmer lines that may be related to long-lasting irradiation after superoutburst.

Low-mass companions to high-mass stars are difficult to detect, which is partly why the binary fraction for high-mass stars is still poorly constrained. Low-mass companions can be detected more easily however, once high-mass stars turn into white dwarfs. These systems are also interesting as the progenitors of a variety of intensely studied interacting binary systems, like novae, CVs, symbiotics, Ba and CH giants, Feige 24-type systems, and dwarf carbon stars. We describe a near-IR photometric search for cool red dwarf companions to hot white dwarfs (WDs). IR photometry offers a sensitive test for low-mass main-sequence (MS) companions. Our sample of EUV-detected WDs offers several advantages over previous (largely proper motion-selected) WD samples: (1) the high WD temperatures (24,000 < T_eff < 70,000 K) insure excellent IR flux contrast with cool dwarfs; (2) the range of evolutionary parameter space occupied by the WDs is considerably narrowed; and (3) the random effects of the intervening ISM provide a complete but reasonably sized sample. While some composite systems have been found optically among WDs detected in recent EUV All-Sky Surveys, we develop an IR technique that probes farther down the main sequence, detecting yet more companions. We use detailed DA model atmosphere fits to optical spectra to predict K magnitudes and distances, against which we contrast our near-IR observations. Our photometric survey reveals 10 DAs with a significant excess in both J and K. Half are newly discovered and are most likely previously unrecognized binary systems. Neither the frequency of infrared excess nor the mass estimate of the red dwarf companion correlate with white dwarf mass, as might be expected if either the EUV detectability or mass of the white dwarfs were significantly affected by a companion. Infrared spectra of these systems should help to determine the mass and spectral type of the cool companions presumably causing the IR excess, leading to better estimates of the mass ratio distribution in binaries. Counting previously known binaries, and resolved pairs, we find the total binary fraction of the sample is at least a third. Since most WD progenitors had initial masses ≥2 M_☉, we thus provide a photometric measure of the binary fraction of high-mass stars that would be difficult to perform in high-mass main-sequence stars. We estimate that 90% of the companions are of type K or later.

We compare the structures of model atmospheres and synthetic spectra calculated using different line lists for TiO and water vapor. We discuss the effects of different line list combinations on the model structures and spectra for both dwarf and giant stars. It is shown that recent improvements result in significantly improved spectra, in particular, in the optical where TiO bands are important. The water vapor-dominated near-infrared region remains problematic as the current water line lists do not yet completely reproduce the shapes of the observed spectra. We find that the AMES TiO list provides more opacity in most bands and that the new, smaller oscillator strengths lead to systematically cooler temperatures for early-type M dwarfs than previous models. These effects combine and will help to significantly improve the fits of models to observations in the optical as well as result in improved synthetic photometry of M stars. We show that the Davis, Littleton, & Phillips f_el-values for the δ and φ bands of TiO best reproduce the observed V-I color indices.

We have obtained images of the Trapezium Cluster (140'' × 140''; 0.3 pc × 0.3 pc) with the Hubble Space Telescope Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). Combining these data with new ground-based K-band spectra (R = 800) and existing spectral types and photometry, we have constructed an H-R diagram and used it and other arguments to infer masses and ages. To allow comparison with the results of our previous studies of IC 348 and ρ Oph, we first use the models of D'Antona & Mazzitelli. With these models, the distributions of ages of comparable samples of stars in the Trapezium, ρ Oph, and IC 348 indicate median ages of ~0.4 Myr for the first two regions and ~1-2 Myr for the latter. The low-mass initial mass functions (IMFs) in these sites of clustered star formation are similar over a wide range of stellar densities (ρ Oph, n = 0.2-1 × 10³ pc^-3; IC 348, n = 1 × 10³ pc^-3; Trapezium, n = 1-5 × 10⁴ pc^-3) and other environmental conditions (e.g., presence or absence of OB stars). With current data, we cannot rule out modest variations in the substellar mass functions among these clusters. We then make the best estimate of the true form of the IMF in the Trapezium by using the evolutionary models of Baraffe et al. and an empirically adjusted temperature scale and compare this mass function to recent results for the Pleiades and the field. All of these data are consistent with an IMF that is flat or rises slowly from the substellar regime to about 0.6 M_☉ and then rolls over into a power law that continues from about 1 M_☉ to higher masses with a slope similar to or somewhat larger than the Salpeter value of 1.35. For the Trapezium, this behavior holds from our completeness limit of ~0.02 M_☉ and probably, after a modest completeness correction, even from 0.01-0.02 M_☉. These data include ~50 likely brown dwarfs. We test the predictions of theories of the IMF against (1) the shape of the IMF, which is not log-normal, in clusters and the field, (2) the similarity of the IMFs among young clusters, (3) the lowest mass observed for brown dwarfs, and (4) the suggested connection between the stellar IMF and the mass function of prestellar clumps. In particular, most models do not predict the formation of the moderately large numbers of isolated objects down to 0.01 M_☉ that we find in the Trapezium.

Interior layers of stars that have been exposed by surface mass loss reveal aspects of their chemical and convective histories that are otherwise inaccessible to observation. It must be significant that the surface hydrogen abundances of luminous blue variables (LBVs) show a remarkable uniformity, specifically X_surf = 0.3-0.4, while those of hydrogen-poor Wolf-Rayet (WN) stars fall, almost without exception, below these values, ranging down to X_surf = 0. According to our stellar model calculations, most LBVs are post-red-supergiant objects in a late blue phase of dynamical instability, and most hydrogen-poor WN stars are their immediate descendants. If this is so, stellar models constructed with the Schwarzschild (temperature-gradient) criterion for convection account well for the observed hydrogen abundances, whereas models built with the Ledoux (density-gradient) criterion fail. At the brightest luminosities, the observed hydrogen abundances of LBVs are too large to be explained by any of our highly evolved stellar models, but these LBVs may occupy transient blue loops that exist during an earlier phase of dynamical instability when the star first becomes a yellow supergiant. Independent evidence concerning the criterion for convection, which is based mostly on traditional color distributions of less massive supergiants on the Hertzsprung-Russell diagram, tends to favor the Ledoux criterion. It is quite possible that the true criterion for convection changes over from something like the Ledoux criterion to something like the Schwarzschild criterion as the stellar mass increases.

We study the correlations between timing and X-ray spectral properties in the low-mass X-ray binary 4U 0614+09 using a large (265 ks) data set obtained with the Rossi X-Ray Timing Explorer. We find strong quasi-periodic oscillations (QPOs) of the X-ray flux, like the kilohertz QPOs in many other X-ray binaries with accreting neutron stars, with frequencies ranging from 1329 Hz down to 418 Hz and perhaps as low as 153 Hz. We report the highest frequency QPO yet from any low-mass X-ray binary at 1329 ± 4 Hz, which has implications for neutron star structure. This QPO has a 3.5 σ single-trial significance; for an estimated 40 trials the significance is 2.4 σ. Besides the kilohertz QPOs, the Fourier power spectra show four additional components: high-frequency noise (HFN), described by a broken power law with a break frequency between 0.7 and 45 Hz, very low frequency noise (VLFN), which is fitted as a power law below 1 Hz, and two broad Lorentzians with centroid frequencies varying from 6 to 38 Hz and from 97 to 158 Hz, respectively. We find strong correlations between the frequencies of the kilohertz QPOs, the frequency of the 6-38 Hz broad Lorentzian, the break frequency of the HFN, the strength of both the HFN and the VLFN, and the position of the source in the hard X-ray color versus intensity diagram. The frequency of the 97-158 Hz Lorentzian does not correlate with these parameters. We also find that the relation between power density and break frequency of the HFN is similar to that established for black hole candidates in the low state. We suggest that the changing mass accretion rate is responsible for the correlated changes in all these parameters.

The hard X-ray bursts observed during both major outbursts of the Bursting Pulsar (GRO J1744-28) show pulsations near the neutron star spin frequency with an enhanced amplitude relative to that of the persistent emission. Consistent with previous work, we find that the pulsations within bursts lag behind their expected arrival times based upon the persistent pulsar ephemeris. For an ensemble of 1293 bursts recorded with the Burst and Transient Source Experiment, the average burst pulse time delay (Δt_FWHM) is 61.0 ± 0.8 ms in the 25-50 keV energy range and 72 ± 5 ms in the 50-100 keV band. The residual time delay (Δt_resid) from 10 to 240 s following the start of the burst is 18.1 ± 0.7 ms (25-50 keV). A significant correlation of the average burst time delay with burst peak flux is found. Our results are consistent with the model of the pulse time lags presented by Miller.

We use the transfer equation in relativistic form to develop an expansion of the one-photon distribution for a medium with constant photon mean free path, . On carrying out appropriate integrations and manipulations, we convert this expansion into one for the frequency-integrated intensity. We regroup the terms of the intensity expansion according to both the power of and the angular structure of the various terms and then carry out angle integrations to obtain the expansions for the components of the stress energy tensor: the radiative energy density, the radiative flux, and the pressure tensor. In leading order, we recover Thomas' results for the viscosity tensor and his expression for the viscosity coefficient, which are correct for short mean free paths. As had been done earlier for the radiative heat equation, we keep at each order in the expansion a dominant portion, but this time one with a richer angular structure. Then, after some rearrangement of the various summations in the expressions for the moments, we replace the sum of the calculated higher order terms by a Padé approximant, or rational approximation, to provide an improved closure approximation for the radiative stress tensor. The resulting radiative viscosity tensor may be expressed either as a simple integral operator acting on the Thomas stress tensor or as the solution of an inhomogenous, linear partial differential equation. The expression obtained for the radiative viscosity tensor applies for media with long, as well as short, photon mean free paths. We also develop results applicable for relatively smooth flows by using the form of the Thomas stress tensor with generalized transport coefficients derived by the application of a suitable operator to the bare Thomas coefficients.

The boundaries of the Uranian , α, and β rings can be fitted by Keplerian ellipses. The pair of ellipses that outline a given ring share a common line of apsides. Apse alignment is surprising because the quadrupole moment of Uranus induces differential precession. We propose that rigid precession is maintained by a balance of forces due to ring self-gravity, planetary oblateness, and interparticle collisions. Collisional impulses play an especially dramatic role near ring edges. Pressure-induced accelerations are maximal near edges because there (1) velocity dispersions are enhanced by resonant satellite perturbations and (2) the surface density declines steeply. Remarkably, collisional forces felt by material in the last ~100 m of a ~10 km wide ring can increase equilibrium masses up to a factor of ~100. New ring surface densities are derived that accord with Voyager radio measurements. In contrast to previous models, collisionally modified self-gravity appears to allow for both negative and positive eccentricity gradients; why all narrow planetary rings exhibit positive eccentricity gradients remains an open question.

The recent discovery of a planetary system around Ups And raises questions concerning the formation process of several planets in the Jupiter-mass range around a single host star. We consider numerically two scenarios involving the interaction of protoplanets with low-viscosity host disks. In the first case, a single protoplanet is assumed to have been formed already, and the development of a tidally induced gap in the disk is calculated. Beyond the outer boundary of the gap, a positive pressure gradient induces the disk gas to attain an azimuthal velocity that is larger than the Keplerian speed. The accumulation of small solid particles at the outer edge of this region provides a favorable location for the formation of an additional protoplanetary core with an orbital radius approximately twice that of the original protoplanet. In the second scenario, we assume that two protoplanets have formed simultaneously, one with twice the orbital radius of the other. Both clear gaps, and the ring of remaining disk material between the planets has a width only a few times the thickness of the disk. The density waves excited by planets on both sides of the ring propagate throughout the ring, and nonlocal dissipation of these waves leads to gas leakage from the ring edges into the gaps. After the ring is depleted, the separation between the planets tends to decline as a result of angular momentum exchange between them and the surrounding inner and outer disks. For a disk with moderate viscosity, the timescale for the planets to approach each other is less than the lifetime of the gas.

The Gamma-Ray Spectrometer (GRS) aboard the Solar Maximum Mission (SMM) discovered a 154 day periodicity in solar flare rates. Subsequently, periodicities in various solar flare activities and in sunspot areas or groups during a few years around solar maxima have been extensively monitored using different diagnostics and at many electromagnetic wavelengths. Notable periods are ~154, 128, 102, 78, and 51 days during maxima of different solar cycles from various data sets. The origin of such quasi periodicities particularly prominent during solar maxima has remained a mystery for nearly two decades. For slow and large-scale photospheric motions, the shallow magnetofluid approximation may be invoked when the Rossby number ₀ ≡ U/(2 Ω_☉L) is small, where U (≲10³ cm s^-1) and L (≳R_☉) are typical horizontal velocity and spatial scales. Physical properties of equatorially trapped Kelvin waves, Poincaré waves, Rossby waves, and mixed Rossby-Poincaré waves are examined. For typical solar parameters, period estimates of Rossby and mixed Rossby-Poincaré waves are ~151-155, 126-127, 101-102, 76-78, and 51-54 days, in good agreement with observed periodicities. The effect of large-scale subsurface magnetic fields is estimated. Two methods of directly detecting solar Rossby-type waves are discussed. It would be of interest to examine whether large-scale coronal mass ejections also carry similar periodicities.

A model-independent reconstruction of mechanical profiles (density, pressure) of the solar interior is outlined using the adiabatic sound speed and buoyancy frequency profiles. These can be inferred from helioseismology if both p- and g-mode frequencies are measured. A simulated reconstruction is presented using a solar model buoyancy frequency and available sound speed data.

We report on large-scale ab initio multiconfiguration Dirac-Fock (MCDF) calculations for spectral lines of Si IX, with emphasis on the forbidden transitions 2p²³P. The J = 0 → J' = 1 transition at 3.9346 μm holds promise as a diagnostic of coronal magnetic fields if and when future coronagraphic instruments can measure the polarized light at this wavelength.

Coronal magnetic flux ropes are closely related to various solar active phenomena such as prominences, flares, and coronal mass ejections. Using a 2.5-dimensional (2.5-D), time-dependent ideal MHD model in Cartesian coordinates, a numerical study is carried out to find the equilibrium solution associated with a magnetic flux rope in the corona, which is assumed to emerge as a whole from the photosphere. The rope in equilibrium is characterized by its geometrical features such as the height of the axis, the half-width of the rope, and the length of the vertical current sheet below the rope, and its magnetic properties such as the axial and annular magnetic fluxes and the magnetic helicity as well, which are conserved quantities of the rope in the frame of ideal MHD. It is shown that, for a given bipolar ambient magnetic field, the magnetic flux rope is detached from the photosphere, leaving a vertical current sheet below, when its axial magnetic flux, annular magnetic flux, or magnetic helicity exceeds a certain critical value. The magnetic field is nearly force free in the rope but not in the prominence region, where the Lorentz force takes an important role in supporting the prominence appearing below the rope axis. The geometrical features of the rope vary smoothly with its magnetic properties, and no catastrophe occurs, a similar conclusion to that reached by Forbes & Isenberg for magnetic flux ropes of large radius.

On 1998 July 14, a class M3 flare occurred at 12:55 UT in AR 8270 near disk center. Kitt Peak line-of-sight magnetograms show that the flare occurred in a δ spot. Mees vector magnetograms show a strong shear localized near a portion of the closed neutral line around the parasitic polarity of the δ spot. Observations of the flare in 171, 195, and 1600 Å have been obtained by TRACE, with ≃40 s temporal and 05 spatial resolutions. They reveal that small-scale preflare loops above the sheared region expanded and disappeared for more than 1 hr before flare maximum. During the flare, bright loops anchored in bright ribbons form and grow. This occurs while large-scale dimmings, associated with large expanding loops, develop on both sides of the active region. This suggests that the flare was eruptive and was accompanied by a coronal mass ejection (CME). Magnetic field extrapolations reveal the presence of a null point in the corona, with its associated "spine" field line, and its "fan" surface surrounding the parasitic polarity. We show that while the whole event occurs, the intersections of the "fan" and the "spine" with the photosphere brighten and move continuously. The interpretation of the event shows that the magnetic evolution of the eruptive flare is strongly coupled with its surrounding complex topology. We discuss evidence supporting a "magnetic breakout" process for triggering this eruptive flare. We finally conclude that multipolar fields cannot be neglected in the study and modeling of the origin of CMEs in the corona.

We analyzed the three-dimensional structure of the linear force-free magnetic field. A longitudinal magnetogram of Active Region NOAA 8375 has been used as the photospheric boundary condition. The 1998 November 5 2B/M8.4 two-ribbon flare can be explained in the framework of quadrupolar reconnection theory: the interaction of two closed magnetic loops that have a small spatial angle. The energy derived from soft X-ray telescope (SXT)/Yohkoh data (3-6 × 10³⁰ ergs) is 1 order of magnitude higher than the lower limit of flare energy predicted by Melrose's model. The latter estimation was made using the linear force-free extrapolation. It was suggested that, taking into account the nonlinear character of the observed magnetic field, we can increase the lower limit of the magnetic energy stored in the studied magnetic configuration. The revealed magnetic configuration allows us to understand the observed location and evolution of the flare ribbons and the additional energy released during the gradual phase of the flare, as well. Besides, reconnection of closed magnetic loops can logically explain the connection between a two-ribbon flare and a giant X-ray postflare arch, which usually is observed after the flare onset. We emphasize that unlike the Kopp and Pneuman configuration, the model discussed here does not necessarily require destabilization and opening of the magnetic field.

A new method for reconstructing force-free magnetic fields from their boundary values, based on minimizing the global departure of an initial field from a force-free and solenoidal state, is presented. The method is tested by application to a known nonlinear solution. We discuss the obstacles to be overcome in the application of this method to the solar case: the reconstruction of force-free fields in the corona from measurements of the vector magnetic field in the low atmosphere.

Recent high-resolution photospheric magnetograms made with the SOHO/Michelson Doppler Imager instrument and the Swedish Vacuum Solar Telescope on La Palma show that concentrations of magnetic flux in the quiet photospheric network of the solar photosphere are highly dynamic objects with small-scale substructure. These observations reveal many details in the dynamics of flux emergence, fragmentation, and cancellation. In order to understand such phenomena we investigate the dynamics of two colliding magnetic flux tubes in weakly ionized plasmas with high plasma beta (β ≃ 1), using the three-dimensional neutral-MHD equations. First we investigate the collision of two parallel flux tubes for the two cases of partial and complete magnetic reconnection. We find that, when one flux tube with weak current and small radius collides with another flux tube with strong current and large radius, the weak-current flux tube splits into two small flux tubes because of magnetic reconnection. We also find that the collision of magnetic flux tubes with weak current leads to the emission of strong fast magnetosonic waves, resulting in shock formation, while the collision site of two strong-current loops shows no strong wave emission. Next we investigate the collision of two noncollinear flux tubes with X-type configuration, taking into account the effect of density inhomogeneity along the flux tubes due to gravity. We find strong upward plasma flows along the flux tubes and also shock wave emission from the X-type collision region. Finally we discuss the application of these simulation results to coronal heating.

A meteoric cloud is the faint glow of sunlight scattered by small meteoroids in the dust trail along the orbit of a comet as seen by an earthbound observer. While these clouds were previously only known from anecdotes of past meteor storms, we now report the detection of a meteoric cloud by modern techniques in the direction of the dust trail of comet 55P/Tempel-Tuttle, the parent of the Leonid meteor stream. Our photometric observations, performed on Mauna Kea, Hawaii, reveal the cloud as a local enhancement in sky brightness during the Leonid shower in 1998. The radius of the trail, deduced from the spatial extent of the cloud, is approximately 0.01 AU and is consistent with the spatial extent mapped out by historic accounts of meteor storms. The brightness of the cloud is approximately ~2%-3% of the background zodiacal light and cannot be explained by simple model calculations based on the zenith hourly rate and population index of the meteor stream in 1998. If the typical size of cloud particles is 10 μm and the albedo is 0.1, the brightness translates into a number density of 1.2 × 10^-10 m^-3. The meteoroid cloud would be the product of the whole dust trail and not only the part that was crossed in 1998.

In 1998, the Optical Gravitational Lensing Experiment (OGLE) successfully implemented automated data reductions for QSO 2237+0305. Using a new image-subtraction method, we achieved a differential photometry scatter of 1%-5% for images A-D, respectively. Combined with a sampling rate of one to two points a week, this is sufficient for early detection of caustic crossings. A nearly real-time photometry of QSO 2237+0305 is available from the OGLE Web site.¹ During the 1999 observing season, the difference between the highest and the lowest apparent V magnitude of the A, B, C, and D images was 0.50, 0.15, 0.65, and 0.35 mag, respectively. However, the most likely interpretation is that none of the recorded microlensing events were a bona fide caustic crossing. The most rapid variation was a 0.25 mag decrease in 30 days, which was observed for image C after it peaked in early July of 1999. The alert system will continue to be active in the current observing season from late April until September of 2000, when OGLE suspends operation for an upgrade. Observations will resume for the season of 2001.

The quasar PKS 0637-752, the first celestial X-ray target of the Chandra X-Ray Observatory, has revealed asymmetric X-ray structure extending from 3'' to 12'' west of the quasar, coincident with the inner portion of the jet previously detected in a 4.8 GHz radio image (Tingay et al. 1998). At a redshift of z = 0.651, the jet is the largest (≳100 kpc in the plane of the sky) and most luminous (~10^44.6 ergs s^-1) of the few so far detected in X-rays. This Letter presents a high-resolution X-ray image of the jet, from 42 ks of data when PKS 0637-752 was on-axis and ACIS-S was near the optimum focus. For the inner portion of the radio jet, the X-ray morphology closely matches that of new Australian Telescope Compact Array radio images at 4.8 and 8.6 GHz. Observations of the parsec-scale core using the very long baseline interferometry space observatory program mission show structure aligned with the X-ray jet, placing important constraints on the X-ray source models. Hubble Space Telescope images show that there are three small knots coincident with the peak radio and X-ray emission. Two of these are resolved, which we use to estimate the sizes of the X-ray and radio knots. The outer portion of the radio jet and a radio component to the east show no X-ray emission to a limit of about 100 times lower flux. The X-ray emission is difficult to explain with models that successfully account for extranuclear X-ray/radio structures in other active galaxies. We think the most plausible is a synchrotron self-Compton model, but this would imply extreme departures from the conventional minimum energy and/or homogeneity assumptions. We also rule out synchrotron or thermal bremsstrahlung models for the jet X-rays, unless multicomponent or ad hoc geometries are invoked.

We have discovered polarized broad emission lines in five type 2 Seyfert galaxies (NGC 424, NGC 591, NGC 2273, NGC 3081, and NGC 4507), establishing that these objects are type 1 Seyferts obscured by dense circumnuclear material. The galaxies are part of a distance-limited sample of 31 Seyfert 2s, for which spectropolarimetric observations are now complete. Combined with published reports, our results indicate that at least 11 of the galaxies in this sample, or ≥35%, possess hidden broad-line regions. As the first reliable estimate of the frequency of polarized broad emission lines in type 2 Seyfert galaxies, this has important implications for the general applicability of Seyfert unification models.

The central bulge of M31 is observed to have two distinct brightness peaks with the separation of ~2 pc. S. Tremaine recently proposed the new idea that M31's nucleus is actually a single thick eccentric disk surrounding the central supermassive black hole. In order to explore the origin of the proposed eccentric disk, we numerically investigate the dynamical evolution of a merger between a central massive black hole with a mass of ~10⁷M_☉ and a compact stellar system with a mass of ~10⁶M_☉ and size of a few parsecs in the central 10 pc of a galactic bulge. We find that the stellar system is destroyed by the strong tidal field of the massive black hole and consequently forms a rotating nuclear thick stellar disk. The orbit of each stellar component in the developed disk is rather eccentric with a mean eccentricity of ~0.5. These results imply that M31's nuclear eccentric disk proposed by Tremaine can be formed by merging between a central massive black hole and a compact stellar system. We furthermore discuss when and how a compact stellar system is transferred into the nuclear region around a massive black hole.

We discuss V- and R-band photometry for 67% of the recent SA 57 ultraviolet-selected galaxy sample of M. Sullivan and collaborators. In a sample of 176 UV-selected galaxies, Sullivan et al. find that 24% have (UV-B) colors too blue for consistency with starburst spectral synthesis models. We propose that these extreme blue, UV excess galaxies are Wolf-Rayet (W-R) galaxies, starburst galaxies with strong UV emission from W-R stars. We measure a median (V-R) = 0.38 ± 0.06 for the UV-selected sample, bluer than a sample optically selected at R but consistent with starburst and W-R galaxy colors. We demonstrate that redshifted W-R emission lines can double or triple the flux through the UV bandpass at high redshifts. Thus, the (UV-B) color of a W-R galaxy can be up to 1.3 mag bluer at high redshift, and the expected selection function is skewed to larger redshifts. The redshift distribution of the extreme blue, UV excess galaxies matches the selection function we predict from the properties of W-R galaxies.

We report the discovery of two distant embedded young stellar clusters located far in the outer Galaxy. The clusters are resolved in near-infrared images and seen as enhancements in the surface density of IR-excess stars centered close to IRAS 07255-2012. The clusters are embedded in a molecular cloud containing a CS dense core. The molecular cloud, as traced by CO (J = 1-0) is elongated and extends over a region of 15 × 6 pc². From the millimeter observations, we derive a kinematic distance of 10.2 kpc and a Galactocentric distance of 16.5 kpc, making these clusters among the most distant embedded clusters in the Galaxy. The main (richer) cluster is well confined to a region of about 1.2 pc in radius. Down to our detection limit of about 1-2 M_☉ at this distance, it contains at least 30 members. The smaller cluster contains at least five stars. They all exhibit near-infrared color excesses consistent with young stellar objects having circumstellar and/or envelope material. We estimate that the star formation efficiency of this molecular cloud is about 4%-10%.

A close, 4th magnitude companion to ζ Orionis A has been resolved with the Navy Prototype Optical Interferometer at Lowell Observatory. This confirms an indication of multiplicity from observations with the stellar intensity interferometer at Narrabri 26 years ago. The new component in the multiple system ADS 4263, ζ Orionis Ab, is 2 mag fainter than Aa, which is a supergiant of type O9.5. During 1998 February and March, the pair had a mean separation of 42 mas. Orbital motion was subsequently detected, but the corresponding arc allows only a preliminary ephemeris.

Recent numerical simulations of self-gravitating protostellar disks have suggested that gravitational instabilities can lead to the production of substellar companions. In these simulations, the disk is typically assumed to be locally isothermal; i.e., the initial, axisymmetric temperature in the disk remains everywhere unchanged. Such an idealized condition implies extremely efficient cooling for outwardly moving parcels of gas. While we have seen disk disruption in our own locally isothermal simulations of a small, massive protostellar disk, no long-lived companions formed as a result of the instabilities. Instead, thermal and tidal effects and the complex interactions of the disk material prevented permanent condensations from forming, despite the vigorous growth of spiral instabilities. In order to compare our results more directly with those of other authors, we here present three-dimensional evolutions of an older, larger, but less massive protostellar disk. We show that potentially long-lived condensations form only for the extreme of local isothermality, and then only when severe restrictions are placed on the natural tendency of the protostellar disk to expand in response to gravitational instabilities. A more realistic adiabatic evolution leads to vertical and radial expansion of the disk but no clump formation. We conclude that isothermal disk calculations cannot demonstrate companion formation by disk fragmentation but only suggest it at best. It will be necessary in future numerical work on this problem to treat the disk thermodynamics more realistically.

We report a detection of water in emission in the spectrum of the M2 supergiant star μ Cephei (M2 Ia) observed by the Short-Wavelength Spectrometer on board Infrared Space Observatory (ISO) and now released in the ISO archives. The emission first appears in the 6 μm region (ν₂ fundamental bands) and then in the 40 μm region (pure rotation lines) despite the rather strong dust emission. The intensity ratios of the emission features are far from those of the optically thin gaseous emission. Instead, we could reproduce the major observed emission features by an optically thick water sphere of the inner radius about 2R_* (≈1300 R_☉), T_ex = 1500 K, and N_col(H₂O) = 3 × 10²⁰ cm^-2. This model also accounts for the H₂O absorption bands in the near-infrared (1.4, 1.9, and 2.7 μm) as well. The detection of water in emission provides strong constraints on the nature of water in the early M supergiant star, and especially, its origin in the outer atmosphere is confirmed against other models such as the large convective cell model. We finally confirm that the early M supergiant star is surrounded by a huge optically thick sphere of the warm water vapor, which may be referred to as MOLsphere for simplicity. Thus, the outer atmosphere of M supergiant stars should have a complicated hierarchical and/or hybrid structure with at least three major constituents, including the warm MOLsphere (T ≈ 10³ K) together with the previously known hot chromosphere (T ≈ 10⁴ K) and cool expanding gas-dust envelope (T ≈ 10² K).

We make the hypothesis that molecular cloud fragments are triaxial bodies with a large-scale magnetic field oriented along the short axis. While consistent with theoretical expectations, this idea is supported by magnetic field strength data, which show strong evidence for flattening along the direction of the mean magnetic field. It is also consistent with early submillimeter polarization data, which show that the projected direction of the magnetic field is often slightly misaligned with the projected minor axis of a molecular cloud core; i.e., the offset angle Ψ is nonzero. We calculate distributions of Ψ for various triaxial bodies, when viewed from a random set of viewing angles. The highest viewing probability always corresponds to Ψ = 0^°, but there is a finite probability of viewing all nonzero Ψ, including even Ψ = 90^°; the average offset typically falls in the range of 10°-30° for triaxial bodies most likely to satisfy observational and theoretical constraints.

Interstellar glycolaldehyde (CH₂OHCHO) has been detected in emission toward the Galactic center source Sagittarius B2(N) by means of millimeter-wave rotational transitions. Glycolaldehyde is an important biomarker since it is structurally the simplest member of the monosaccharide sugars that heretofore have gone undetected in interstellar clouds. There is no consensus as to how any such large complex molecules are formed in the interstellar clouds. It may be that the typical environment of dense interstellar clouds is favorable to glycolaldehyde synthesis by means of the polymerization of formaldehyde (H₂CO) molecules either on grain surfaces or in the gas phase. Alternatively, we speculate that glycolaldehyde and other complex molecules may undergo assembly from functional molecular groups on grain surfaces. Utilizing common chemical precursors, a chance process could account for the high degree of isomerism observed in complex interstellar molecules (e.g., methyl formate, acetic acid, and glycolaldehyde). This work suggests that the phenomenon of isomerism be investigated further as a means of potentially constraining interstellar chemistry routes for those individual sources where the condition of good source-beam coupling can be achieved.

We report the first comprehensive observation of the abundances of heavy elements of atomic number Z in the range 34 ≤ Z ≤ 82 in solar energetic particle (SEP) events as observed on the Wind spacecraft. In large gradual SEP events, abundances of the element groups 34 ≤ Z ≤ 40, 50 ≤ Z ≤ 56, and 70 ≤ Z ≤ 82, relative to Fe, are similar to corresponding coronal abundances within a factor of ~2 and vary little with time during the events. However, in sharp contrast, abundances of these ions from impulsive flares increase dramatically with Z so that abundances of Fe, 34 ≤ Z ≤ 40, and 50 ≤ Z ≤ 56, relative to O, are seen at ~10, ~100, and ~1000 times their coronal values, respectively.

A solar prominence has either dextral or sinistral chirality depending on its axial field direction. We determine the magnetic helicity sign of filaments using high-resolution observations performed by Transition Region And Coronal Explorer. At EUV wavelengths, filaments sometimes appear as mixtures of bright threads and dark threads. This characteristic has enabled us to discern overlying threads and underlying ones and to determine the sign of magnetic helicity based on the assumption that the helicity sign of two crossing thread segments is the same as that of the filament. Our results support the notion that dextral filaments have negative magnetic and that sinistral filaments have positive helicity.