Showing papers in "Astrophysical Journal Supplement Series in 2001"

Toward Better Age Estimates for Stellar Populations: The Y2 Isochrones for Solar Mixture

Abstract: We have constructed a new set of isochrones, called the Y2 Isochrones, that represent an update of the Revised Yale Isochrones (RYI), using improved opacities and equations of state. Helium diffusion and convective core overshoot have also been taken into consideration. This first set of isochrones is for the scaled solar mixture. A subsequent paper will consider the effects of α-element enhancement, believed to be relevant in many stellar systems. Two additionally significant features of these isochrones are that (1) the stellar models start their evolution from the pre-main-sequence birthline instead of from the zero-age main sequence and (2) the color transformation has been performed using both the latest table of Lejeune et al., and the older, but now modified, Green et al. table. The isochrones have performed well under the tests conducted thus far. The reduction in the age of the Galactic globular clusters caused by this update in stellar models alone is approximately 15% relative to RYI-based studies. When the suggested modification for the α-element enhancement is made as well, the total age reduction becomes approximately 20%. When post-RGB evolutionary stages are included, we find that the ages of globular clusters derived from integrated colors are consistent with the isochrone fitting ages.

An Empirical Ultraviolet Template for Iron Emission in Quasars as Derived from I Zwicky 1

Abstract: We present an empirical template spectrum suitable for fitting and subtracting/studying the Fe II and Fe III emission lines in the rest frame ultraviolet spectra of quasars and active galactic nuclei, the first empirical ultraviolet iron template to cover the full range of 1250-3090 A. Iron emission is often a severe contaminant in optical-ultraviolet spectra of active galactic nuclei and quasars. Its presence complicates and limits the accuracy of measurements of both strong and weak emission lines and the continuum emission, affecting studies of line and continuum interrelations, the ionization structure, and elemental abundances in active galaxies and quasars. Despite the wealth of work on modeling the quasar Fe II emission and the need to account for this emission in observed quasar spectra, there is no ultraviolet template electronically available to aid this process. The iron template we present is based on Hubble Space Telescope spectra of the narrow line Seyfert 1 galaxy, I Zwicky 1 (I Zw 1, z = 0.061). The intrinsic narrow lines ( 900 km s-1) of this source and its rich iron spectrum make the template particularly suitable for use with most active galactic nuclei and quasar spectra. The iron emission spectrum, the line identifications, and the measurements of absorption and emission lines are presented and compared with the work of Laor et al. Comments on each individual line feature and the line fitting are available in Appendix A. The methods used to develop and apply the template are also described. We illustrate the application of the derived Fe II and Fe III templates by fitting and subtracting iron emission from the spectra of four high-redshift quasars and of the nearby quasar, 3C 273, confirming their general applicability to active galaxies despite the somewhat unusual properties of I Zw 1. We briefly discuss the small discrepancies between the observed iron emission of these quasars and the ultraviolet template, and compare the template with previously published ones. We discuss the advantages and limitations of the UV Fe II and Fe III templates and of the template fitting method. We conclude that the templates work sufficiently well to be a valuable and important tool for eliminating and studying the iron emission in active galaxies, at least until accurate theoretical iron emission models are developed. The Si IV+O IV] λ1400 feature in I Zw 1 is clearly strong relative to C IV λ1549, and C IV and C III] λ1909 are both relatively weak. This may partially be due to the higher densities and lower ionization parameter prevailing in narrow line Seyfert 1 galaxies and to the big blue bump shifting toward lower energies in more luminous Seyferts, such as I Zw 1. In I Zw 1 the narrow line width reveals that C III] is heavily blended with Si III] λ1892, Al III λλ1854,1863, and Fe III transitions. This suggests that the C III] line strength and width may be overestimated in many quasar line studies where the lines are broader and deblending is not possible. This affects density estimates of the broad line region. Photoionization modeling, including all these line features, and subsequent fitting to the spectra are required to estimate the true C III] strength. We also argue, based on earlier work, that (strong) iron emission may be connected with high densities and associated with outflows.

Photoelectric Emission from Interstellar Dust: Grain Charging and Gas Heating

Abstract: We model the photoelectric emission from and charging of interstellar dust and obtain photoelectric gas heating efficiencies as a function of grain size and the relevant ambient conditions. We employ improved estimates for photoelectric thresholds, yields, and electron capture rates. Using realistic grain size distributions, we evaluate the net gas heating rate for various interstellar environments and find less heating for dense regions characterized by RV = 5.5 than for diffuse regions with RV = 3.1. We provide fitting functions that reproduce our numerical results for photoelectric heating and recombination cooling for a wide range of interstellar conditions. Finally, we investigate the potential importance of photoelectric heating in H II regions, including the warm ionized medium. We find that photoelectric heating could be comparable to or exceed heating due to photoionization of H for high ratios of the radiation intensity to the gas density. We also find that photoelectric heating by dust can account for the observed variation of temperature with distance from the Galactic midplane in the warm ionized medium.

Optical Classification of Southern Warm Infrared Galaxies

Abstract: In this paper, we present high-resolution optical spectra and optical classifications from our large sample of 285 warm infrared galaxies 108 < LIR < 1012.5 L☉. We have classified these galaxies using new theoretical lines on the standard optical diagnostic diagrams. We use a theoretical extreme mixing line between the starburst and AGN regions to classify LINER galaxies and we define a theoretical boundary separating AGNs from starbursts. We find that many galaxies previously classified as LINERs appear to lie on a mixing sequence between starburst and AGN type galaxies. These are likely to be of a composite nature with their excitation being a combination of photoionization due to hot stars plus either ionization by a power-law radiation field associated with an AGN or shock excitation where the shock may result from such processes as cooling flows, superwind activity, or an accretion disk around an AGN. We compare our theory-based classification scheme with the previous semiempirical scheme of Veilleux & Osterbrock . We find that our classification method results in 6% ambiguity in classifications between the different diagnostic diagrams compared with 16% ambiguity using the traditional Veilleux & Osterbrock method. We find that 70% of the galaxies in our sample are classified optically as starburst, 17% are Seyfert 2, 4% are Seyfert 1, and 0.4% are LINERs. A further 2% of our sample are certainly composite galaxies. A fraction (20%) of the Seyfert galaxies, 3% of the starburst galaxies, and 71% of the ambiguous galaxies are possibly composite in nature (11% of the total sample).

Correlated X-Ray Spectral and Timing Behavior of the Black Hole Candidate XTE J1550–564: A New Interpretation of Black Hole States

Abstract: We present an analysis of data of the black hole candidate and X-ray transient XTE J1550-564, taken with the Rossi X-Ray Timing Explorer between 1998 November 22 and 1999 May 20. During this period the source went through several different states, which could be divided into soft and hard states based on the relative strength of the high-energy spectral component. These states showed up as distinct branches in the color-color and hardness-intensity diagrams, connecting to form a structure with a comblike topology, the branch corresponding to the soft state forming the spine and the branches corresponding to the various hard states forming the teeth of the comb. The power spectral properties of the source were strongly correlated with its position on the branches. The broadband noise became stronger and changed from power law-like to band-limited, as the spectrum became harder. Three types of quasi-periodic oscillations (QPOs) were found: 1-18 Hz and 102-284 Hz QPOs on the hard branches, and 16-18 Hz QPOs on and near the soft branch. The 1-18 Hz QPOs on the hard branches could be divided into three subtypes. The frequencies of the high- and low-frequency QPOs on the hard branches were correlated with each other and were anticorrelated with spectral hardness. The changes in QPO frequency suggest that the inner disk radius only increases by a factor of 3-4 as the source changes from a soft to a hard state. Our results on XTE J1550-564 strongly favor a two-dimensional description of black hole behavior, where the regions near the spine of the comb in the color-color diagram can be identified with the high state, and the teeth with transitions from the high state, via the intermediate state (which includes the very high state) to the low state, and back. The two physical parameters underlying this two-dimensional behavior vary to a large extent independently and could for example be the accretion rate through the disk and the size of the Comptonizing region causing the hard tail. The difference between the various teeth is then associated with the mass accretion rate through the disk, suggesting that high state ↔ low state transitions can occur at any disk mass accretion rate and that these transitions are primarily caused by another, independent parameter. We discuss how this picture could tie in with the canonical, one-dimensional behavior of black hole candidates that has usually been observed.

Photometric and Spectroscopic Analysis of Cool White Dwarfs with Trigonometric Parallax Measurements

Abstract: A detailed photometric and spectroscopic analysis of cool (Teff 12,000 K) white dwarf stars is presented. The sample has been drawn from the Yale Parallax Catalog and from a proper motion survey in the southern hemisphere. Optical BVRI and infrared JHK photometry, as well as spectroscopy at Hα, have been secured for a sample of 152 white dwarfs. The discovery of seven new DA white dwarfs, two new DQ white dwarfs, one new magnetic white dwarf, and three weak magnetic white dwarf candidates, is reported. Our sample also identifies 19 known or suspected double degenerates. The photometric energy distributions, the Hα line profiles, and the trigonometric parallax measurements are combined and compared against the predictions of model atmosphere calculations to determine the effective temperature and the radius of each object in the sample and also to constrain the atmospheric composition. New evolutionary sequences with carbon/oxygen cores with thin and thick hydrogen layers are used to derive stellar masses and ages. The results are used to improve our understanding of the chemical evolution of cool white dwarfs. We confirm the existence of a range in effective temperature between ~5000 and 6000 K where almost all white dwarfs have hydrogen-rich atmospheres. Our sample shows little evidence for mixed H/He white dwarfs, with the exception of two helium-rich DA stars, and four (possibly five) C2H white dwarfs which have been interpreted as having mixed H/He/C atmospheres. The observed sequence of DQ stars is found to terminate abruptly near 6500 K, below which they are believed to turn into C2H stars. True DC stars slightly above this temperature are found to exhibit hydrogen-like energy distributions despite the lack of Hα absorption features. The mean mass of our complete sample is 0.65 M☉ with a dispersion of σ ~ 0.20 M☉. Attempts to interpret the chemical evolution of cool white dwarfs show the problem to be complex. Convective mixing is called upon to account for the increase of the non-DA to DA ratio below 12,000 K, as well as the reappearance of helium-rich stars below ~5000 K. The possible presence of helium in cool DA stars, the existence of the non-DA gap, and the nature of the peculiar DC stars are also explained in terms of convective mixing, although our understanding of how this mechanism works needs to be revised in order to account for these observations. Given this chemical evolution uncertainty, it is not clear whether thick or thin hydrogen layer models should be used to determine cooling ages. The oldest object in our sample is ~7.9 Gyr or ~9.7 Gyr old depending on whether thin or thick hydrogen layer models are used, respectively.

Distances and Metallicities of High- and Intermediate-Velocity Clouds

Abstract: A table is presented that summarizes published absorption line measurements for the high- and intermediate-velocity clouds (HVCs and IVCs). New values are derived for N(H I) in the direction of observed probes, in order to arrive at reliable abundances and abundance limits (the H I data are described in Paper II). Distances to stellar probes are revisited and calculated consistently, in order to derive distance brackets or limits for many of the clouds, taking care to properly interpret nondetections. The main conclusions are the following. (1) Absolute abundances have been measured using lines of S II, N I, and O I, with the following resulting values: ~0.1 solar for one HVC (complex C), ~0.3 solar for the Magellanic Stream, ~0.5 solar for a southern IVC, and ~solar for two northern IVCs (the IV Arch and LLIV Arch). Finally, approximate values in the range 0.5-2 solar are found for three more IVCs. (2) Depletion patterns in IVCs are like those in warm disk or halo gas. (3) Most distance limits are based on strong UV lines of C II, Si II, and Mg II, a few on Ca II. Distance limits for major HVCs are greater than 5 kpc, while distance brackets for several IVCs are in the range 0.5-2 kpc. (4) Mass limits for major IVCs are 0.5-8 × 105 M☉, but for major HVCs they are more than 106 M☉. (5) The Ca II/H I ratio varies by up to a factor 2-5 within a single cloud, somewhat more between clouds. (6) The Na I/H I ratio varies by a factor of more than 10 within a cloud, and even more between clouds. Thus, Ca II can be useful for determining both lower and upper distance limits, but Na I only yields upper limits.

Multiepoch Very Long Baseline Array Observations of EGRET-detected Quasars and BL Lacertae Objects: Superluminal Motion of Gamma-Ray Bright Blazars

Abstract: We present the results of a program to monitor the structure of the radio emission in 42 ?-ray bright blazars (31 quasars and 11 BL Lac objects) with the Very Long Baseline Array (VLBA) at 43, 22, and occasionally 15 and 8.4 GHz, over the period from 1993 November to 1997 July. We determine proper motions in 33 sources and find that the apparent superluminal motions in ?-ray sources are much faster than for the general population of bright compact radio sources. This follows the strong dependence of the ?-ray flux on the level of relativistic beaming for both external radiation Compton and synchrotron self-Compton emission. There is a positive correlation (correlation coefficient r = 0.45) between the flux density of the VLBI core and the ?-ray flux and a moderate correlation (partial correlation coefficient r = 0.31) between ?-ray apparent luminosity and superluminal velocities of jet components, as expected if the ?-ray emission originates in a very compact region of the relativistic jet and is highly beamed. In 43% of the sources the jet bends by more than 20? on parsec scales, which is consistent with amplification by projection effects of modest actual changes in position angle. In 27 of the sources in the sample there is at least one non-core component that appears to be stationary during our observations. Different characteristics of stationary features close to and farther from the core lead us to suggest two different classes of stationary components: those within about 2 mas of the core, probably associated with standing hydrodynamical compressions, and those farther down the jet, which tend to be associated with bends in the jet.

Multiscale Gaussian Random Fields and Their Application to Cosmological Simulations

Abstract: This paper describes the generation of Gaussian random fields with multiple levels of resolution. We present the theory of adaptive mesh refinement of Gaussian random fields followed by the implementation and testing of a computer code package performing this refinement called "GRAFIC2." This package is available to the computational cosmology community at http://arcturus.mit.edu/grafic/ or by e-mail from the author.

Radio Continuum Survey of an Optically Selected Sample of Nearby Seyfert Galaxies

Abstract: We have used the Very Large Array (VLA) to conduct a survey for radio continuum emission in the sample of 52 Seyfert nuclei selected from the optical spectroscopic galaxy catalog of Ho, Filippenko, & Sargent. This Seyfert sample is the most complete and least biased available, and, as such, it will be useful for a variety of statistical analyses. Here we present the observations, measurements, and an atlas of radio maps. The observations were made at 6 cm in the B array and at 20 cm in the A array, yielding matched angular resolutions of ~1''. We detected 44 objects (85%) at 6 cm and 37 objects (71%) at 20 cm above a 3 σ threshold of 0.12 mJy beam-1. The sources have a wide range of radio powers (P ≈ 1018-1025 W Hz-1), spectral indices (α ≈ +0.5 to -1), and linear sizes (L ≈ few tens pc-15 kpc). The morphology of the radio emission is predominantly that of a compact core, either unresolved or slightly resolved, occasionally accompanied by elongated, jetlike features. Linearly polarized emission was detected at 6 cm in 12 sources, nine of which were also detected at 20 cm.

Faint X-Ray Sources Resolved in the ASCA Galactic Plane Survey and Their Contribution to the Galactic Ridge X-Ray Emission

Abstract: The X-ray emission from the central region of the Galactic plane, |l| 45° and |b| 04, was studied in the 0.7-10 keV energy band with a spatial resolution of ~3' with the Advanced Satellite for Cosmology and Astrophysics (ASCA) observatory. We developed a new analysis method for the ASCA data to resolve discrete sources from the extended Galactic ridge X-ray emission (GRXE). We successfully resolved 163 discrete sources with an X-ray flux down to 10-12.5 ergs cm-2 s-1 and determined the intensity variations of the GRXE as a function of the Galactic longitude with a spatial resolution of about 1°. The longitudinal intensity variation in the energy band above 4 keV, for which there is little absorption in the Galactic plane, shows a large enhanced feature within |l| 30°. This suggests a strong enhancement of X-ray emissivity of the GRXE inside the 4 kpc arm of the Galaxy. Searches for identifications of the resolved X-ray sources with cataloged X-ray sources and optical stars show that the 66% are unidentified. Spectral analysis of each source shows that a large number of the unidentified sources have hard X-ray spectra obscured by the Galactic interstellar medium. We classified the sources into several groups by the flux, the hardness and the softness of the spectra, and performed further detailed analysis for the spectra summed within each group. Possible candidates of X-ray origins of these unidentified sources are discussed based on the grouping spectral analysis. Also, we derived the log N-log S relations of the resolved sources in the energy bands below and above 2 keV separately. The log N-log S relation of the Galactic X-ray sources above 2 keV was obtained for the first time with this study. It is represented by a power-law with an index of -0.79 ± 0.07 after correction for the contribution of extragalactic X-ray sources. This flat power-law relation suggests that the spatial distribution of the X-ray sources should have an armlike structure in which the solar system is included. The integrated surface brightness of the resolved sources is about 10% of the total GRXE in both energy bands. The approximately 90% of the emission remaining is still unresolved.

A Survey for Infall Motions toward Starless Cores. II. CS (2-1) and N 2 H + (1-0) Mapping Observations

Abstract: We present the results of an extensive mapping survey of starless cores in the optically thick line of CS (2-1) and the optically thin lines of N2H+ (1-0) and C18O (1-0). The purpose of this survey was to search for signatures of extended inward motions. A total of 53 targets were observed in the three lines with the FCRAO 14 m telescope. Thirty-three regions were mapped in both CS and N2H+, and thirty seven well-defined N2H+ cores have been identified. The N2H+ emission is generally compact enough to find a peak, while the CS and C18O emissions are more diffuse. For each core, we have derived the normalized velocity difference (?VCS) between the thick CS and thin N2H+ peak velocities. We define 10 strong and nine probable infall candidates, based on ?VCS analysis and on the spectral shapes of CS lines. From our analysis of the blue-skewed CS spectra and the ?VCS parameter, we find typical infall radii of 0.06-0.14 pc. Also, using a simple two-layer radiative transfer model to fit the profiles, we derive one-dimensional infall speeds, the values of half of which lie in the range of 0.05-0.09 km s-1. These values are similar to those found in L1544 by Tafalla et al., and this result confirms that infall speeds in starless cores are generally faster than expected from ambipolar diffusion in a strongly subcritical core. In addition, the observed infall regions are too extended to be consistent with the inside-out collapse model applied to a very low mass star. In the largest cores, the spatial extent of the CS spectra with infall asymmetry is larger than the extent of the N2H+ core by a factor of 2-3. All these results suggest that extended inward motions are a common feature in starless cores, and that they could represent a necessary stage in the condensation of a star-forming dense core.

A Line Survey of Orion-KL from 607 to 725 GHz

Abstract: With the Caltech Submillimeter Telescope, we have performed an unbiased line survey of Orion-KL in the frequency range 607-725 GHz. We were able to identify lines down to a threshold of 1-2 K in main beam brightness temperature units, and we found 1064 spectral features consisting of 2032 lines, partially blended. Apart from the abundant diatomic rotors such as CO, CS, SO, the spectrum is dominated by CH3OH and SO2, both in terms of numbers of lines and integrated flux. The number of unidentified lines is 155 or 14%. We also report the tentative identification of the new molecule SiH in our data. For some complex organic molecules, we find rotation temperatures and column densities which are much higher than those found in earlier studies. It is likely the cause of this is a significant contribution from a very compact, hot component of the Orion molecular cloud core which was not visible in lower frequency surveys.

The XSTAR Atomic Database

Abstract: We report on the XSTAR atomic database that contains a large quantity of atomic rates for use in spectral modeling of astrophysical plasmas. The database includes atomic energy levels, line wavelengths, radiative transition probabilities, electron impact excitation rates, photoionization cross section, recombination rate coefficients, electron impact ionization rates, and fluorescence and Auger yields. The species considered are all the ions of H, He, C, N, O, Ne, Mg, Si, S, Ar, Ca, Fe, and Ni. The database collects recent data from many sources including CHIANTI, TOPbase, ADAS, NIST, and the IRON Project. Two particular features of the database are the incorporation of photoionization cross sections from the Opacity Project for all levels of every ion, and state-specific recombination rates. State-specific collisional ionization and three-body recombination are also included. The collection of data is used to build excitation-ionization models of each ion for calculation of their spectra for electron densities of up to 1018 cm-3 and temperatures between 100 and 109 K. In addition, every ion model is designed to converge to LTE under appropriate conditions. These models and data are implemented in the photoionization modeling code XSTAR v.2.

The Distribution of Thermal Pressures in the Interstellar Medium from a Survey of C I Fine-Structure Excitation

Abstract: We used the Space Telescope Imaging Spectrograph (STIS) with its smallest entrance aperture (003 wide slit) and highest resolution echelle gratings (E140H and E230H) to record the interstellar absorption features for 10 different multiplets of neutral carbon at a resolving power of ?/?? = 200,000 in the UV spectra of 21 early-type stars. Our objective was to measure the amount of C I in each of its three fine-structure levels of the ground electronic state, so that we could determine the thermal pressures in the absorbing gas and how much they vary in different regions. Our observations are principally along directions out to several kiloparsecs in the Galactic plane near longitudes l = 120? and 300?, with the more distant stars penetrating nearby portions of the Perseus and Sagittarius-Carina arms of the Galaxy. We devised a special analysis technique to decipher the overlapping absorption features in the different multiplets, each with different arrangements of the closely spaced transitions. In order to derive internally consistent results for all multiplets, we found that we had to modify the relative transition f-values in a way that made generally weak transitions stronger than amounts indicated in the current literature. We compared our measured relative populations of the excited fine-structure levels to those expected from equilibria calculated with collisional rate constants for various densities, temperatures, and compositions. The median thermal pressure for our entire sample was p/k = 2240 cm-3 K, or slightly higher if the representative temperatures of the material are much above or below a most favorable temperature of 40 K for the excitation of the first excited level at a given pressure. For gas that is moving outside the range of radial velocities permitted by differential Galactic rotation between us and the targets, about 15% of the C I indicates a thermal pressure p/k > 5000 cm-3 K. For gas within the allowed velocities, this fraction is only 1.5%. This contrast reveals a relationship between pressure enhancements and the kinematics of the gas. Regardless of velocity, we usually can register the presence of a very small proportion of the gas that seems to be at p/k 105 cm-3 K. We interpret these ubiquitous wisps of high-pressure material to arise either from small-scale density enhancements created by converging flows in a turbulent medium or from warm turbulent boundary layers on the surfaces of dense clouds moving through an intercloud medium. For turbulent compression, our C I excitations indicate that the barytropic index ?eff 0.90 must apply if the unperturbed gas starts out with representative densities and temperatures n = 10 cm-3 and T = 100 K. This value for ?eff is larger than that expected for interstellar material that remains in thermal equilibrium after it is compressed from the same initial n and T. However, if regions of enhanced pressure reach a size smaller than ~0.01 pc, the dynamical time starts to become shorter than the cooling time, and ?eff should start to approach the adiabatic value cp/cv = 5/3. Some of the excited C I may arise from the target stars' H II regions or by the effects of optical pumping from the submillimeter line radiation emitted by them. We argue that these contributions are small, and our comparisons of the velocities of strongly excited C I to those of excited Si II seem to support this outlook. For six stars in the survey, absorption features from interstellar excited O I could be detected at velocities slightly shifted from the persistent features of telluric origin. These O I* and O I** features were especially strong in the spectra of HD 93843 and HD 210839, the same stars that show exceptionally large C I excitations. In appendices of this paper, we present evidence that (1) the wavelength resolving power of STIS in the E14OH mode is indeed about 200,000, and (2) the telluric O I* and O I** features exhibit some evidence for macroscopic motions, since their broadenings are in excess of that expected for thermal Doppler broadening at an exospheric temperature T = 1000 K.

A Module for Radiation Hydrodynamic Calculations with ZEUS-2D Using Flux-limited Diffusion

Abstract: A module for the ZEUS-2D code is described that may be used to solve the equations of radiation hydrodynamics to order unity in v/c, in the flux-limited diffusion (FLD) approximation. In this approximation, the factor Eddington tensor f, which closes the radiation moment equations, is chosen to be an empirical function of the radiation energy density. This is easier to implement and faster than full-transport techniques, in which f is computed by solving the transfer equation. However, FLD is less accurate when the flux has a component perpendicular to the gradient in radiation energy density and in optically thin regions when the radiation field depends strongly on angle. The material component of the fluid is here assumed to be in local thermodynamic equilibrium. The energy equations are operator split, with transport terms, radiation diffusion term, and other source terms evolved separately. Transport terms are applied using the same consistent transport algorithm as in ZEUS-2D. The radiation diffusion term is updated using an alternating direction-implicit method with convergence checking. Remaining source terms are advanced together implicitly using numerical root finding. However, when absorption opacity is zero, accuracy is improved by instead treating the compression and expansion source terms using a time-centered differencing scheme. Results are discussed for test problems including radiation-damped linear waves, radiation fronts propagating in optically thin media, subcritical and supercritical radiating shocks, and an optically thick shock in which radiation dominates downstream pressure.

A Method for Determining the Star Formation History of a Mixed Stellar Population

Abstract: We present a method to determine the star formation history of a mixed stellar population from its photometry. We perform a χ2 minimization between the observed photometric distribution and a model photometric distribution, based on theoretical isochrones. The initial mass function, distance modulus, interstellar reddening, binary fraction, and photometric errors are incorporated into the model, making it directly comparable to the data. The model is a linear combination of individual synthetic color-magnitude diagrams (CMDs), each of which represents the predicted photometric distribution of a stellar population of a given age and metallicity. While the method is similar to existing synthetic CMD algorithms, we describe several key improvements in our implementation. In particular, we focus on the derivation of accurate error estimates on the star formation history to enable comparisons between such histories, either from different objects or from different regions of a single object. We present extensive tests of the algorithm, using both simulated and actual photometric data. From a preliminary application of the algorithm to a subregion of the Large Magellanic Cloud (LMC), we find that the that the lull in star formation observed among the LMC's cluster population between 3 and 8 Gyr ago is also present in the field population. The method was designed with flexibility and generality in mind, and we make the code available for use by the astronomical community.

Proton-induced Thermonuclear Reaction Rates for A = 20–40 Nuclei

Abstract: Proton-induced reaction rates on 26 stable and 29 unstable target nuclei in the mass A = 20–40 region have been evaluated and compiled. Recommended reaction rates, assuming that all interacting nuclei are in the ground state, are presented in tabular form on a temperature grid in the range T = 0.01–10.0 GK. Most reaction rates involving stable targets were normalized to a set of measured standard resonance strengths in the sd shell. For the majority of reaction rates, experimental information from transfer reaction studies has been used consistently. Our results are compared with recent statistical model (Hauser-Feshbach) calculations. Reaction rate uncertainties are presented and amount to several orders of magnitude for many of the reactions. Several of these reaction rates and/or their corresponding uncertainties deviate from results of previous compilations. In most cases, the deviations are explained by the fact that new experimental information became available recently. Examples are given for calculating reaction rates and reverse reaction rates for thermally excited nuclei from the present results. The survey of literature for this review was concluded in 2000 August.

CHIANTI-An Atomic Database for Emission Lines. IV. Extension to X-Ray Wavelengths

Abstract: CHIANTI provides a database of atomic energy levels, wavelengths, radiative transition probabilities, and electron excitation data for a large number of ions of astrophysical interest. It also includes a suite of Interactive Data Language programs to calculate optically thin synthetic spectra and to perform spectral analysis and plasma diagnostics. This database allows the calculation of theoretical line emissivities necessary for the analysis of optically thin emission-line spectra. The first version (1.01) of the CHIANTI database was released in 1996 and published by Dere et al. in 1997 as Paper I in this series. The second version, released in 1999 by Landi et al., included continuum emission and data for additional ions. Both versions of the CHIANTI database have been used extensively by the astrophysical and solar communities to analyze emission-line spectra from astrophysical sources. Now the CHIANTI database has been extended to wavelengths shorter than 50 A by including atomic data for the hydrogen and helium isoelectronic sequences, inner-shell transitions and satellite lines, and several other ions. In addition, some of the ions already present in the database have been updated and extended with new atomic data from published calculations.

The UCSD HIRES/Keck I Damped Lyα Abundance Database. I. The Data

Abstract: We present new chemical abundance measurements of 16 damped Lyα systems at z > 1.5 and update our previous abundance analyses. The entire database presented here was derived from HIRES observations on the Keck I telescope, reduced with the same software package, and analyzed with identical techniques. Altogether, we present a large, homogeneous database of chemical abundance measurements for protogalaxies in the early universe, ideal for studying a number of important aspects of galaxy formation. In addition, we have established an on-line directory for this database and will continuously update the results.

A Quantitative Evaluation of Potential Radio Identifications for 3EG EGRET Sources

Abstract: The method of Mattox et al. for identifying EGRET sources with 5 GHz radio sources has been applied to the 3EG EGRET catalog of Hartman et al. Complete results are tabulated. We tabulate separately 46 EGRET sources with radio identifications which we expect to have a high probability of being correct. We suggest that these sources are appropriate for most studies of the properties of γ-ray blazars as a class. All but one of these 46 sources were classified by Hartman et al. as high-confidence identifications; and the additional source was classified by Hartman et al. as a plausible identification. We also tabulate separately 37 additional "plausible identifications of EGRET sources with radio sources." These less secure possibilities include the remaining 21 "high-confidence identifications" of Hartman et al., three of the 27 "lower confidence potential blazar identifications" of Hartman et al., and an additional 15 plausible identifications which have not been suggested previously. Many of these sources require new radio and optical observations to establish them as blazars. We suggest that the 23 "lower-confidence potential blazar identifications" of Hartman et al. which we do not find plausible should not be used in studies of the properties of the EGRET blazars. For this analysis, we have made elliptical fits to the 95% confidence contours of the position uncertainty regions of the 3EG sources which are tabulated in an Appendix.

Total Variation Diminishing Scheme for Relativistic Magnetohydrodynamics

Abstract: In this paper we present a total variation diminishing (TVD) scheme for numerical relativistic magnetohydrodynamics (MHD). The eigenstructure of the equations of relativistic MHD has been cataloged here. We also describe a strategy for obtaining the physically relevant eigenvectors. These eigenvectors are then used to build an interpolation strategy that operates on the characteristic fields. The design of a linearized Riemann solver for relativistic MHD is also described. The resulting higher order Godunov scheme has been implemented in the author's RIEMANN code for astrophysical fluid dynamics. The resulting code is second-order accurate both in space and time. A number of design features have been used to make it a high-resolution scheme. It shows efficient and robust performance for several stringent test problems.

Deuterated molecules in translucent and dark clouds

Abstract: We present observations of 10 deuterated molecular species in the dark clouds TMC-1, L183, and the translucent object CB 17, as well as a subset of species in and other objects. With sensitive TMCNH 3 observations of the J \ 1¨0 and 2¨1 transitions of DCN, and DNC, we have been able to derive N 2 D‘, molecular constants that include the complex nuclear quadrupole hyper—ne splitting in these species, which is essential to determine accurate abundances. The spectroscopic results have required, in turn, new radiative transport techniques to handle the hyper—ne eUects. Our abundance determinations also utilize sensitive observations of secondary isotopomers involving 13C, 18O, and 15N. Compared with earlier molecular D/H ratios in the literature, these innovations have resulted in radically diUerent values in some cases in TMC-1 and in TMC-1; DCN/HCN in CB 17), (N 2 D‘/N 2 H‘ TMCNH 3 ;N H 2 D/NH 3 and important modi—cations in others in TMC-1). The new techniques usually produce (C 3 HD/C 3 H 2 deuteration ratios lower than those obtained earlier by simpler methods. Thus, in addition to the special cases noted above, our results are generally lower than previous ones by factors of typically 2. We also —nd that deuteration occurs only in regions of high density, while nondeuterated species generally reside at lower densities. A recently proposed model of translucent clouds as low-density objects containing embedded small, high-density fragments explains the observations. To study the chemistry of deuterated species, we have used the New Standard Model, modi—ed to include all monodeuterated species, and now containing 9930 reactions and 610 species. Our models explore the dependence of the molecular D/H ratios upon temperature, density, ionization rate, extinction, epoch, and elemental abundances. Within the uncertainties, we —nd agreement between observed and modeled ratios for nearly all species in nearly all sources. Our results generally agree with those of Roberts & Millar in a recent, similar study. We —nd signi—cantly higher ratios in L183 than in TMC, and intermediate values in CB 17. With our lower values in general, however, we believe that L183 is ii normal ˇˇ for a cold dark cloud, CB 17 is typical of a slightly warmer translucent object, and the TMC region is perhaps underdeuterated in general, certainly strongly so in the case of and These N 2 H‘ NH 3 . ii anomalous ˇˇ cases have no plausible single explanation in terms of gas-phase chemistry at this time. Grain processes are implicated. . .. . .. . ..

An Optical Survey of Outlying Ejecta in Cassiopeia A: Evidence for a Turbulent, Asymmetric Explosion

Abstract: A deep optical survey of the Cassiopeia A supernova remnant has revealed dozens of new emission-line ejecta knots out beyond the remnant's bright nebular shell. Most of the newly detected knots exhibit a 4500-7500 A spectrum dominated by [N II] λλ6548,6583 line emissions. After accounting for possible decelerations, the estimated space velocities for about four dozen of these [N II] knots suggest a nearly isotropic 10,000 km s-1 ejection velocity. However, a small group along the southwestern limb show significantly higher velocities of up to 12,000 km s-1. Over 20 outlying O + S emission knots were also discovered, mostly along the remnant's western limb. These knots have optical spectral properties like those seen in the main shell's metal-rich fast-moving knots but with much higher estimated space velocities of between 7600 and 12,600 km s-1. Discovery of these knots means that the remnant's highest-velocity, O + S debris are not confined to just the remnant's northeast jet. [S II] λλ6716,6731 emissions dominate the spectra of these knots above an expansion velocity of 11,000 km s-1. A few mixed emission knots, which show both strong nitrogen and sulfur line emissions, were also detected along the remnant's western rim. The properties of these outlying debris knots suggest a turbulent supernova expansion in which the innermost S-rich layers were ejected up through overlying material in certain regions, attaining final outward velocities greater than the star's N and He-rich surface layers. The detection of such high-velocity, sulfur-rich ejecta only along the remnant's northeast and southwest limbs further suggests an asymmetric expansion, possibly bipolar. A turbulent expansion may help explain the creation of the observed mixed emission knots. It is unclear, however, if mixed knots represent truly microscopically mixed debris or are simply small, comoving clusters of chemically distinct ejecta.

Multiband analysis of a sample of blue compact dwarf galaxies. i. surface brightness distribution, morphology, and structural parameters

Abstract: Broadband observations in B, V, R, and I have been performed for a sample of 28 galaxies cataloged as blue compact dwarf galaxies (BCDs). Our deep imaging, reaching surface brightness levels of a few percent of the sky brightness, has allowed the detection of underlying emission or low surface brightness features for a substantial fraction of the sample. In this paper we present the first results of the program: deep contour maps in the B band, surface brightness profiles, and color profiles for all the galaxies. The information derived for this sample of galaxies has allowed us to analyze the morphology and the structural components of BCDs. Over 70% of the galaxies show complex profiles that preclude fitting by a single standard law, with extra structure at high to intermediate intensity levels. In 21 galaxies of the sample, an underlying low surface brightness component has been detected.

An Atlas of Hα and R Images and Radial Profiles of 63 Bright Virgo Cluster Spiral Galaxies

Abstract: Narrowband Hα and broadband R images and radial profiles are presented for 63 bright spiral galaxies in the Virgo Cluster. The sample is complete for Sb-Scd galaxies with B ≤ 12 and inclination ≤75°. Isophotal radii, disk scale lengths, concentration parameters, and integrated fluxes are derived for the sample galaxies.

Hydrogenation and Charge States of PAHs in Diffuse Clouds. I. Development of a Model

Abstract: A model of the hydrogenation and charge states of polycyclic aromatic hydrocarbons (PAHs) in diffuse clouds is presented. The main physical and chemical processes included in the model are ionization and photodissociation in the interstellar UV field, electron recombination with PAH cations, and chemistry between PAH cations and major interstellar species present in the diffuse medium, such as H2, H, O, and N atoms. A statistical model of photodissociation is presented, which is a simplified version of the Rice-Ramsperger-Kassel-Marcus theory. The predictions of this new approach have been successfully compared to experimental results for small PAH cations, justifying the application of the model to larger PAHs for which no experimental data are available. This simplified statistical theory has also been used to estimate the importance of the dissociative recombination channel in the reaction between PAH cations and electrons. Recent experimental results obtained on the chemistry between PAH cations and H2, H, O, and N atoms are discussed and included in the model. Finally, a discussion is presented on other important processes which may affect the PAH distribution in the interstellar medium, such as electron attachment, photodetachment, photofragmentation with carbon loss, double ionization, and chemistry between PAH cations and minor species present in diffuse clouds. The results obtained with this model for compact PAHs ranging from benzene to species bearing up to 200 carbon atoms are discussed in a separate paper.

Multiband Analysis of a Sample of Blue Compact Dwarf Galaxies. II. Spatially Resolved and Integrated Photometry

Abstract: This is the second in a series of papers devoted to the study of a sample of 28 blue compact dwarf galaxies (BCDs). In the first paper, we presented the broadband observations in B, V, R, and I. Deep contour maps, surface-brightness profiles, and color profiles were used to analyze and discuss their morphology and structure. Here, we present new U-band and Hα observations. We compute isophotal and asymptotic magnitudes of the sample galaxies and total Hα fluxes. Comparison with previous works is done to assess the quality of the data available for BCDs. Finally, we produce an atlas of detailed color and Hα maps, the first systematic atlas of this kind published so far. The high quality of the data allows us to identify the different star-forming knots in the maps and to discriminate between them and the regions where star formation activity has already ceased. We supplement the atlas with a morphological description of each galaxy and group the galaxies in four classes according to the distribution of their star-forming regions.

On the Similarity of FU Orionis Stars to Class I Protostars: Evidence from the Submillimeter

Abstract: On the basis of analysis of new submillimeter maps of a sample of most known FU Orionis stars, we confirm that these stars are extremely young, more similar to Class I protostars than to Class II T Tauri stars. The submillimeter maps reveal that FU Orionis stars have accretion disks that are larger and contain an order of magnitude more mass than those of T Tauri stars but are comparable in mass to those seen around Class I sources. The disks around the FU Orionis stars are sufficiently massive to act as reservoirs of material for replenishing the mass of the disks after eruptions, in which large amounts of material are believed to fall onto the underlying stars. We have identified a number of FU Orionis stars that are surrounded by extended cloud emission, and in several cases we find nearby submillimeter sources that have no optical or near-infrared counterparts. These sources most likely are young protostellar Class I or Class 0 sources. In at least in one case, V1735 Cyg, the dominant submillimeter source is the protostar, not the optically visible FU Orionis star.

The las campanas distant cluster survey - the catalog

Abstract: We present an optically selected catalog of 1073 galaxy cluster and group candidates at 0.3 z 1. These candidates are drawn from the Las Campanas Distant Clusters Survey (LCDCS), a drift-scan imaging survey of a 130 square degree strip of the southern sky. To construct this catalog we utilize a novel detection process in which clusters are detected as positive surface brightness fluctuations in the background sky. This approach permits us to find clusters with significantly shallower data than other matched-filter methods that are based upon number counts of resolved galaxies. Selection criteria for the survey are fully automated so that this sample constitutes a well-defined, homogeneous sample that can be used to address issues of cluster evolution and cosmology. Estimated redshifts are derived for the entire sample, and an observed correlation between surface brightness and velocity dispersion, σ, is used to estimate the limiting velocity dispersion of the survey as a function of redshift. We find a net surface density of 15.5 candidates per square degree at zest ≥ 0.3, with a false-detection rate of ~30%. At z ~ 0.3 we probe down to the level of poor groups while by z ~ 0.8 we detect only the most massive systems (σ 1000 km s-1). We also present a supplemental catalog of 112 candidates that fail one or more of the automated selection criteria, but appear from visual inspection to be bona fide clusters.