Table of contents

In the context of a cold big bang (CBB) cosmological model, I estimate the temperature, calculate the evolution, and discuss the anisotropies of a homogeneous radiation background emitted at high redshift by Population III objects and thermalized by a mixture of carbon/silicate dust and iron or carbon whiskers. Assuming that Population III objects supply dark matter remnants and produce the universal helium abundance of ~24%, the resulting radiation, if thermal, should have temperature 0.5 K ≲ T₀ ≲ 9 K. To calculate its thermalization I limit standard dust density using constraints from maximum allowed metallicity, quasar reddening, Type Ia supernova observations, and high-redshift object visibility and assume a small mass ratio of whiskers to standard dust. For high redshift of generation (z_i ≳ 100) and highly conducting, high aspect ratio (length/diameter ≳1000) iron whiskers, the final spectrum meets the FIRAS spectral distortion limits and does not exceed limits on IR-background light. Energetic considerations probably require z_i ≲ 40Ω_b if the observed cosmic microwave background (CMB) is star generated. Such a model is marginally viable with baryon density Ω_b = 1, but probably ruled out if Ω_b ≲ 0.5. I comment briefly on possible other thermalization mechanisms. In whisker-thermalized models, CMB anisotropies are imprinted at lower redshift and through somewhat different physical processes than in the hot big bang (HBB). Nonlocal thermal equilibrium of the dust causes the Sachs-Wolfe effect to disappear on small angular scales. Radiation pressure probably ejects whiskers from luminous regions such as galaxies. If significant intergalactic magnetic fields are absent, CMB radiation drag probably fixes whiskers in the CMB frame, hence large-scale Doppler anisotropies (other than the dipole) do not exist. "Intrinsic" temperature fluctuations and the integrated Sachs-Wolfe effect operate essentially as in the HBB. Numerical results for the horizon, diffusion, and other length scales relevant to CMB anisotropies for fiducial thermalization models are provided. Finally, I note that CMB polarization may be higher in the whisker model than in the HBB and that acoustic oscillations should not appear in the CBB power spectrum, and I comment on the amplitude and possible frequency dependence of the anisotropies.

Powerful constraints on theories can already be inferred from existing CMB anisotropy data. But performing an exact analysis of available data is a complicated task and may become prohibitively so for upcoming experiments with ≳10⁴ pixels. We present a method for approximating the likelihood that takes power spectrum constraints, e.g., "band-powers," as inputs. We identify a bias which results if one approximates the probability distribution of the band-power errors as Gaussian—as is the usual practice. This bias can be eliminated by using specific approximations to the non-Gaussian form for the distribution specified by three parameters (the maximum likelihood or mode, curvature or variance, and a third quantity). We advocate the calculation of this third quantity by experimenters, to be presented along with the maximum-likelihood band-power and variance. We use this non-Gaussian form to estimate the power spectrum of the CMB in 11 bands from multipole moment ℓ = 2 (the quadrupole) to ℓ = 3000 from all published band-power data. We investigate the robustness of our power spectrum estimate to changes in these approximations as well as to selective editing of the data.

We determine the distances to the z ≃ 0.55 galaxy clusters MS 0451.6-0305 and Cl 0016+16 from a maximum-likelihood joint fit to interferometric Sunyaev-Zeldovich effect (SZE) and X-ray observations. We model the intracluster medium (ICM) using a spherical isothermal β model. We quantify the statistical and systematic uncertainties inherent to these direct distance measurements, and we determine constraints on the Hubble parameter for three different cosmologies. For an Ω_M = 0.3, Ω_Λ = 0.7 cosmology, these distances imply a Hubble constant of 63 km s^-1 Mpc^-1, where the uncertainties correspond to statistical followed by systematic at 68% confidence. The best-fit H₀ is 57 km s^-1 Mpc^-1 for an open (Ω_M = 0.3) universe and 52 km s^-1 Mpc^-1 for a flat (Ω_M = 1) universe.

Formulating the equations of motion for cosmological bodies (such as galaxies) in an integral, rather than differential, form has several advantages. Using an integral, the mathematical instability at early times is avoided, and the boundary conditions of the integral correspond closely with available data. Here it is shown that such a least-action calculation for a number of bodies interacting through gravity has a finite number of solutions, possibly only one. Characteristics of the different possible solutions are explored. The results are extended to cover the motion of a continuous fluid. We give a method for generalizing an action so as to use velocities, instead of positions, in boundary conditions, which reduces in particular cases to those given by Giavalisco et al. and Schmoldt & Saha. The velocity boundary condition is shown to have no effect on the number of solutions.

We present an analysis of the orbital properties of nine intermediate-redshift clusters of the Canadian Network for Observational Cosmology (CNOC1) survey and compare them to a control sample of 12 nearby clusters. Similar to the nearby elliptical galaxies, the bulge-dominated galaxies in clusters at redshifts ~0.1-0.4 present orbits that are more eccentric than those of disk-dominated galaxies. However, the orbital segregation is less significant than that found for elliptical and spiral galaxies in nearby clusters. The strongest orbital segregation is found when galaxies are separated by colors [red galaxies with colors in the rest frame (U-V)₀ > 1.4, blue galaxies with (U-V)₀ ≤ 1.4]. Therefore, the segregation we find seems to modify the star formation activity more efficiently than the internal shape of the galaxies. When we compare the orbits of early-type galaxies at intermediate redshift with those for z = 0, they seem to develop significant changes, becoming much more eccentric. A different behavior is observed in the late-type galaxies, which present no significant evolution in their orbit shapes.

We have calculated nonthermal bremsstrahlung (NTB) models for the hard X-ray (HXR) tails recently observed by BeppoSAX in clusters of galaxies. In these models, the HXR emission is caused by suprathermal electrons with energies of ~10-200 keV. We consider models in which these transrelativistic suprathermal particles are the low energy end of a population of electrons being accelerated to high energies by shocks or turbulence ("accelerating electron" models). We also consider a model in which these electrons are the remnant of an older nonthermal population that is losing energy and rejoining the thermal distribution as a result of Coulomb interactions ("cooling electron" models). The suprathermal populations are assumed to start at an electron kinetic energy 3kT, where T is the temperature of the thermal intracluster medium (ICM). The nonthermal bremsstrahlung spectra flatten at low photon energies because of the lack of low-energy nonthermal particles. The accelerating electron models have HXR spectra that are nearly power laws from ~20-100 keV. However, the spectra are brighter and flatter than given by the nonrelativistic bremsstrahlung cross section because of transrelativistic effects. The HXR spectrum of the cooling electron model is very flat, and most of the X-ray emission in the HXR energy range (10-100 keV) actually arises from electrons with much higher energies (~100 MeV). Under the assumption that the suprathermal electrons form part of a continuous spectrum of electrons including highly relativistic particles, we have calculated the inverse Compton (IC) extreme-ultraviolet (EUV), HXR, and radio synchrotron emission by the extensions of the same populations. For accelerating electron models with power-law momentum spectra (N[p] ∝ p^-μ) with μ ≲ 2.7, which are those expected from strong shock acceleration, the IC HXR emission exceeds that caused by NTB. Thus, these models are of interest only if the electron population is cut off at some upper energy ≲1 GeV. Similarly, flat-spectrum accelerating electron models produce more radio synchrotron emission than is observed from clusters if the ICM magnetic field is B ≳ 1 μG. The cooling electron model produces vastly too much EUV emission as compared to the observations of clusters. We have compared these NTB models to the observed HXR tails in Coma and Abell 2199. The NTB models require a nonthermal electron population that contains about 3% of the number of electrons in the thermal ICM. If the suprathermal electron population is cut off at some energy above 100 keV, then the models can fit the observed HXR fluxes and spectral indices in both clusters easily. For accelerating electron models without a cutoff, the electron spectrum must be rather steep, ≳2.9, to avoid producing too much IC HXR emission. The model HXR spectra are then rather steep but are marginally consistent with observations of the HXR spectrum in Abell 2199 and Coma or the radio spectrum in Coma. These models can account for the HXR and radio properties of these two clusters but do not produce enough EUV emission.

We have shown that there exist low-frequency growing modes driven by a global temperature gradient in electron and ion plasmas by linear perturbation analysis within the framework of plasma kinetic theory. The driving force of the instability is the local deviation of the distribution function from the Maxwell-Boltzmann distribution due to a global temperature gradient. Application of the results to the intracluster medium is being reduced to 5-7 orders of magnitude less than the mean free paths due to Coulomb collisions. This may provide a hint in explaining why hot and cool gas can coexist in the intracluster medium in spite of the very short evaporation timescale due to thermal conduction if the conductivity is the classical Spitzer value. Our results suggest that the realization of the global thermal equilibrium is postponed by the local instability, which is induced for a quicker realization of the local thermal equilibrium state in plasmas. The instability provides a new possibility to create and grow cosmic magnetic fields without any seed magnetic field.

Multifrequency polarimetry with the Very Long Baseline Array telescope has revealed absolute Faraday rotation measures (RMs) in excess of 1000 rad m^-2 in the central regions of seven out of eight strong quasars studied (e.g., 3C 273, 3C 279, and 3C 395). Beyond a projected distance of ~20 pc, however, the jets are found to have |RM| < 100 rad m^-2. Such sharp RM gradients cannot be produced by cluster or galactic-scale magnetic fields, but rather must be the result of magnetic fields organized over the central 1-100 pc. The RMs of the sources studied to date and the polarization properties of BL Lacs, quasars, and galaxies are shown to be consistent so far with the predictions of unified schemes. The direct detection of high RMs in these quasar cores can explain the low fractional core polarizations usually observed in quasars at centimeter wavelengths as the result of irregularities in the Faraday screen on scales smaller than the telescope beam. Variability in the RM of the core is reported for 3C 279 between observations taken 1.5 yr apart, indicating that the Faraday screen changes on that timescale or that the projected superluminal motion of the inner jet components samples a new location in the screen with time. Either way, these changes in the Faraday screen may explain the dramatic variability in core polarization properties displayed by quasars.

We have carried out a series of model calculations of the photoionized intergalactic medium (IGM) to determine the effects on the predicted ionic column densities due to uncertainties in the published dielectronic recombination (DR) rate coefficients. Based on our previous experimental work and a comparison of published theoretical DR rates, we estimate there is in general a factor of 2 uncertainty in existing DR rates used for modeling the IGM. We demonstrate that this uncertainty results in factors of ~1.9 uncertainty in the predicted N V and Si IV column densities, ~2.0 for O VI, and ~1.7 for C IV. We show that these systematic uncertainties translate into a systematic uncertainty of up to a factor of ~3.1 in the Si/C abundance ratio inferred from observations. The inferred IGM abundance ratio could thus be less than (Si/C)_☉ or greater than 3(Si/C)_☉. If the latter is true, then it suggests the metagalactic radiation field is not due purely to quasars but includes a significant stellar component. Lastly, column density ratios of Si IV to C IV versus C II to C IV are often used to constrain the decrement in the metagalactic radiation field at the He II absorption edge. We show that the variation in the predicted Si IV to C IV ratio due to a factor of 2 uncertainty in the DR rates is almost as large as that due to a factor of 10 change in the decrement. Laboratory measurements of the relevant DR resonance strengths and energies are the only unambiguous method of removing the effects of these atomic physics uncertainties from models of the IGM.

Despite the fact that quasars are generally strong X-ray emitters, ROSAT discovered several objects with only very weak X-ray emission. In this paper, the X-ray data from ASCA and ROSAT and the UV spectra from Hubble Space Telescope (HST) and IUE of one of these quasars, PG 0844+349, are analyzed. The ROSAT spectrum can be well fitted by a single power law with Galactic absorption. No spectral variations were observed during changes of the 0.1-2.4 keV X-ray flux by a factor of 10 between the ROSAT All-Sky Survey and pointed observations, separated by 6 months. The ASCA satellite found the object in a high state with a photon index of 1.98 and an Fe Kα line with EW ~ 300 eV. The X-ray flux in the 2-10 keV band is highly variable; the fastest variation detected is 60% in less than 2 × 10⁴ s. The measured excess variance fits well the excess variance versus L_{2-10 keV} relation for Seyfert 1 galaxies; the flux variability in the 0.5-2.0 keV band shows a slightly higher amplitude than in the 2-10 keV band. We show that the optical microvariability of this object can actually be driven by reprocessing of the variable X-ray flux if one-half of the absorbed X-rays are reradiated in the optical-to-UV band. A weak broad intrinsic absorption line (FWHM ≃ 800 km s^-1), most likely Lyα absorption blueshifted by a velocity ≃-6000 km s^-1 relative to the quasar's rest frame, is found in the HST Faint Object Spectrograph spectrum. A similar C IV broad absorption line may also be present in the low-resolution IUE spectrum. Historic light curves in the X-ray, UV, and optical bands indicate that the variability amplitude in the UV and optical bands is much smaller than in the X-ray band. The QSO can be classified as X-ray weak only on one occasion out of five X-ray observations. An analysis of the long-term behavior of several other X-ray-weak objects shows no indications of a similar large X-ray variability.

The Hubble Deep Field images have provided us with a unique chance to relate statistical properties of high-redshift galaxies to statistical properties of Lyα absorption systems. Combining an empirical measure of the galaxy surface density versus redshift with an empirical measure of the gaseous extent of galaxies, we predict the number density of Lyα absorption systems that originate in extended gaseous envelopes of galaxies versus redshift. We show that at least 50% and as much as 100% of observed Lyα absorption systems of W ≳ 0.32 Å can be explained by extended gaseous envelopes of galaxies. Therefore, we conclude that known galaxies of known gaseous extent must produce a significant fraction and perhaps all of Lyα absorption systems over a large redshift range.

We have used deep Hubble Space Telescope WFPC2 images in V (F606W) and I (F814W) to measure the luminosity distribution of the globular clusters in NGC 4874, the central cD galaxy of the Coma cluster. We find the "turnover" point of the globular cluster luminosity function (GCLF) to lie at V = 27.88 ± 0.12, while the overall GCLF shape matches the standard Gaussian-like form with dispersion σ_V = 1.49 ± 0.12. We use the GCLF as a standard candle by matching the turnover points in NGC 4874 and another Coma elliptical, IC 4051, with those of the giant ellipticals in the Virgo cluster (M87 and five others). The result is Δ(m - M) (Coma-Virgo) = 4.06 ± 0.11 mag, which converts to a Coma distance d = 102 Mpc if the Virgo distance modulus is (m - M)₀ = 30.99 ± 0.04. The Hubble constant which emerges from our GCLF measurement is then H₀ = (69 ± 9) km s^-1 Mpc^-1. We confirm this H₀ value with a novel presentation of the "Hubble diagram" for GCLFs in giant E galaxies. Measurements of additional GCLFs in the Coma ellipticals, as well as calibrating galaxies in Virgo and Fornax, have excellent potential to refine this result in the near future.

Deep Hubble Space Telescope WFPC2 images in V and I are used to investigate the globular cluster system (GCS) in NGC 4874, the central cD galaxy of the Coma cluster. Although the luminosity function of the clusters displays its normal Gaussian-like shape and turnover level, other features of the system are surprising. We find the GCS to be (a) spatially extended, with core radius r_c ~ 22 kpc, (b) entirely metal poor (a narrow, unimodal metallicity distribution with ⟨[Fe/H]⟩ ~ -1.5), and (c) modestly populated for a cD-type galaxy, with specific frequency S_N = 3.7 ± 0.5. Model interpretations suggest to us that as much as half of this galaxy might have accreted from low-mass satellites, but no single one of the three classic modes of galaxy formation (accretion, disk mergers, in situ formation) can supply a fully satisfactory model for the formation of NGC 4874. Even when they are used in combination, strong challenges to these models remain. We suggest that the principal anomaly in this GCS is essentially the complete lack of metal-rich clusters. If these were present in normal (M87-like) numbers in addition to the metal-poor ones that are already there, then the GCS in total would more closely resemble what we see in many other giant E galaxies. This supergiant galaxy appears to have avoided forming globular clusters during the main metal-rich stage of star formation that built the bulk of the galaxy.

We report high-resolution CO(1-0) observations in the central 6 kpc (1') of the LINER galaxy NGC 5005 with the Owens Valley Radio Observatory millimeter array. Molecular gas is distributed in three components—a ring at a radius of about 3 kpc, a strong central condensation, and a stream to the northwest of the nucleus but inside the 3 kpc ring. The ring shows systematic noncircular motions, with apparent inward velocities of ~50 km s^-1 on the minor axis. The central condensation is a disk of ~1 kpc radius with a central depression of ~50 pc radius. This disk has a molecular gas mass of ~2 × 10⁹M_☉; it shows a steep velocity gradient and a velocity range (~750 km s^-1) 30% larger than the velocity width of the rest of the galaxy. The stream between the 3 kpc ring and the nuclear disk lies on a straight dust lane seen in the optical. If this material moves in the plane of the galaxy, it lies at a radius of ~1 kpc but has a velocity offset by up to ~150 km s^-1 from galactic rotation. We suggest that an optically inconspicuous stellar bar lying within the 3 kpc ring can explain the observed gasdynamics. This bar is expected to connect the nuclear disk and the ring along the position angle of the northwest stream. A position-velocity cut in this direction reveals features that match the characteristic motions of gas in a barred potential. Our model indicates that gas in the northwest stream is on an x₁ orbit at the bar's leading edge; it is falling into the nucleus with a large noncircular velocity and will eventually contribute ~2 × 10⁸M_☉ to the nuclear disk. If most of this material merges with the disk on its first passage of pericenter, the gas accretion rate during the collision will be ~50 M_☉ yr^-1. We associate the disk with an inner 2 : 1 Lindblad resonance and attribute its large line width to favorably oriented elliptical orbits rather than (necessarily) to a large central mass. The 3 kpc ring is likely an inner 4 : 1 Lindblad resonance ring—or a pair of tightly wound spiral arms—arising at the bar ends. Both scenarios can explain the apparent noncircular motions in the ring. The high rate of bar-driven inflow and the irregular appearance of the northwest stream suggest that a major fueling event is in progress in NGC 5005. Such episodic (rather than continuous) gas supply can regulate the triggering of starburst and accretion activity in galactic nuclei.

Using a two-dimensional galaxy image decomposition technique, we extract global bulge and disk parameters for a complete sample of early-type disk galaxies in the near-infrared K band. We find significant correlation of the bulge parameter n with the central bulge surface brightness μ_b(0) and with effective radius r_e. Using bivariate analysis techniques, we find that log n, log r_e, and μ_b(0) are distributed in a plane with small scatter. We do not find a strong correlation of n with bulge-to-disk luminosity ratio, contrary to earlier reports. For these early-type disk galaxies, r_e and the disk scale length r_d are well correlated, but with large scatter. We examine the implications of our results for various bulge formation scenarios in disk galaxies.

We here estimate the mass M of the central object in five active galactic nuclei (AGNs), using the most recent reverberation data obtained by the AGN Watch Consortium. The cross-correlation function (CCF) centroids of the broad Lyα λ1216 and C IV λ1549 lines are used to estimate the size of the broad-line region (BLR) in these sources. We calculate the velocity dispersions of these lines in the root mean square (rms) spectra and then use our results to estimate M. We argue that our technique of calculating the velocity dispersion should work in the general case of an arbitrary line profile, unlike methods that depend on the measurement of the full-width at half-maximum (FWHM) of the broad line. We also show that our results agree with the FWHM method in the limit of a normal (Gaussian) line profile. The masses calculated here are considerably smaller than those calculated with the previous generation of reverberation data.

The appearance of wavelike helical structures on steady relativistic jets is studied using a normal mode analysis of the linearized fluid equations. Helical structures produced by the normal modes scale relative to the resonant (most unstable) wavelength and not with the absolute wavelength. The resonant wavelength of the normal modes can be less than the jet radius even on highly relativistic jets. High-pressure regions helically twisted around the jet beam may be confined close to the jet surface, penetrate deeply into the jet interior, or be confined to the jet interior. The high-pressure regions range from thin and ribbon-like to thick and tubelike depending on the mode and wavelength. The wave speeds can be significantly different at different wavelengths but are less than the flow speed. The highest wave speed for the jets studied has a Lorentz factor somewhat more than half that of the underlying flow speed.

A maximum pressure fluctuation criterion found through comparison between theory and a set of relativistic axisymmetric jet simulations is applied to estimate the maximum amplitudes of the helical, elliptical, and triangular normal modes. Transverse velocity fluctuations for these asymmetric modes are up to twice the amplitude of those associated with the axisymmetric pinch mode. The maximum amplitude of jet distortions and the accompanying velocity fluctuations at, for example, the resonant wavelength decreases as the Lorentz factor increases. Long-wavelength helical surface mode and shorter wavelength helical first body mode generated structures should be the most significant.

Emission from high-pressure regions as they twist around the jet beam can vary significantly as a result of angular variation in the flow direction associated with normal mode structures if they are viewed at about the beaming angle θ = 1/γ. Variation in the Doppler boost factor can lead to brightness asymmetries by factors up to 6 as long-wavelength helical structure produced by the helical surface mode winds around the jet. Higher order surface modes and first body modes produce less variation. Angular variation in the flow direction associated with the helical mode appears consistent with precessing jet models that have been proposed to explain the variability in 3C 273 and BL Lac object AO 0235+164. In particular, cyclic angular variation in the flow direction produced by the normal modes could produce the activity seen in BL Lac object OJ 287. Jet precession provides a mechanism for triggering the helical modes on multiple length scales, e.g., the galactic superluminal GRO J1655-40.

The degeneracy between the disk and the dark matter contribution to galaxy rotation curves remains an important uncertainty in our understanding of disk galaxies. Here we discuss a new method for breaking this degeneracy using gravitational lensing by spiral galaxies, and apply this method to the spiral lens B1600+434 as an example. The combined image and lens photometry constraints allow models for B1600+434 with either a nearly singular dark matter halo or a halo with a sizable core. If the dark halo has a core, then the bulge dominates the gravitational potential in the inner part of the galaxy, its mass is between 1.3 and 1.5 × 10¹¹M_☉, and the disk mass is less then 5 × 10¹⁰M_☉. If the dark halo is singular, there is a degeneracy between the disk mass and the halo ellipticity. The dark halo flattening (c/a) can be as low as 0.53 if there is no disk mass, while the maximum allowed disk mass, 1.3 × 10¹¹M_☉, is reached with a spherical halo. A maximum disk model is ruled out with high confidence. Further information, such as the circular velocity of this galaxy, will help break the degeneracies. Future studies of spiral galaxy lenses will be able to determine the relative contribution of disk, bulge, and halo to the mass in the inner parts of galaxies.

We report the result of a study in which we have used very deep broadband V and I Wide Field Planetary Camera 2 images of the R136 cluster in the Large Magellanic Cloud from the Hubble Space Telescope archive to sample the luminosity function below the detection limit of 2.8 M_☉ previously reached. In these new deeper images, we detect stars down to a limiting magnitude of m_F555W = 24.7 (≃1 mag deeper than previous works) and identify a population of red stars evenly distributed in the surrounding of the R136 cluster. A comparison of our color-magnitude diagram with recently computed evolutionary tracks indicates that these red objects are pre-main-sequence stars in the mass range 0.6-3 M_☉. We construct the initial mass function (IMF) in the 1.35-6.5 M_☉ range and find that, after correcting for incompleteness, the IMF shows a definite flattening below ≃2 M_☉. We discuss the implications of this result for the R136 cluster and for our understanding of starburst galaxy formation and evolution in general.

We review the results on distances and absolute ages of Galactic globular clusters (GCs) obtained after the release of the Hipparcos catalog. Several methods aimed at the definition of the Population II local distance scale are discussed, and their results compared, exploiting new results for RR Lyraes in the Large Magellanic Cloud (LMC). We find that the so-called short distance and long distance scales may be reconciled whether or not a consistent reddening scale is adopted for Cepheids and RR Lyrae variables in the LMC. Emphasis is given in the paper to the discussion of distances and ages of GCs derived using Hipparcos parallaxes of local subdwarfs. We find that the selection criteria adopted to choose the local subdwarfs, as well as the size of the corrections applied to existing systematic biases, are the main culprit for the differences found among the various independent studies that first used Hipparcos parallaxes and the subdwarf fitting technique. We also caution that the absolute age of M92 (usually considered one of the oldest clusters) still remains uncertain due to the lack of subdwarfs of comparable metallicity with accurate parallaxes. Distances and ages for the nine clusters discussed in a previous paper by Gratton et al. are rederived using an enlarged sample of local subdwarfs, which includes about 90% of the metal-poor dwarfs with accurate parallaxes (Δπ/π ≤ 0.12) in the whole Hipparcos catalog. On average, our revised distance moduli are decreased by 0.04 mag with respect to Gratton et al. The corresponding age of the GCs is t = 11.5 ± 2.6 Gyr, where the error bars refer to the 95% confidence range. The relation between the zero-age horizontal branch (ZAHB) absolute magnitude and metallicity for the nine program clusters turns out to be M_V(ZAHB) = (0.18 ± 0.09)([Fe/H] + 1.5) + (0.53 ± 0.12) Thanks to Hipparcos the major contribution to the total error budget associated with the subdwarf fitting technique has been moved from parallaxes to photometric calibrations, reddening, and metallicity scale. This total uncertainty still amounts to about ±0.12 mag.

We then compare the corresponding (true) LMC distance modulus μ_LMC = 18.64 ± 0.12 mag with other existing determinations. We conclude that at present the best estimate for the distance of the LMC is μ_LMC = 18.54 ± 0.03 ± 0.06, suggesting that distances from the subdwarf fitting method are ~1 σ too long. Consequently, our best estimate for the age of the GCs is revised to Age = 12.9 ± 2.9 Gyr (95% confidence range). The best relation between ZAHB absolute magnitude and metallicity is M_V(ZAHB) = (0.18 ± 0.09)( + 1.5) + (0.63 ± 0.07). Finally, we compare the ages of the GCs with the cosmic star formation rate recently determined by studies of the Hubble Deep Field (HDF), exploiting the determinations of Ω_M = 0.3 and Ω_Λ = 0.7 provided by Type Ia supernovae surveys. We find that the epoch of formation of the GCs (at z ~ 3) matches well the maximum of the star formation rate for elliptical galaxies in the HDF as determined by Franceschini et al.

The presence of dust in starburst galaxies complicates the study of their stellar populations as the dust's effects are similar to those associated with changes in the galaxies' stellar ages and metallicities. This degeneracy can be overcome for starburst galaxies if UV/optical/near-infrared observations are combined with far-infrared observations. We present the calibration of the flux ratio method for calculating the dust attenuation at a particular wavelength, Att(λ), based on the measurement of the F(IR)/F(λ) flux ratio. Our calibration is based on spectral energy distributions from the PEGASE stellar evolutionary synthesis model and the effects of dust (absorption and scattering) as calculated from our Monte Carlo radiative transfer model. We tested the attenuations predicted from this method for the Balmer emission lines of a sample of starburst galaxies against those calculated using radio observations and found good agreement. The UV attenuation curves for a handful of starburst galaxies were calculated using the flux ratio method, and they compare favorably with past work. The relationship between Att(λ) and F(IR)/F(λ) is almost completely independent of the assumed dust properties (grain type, distribution, and clumpiness). For the UV, the relationship is also independent of the assumed stellar properties (age, metallicity, etc.) except for the case of very old burst populations. However, at longer wavelengths the relationship is dependent on the assumed stellar properties.

We report the results of a five-field mosaic of the central 15 pc of the Galaxy in the (1, 1) and (2, 2) lines of NH₃. Two narrow filaments or streamers are seen running parallel to the Galactic plane. The southern streamer appears to carry gas directly toward the nuclear region from the 20 km s^-1 cloud. The eastern streamer, which we will denote the molecular ridge, appears to be the denser part of the 50 km s^-1 cloud that lies immediately east of the Sgr A East complex and extends in the south toward the 20 km s^-1 cloud. This ridge of gas carries the kinematical signatures of interactions with Sgr A East as well as a supernova remnant that lies south of the Galactic center. The bulk motion of the gas, the enhanced line widths, and the heating of the molecular material all suggest an active evolutionary phase for the gas immediately adjacent to the nucleus.

Isotopic abundance ratios ²⁴Mg : ²⁵Mg : ²⁶Mg are derived for 20 stars from high-resolution spectra of the MgH A-X 0-0 band at 5140 Å. With the exception of the weak G-band giant HR 1299, the stars are dwarfs that sample the metallicity range -1.8 < [Fe/H] < 0.0. The abundance of ²⁵Mg and ²⁶Mg relative to the dominant isotope ²⁴Mg decreases with decreasing [Fe/H] in fair accord with predictions from a recent model of Galactic chemical evolution in which the Mg isotopes are synthesized by massive stars. Several stars appear especially enriched in the heavier Mg isotopes, suggesting contamination by material from the envelopes of intermediate-mass asymptotic giant branch stars.

We present OH and H I Zeeman observations of the NGC 6334 complex taken with the Very Large Array. The OH absorption profiles associated with the complex are relatively narrow (Δv_FWHM ~ 3 km s^-1) and single-peaked over most of the sources. The H I absorption profiles contain several blended velocity components. One of the compact continuum sources in the complex (source A) has a bipolar morphology. The OH absorption profiles toward this source display a gradient in velocity from the northern continuum lobe to the southern continuum lobe; this velocity gradient likely indicates a bipolar outflow of molecular gas from the central regions to the northern and southern lobes. Magnetic fields of the order of 200 μG have been detected toward three discrete continuum sources in the complex. Virial estimates suggest that the detected magnetic fields in these sources are of the same order as the critical magnetic fields required to support the molecular clouds associated with the sources against gravitational collapse.

The cosmic-ray proton and helium spectra from 0.2 GeV nucleon^-1 to about 200 GeV nucleon^-1 have been measured with the balloon-borne experiment Isotope Matter-Antimatter Experiment (IMAX) launched from Lynn Lake, Manitoba, Canada, in 1992. IMAX was designed to search for antiprotons and light isotopes using a superconducting magnet spectrometer together with scintillators, a time-of-flight system, and Cherenkov detectors. Using redundant detectors, an extensive examination of the instrument efficiency was carried out. We present here the absolute spectra of protons and helium corrected to the top of the atmosphere and to interstellar space. If demodulated with a solar modulation parameter of ϕ = 750 MV, the measured interstellar spectra between 20 and 200 GV can be represented by a power law in rigidity, with (1.42 ± 0.21) × 10⁴R^-2.71±0.04 (m² GV s sr)^-1 for protons and (3.15 ± 1.03) × 10³R^-2.79±0.08 (m² GV s sr)^-1 for helium.

If aspherical dust grains are immersed in an anisotropic radiation field, their temperature depends on the cross sections projected in the direction of the anisotropy. It was shown that the temperature difference produces polarized thermal emission even without alignment, if the observer looks at the grains from a direction different from the anisotropic radiation. When the dust grains are aligned, the anisotropy in the radiation produces various effects on the polarization of the thermal emission, depending on the relative angle between the anisotropy and alignment directions. If both directions are parallel, the anisotropy produces a steep increase in the degree of polarization at short wavelengths. If they are perpendicular, the polarization reversal occurs at a wavelength shorter than the emission peak. The effect of the anisotropic radiation will produce a change of more than a few percent in the degree of polarization for short wavelengths, and the effect must be taken into account in the interpretation of the polarization in the thermal emission. The anisotropy in the radiation field produces a strong spectral dependence of the degree of polarization and position angle, which is not seen under isotropic radiation. The dependence changes with the grain shape to a detectable level, and thus it will provide a new tool to investigate the shapes of dust grains. This paper presents examples of numerical calculations of the effects and demonstrates the importance of an anisotropic radiation field on the polarized thermal emission.

Weak interstellar scintillations of pulsar B0809+74 were observed at two epochs using a 30 m EISCAT antenna at 933 MHz. These have been used to constrain the spectrum, the distribution, and the transverse velocity of the scattering plasma with respect to the LSR. The Kolmogorov power law is a satisfactory model for the electron density spectrum at scales between 2 × 10⁷ and 10⁹ m. We compare the observations with model calculations from weak scintillation theory and the known transverse velocities of the pulsar and the Earth. The simplest model is that the scattering is uniformly distributed along the 310 pc line of sight (l = 140°, b = 32°) and is stationary in the LSR. With the scattering measure as the only free parameter, this model fits the data within the errors, and a range of about ±10 km s^-1 in velocity is also allowed. The integrated level of turbulence is low, being comparable to that found toward PSR B0950+08, and suggests a region of low local turbulence over as much as 90° in longitude, including the Galactic anticenter. If, on the other hand, the scattering occurs in a compact region, the observed timescales require a specific velocity-distance relation. In particular, enhanced scattering in a shell at the edge of the local bubble, proposed by Bhat et al. in 1998, near 72 pc toward the pulsar, must be moving at about ~17 km s^-1; however, the low scattering measure argues against a shell of enhanced scattering in this direction. The analysis also excludes scattering in the termination shock of the solar wind or in a nebula associated with the pulsar.

We present observations of SN 1997cy, a supernova (SN) discovered as part of the Mount Stromlo Abell Cluster SN Search (Reiss et al.), which does not easily fit into the traditional classification scheme for supernovae. This object's extraordinary optical properties and coincidence with GRB 970514, a short duration gamma-ray burst (GRB), suggest a second case, after SN 1998bw/GRB 980425, for a SN-GRB association. SN 1997cy is among the most luminous SNe yet discovered (M_R ≪ -20.1, H₀ = 65) and has a peculiar spectrum. We present evidence that SN 1997cy ejected approximately 2.6 M_☉ of ⁵⁶Ni, supported by its late-time light curve and Fe II/[Fe III] lines in its spectrum, although it is possible that both these observations can be explained via circumstellar interaction. While SN 1998bw and SN 1997cy appear to be very different objects with respect to both their gamma-ray and optical properties, SN 1997cy and the optical transient associated with GRB 970508 have roughly similar late-time optical behavior. This similarity may indicate that the late-time optical output of these two intrinsically bright transient events have a common physical process. Although the connection between GRB 970514 and SN 1997cy is suggestive, it is not conclusive. However, if this association is real, follow-up of short duration GRBs detected with BATSE or HETE2 should reveal objects similar to SN 1997cy.

We report on Rossi X-Ray Timing Explorer observations of four type I X-ray bursters, namely, 1E 1724-3045, GS 1826-238, SLX 1735-269, and KS 1731-260. The first three were in a low state, with 1-200 keV X-ray luminosities in the range ~0.05-0.1L_Edd (L_Edd: Eddington luminosity for a neutron star, L_Edd = 2.5 × 10³⁸ ergs s^-1), whereas KS 1731-260 was in a high state, with luminosity ~0.35L_Edd. The low-state sources have very similar power spectra, displaying high-frequency noise up to ~200 Hz. For KS 1731-260, its power spectrum is dominated by noise at frequencies ≲20 Hz; in addition a quasi-periodic oscillation at 1200 Hz is detected in a segment of the observation. The 1-200 keV spectra of the low-state sources are all consistent with resulting from thermal Comptonization with an electron temperature (kT_e) around 25-30 keV. For KS 1731-260, the spectrum is also dominated by thermal Comptonization, but with a much lower kT_e ~ 3 keV and no significant hard X-ray emission. With the exception of GS 1826-238, they each have an underlying soft component, carrying at most ~25% of the total 1-200 keV luminosity. For all sources, we have detected an iron Kα line at 6.4 keV (although it is weak and marginal in 1E 1724-3045). A reflection component is present in the spectra of GS 1826-238 and SLX 1735-269, and for both we find that the reflecting medium subtends only a small solid angle (Ω/2π ~ 0.15, 0.28). The origin of the line and the reflection component is most likely to be irradiation of the accretion disk by the X-ray source. We suggest a model in which the region of main energy release, where hard X-rays are produced, would be an optically thin boundary layer merged with an advection-dominated accretion flow (ADAF) and would be responsible for the rapid variability observed. The soft component observed probably represents the unscattered emission from an optically thick accretion disk of variable inner radius. When the accretion rate increases, the inner disk radius shrinks and the strength of the reflected component and associated iron line increase. At the same time, the Comptonization region cools off in response to an increased cooling flux from the accretion disk and from the reprocessed/reflected component, thus leading progressively to a quenching of the hard X-ray emission. If low-state neutron stars (NSs) accrete via ADAFs, the observation of X-ray bursts, indicating that all the accreting matter actually accumulates onto the NS surface, argues against the existence of strong winds from such accretion flows. Finally, we discuss two criteria recently proposed to distinguish between nonquiescent black holes (BHs) and NSs that are not contradicted by existing observations. The first one states that, when thermal Comptonization is responsible for the hard X-ray emission, only BHs have kT_e larger than ~50 keV. However, this criterion is weakened by the fact that there are NSs displaying nonattenuated power laws extending up to at least 200 keV, possibly implying nonthermal Comptonization or thermal Comptonization with kT_e larger than 50 keV. The second criterion stipulates that only BHs are capable of emitting hard X-ray tails with 20-200 keV luminosities ≳1.5 × 10³⁷ ergs s^-1.

The low-mass X-ray binary (LMXB) X1832-330 in NGC 6652 is one of 12 bright, or transient, X-ray sources to have been discovered in globular clusters. We report on a serendipitous ASCA observation of this globular cluster LMXB, during which a type I burst was detected and the persistent, nonburst emission of the source was at its brightest level recorded to date. No orbital modulation was detected, which argues against a high inclination for the X1832-330 system. The spectrum of the persistent emission can be fitted with a power law plus a partial covering absorber, although other models are not ruled out. Our time-resolved spectral analysis through the burst shows, for the first time, clear evidence for spectral cooling from kT = 2.4 ± 0.6 keV to kT = 1.0 ± 0.1 keV during the decay. The measured peak flux during the burst is ~10% of the Eddington luminosity for a 1.4 M_☉ neutron star. These quantities are characteristic of a type I burst, in the context of the relatively low quiescent luminosity of X1832-330.

We present the results of numerical experiments designed to evaluate the usefulness of near-infrared (NIR) luminosity functions for constraining the initial mass function (IMF) of young stellar populations. We test the sensitivity of the NIR K-band luminosity function (KLF) of a young stellar cluster to variations in the underlying IMF, star-forming history, and pre-main-sequence mass-to-luminosity relations. Using Monte Carlo techniques, we create a suite of model luminosity functions systematically varying each of these basic underlying relations. From this numerical modeling, we find that the luminosity function of a young stellar population is considerably more sensitive to variations in the underlying initial mass function than to either variations in the star-forming history or assumed pre-main-sequence (PMS) mass-to-luminosity relation. Variations in a cluster's star-forming history are also found to produce significant changes in the KLF. In particular, we find that the KLFs of young clusters evolve in a systematic manner with increasing mean age. Our experiments indicate that variations in the PMS mass-to-luminosity relation, resulting from differences in adopted PMS tracks, produce only small effects on the form of the model luminosity functions and that these effects are mostly likely not detectable observationally. To illustrate the potential effectiveness of using the KLF of a young cluster to constrain its IMF, we model the observed KLF of the nearby Trapezium cluster. With knowledge of the star-forming history of this cluster obtained from optical spectroscopic studies, we derive the simplest underlying IMF whose model luminosity function matches the observations. Our derived mass function for the Trapezium spans 2 orders of magnitude in stellar mass (5 > M_☉ > 0.02) and has a peak near the hydrogen-burning limit. Below the hydrogen-burning limit, the mass function steadily decreases with decreasing mass throughout the brown dwarf regime. Comparison of our IMF with that derived by optical and spectroscopic methods for the entire Orion Nebula Cluster suggests that modeling the KLF is indeed a useful tool for constraining the mass function in young stellar clusters particularly at and below the hydrogen-burning limit.

We have detected an X-ray flare on the very low mass star VB 10 (GL 752 B; M8V) using the ROSAT High Resolution Imager. VB 10 is the latest type (lowest mass) main-sequence star known to exhibit coronal activity. X-rays were detected from the star during a single 1.1 ks segment of an observation that lasted 22 ks in total. The energy released by this flare is on the order of 10²⁷ ergs s^-1. This is at least 2 orders of magnitude greater than the quiescent X-ray luminosity of VB 10, which has yet to be measured. This X-ray flare is very similar in nature to the far-ultraviolet flare that was observed by Linsky et al. using the Goddard High Resolution Spectrograph onboard the Hubble Space Telescope. We discuss reasons for the extreme difference between the flare and quiescent X-ray luminosities, including the possibility that VB 10 has no quiescent (10⁶ K) coronal plasma at all.

We present observations of the microlensing event MACHO 98-BLG-35, which reached a peak magnification factor of almost 80. These observations by the Microlensing Planet Search (MPS) and MOA collaborations place strong constraints on the possible planetary system of the lens star and show intriguing evidence for a low-mass planet with a mass fraction 4 × 10^-5 ≤ ≤ 2 × 10^-4. A giant planet with = 10^-3 is excluded from 95% of the region between 0.4 and 2.5 R_E from the lens star, where R_E is the Einstein ring radius of the lens. This exclusion region is more extensive than the generic "lensing zone," which is 0.6-1.6 R_E. For smaller mass planets, we can exclude 57% of the "lensing zone" for = 10^-4 and 14% of the lensing zone for = 10^-5. The mass fraction = 10^-5 corresponds to an Earth-mass planet for a lensing star of mass ~0.3 M_☉. A number of similar events will provide statistically significant constraints on the prevalence of Earth-mass planets. In order to put our limits in more familiar terms, we have compared our results to those expected for a solar system clone, averaging over possible lens system distances and orientations. We find that such a system is ruled out at the 90% confidence level. A copy of the solar system with Jupiter replaced by a second Saturn-mass planet can be ruled out at 70% confidence. Our low-mass planetary signal (few Earth masses to Neptune mass) is significant at the 4.5 σ confidence level. If this planetary interpretation is correct, the MACHO 98-BLG-35 lens system constitutes the first detection of a low-mass planet orbiting an ordinary star without gas giant planets.

Tensor-scalar theory of gravity allows the generation of gravitational waves from astrophysical sources, like supernovae, even in the spherical case. That motivated us to study the collapse of a degenerate stellar core, within tensor-scalar gravity, leading to the formation of a neutron star through a bounce and the formation of a shock. This paper discusses the effects of the scalar field on the evolution of the system, as well as the appearance of strong nonperturbative effects of this scalar field (the so-called spontaneous scalarization). As a main result, we describe the resulting gravitational monopolar radiation (form and amplitude) and discuss the possibility of its detection by the gravitational detectors currently under construction, taking into account the existing constraints on the scalar field. From the numerical point of view, it is worthy to point out that we have developed a combined code that uses pseudo-spectral methods for the evolution of the scalar field and High-Resolution Shock-Capturing schemes, as well as for the evolution of the hydrodynamical system. Although this code has been used to integrate the field equations of that theory of gravity, in the spherically symmetric case, a by-product of the present work is to gain experience for an ulterior extension to multidimensional problems in Numerical Relativity of such numerical strategy.

The effect of color superconductivity on the cooling of quark stars and neutron stars with large quark cores is investigated. Various known and new quark-neutrino processes are studied. As a result, stars in the color-flavor-locked color superconducting phase cool down extremely fast. Quark stars with no crust cool down too rapidly, in disagreement with X-ray data. The cooling of stars in the N_f = 2 color superconducting phase with a crust is compatible with existing X-ray data. In addition, the cooling histories of stars with hypothetical pion condensate nuclei and a crust do not contradict the data.

We investigate the evolution of low-mass metal-free Population III stars. Emphasis is laid upon the question of internal and external sources for CNO elements, which—if present in sufficient amounts in the hydrogen-burning regions—lead to a strong modification of the stars' evolutionary behavior. For the production of carbon due to nuclear processes inside the stars, we use an extended nuclear network, demonstrating that hot p-p chains do not suffice to produce enough carbon or are less effective than the 3α process. As an external source of CNO elements we test the efficiency of pollution by a nearby massive star combined with particle diffusion. For all cases investigated, the additional metals fail to reach nuclear-burning regions before deep convection on the red giant branch obliterates the previous evolution. The surface abundance history of the polluted Population III stars is presented. The possibilities to discriminate between a Population II and a polluted Population III field star are also discussed.

Neutrino-driven winds from young hot neutron stars, which are formed by supernova explosions, are the most promising candidate site for r-process nucleosynthesis. We study general relativistic effects on this wind in Schwarzschild geometry in order to look for suitable conditions for successful r-process nucleosynthesis. It is quantitatively demonstrated that general relativistic effects play a significant role in increasing the entropy and decreasing the dynamic timescale of the neutrino-driven wind. Exploring the wide parameter region that determines the expansion dynamics of the wind, we find interesting physical conditions that lead to successful r-process nucleosynthesis. The conditions that we found are realized in a neutrino-driven wind with a very short dynamic timescale, τ_dyn ~ 6 ms, and a relatively low entropy, S ~ 140. We carry out α-process and r-process nucleosynthesis calculations on these conditions with our single network code, which includes over 3000 isotopes, and confirm quantitatively that the second and third r-process abundance peaks are produced in neutrino-driven winds.

We have observed HCO⁺J = 3-2 toward 16 class I sources and 18 class 0 sources, many of which were selected from Mardones et al.'s recent observations. Eight sources have profiles significantly skewed to the blue relative to optically thin lines. We suggest six sources as new infall candidates. We find an equal "blue excess" among class 0 and class I sources after combining this sample with that of Gregersen et al. We used a Monte Carlo code to simulate the temporal evolution of line profiles of optically thick lines of HCO⁺, CS, and H₂CO in a collapsing cloud and found that HCO⁺ had the strongest asymmetry at late times. If a blue-peaked line profile implies infall, then the dividing line between the two classes does not trace the end of the infall stage.

In the early solar nebula, the formation of planetesimals and cometesimals is believed to be due to inelastic collisions of initially micron-sized grains. The collisions are caused by relative velocities due to size-dependent interactions with the surrounding dilute gas. The grain growth process is determined by the velocity-dependent sticking efficiency upon collisions. Therefore, we performed experiments with eight samples of micron-sized particles consisting of monodisperse silica spheres, of irregularly shaped diamond, enstatite, and silicon carbide grains, and of silicon carbide whiskers. We determined the sticking probability and the energy loss upon bouncing collisions by studying individual grain-target collisions in vacuum. We found a sticking probability higher than predicted by previous theoretical work. Grain size, roughness, and primarily grain shape, i.e., the difference of spherical versus irregular grain shape, is important for the collisional behavior, whereas the material properties are rather unimportant. Our results indicate that the preplanetary dust aggregation is more effective than previously thought.

Collisions between micron-sized grains and larger objects with velocities up to several 10 m s^-1 are believed to be an important physical process in the solar nebula with respect to the preplanetary dust aggregation. Former collision experiments demonstrated that grain-target collisions of micron-sized particles were marked by obvious electrostatic effects. Among those were the observation of particles which, after mechanical rebound, returned to the target and finally stuck, and of particle deposition on targets influenced by the presence of conducting materials. Therefore, it is clear that the dust aggregation process cannot adequately be described without investigating collisional grain charging experimentally. We present experiments on the collisional grain charging of micron-sized grains impacting target surfaces which, in contrast to former work, consist both of nonconducting material and the experiments involving smaller particles than before. Collisional grain charging is stronger than previously discussed with respect to preplanetary grains and should be considered concerning the preplanetary dust aggregation, the formation of lightning in the solar nebula, and a coupling of charged grains to magnetic fields.

A coronal mass ejection (CME) was observed on 1997 September 9 by the Mauna Loa Solar Observatory Mark III K-coronameter (MK3) and by the LASCO C2/C3 and EIT instruments on board the SOHO spacecraft. Magnetograms and EIT images obtained on days leading up to the eruption show a neutral line that appears to correspond to the site of the eruption. Taken together, the data from these instruments provide a comprehensive, beginning-to-end record of the event within the 32 R_☉ field of view. The motion of several features are tracked through the fields of view of MK3, C2, and C3. The CME exhibits the previously identified morphological features and dynamical properties consistent with those of an erupting magnetic flux rope with its legs connected to the Sun. The LASCO images and magnetograms indicate that the flux rope axis was aligned with the neutral line approximately 2 days behind the west limb. Its apparent orientation provides an oblique view of an erupting flux rope, a view that has not been discussed previously. A theoretical flux rope model is used to understand the forces responsible for the observed CME dynamics. Synthetic coronagraph images based on the model flux rope are constructed.

A mechanism for chromospheric network heating and a necessary and sufficient condition for its onset are presented. The heating mechanism consists of resistive dissipation of proton Pedersen currents, which flow orthogonal to the magnetic field in weakly ionized chromospheric plasma. The currents are driven by a convection electric field generated by velocity oscillations of linear, slow, longitudinal magnetoacoustic waves with frequencies ν ≲ 3.5 mHz in the lower chromosphere. The heating occurs in thin magnetic flux tubes and begins lower in the chromosphere in flux tubes with higher photospheric field strength. The lower chromosphere, which emits most of the net radiative loss in the network, is heated by flux tubes with photospheric field strengths ~700-1500 G. A typical field strength and core diameter for a flux tube in the lower chromosphere with a core heating rate of 10⁷ ergs cm^-2 s^-1 are 170 G and 10 km. This core region is contained in a region with a diameter ~100 km in which the heating rate is an order of magnitude smaller. About N ~ 10² of these flux tubes distributed over the boundary region of a granule with a diameter ~10³ km provide an average heating rate over the entire granule ~10⁷ ergs cm^-2 s^-1. If the core heating rate is changed by a factor f, then N ~ f^-1/210². The condition for the onset of heating is that the ratio of the proton cyclotron frequency to the proton-hydrogen collision frequency equal unity. This ratio increases with height, and the condition is satisfied at a single height in a given flux tube. At this height, control of the proton dynamics begins to be dominated by the magnetic field rather than by collisions with hydrogen, and the anisotropic nature of the electrical conductivity begins to play a critical role in resistive dissipation. The protons become magnetized. Heating by dissipation of heavy ion and, to a lesser extent, proton Pedersen currents causes the temperature to start increasing. The heating increases hydrogen ionization. With increasing height, and hence proton magnetization, the Pedersen current density rapidly increases with hydrogen ionization via positive feedback, and the proton number density rapidly reaches and exceeds the heavy ion number density, resulting in an increase in heating rate by an order of magnitude over only 1 pressure scale height. During this process the protons rapidly dominate the Pedersen current. Heating by dissipation of magnetic field aligned currents is insignificant. Below the height in the atmosphere at which the onset condition is satisfied, any current orthogonal to the magnetic field must be primarily a Hall current, which is nondissipative. Heating by this mechanism must occur to some degree in the chromospheric network of all solar-type stars. It is proposed to be the dominant mechanism of chromospheric network heating, although viscous dissipation may also be important if the core heating rate is much larger than ~10⁷ ergs cm^-2 s^-1 or if the linear MHD waves studied here evolve into shock waves with increasing height. Flux tubes in the quiet chromosphere are predicted to have two possible core diameters: ~10 km, corresponding to flux tubes in which network heating occurs, and ~10⁴-10⁵ km, perhaps corresponding to flux tubes in which active region heating might occur. The model has a singularity at the acoustic cutoff frequency, corresponding to periods near 3 minutes. Therefore, unless nonresistive damping mechanisms such as viscous dissipation and thermal conduction provide sufficiently strong damping, MHD oscillations with periods near 3 minutes in chromospheric magnetic flux tubes must be nonlinear.

The propagation and the dissipation of small-amplitude Alfvénic wave packets in a three-dimensional magnetic field is studied in the WKB approximation. In a chaotic magnetic field nearby lines exponentially diverge; thus, in propagating packets small scales are formed exponentially in time. Related to this phenomenon, a dissipation time t_d proportional to log S is obtained, S being the Reynolds and/or the Lundquist number. This scaling corresponds to a dissipation much faster than that of phase mixing (t_d ∝ S^1/3). In the present work we consider force-free magnetic fields in which both phase mixing and exponential divergence are present, and we study both the competition between the two scalings and the transition between phase mixing and the three-dimensional fast dissipation regimes. In a simpler equilibrium structure (the Arnold-Beltrami-Childress field) we found that both phenomenologies take place, in spatially separated regions. So, a fraction of the initial wave energy, proportional to the relative amplitude of the chaotic regions, is dissipated by the faster mechanism. For more complex fields (two-dimensional flux tubes perturbed by three-dimensional small-amplitude components) two different regimes exist: the dissipation time follows either the three-dimensional or the phase-mixing scaling, when S is above or below a threshold. The threshold value decreases with increasing the amplitude of the three-dimensional force-free component. These results are discussed with reference to the problem of coronal heating.

We study the emission and dynamical characteristics of transition region temperature plasmas in magnetic loops by analyzing a high-resolution, limb observation of the active region NOAA 7962. The observations were performed by the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) instrument on board the Solar and Heliospheric Observatory (SOHO). The SUMER observation produced a set of raster scans of the region, in the four lines, H I Lyβ λ1025, O VI λλ1032, 1038, and C II λ1037. The data are used to construct intensity, velocity, and line width maps of the active region, from which more than 10 well-resolved loops are identified and classified into four different groups. We determine several physical parameters of the loops in each group such as diameter, length, temperature, line-of-sight plasma velocity, and nonthermal line broadening. Our results indicate that both kinds of temperature variations exist in active region loops: variations from loop to loop and variations along each loop. It is also found that there is a distinction between stationary loops and dynamic loops. The dynamic loops have large bulk motions and large nonthermal line broadenings. Some of the dynamic loops display large velocity shears with the sign of line-of-sight velocities changing across the loop axes. These velocity shears appear to represent rotational motions around the loop axes with velocities of up to 50 km s^-1. There are indications that nonthermal line broadening is the result of magnetohydrodynamic turbulence inside the loops. Based on our observations, we postulate that when loops erupt, some of the kinetic and magnetic energy cascades down to turbulent energy which would be dissipated as heat.

The dynamics of the vigorous convection in the outer envelope of the Sun must determine the transport of energy, angular momentum, and magnetic fields and must therefore be responsible for the observed surface activity and the angular velocity profile inferred helioseismically from SOI-MDI p-mode frequency splittings. Many different theoretical treatments have been applied to the problem, ranging from simple physical models such as mixing-length theory to sophisticated numerical simulations. Although mixing-length models provide a good first approximation to the structure of the convection zone, recent progress has mainly come from numerical simulations. Computational constraints have until now limited simulations in full spheres to essentially laminar convection. The angular velocity profiles have shown constancy on cylinders, in striking contrast to the approximately constant angular velocity on radial lines inferred for the Sun. In an effort to further our understanding of the dynamics of the solar convection zone, we have developed a new computer code that, by exploiting massively parallel architectures, enables us to study fully turbulent spherical shell convection. Here we present five fully evolved solutions. Motivated by the fact that a constant entropy upper boundary condition produces a latitudinal modulation of the emergent energy flux (of about 10%, i.e., far larger than is observed for the Sun), three of these solutions have a constant energy flux upper boundary condition. This leads to a latitudinal modulation of the specific entropy that breaks the constancy of the angular velocity on cylinders, making it more nearly constant on radial lines at midlatitudes. The effect of lowering the Prandtl number is also considered—highly time-dependent, vortical convective motions are revealed, and the Reynolds stresses are altered, leading to a reduced differential rotation. The differential rotation in all of our simulations shows a balance between driving by Reynolds stresses and damping by viscosity. This contrasts with the situation in the Sun, where the effect of viscosity on the mean differential rotation is almost negligible.

We present microwave (17 GHz) observations of eruptive activity in four solar coronal events with the Nobeyama Radioheliograph. These are weak events occurring at or near the solar limb associated with several types of activity: polar crown activity, prominence eruptions, and arcade flares. The morphological evolutions of the microwave sources in these events show the following characteristic features in common. The activity starts as a mound-shaped source (1.0-4.5 × 10⁴ km in height), above which a compact blob (≤1.0 × 10⁴ km in size) appears later and expands horizontally toward the top of another low-lying mound. Finally a faint arch with a filamentary structure is formed, bridging the two mounds (0.2-2.0 × 10⁵ km in horizontal scale size). Thus, the activity seems to propagate through the arch corridor from the initially activated mound to the secondary. During this process, the activity level increases, as indicated by an increase of the brightness temperature of the mound and the blob as well as by the formation of the arch and the eruption of the blob. These common features suggest that basically the same energy buildup and release process takes place in all four events, in spite of the large difference in the total released energy. Here we propose magnetic reconnection progressing in between the blob and the mound as the basic process, as indicated by high-temperature plasma there, seen in soft X-rays. Our findings support a theoretically predicted analogy between filament (or prominence) activity and flare onset.

Time-distance analysis and acoustic imaging are two related techniques for probing the local properties of the solar interior. In this study, we discuss the relation of phase time and envelope time between the two techniques. The location of the envelope peak of the cross-correlation function in time-distance analysis is identified as the travel time of the wave packet formed by modes with the same horizontal phase velocity. The phase time of the cross-correlation function provides information on the phase change accumulated along the wave path, including the phase change at the boundaries of the mode cavity. The acoustic signals constructed with the technique of acoustic imaging contain both phase and intensity information. The phase of constructed signals can be studied by computing the cross-correlation function between time series constructed with ingoing and outgoing waves. We use a simple theory of wave packets to obtain two predictions about the cross-correlation function of constructed ingoing and outgoing time series. First, if the envelope time measured in time-distance analysis is used to construct signals in acoustic imaging, the envelope time of the cross-correlation is zero. Second, the phase time of the cross-correlation is twice the difference between the phase time and envelope time measured in time-distance analysis. In this study, we use data taken with the Taiwan Oscillation Network (TON) instrument and the Michelson Doppler Imager (MDI) instrument. The analysis is carried out for the quiet Sun. We use the relation of envelope time versus distance measured in time-distance analysis to construct the acoustic signals in acoustic imaging analysis. The phase time of the cross-correlation function of constructed ingoing and outgoing time series is twice the difference between phase time and envelope time in time-distance analysis, as predicted. The envelope peak of the cross-correlation function between constructed ingoing and outgoing time series is located at zero time, as predicted for one-bounce results at 3 mHz for all four data sets and two-bounce results at 3 mHz for two TON data sets, but it is different from zero for other cases. The deviation of the envelope peak from zero has the same sign for all these cases. The cause is not known.

Erratum to ApJ, 522, 524 [1999]

The formation rate of luminous galaxies seems to be roughly constant from z ~ 2 to ~4 from the recent observations of Lyman break galaxies (LBGs). The abundance of luminous quasars, on the other hand, appears to drop off by a factor of more than 20 from z ~ 2 to z ~ 5. The difference in evolution between the two classes of objects in the overlapping, observed redshift range (z = 2-4) can be explained naturally if we assume that quasar activity is triggered by mergers of luminous LBGs and one quasar lifetime is ~10⁷-10⁸ yr. If this merger scenario holds at higher redshift, for the evolutions of these two classes of objects to be consistent at z > 4, the formation rate of luminous LBGs is expected to drop off at least as rapidly as exp at z > 4.

Observations of the damped Lyα systems provide direct measurements on the chemical enrichment history of neutral gas in the early universe. In this Letter, we present new measurements for four damped Lyα systems at high redshift. Combining these data with [Fe/H] values culled from the literature, we investigate the metallicity evolution of the universe from z ≈ 1.5 to 4.5. Contrary to our expectations and the predictions of essentially every chemical evolution model, the N(H I)-weighted mean [Fe/H] metallicity exhibits minimal evolution over this epoch. For the individual systems, we report tentative evidence for an evolution in the unweighted [Fe/H] mean and the scatter in [Fe/H], with the higher redshift systems showing lower scatter and lower typical [Fe/H] values. We also note that no damped Lyα system has [Fe/H] < -2.7 dex. Finally, we discuss the potential impact of small number statistics and dust on our conclusions and consider the implications of these results on chemical evolution in the early universe.

Integral field optical spectroscopy with the INTEGRAL fiber-fed system and Hubble Space Telescope optical imaging are used to map the complex stellar and warm ionized gas structure in the ultraluminous infrared galaxy IRAS 12112+0305. Images reconstructed from wavelength-delimited extractions of the integral field spectra reveal that the observed ionized gas distribution is decoupled from the stellar main body of the galaxy, with the dominant continuum and emission-line regions separated by projected distances of up to 7.5 kpc. The two optical nuclei are detected as apparently faint emission-line regions, and their optical properties are consistent with being dust-enshrouded weak [O I] LINERs. The brightest emission-line region is associated with a faint (m_I = 20.4), giant H II region of 600 pc diameter, in which a young (~5 Myr) massive cluster of about 2 × 10⁷M_☉ dominates the ionization. Internal reddening toward the line-emitting regions and the optical nuclei ranges from 1 to 8 mag in the visual. Taking the reddening into account, the overall star formation in IRAS 12112+0305 is dominated by starbursts associated with the two nuclei and corresponds to a star formation rate of 80 M_☉ yr^-1.

We present detections of emission at 250 GHz (1.2 mm) from two high-redshift QSOs from the Sloan Digital Sky Survey sample using the bolometer array at the IRAM 30 m telescope. The sources are SDSSp 015048.83+004126.2 at z = 3.7 and SDSSp J033829.31+002156.3 at z = 5.0; the latter is the third highest redshift QSO known and the highest redshift millimeter-emitting source yet identified. We also present deep radio continuum imaging of these two sources at 1.4 GHz using the Very Large Array. The combination of centimeter and millimeter observations indicate that the 250 GHz emission is most likely thermal dust emission, with implied dust masses ≈10⁸M_☉. We consider possible dust heating mechanisms, including UV emission from the active galactic nucleus (AGN) and a massive starburst concurrent with the AGN, with implied star formation rates greater than 10³M_☉ yr^-1.

Deep I-band imaging to I ≈ 26.5 of the soft gamma-ray repeater SGR 1900+14 region has revealed a compact cluster of massive stars located only a few arcseconds from the fading radio source thought to be the location of the soft gamma-ray repeater (SGR). This cluster was previously hidden in the glare of the pair of M5 supergiant stars (whose light was removed by point-spread function subtraction) proposed by Vrba et al. as likely associated with SGR 1900+14. The cluster has at least 13 members within a cluster radius of ≈0.6 pc based on an estimated distance of 12-15 kpc. It is remarkably similar to a cluster found associated with SGR 1806-20. That similar clusters have now been found at or near the positions of the two best studied SGRs suggests that young neutron stars, which are thought to be responsible for the SGR phenomenon, have their origins in proximate compact clusters of massive stars.

The galactic environment of gamma-ray bursts can provide good evidence about the nature of the progenitor system, with two old arguments implying that the burst host galaxies are significantly subluminous. New data and new analysis have now reversed this picture: (1) Even though the first two known host galaxies are indeed greatly subluminous, the next eight hosts have absolute magnitudes typical for a population of field galaxies. A detailed analysis of the 16 known hosts (10 with redshifts) shows them to be consistent with a Schechter luminosity function with R* = -21.8 ± 1.0, as expected for normal galaxies. (2) Bright bursts from the Interplanetary Network are typically 18 times brighter than the faint bursts with redshifts; however, the bright bursts do not have galaxies inside their error boxes to limits deeper than expected based on the luminosities for the two samples being identical. A new solution to this dilemma is that a broad burst luminosity function along with a burst number density varying as the star formation rate will require the average luminosity of the bright sample (>6 × 10⁵⁸ photons s^-1 or >1.7 × 10⁵² ergs s^-1) to be much greater than the average luminosity of the faint sample (~10⁵⁸ photons s^-1 or ~3 × 10⁵¹ ergs s^-1). This places the bright bursts at distances for which host galaxies with a normal luminosity will not violate the observed limits. In conclusion, all current evidence points to gamma-ray burst host galaxies being normal in luminosity.

The Dominion Radio Astrophysical Observatory's Synthesis Telescope provides the highest resolution data (1' and 0.82 km s^-1) to date of an H I worm candidate. Observed as part of the Canadian Galactic Plane Survey, mushroom-shaped GW 123.4-1.5 extends only a few hundred parsecs, contains ~10⁵M_☉ of neutral hydrogen, and appears unrelated to a conventional shell or chimney structure. Our preliminary Zeus two-dimensional models use a single off-plane explosion with a modest (~10⁵¹ ergs) energy input. These generic simulations generate, interior to an expanding outer blast wave, a buoyant cloud whose structure resembles the morphology of the observed feature. Unlike typical model superbubbles, the stem can be narrow because its width is not governed by the pressure behind the blast wave or the disk scale height. Using this type of approach, it should be possible to more accurately model the thin stem and other details of GW 123.4-1.5 in the future.

Chandra observations of the Crab-like supernova remnant G21.5-0.9 reveal a compact central core and spectral variations indicative of synchrotron burn-off of higher energy electrons in the inner nebula. The central core is slightly extended, perhaps indicating the presence of an inner wind-shock nebula surrounding the pulsar. No pulsations are observed from the central region, yielding an upper limit of ~40% for the pulsed fraction. A faint outer shell may be the first evidence of the expanding ejecta and blast wave formed in the initial explosion, indicating a composite nature for G21.5-0.9.

We report the detection of a broad 22 μm emission feature in the Carina Nebula H II region by the Infrared Space Observatory (ISO) short-wavelength spectrometer. The feature shape is similar to that of the 22 μm emission feature of newly synthesized dust observed in the Cassiopeia A supernova remnant. This finding suggests that both of the features are arising from the same carrier and that supernovae are probably the dominant production sources of this new interstellar grain. A similar broad emission dust feature is also found in the spectra of two starburst galaxies from the ISO archival data. This new dust grain could be an abundant component of interstellar grains and can be used to trace the supernova rate or star formation rate in external galaxies. The existence of the broad 22 μm emission feature complicates the dust model for starburst galaxies and must be taken into account correctly in the derivation of dust color temperature. Mg protosilicate has been suggested as the carrier of the 22 μm emission dust feature observed in Cassiopeia A. The present results provide useful information in studies on the chemical composition and emission mechanism of the carrier.

We present subarcsecond thermal infrared imaging of HD 98800, a young quadruple system composed of a pair of low-mass spectroscopic binaries separated by 08 (38 AU), each with a K-dwarf primary. Images at wavelengths ranging from 5 to 24.5 μm show unequivocally that the optically fainter binary, HD 98800B, is the sole source of a comparatively large infrared excess on which a silicate emission feature is superposed. The excess is detected only at wavelengths of 7.9 μm and longer, peaks at 25 μm, and has a best-fit blackbody temperature of 150 K, indicating that most of the dust lies at distances greater than the orbital separation of the spectroscopic binary. We estimate the radial extent of the dust with a disk model that approximates radiation from the spectroscopic binary as a single source of equivalent luminosity. Given the data, the most likely values of disk properties in the ranges considered are R_in = 5.0 ± 2.5 AU, ΔR = 13 ± 8 AU, λ₀ = 2 μm, γ = 0 ± 2.5, and σ_total = 16 ± 3 AU², where R_in is the inner radius, ΔR is the radial extent of the disk, λ₀ is the effective grain size, γ is the radial power-law exponent of the optical depth τ, and σ_total is the total cross section of the grains. The range of implied disk masses is 0.001-0.1 times that of the Moon. These results show that, for a wide range of possible disk properties, a circumbinary disk is far more likely than a narrow ring.

We have estimated the ages of eight late-type Vega-like stars by using standard age-dating methods for single late-type stars, e.g., location on the color-magnitude diagram, Li λ6708 absorption, Ca II H and K emission, X-ray luminosity, and stellar kinematic population. With the exception of the very unusual pre-main-sequence star system HD 98800, all the late-type Vega-like stars are the same age as the Hyades cluster (600-800 Myr) or older.

Near-infrared spectroscopic observations of a sample of very cool, low-mass objects are presented with higher spectral resolution than in any previous studies. Six of the objects are L dwarfs, ranging in spectral class from L2 to L8/9, and the seventh is a methane or T dwarf. These new observations were obtained during commissioning of the near-infrared spectrometer (NIRSPEC), the first high-resolution near-infrared cryogenic spectrograph for the Keck II 10 m telescope on Mauna Kea, Hawaii. Spectra with a resolving power of R ≈ 2500 from 1.135 to 1.360 μm (approximately J band) are presented for each source. At this resolution, a rich spectral structure is revealed, much of which is due to blending of unresolved molecular transitions. Strong lines due to neutral potassium (K I) and bands due to iron hydride (FeH) and steam (H₂O) change significantly throughout the L sequence. Iron hydride disappears between L5 and L8, the steam bands deepen, and the K I lines gradually become weaker but wider because of pressure broadening. An unidentified feature occurs at 1.22 μm that has a temperature dependence like FeH but has no counterpart in the available FeH opacity data. Because these objects are 3-6 mag brighter in the near-infrared compared with the I band, spectral classification is efficient. One of the objects studied (2MASSW J1523+3014) is the coolest L dwarf discovered so far by the 2 Micron All-Sky Survey (2MASS), but its spectrum is still significantly different from the methane-dominated objects such as Gl 229B or SDSS 1624+0029.

We present moderate- (R ≈ 2700) and high-resolution (R ≈ 22,400) 2.0-2.4 μm spectroscopy of the central 0.1 arcsec² of the Galaxy obtained with the facility near-infrared spectrometer (NIRSPEC) for the Keck II telescope. The composite spectra do not have any features attributable to the brightest stars in the central cluster; i.e., after background subtraction, W < 2 Å. This stringent limit leads us to conclude that the majority, if not all, of the stars are hotter than typical red giants. Coupled with previously reported photometry, we conclude that the sources are likely OB main-sequence stars. In addition, the continuum slope in the composite spectrum is bluer than that of a red giant and is similar to that of the nearby hot star IRS 16NW. It is unlikely that they are late-type giants stripped of their outer envelopes because such sources would be much fainter than those observed. Given their inferred youth (τ_age < 20 Myr), we suggest the possibility that the stars have formed within 0.1 pc of the supermassive black hole. We find a newly identified broad-line component (V_FWHM ≈ 1000 km s^-1) toward the 2.2178 μm [Fe III] line located within a few arcseconds of Sagittarius A*. A similar component is not seen in the Brγ emission.

We present high-resolution spectroscopy and images of a photodissociation region (PDR) in M16 obtained during commissioning of the near-infrared spectrometer (NIRSPEC) on the Keck II telescope. PDRs play a significant role in regulating star formation, and M16 offers the opportunity to examine the physical processes of a PDR in detail. We simultaneously observe both the molecular and ionized phases of the PDR and resolve the spatial and kinematic differences between them. The most prominent regions of the PDR are viewed edge-on. Fluorescent emission from nearby stars is the primary excitation source, although collisions also preferentially populate the lowest vibrational levels of H₂. Variations in density-sensitive emission-line ratios demonstrate that the molecular cloud is clumpy, with an average density n = 3 × 10⁵ cm^-3. We measure the kinetic temperature of the molecular region directly and find that T = 930 K. The observed density, temperature, and UV flux imply a photoelectric heating efficiency of 4%. In the ionized region, n_i = 5 × 10³ cm^-3 and T = 9500 K. In the brightest regions of the PDR, the recombination line widths include a nonthermal component, which we attribute to viewing geometry.

We present infrared spectroscopy of the Antennae galaxies (NGC 4038/9) with the near-infrared spectrometer (NIRSPEC) at the W. M. Keck Observatory. We imaged the star clusters in the vicinity of the southern nucleus (NGC 4039) with 039 seeing in the K band using NIRSPEC's slit-viewing camera. The brightest star cluster revealed in the near-IR [M_K(0) ≃ -17.9] is insignificant optically but is coincident with the highest surface brightness peak in the mid-IR (12-18 μm) Infrared Space Observatory image presented by Mirabel et al. We obtained high signal-to-noise ratio 2.03-2.45 μm spectra of the nucleus and the obscured star cluster at R ~ 1900. The cluster is very young (~4 Myr), massive (M ~ 16 × 10⁶M_☉), and compact (with a density of ~115 M_☉ pc^-3 within a 32 pc half-light radius), assuming a Salpeter initial mass function (0.1-100 M_☉). Its hot stars have a radiation field characterized by T_eff ~ 39,000 K, and they ionize a compact H II region with n_e ~ 10⁴ cm^-3. The stars are deeply embedded in gas and dust (A_V ~ 9-10 mag), and their strong far-ultraviolet field powers a clumpy photodissociation region with densities n_H ≳ 10⁵ cm^-3 on scales of ~200 pc, radiating L = 9600 L_☉.

This Letter presents infrared spectra taken with the newly commissioned near-infrared spectrometer (NIRSPEC) on the Keck II telescope of the high-redshift radio galaxy MRC 2025-218 (z = 2.63). These observations represent the deepest infrared spectra of a radio galaxy to date and have allowed for the detection of Hβ, [O III] λλ4959, 5007, [O I] λ6300, Hα, [N II] λλ6548, 6583, and [S II] λλ6716, 6713. The Hα emission is very broad (FWHM = 9300 km s^-1) and luminous (2.6 × 10⁴⁴ ergs s^-1), and it is very comparable to the line widths and strengths of radio-loud quasars at the same redshift. This strongly supports active galactic nucleus unification models linking radio galaxies and quasars, although we discuss some of the outstanding differences. The line [O III] λ5007 is extremely strong and has extended emission with large relative velocities toward the nucleus. We also derive that if the extended emission is due to star formation, each knot has a star formation rate comparable to a Lyman-break galaxy at the same redshift.

Moderate-resolution, near-IR spectroscopy of MS 1512-cB58 is presented, obtained during commissioning of the near-infrared spectrometer (NIRSPEC) on the Keck II telescope. The strong lensing of this z = 2.72 galaxy by the foreground cluster MS 1512+36 makes it the best candidate for detailed study of the rest-frame optical properties of Lyman-break galaxies. In 80 minutes of on-source integration, we have detected Hα, [N II] λλ6583, 6548, [O I] λ6300, He I λ5876, [O III] λλ5007, 4959, Hβ, Hγ, [O II] λ3727, and a strong continuum signal in the range of 1.29-2.46 μm. A redshift of z = 2.7290 ± 0.0007 is inferred from the emission lines, in contrast to the z = 2.7233 calculated from UV observations of interstellar absorption lines. Using the Balmer line ratios, we find an extinction of E(B-V) = 0.27. Using the line strengths, we infer a star formation rate (SFR) of 620 ± 18 M_☉ yr^-1 (H₀ = 75, q₀ = 0.1, and Λ = 0), which is a factor of 2 higher than that measured from narrowband imaging observations of the galaxy but is a factor of almost 4 lower than the SFR inferred from the UV continuum luminosity. The width of the Balmer lines yields a mass of M_vir = 1.2 × 10¹⁰M_☉. We find that the oxygen abundance is solar, in good agreement with other estimates of the metallicity. However, we infer a high nitrogen abundance, which may argue for the presence of an older stellar population.