Showing papers in "Astrophysical Journal Supplement Series in 1998"

Old Stellar Populations. VI. Absorption-Line Spectra of Galaxy Nuclei and Globular Clusters

Abstract: We present absorption-line strengths on the Lick/IDS line-strength system of 381 galaxies and 38 globular clusters in the 4000-6400 A region. All galaxies were observed at Lick Observatory between 1972 and 1984 with the Cassegrain Image Dissector Scanner spectrograph, which makes this study one of the largest homogeneous collections of galaxy spectral line data to date. We also present a catalog of nuclear velocity dispersions used to correct the absorption-line strengths onto the stellar Lick/IDS system. Extensive discussion of both random and systematic errors of the Lick/IDS system is provided. Indices are seen to fall into three families: α-element-like indices (including CN, Mg, Na D, and TiO2) that correlate positively with velocity dispersion; Fe-like indices (including Ca, the G band, TiO1, and all Fe indices) that correlate only weakly with velocity dispersion and the α indices; and Hβ that anticorrelates with both velocity dispersion and the α indices. C24668 seems to be intermediate between the α and Fe groups. These groupings probably represent different element abundance families with different nucleosynthesis histories.

A Hubble Space Telescope Imaging Survey of Nearby Active Galactic Nuclei

Abstract: We have obtained WFPC2 images of 256 of the nearest (z ≤ 0.035) Seyfert 1, Seyfert 2, and starburst galaxies. Our 500 s broadband (F606W) exposures reveal much fine-scale structure in the centers of these galaxies, including dust lanes and patches, bars, rings, wisps, and filaments, and tidal features such as warps and tails. Most of this fine structure cannot be detected in ground-based images. We have assigned qualitative classifications for these morphological features and a Hubble type for the inner region of each galaxy, and we have also measured quantitative information such as 0″.18 and 0″.92 aperture magnitudes, position angles, and ellipticities, where possible. There is little direct evidence for unusually high rates of interaction in the Seyfert galaxies. Slightly less than 10% of all the galaxies show tidal features or multiple nuclei. The incidence of inner starburst rings is about 10% in both classes of Seyfert galaxies. In contrast, galaxies with H II region emission-line spectra appear substantially more irregular and clumpy because of their much higher rates of current star formation per unit of galactic mass. The presence of an unresolved central continuum source in our Hubble Space Telescope images is a virtually perfect indicator of a Seyfert 1 nucleus as seen by ground-based spectroscopy. Fifty-two percent of these Seyfert 1 point sources are saturated in our images; we use their wings to estimate magnitudes ranging from 15.8 to 18.5. The converse is not universally true, however, as over one-third of Seyferts with direct spectroscopic evidence for broad Balmer wings show no nuclear point source. These 34 resolved Seyfert 1's have fainter nonstellar nuclei, which appear to be more extinguished by dust absorption. Like the Seyfert 2's, they have central surface brightnesses consistent with those expected for the bulges of normal galaxies. The rates for the occurrences of bars in Seyfert 1's and 2's and non-Seyferts are the same. We found one significant morphological difference between the host galaxies of Seyfert 1 and Seyfert 2 nuclei. The Seyfert 2 galaxies are significantly more likely to show nuclear dust absorption, especially in lanes and patches that are irregular or reach close to the nucleus. A few simple tests show that the difference cannot be explained by different average redshifts or selection techniques. It is confirmed by our galaxy morphology classifications, which show that Seyfert 1 nuclei reside in earlier type galaxies than Seyfert 2 nuclei. If, as we believe, this is an intrinsic difference in host galaxy properties, it undermines one of the postulates of the strong unification hypothesis for Seyfert galaxies, that they merely appear different because of the orientation of their central engine. The excess galactic dust we see in Seyfert 2's may cause substantial absorption that obscures their hypothesized broad emission line regions and central nonstellar continua. This galactic dust could produce much of the absorption in Seyfert 2 nuclei that had instead been attributed to a thick dusty accretion torus forming the outer part of the central engine.

ASCA Observations of Seyfert 1 Galaxies. III. The Evidence for Absorption and Emission Due to Photoionized Gas

Abstract: We present the results from a detailed analysis of the 0.6-10 keV spectra of 23 ASCA observations of 18 objects. We find that in most cases the underlying continuum can be well represented by a power law with a photon index Γ ~ 2. However, we find strong evidence for photoionized gas in the line of sight to 13/18 objects. We present detailed modeling of this gas based upon the ION photoionization code. Other studies have been made of the "warm absorber" phenomenon, but this paper contains the first consideration of the importance of the covering fraction of the ionized gas and a direct comparison between models of attenuation by ionized versus neutral material. We find the X-ray ionization parameter for the ionized material is strongly peaked at UX ~ 0.1. The column densities of ionized material are typically in the range NH, z ~ 1021-1023 cm-2, although highly ionized (and hence pseudotransparent) column densities up to 1024 cm-2 cannot be excluded in some cases. We also investigate the importance of the emission spectrum from the ionized gas, finding that it significantly improves the fit to many sources with an intensity consistent with material subtending a large solid angle at the central source. Allowing a fraction of the continuum to be observed without attenuation also improves the fit to many sources and is definitely required in the case of NGC 4151. A deficit of counts is observed at ~1 keV in the sources exhibiting the strongest absorption features. We suggest this is likely to be the signature of a second zone of (more highly) ionized gas, which might have been seen previously in the deep Fe K-shell edges observed in some Ginga observations. We find evidence that the ionized material in NGC 3227 and MCG -6-30-15 contains embedded dust, while there is no such evidence in the other sources We discuss these results in the context of previous studies and briefly explore the implications in other wave bands.

The Five College Radio Astronomy Observatory CO Survey of the Outer Galaxy

Abstract: Images from the FCRAO CO Survey of the Outer Galaxy and a description of the calibration and processing of the data are presented. The survey is comprised of 1,696,80012CO J = 1-0 spectra sampled every 50'' between Galactic longitudes 10249 and 14154 and latitudes -303 and 541. The VLSR ranges from -153 to +40 km s-1 sampled every 0.81 km s-1 with a velocity resolution of 0.98 km s-1. The survey provides the highest spatial dynamic range imaging of the molecular interstellar medium ever obtained. We describe the observing procedure and characterize the random and systematic noise of the survey. The images reveal a wide diversity of spatial structure within the molecular interstellar medium. We identify several voids with angular extents of 2°-5° in diameter that may result from the cumulative interactions of ultraviolet radiation fields and stellar winds from newborn massive stars with ambient interstellar gas. It is demonstrated that a significant fraction of the integrated intensity summed over the survey field originates from regions with low peak antenna temperature and low column density assuming a conversion between integrated CO emission and molecular hydrogen column density. Assuming kinematic distances to the observed emission, we derive the radial profile of mass surface density and the variations of the z midplane and scale height of the molecular gas distribution with galactocentric radius.

The IRAS 1 Jy Survey of Ultraluminous Infrared Galaxies. I. The Sample and Luminosity Function

Abstract: A complete flux-limited sample of 118 ultraluminous infrared galaxies (ULIGs) has been identified from the IRAS Faint Source Catalog (FSC). The selection criteria were a 60 ?m flux density greater than 1 Jy in a region of the sky ? > -40?, |b| > 30?. All sources were subsequently reprocessed using co-added IRAS maps in order to obtain the best available flux estimates in all four IRAS wavelength bands. The maximum observed infrared luminosity is Lir = 1012.90 L?, and the maximum redshift is z = 0.268. The luminosity function for ULIGs over the decade luminosity range Lir = 1012-1013 L? can be approximated by a power law ?(L) L-2.35[Mpc-3 mag-1]. In the local universe (z 0.1), the space density of ULIGs appears to be comparable to or slightly larger than that of optically selected QSOs at comparable bolometric luminosities. A maximum likelihood test suggests strong evolution for our sample; assuming density evolution proportional to (1 + z)? we find ? = 7.6 ? 3.2. Examination of the two-point correlation function shows a barely significant level of clustering, ?(r) = 1.6 ? 1.2, on size scales r ~ 22 h-1 Mpc.

Patterns of Variation among Sun-like Stars

Abstract: We examine the patterns of variation among a sample of 35 stars that includes the Sun, particularly on the timescale of the 11 yr solar activity cycle. Our investigation uses contemporaneous photometric and chromospheric HK emission time series measurements from the Lowell and Mount Wilson Observatories, and comparable solar data. We find that the photometric and HK variability of the stars in our sample can be related to their average level of chromospheric activity by power laws. The photometric variability of the Sun may be somewhat subdued for its average activity level. We find that the younger, more active stars in our sample tend to become fainter as their HK emission increases, whereas the older, less active stars tend to become brighter as their HK emission increases, as the Sun does during its activity cycle.

A large-scale CO survey of the galactic center

Abstract: We present high-resolution CO images of the Galactic center region taken with the 2 × 2 focal-plane array receiver mounted on the 45 m telescope of Nobeyama Radio Observatory. We have collected about 44,00012C16O (J = 1-0) spectra and over 13,00013C16O (J = 1-0) spectra with a 34'' (1.4 pc) grid spacing. The 12CO mapping area is roughly -15 ≤ l ≤ +34 and -06 ≤ b ≤ +06, which covers almost the full extent of the molecular gas concentration in the Galactic center. These CO images demonstrate extremely complex distribution and kinematics of molecular gas in the Galactic center. While its large-scale behavior can be attributed to the well-known coherent features, bright CO emission arises from a number of compact (d ≤ 10 pc) clouds with large velocity widths (ΔV ≥ 30 km s-1). The small-scale structure of molecular gas is characterized by filaments, arcs, and shells. The boisterous molecular gas kinematics there may be a result of violent release of kinetic energy by a number of supernova explosions and/or Wolf-Rayet stellar winds.

Quadrupole Transition Probabilities for the Excited Rovibrational States of H 2

Abstract: Accurate calculations of the quadrupole moment of H2, carried out with a 494 term variational representation of the electronic eigenfunction, are reported, and the quadrupole transition probabilities connecting all the bound rovibrational levels of H2 are presented.

An X-Ray Minisurvey of Nearby Edge-on Starburst Galaxies. I. The Data

Abstract: We have analyzed all available ROSAT PSPC and HRI and ASCA data for a small far-infrared flux-limited sample of seven nearby edge-on starburst galaxies in order to search for hot gas in their halos We find that all five normal-sized spiral galaxies (NGC 253, NGC 3079, NGC 3628, NGC 4631, and NGC 4666) have hot gas in their halos, as does the small peculiar galaxy M82 NGC 55, a nearby Magellanic irregular, shows signs of hot gas beyond its thin disk only near the most actively star-forming region that is associated with a giant H? bubble All fits to joint PSPC + ASCA spectra, except NGC 55, indicate the presence of two gas phases, one at 02-04 keV and another at 065-09 keV, which in general appear to dominate the emission of the hot gas In NGC 4666, for which only ROSAT data are available, we currently find only a gas component near 03 keV The data of NGC 4631 hint at the presence of a third gas component in this galaxy, at a very low temperature of kT 005 keV One of our general results is that the joint PSPC + ASCA spectra allow us to identify different spectral components Based on the additional imaging information and on consistency checks, we could fit all spectra with similar composite models including both the above thermal plasmas and a harder power-law component The results of these fits are statistically good, consistent with each other, and physically meaningful However, they are not unique, and it is extremely difficult to constrain the metallicities of the different components of hot gas Our results show that all data can be fitted by composite model spectra with near-solar metallicities Extremely low metallicities, as previously fitted by others based on individual ROSAT or ASCA observations, are thus not required As a by-product of our investigations, spectra of other X-ray emitters in the target galaxies were obtained We determined the spectral properties of the hot gas and compact sources (presumably mostly high-mass X-ray binaries) in their disks Integral spectra, indicating thermal emission characteristics, of those compact sources outside the galaxy disks with soft hardness ratios, suggest that they might be associated with the halo gas

Multiple Molecular Winds in Evolved Stars. I. A Survey of CO(2-1) and CO(3-2) Emission from 45 Nearby Asymptotic Giant Branch Stars

Abstract: This paper describes observations of a new phenomenon in evolved mass-losing asymptotic giant branch (AGB) stars: the presence of two winds with different expansion velocities. CO(2-1) and CO(3-2) line emission was observed for 45 AGB stars at high velocity resolution and double winds found in 20% of the sample. Highly asymmetric lines were found in six other stars. The data tentatively suggest that double winds occur when the star undergoes a change (pulsational mode, chemical composition) and that the very narrow components represent the onset of a new phase of mass loss.

Adaptive Smoothed Particle Hydrodynamics: Methodology. II.

Abstract: Further development and additional details and tests of adaptive smoothed particle hydrodynamics (ASPH), the new version of smoothed particle hydrodynamics (SPH) described in the first paper in this series (Shapiro et al.), are presented. The ASPH method replaces the isotropic smoothing algorithm of standard SPH, in which interpolation is performed with spherical kernels of radius given by a scalar smoothing length, with anisotropic smoothing involving ellipsoidal kernels and tensor smoothing lengths. In standard SPH, the smoothing length for each particle represents the spatial resolution scale in the vicinity of that particle and is typically allowed to vary in space and time so as to reflect the local value of the mean interparticle spacing. This isotropic approach is not optimal, however, in the presence of strongly anisotropic volume changes such as occur naturally in a wide range of astrophysical flows, including gravitational collapse, cosmological structure formation, cloud-cloud collisions, and radiative shocks. In such cases, the local mean interparticle spacing varies not only in time and space but also in direction as well. This problem is remedied in ASPH, where each axis of the ellipsoidal smoothing kernel for a given particle is adjusted so as to reflect the different mean interparticle spacings along different directions in the vicinity of that particle. By deforming and rotating these ellipsoidal kernels so as to follow the anisotropy of volume changes local to each particle, ASPH adapts its spatial resolution scale in time, space, and direction. This significantly improves the spatial resolving power of the method over that of standard SPH at fixed particle number per simulation. This paper presents an alternative formulation of the ASPH algorithm for evolving anisotropic smoothing kernels, in which the geometric approach of the first paper in this series, based upon the Lagrangian deformation of ellipsoidal fluid elements surrounding each particle, is replaced by an approach involving a local transformation of coordinates to those in which the underlying anisotropic volume changes appear to be isotropic. Using this formulation the ASPH method is presented in two and three dimensions, including a number of details not previously included in the earlier paper, some of which represent either advances or different choices with respect to the ASPH method detailed in the earlier paper. Among the advances included here are an asynchronous time-integration scheme with different time steps for different particles and the generalization of the ASPH method to three dimensions. In the category of different choices, the shock-tracking algorithm described in the earlier paper for locally adapting the artificial viscosity to restrict viscous heating just to particles encountering shocks is not included here. Instead, we adopt a different interpolation kernel for use with the artificial viscosity, which has the effect of spatially localizing effects of the artificial viscosity. This version of the ASPH method in two and three dimensions is then applied to a series of one-, two-, and three-dimensional test problems, and the results are compared to those of standard SPH applied to the same problems. These include the problem of cosmological pancake collapse, the Riemann shock tube, cylindrical and spherical Sedov blast waves, the collision of two strong shocks, and problems involving shearing disks intended to test the angular momentum conservation properties of the method. These results further support the idea that ASPH has significantly better resolving power than standard SPH for a wide range of problems, including that of cosmological structure formation.

A three-position spectral line survey of Sagittarius B2 between 218 and 263 GHz. I. The observational data

Abstract: We have surveyed the frequency band 218.30-263.55 GHz toward the core positions N and M and the quiescent cloud position NW in the Sgr B2 molecular cloud using the Swedish-ESO Submillimetre Telescope. In total 1730, 660, and 110 lines were detected in N, M, and NW, respectively, and 42 different molecular species were identified. The number of unidentified lines are 337, 51, and eight. Toward the N source, spectral line emission constitutes 22% of the total detected flux in the observed band, and complex organic molecules are the main contributors. Toward M, 14% of the broadband flux is caused by lines, and SO2 is here the dominant source of emission. NW is relatively poor in spectral lines and continuum. In this paper we present the spectra together with tables of suggested line identifications.

Total Variation Diminishing Scheme for Adiabatic and Isothermal Magnetohydrodynamics

Abstract: In this paper a total variation diminishing (TVD) scheme is constructed for solving the equations of ideal adiabatic and isothermal MHD. It is based on an extremely efficient formulation of the MHD Riemann problems for either case. Piecewise linear interpolation is applied to the characteristic variables along with steepening of linearly degenerate characteristic fields. A predictor-corrector formulation is used to achieve second-order-accurate temporal update. An artificial viscosity and hyperviscosity are formulated using the characteristic variables. The viscosity and hyperviscosity are designed so that they never damage the TVD property. An accurate formulation of the divergence cleaning step is presented. This formulation is more accurate than the one that has been used so far. The scheme designed is second-order accurate in space and time. It has been implemented in the author's RIEMANN code for numerical MHD. A variety of test problems are presented. They test all aspects of numerical MHD including (1) handling of exotic wave structures that occur in MHD, (2) treatment of multiple discontinuities, (3) handling of very strong shocks, and (4) multidimensional problems. The scheme displays robust and accurate behavior in each case. An extremely efficient implementation has been achieved for massively parallel processor (MPP) machines displaying the ability of this scheme to sustain scalable, load-balanced performance in MPP environments.

The Hubble Space Telescope Quasar Absorption Line Key Project. XIII. A Census of Absorption-Line Systems at Low Redshift

Abstract: We present a catalog of absorption lines obtained from the analysis of the ultraviolet spectra of 66 quasars. The data were acquired with the Faint Object Spectrograph of the Hubble Space Telescope (HST) as part of the Quasar Absorption Line Survey, a Key Project for the first four cycles of HST observations. This is the third of a series of catalogs of absorption lines produced from the survey and increases the number of quasars whose higher resolution (R = 1300) spectra we have published from 17 to 83. The general properties and execution of the survey are reviewed, including descriptions of the final sample of observed objects and the algorithmic processes used to construct the catalog. This database is suitable for a wide variety of studies of gaseous systems in the nearby universe. This third catalog includes 2594 absorption lines and brings the total number of absorption lines in the combined catalog to 3238. The third catalog has 878 identified Lyα lines, 27 extensive metal line systems (detected absorption lines from four or more metal ions), 88 C IV systems, and 34 O VI systems. The combined catalog contains the following numbers of extragalactic absorption lines: 1129 Lyα lines, 107 C IV systems, 41 O VI systems, 16 Lyman limit systems, and one damped Lyα system (in the spectrum of PG 0935+416). In addition, there are 25 pairs of identified Lyα lines that are candidate C IV doublets. Of the 122 identified C IV and candidate C IV systems in the completely identified sample of absorption lines, 24 ± 5 are expected to be chance coincidences of other lines (based upon Monte Carlo simulations). The detection of a single damped Lyα system in a path length of Δz = 49 yields an observed number of damped systems per unit redshift of (dN/dz)damp(z = 0.58) = 0.020 with 95% confidence boundaries of 0.001-0.096 systems per unit redshift. We include notes on our analysis of each of the observed quasars and the absorption systems detected in each spectrum. Some especially interesting systems include low-redshift Lyα absorbers suitable for extensive follow-up observations (e.g., in the spectra of TON 28 and PG 1216+069), possibly physically associated pairs of extensive metal line absorption systems (e.g., in the spectrum of PG 0117+213), and systems known to be associated with galaxies (e.g., in the spectrum of 3C 232). The spectra of five broad absorption line (BAL) quasars (UM 425, PG 1254+047, PG 1411+442, PG 1700+518, and PG 2112+059) can be found in this third catalog, bringing the total number of BAL quasars in the combined catalog to six (including PG 0043+039).

Temperature Dependence of Ultraviolet Line Average Doppler Shifts in the Quiet Sun

Abstract: The existence of prevailing redshifts in the UV lines formed in the solar transition region raises an important question concerning its physical origin and its role in the mass and energy balance of the outer solar atmosphere. A series of UV spectral lines observed by SUMER has been analyzed to obtain the spatial average of Doppler shifts in the quiet Sun as a function of temperature. The UV lines used for the analysis cover temperatures ranging from 104 to 106 K. The wavelength calibration has been done in reference to the coolest chromospheric lines such as neutral lines of silicon and sulfur. The positioning of the line center in blended lines has been made by employing a constrained multi-Gaussian fitting technique. The error in the measured average of the Doppler shifts is estimated to be smaller than 1 km s-1. Our results show that the average Doppler shift at the base of the transition region is about 1-2 km s-1, increasing with temperature with a peak value of 11 km s-1 near T = 2.3 × 105 K. Then it decreases but remains still above zero (5 km s-1 in Ne VIII lines and 4 km s-1 in Mg X lines). We find that this behavior can be explained by the dominance of emission from plasma flowing downward from the upper hot region to the lower cool region along flux tubes with varying cross section. Assuming that pressure and mass flux are constant along a flux tube, the cross section of a typical flux tube has been estimated as a function of temperature. It turns out that the cross section is nearly constant below T = 105 K and then expands by a factor of about 30 at T = 106 K. This behavior is fairly well represented by an analytical functional form, A(T)/A(Th) = [1 + (Γ2 - 1)(T/Th)ν]1/2/Γ with parameters of Th = 106 K, Γ = 30, and ν = 3.6.

A 13 CO Survey of Molecular Clouds in Gemini and Auriga

Abstract: A large-scale survey for molecular clouds in 13CO(J = 1-0) was performed toward the Gemini and Auriga regions (170° < l ≤ 196° and -10° ≤ b < 10°) with velocity coverages of -30 < VLSR < +30 km s-1 and -20 < VLSR < +40 km s-1 for 170° < l ≤ 188° and 188° < l ≤ 196°, respectively, by using the two 4 m millimeter-wave telescopes at Nagoya University. An area of 520 deg2 was covered at an 8' grid spacing with a 27 beam, and 29,250 positions were observed. Significant 13CO emission (≥1.2 K km s-1 = 3 σ) is detected at 1032 positions, and 139 distinct 13CO clouds are identified. Physical properties such as molecular column density, size, and mass are derived for each cloud. Comparison with known H II regions and other associated visible objects indicates that 98 of the 139 clouds are located at ~2 kpc, while the rest lie at 1 kpc. The candidates for protostars are selected from IRAS point sources and 141 sources are identified as protostellar candidates associated with the 13CO clouds. A statistical study of the 13CO clouds and of the candidates for protostars shows the following results: 1. The mass spectra of the 13CO clouds are well represented by a power law, Ncloud(≥Mcloud) = 1.4 × 104(Mcloud/M☉)-0.83 - 1.4 for the clouds at ~2 kpc and Ncloud(≥Mcloud) = 3.5 × 102(Mcloud/M☉)-0.64 - 2.4 for the rest. 2. The line width, ΔVcomp, and the size, R, of the 13CO clouds show a power-law relation with an index 0.24 ± 0.06, where the dispersion in the fitting is fairly large. 3. A virial analysis made for the 13CO clouds indicates that the relation between the virial mass, Mvir, and the mass measured in 13CO, Mcloud, is aproximated well by (Mvir/M☉) = 2.0 × 101(Mcloud/M☉)0.72, which suggests that smaller clouds tend to be more weakly bound gravitationally than larger clouds or are dispersing if the external pressure is negligible. This is probably the cuase of a small index value of the line width-size relation. 4. The luminosity function of the IRAS point sources associated with 13CO clouds are well represented by a power law with Nstar(≥Lstar) = 4.0 × 102L-1.9×10 for those at ~2 kpc and Nstar(≥Lstar) = 2.3 × 10L-3.2 for the rest. 5. The luminosity of the most luminous IRAS point source in a given molecular cloud increases systematically with the mass of the associated cloud. 6. The 13CO clouds associated with IRAS point sources, which are regarded as ongoing star-forming clouds, tend to be more massive and larger in size and to have higher column densities than those without any sign of star formation. These relations are found to be consistent with those derived in Cygnus by Dobashi, Bernard, & Fukui and in Cepheus-Cassiopeia by Yonekura et al. In order to study the star formation activities, we studied the ratio of virial mass and the mass measured in 13CO, Mvir/Mcloud, and its relation with star formation. It is clearly seen that the star-forming 13CO clouds have low Mvir/Mcloud, and all the clouds with high Mvir/Mcloud exhibit no sign of star formation. This suggests that star formation rarely occurs in clouds with overall kinetic energy higher than the gravitational energy.

A Near-Infrared Spectroscopic Survey of LINER Galaxies

Abstract: This paper reports the results of a near-infrared spectroscopic survey of LINER galaxies undertaken with a new infrared spectrograph at the 5 m Hale telescope. The galaxy sample includes 11 LINERs with spectra covering the [Fe II] (1.2567 μm), Paβ (1.2818 μm), H2 [1-0 S(1), 2.1218 μm], and Brγ (2.1655 μm) near-infrared emission lines and one additional galaxy with only [Fe II] and Paβ line coverage. All of the LINERs with infrared line detections have strong [Fe II] and/or H2 emission, with about half (four of nine) having extremely high ratios (>2) of [Fe II] to Paβ. The strength of the H2 and [Fe II] lines is well correlated with the optical [O I] line, with many LINERs having higher ratios of [Fe II]/Paβ, H2/Brγ, and [O I]/Hα than other galaxy types. The LINERs with the highest [Fe II]/Paβ ratios (termed "strong" [Fe II] LINERs) show evidence for recent star formation. Shocks from compact supernova remnants may enhance the [Fe II] emission in these "strong" [Fe II] LINERs. The LINERs with lower [Fe II]/Paβ ratios (termed "weak" [Fe II] LINERs) are more consistent with Seyfert-like activity, including higher ionization states, some strong X-ray sources, and some broad Hα detections. The [Fe II] luminosity and the [Fe II]/Paβ ratio in these objects are more easily explained by hard X-ray excitation than in the "strong" [Fe II] LINERs. These "weak" [Fe II] LINERs are considered prime candidates for being low-luminosity Seyfert nuclei.

Photometric separation of stellar properties using sdss filters

Abstract: Using synthetic photometry of Kurucz model spectra, we explore the colors of stars as a function of temperature, metallicity, and surface gravity with Sloan Digital Sky Survey (SDSS) filters, u'g'r'i'z'. The synthetic colors show qualitative agreement with the few published observations in these filters. We find that the locus of synthetic stars is basically two-dimensional for 4500 < T < 8000 K, which precludes simultaneous color separation of the three basic stellar characteristics we consider. Colors including u' contain the most information about normal stellar properties; measurements in this filter are also important for selecting white dwarfs. We identify two different subsets of the locus in which the loci separate by either metallicity or surface gravity. For 0.5 < g' - r' < 0.8 (corresponding roughly to G stars), the locus separates by metallicity; for photometric error of a few percent, we estimate metallicity to within ~0.5 dex in this range. In the range -0.15 < g' - r' < 0.00 (corresponding roughly to A stars), the locus shows separation by surface gravity. In both cases, we show that it is advantageous to use more than two colors when determining stellar properties by color. Strategic observations in SDSS filters are required to resolve the source of a ~5% discrepancy between synthetic colors of Gunn-Stryker stars, Kurucz models, and external determinations of the metallicities and surface gravities. The synthetic star colors can be used to investigate the properties of any normal star and to construct analytic expressions for the photometric prediction of stellar properties in special cases.

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. XII. Ground-based monitoring of 3C 390.3

Abstract: Results of a ground-based optical monitoring campaign on 3C 390.3 in 1994-1995 are presented. The broadband fluxes (B, V, R, and I), the spectrophotometric optical continuum flux Fλ(5177 A), and the integrated emission-line fluxes of Hα, Hβ, Hγ, He I λ5876, and He II λ4686 all show a nearly monotonic increase with episodes of milder short-term variations superposed. The amplitude of the continuum variations increases with decreasing wavelength (4400-9000 A). The optical continuum variations follow the variations in the ultraviolet and X-ray with time delays, measured from the centroids of the cross-correlation functions, typically around 5 days, but with uncertainties also typically around 5 days; zero time delay between the high-energy and low-energy continuum variations cannot be ruled out. The strong optical emission lines Hα, Hβ, Hγ, and He I λ5876 respond to the high-energy continuum variations with time delays typically about 20 days, with uncertainties of about 8 days. There is some evidence that He II λ4686 responds somewhat more rapidly, with a time delay of around 10 days, but again, the uncertainties are quite large (~8 days). The mean and rms spectra of the Hα and Hβ line profiles provide indications for the existence of at least three distinct components located at ±4000 and 0 km s-1 relative to the line peak. The emission-line profile variations are largest near line center.

Radio-loud Active Galaxies in the Northern ROSAT All-Sky Survey. III. New Spectroscopic Identifications from the RASS-Green Bank BL Lacertae Survey

Abstract: We present new spectroscopic identifications for 169 objects in the RASS-Green Bank (RGB) catalog of radio- and X-ray-emitting active galactic nuclei (AGNs). The data presented here significantly increase the fraction of bright RGB objects with classifications. Specifically, we report and discuss the classification of 57 radio-loud quasars, eight radio-quiet quasars (QSOs), 53 BL Lacs, 32 broad-line radio galaxies, five narrow-line radio galaxies, three Seyfert I galaxies, and 11 galaxies or galaxies in clusters. Over 78% of the identifications we present here are the first published classifications for these sources. The observations we report were undertaken as part of our targeted search program to identify a new, large unbiased sample of BL Lac objects, and we therefore discuss the BL Lac sample extensively. Unlike many previous surveys, we impose no selection criteria based on optical morphology, color, or broadband spectral energy distribution. Our classifications are based solely on a carefully defined set of self-consistent spectroscopic classification criteria. These criteria are then carefully evaluated, and particular attention is paid to issues involving the classification and description of BL Lacs. The criteria yielded BL Lac classifications for 53 RGB objects, 38 of which were are newly discovered BL Lacs. We show these RGB BL Lacs exhibit transitional properties between normal galaxies and BL Lacs discovered in previous radio and X-ray surveys. We briefly discuss the broadband flux distributions of these new RGB BL Lacs and the range of Ca II H and K break contrasts (Br4000) they exhibit. We show that there is no clear separation in Br4000 between BL Lacs and galaxies detected in the RGB survey, with the distribution of break strengths varying smoothly between 0% and 50%. We also present and use a simple method based on the break strength to estimate the contribution of both the host galaxy and AGN to the 4000 A flux. We also show that the newly discovered RGB BL Lacs reside in a "zone of avoidance" in the log (Sx/Sr) versus log (So/Sr) diagram. This has important implications for BL Lac search strategies since it shows that RASS BL Lac samples will be severely incomplete if candidates are chosen only from among those objects with the highest Sx/Sr flux ratios.

Linearized Formulation of the Riemann Problem for Adiabatic and Isothermal Magnetohydrodynamics

Abstract: In this paper we present a linearized formulation of the Riemann problem for ideal adiabatic and isothermal magnetohydrodynamic (MHD). Roe's property U is obtained. This ensures good capturing of shocks of arbitrary strength. A parameter vector is found which gives an almost consistent mean state, where consistency is defined in the sense of Roe. The eigenvalues and eigenvectors for the linearized Riemann problem for MHD have been explicitly constructed further facilitating numerical implementation. The method has been tested in the author's RIEMANN code for numerical MHD. In a companion paper on TVD methods for MHD, where this Riemann solver was used, we have shown that excellent results can be obtained. The consistency for this formulation is illustrated by the fact that the eigenvalues have the same symmetrical distribution around the Roe-averaged x-component of the velocity that the eigenvalues for ideal MHD have around the physical x-component of the velocity. The accuracy, simplicity, and noniterative nature of the linearized formulation of the MHD Riemann problem presented here make it especially suitable for use on massively parallel processor machines.

The Infrared Spectral Classification of Oxygen-rich Dust Shells

Abstract: This paper presents infrared spectral classifications for a flux-limited sample of 635 optically identified oxygen-rich variables including supergiants and sources on the asymptotic giant branch (AGB). Several classes of spectra from oxygen-rich dust exist, and these can be arranged in a smoothly varying sequence of spectral shapes known as the silicate dust sequence. Classification based on this sequence reveals several dependencies of the dust emission on the properties of the central star. Nearly all S stars show broad emission features from alumina dust, while most of the supergiants exhibit classic features from amorphous silicate dust. Mira variables with symmetric light curves generally show broad alumina emission, while those with more asymmetric light curves show classic silicate emission. These differences may arise from differences in the photospheric C/O ratio.

Planetary Nebulae in the NRAO VLA Sky Survey

Abstract: The 1.4 GHz NRAO VLA Sky Survey (NVSS) images and source catalog were used to detect radio emission from the 885 planetary nebulae north of J2000 declination δ = -40° in the Strasbourg-ESO Catalogue of Galactic Planetary Nebulae. We identified 680 radio sources brighter than about S = 2.5 mJy beam-1 (equivalent to T ≈ 0.8 K in the 45'' FWHM NVSS beam) with planetary nebulae by coincidence with accurate optical positions measured from Digitized Sky Survey (DSS) images. Total extinction coefficients c at λ = 4861 A were calculated for the 429 planetary nebulae with available Hβ fluxes and low free-free optical depths at 1.4 GHz. The variation of c with Galactic latitude and longitude is consistent with the extinction being primarily interstellar and not intrinsic.

Star Formation in the Cepheus Flare Molecular Clouds. I. Distance Determination and the Young Stellar Object Candidates

Abstract: Results of an objective prism Schmidt survey are combined with IRAS survey data in order to assess the star-forming activity in the Cepheus Flare, a nearby giant molecular cloud complex at ~+15° above the Galactic equator. The distribution of absorbing matter along the line of sight was also studied. The Wolf diagrams, displaying the cumulative distribution of field star distance moduli, show that the interstellar matter in this region is concentrated at three characteristic distances: 200, 300, and 450 pc. The three components, though partly overlapping, can be separated along the Galactic latitude. Within the area of the Cepheus Flare, distances are determined for 14 Lynds dark clouds and for some other clouds. In order to compile a sample of young stellar object (YSO) candidates, three types of star formation signposts were searched for the following: 1. Prestellar cores in the IRAS 100 μm optical depth image of the region, 2. Far-infrared sources representing embedded YSOs and optically visible pre-main-sequence star candidates using IRAS Point Source Catalog, Faint Source Catalog, and calibrated IRAS detector scans, 3. Hα emission stars appearing on low-dispersion objective prism Schmidt plates as candidate pre-main-sequence stars. The IRAS 100 μm optical depth image of the region revealed 107 dense cores. Most of them are probably "starless" cores and, as such, potential sites of future star formation. One hundred twenty-two IRAS point sources were selected as probable YSOs at various evolutionary stages. An objective prism search for Hα emission stars covering an area of about 150 deg2 resulted in the detection of 142 Hα emission stars. Infrared fluxes taken from the IRAS catalogs or determined from the detector scans are listed for 95 of them. This sample is expected to consist of mostly T Tauri stars associated with the cloud complex. Finding charts and catalogs of the young stellar object candidates are also given. These results may serve as a basis for further dedicated studies of the region and will be useful for comparison when star-forming activity in other cloud complexes is investigated.

Solar Active Region and Quiet-Sun Extreme-Ultraviolet Spectra from SERTS-95

Abstract: Goddard Space Flight Center's Solar EUV Rocket Telescope and Spectrograph was flown on 1995 May 15 (SERTS-95), carrying a multilayer-coated toroidal diffraction grating that enhanced the instrumental sensitivity in its second-order wave band (171-225 A). Spectra and spectroheliograms of NOAA active region 7870 (N09 W22) were obtained in this wave band with a spectral resolution (instrumental FWHM) ~30 mA and in the first-order wave band (235-335 A) with a spectral resolution ~55 mA. Spectra and spectroheliograms of quiet-Sun areas northeast of the active region were also obtained. We derived the SERTS-95 relative radiometric calibration directly from flight data by means of density- and temperature-insensitive line intensity ratios. Most theoretical values for such ratios were obtained from the CHIANTI database. A total of 44 different lines were used to derive the relative radiometric calibration in the two spectral orders, most of them coming from seven (Fe X-Fe XVI) of the nine (Fe IX-Fe XVII) observed ionization stages of iron. The resulting relatively calibrated line intensities agree well with their corresponding normalized theoretical values. This supports the overall accuracy of the atomic physics parameters and demonstrates the power of the technique. The present work extends earlier work by Brosius, Davila, & Thomas, who determined the SERTS-95 second-order response using this technique. Many of the ratios employed here can be used to carry out a similar calibration exercise on spectra from the Coronal Diagnostic Spectrometer (CDS) aboard the Solar and Heliospheric Observatory (SOHO). We placed the line intensities onto an absolute scale by forcing our quiet-Sun He II λ303.8 + Si XI λ303.3 intensity to match that from previous observations. The resulting active region and quietSun absolutely calibrated line lists contain 127 and 20 lines, respectively. Active region densities derived from density-sensitive line intensity ratios of Fe X, XI, XIII, and XIV are mutually consistent with log ne ~ 9.4 ± 0.2; densities derived from Fe XII are significantly greater (log ne ~ 10).

Panoramic Views of the Cygnus Loop

Abstract: We present a complete atlas of the Cygnus Loop supernova remnant in the light of [O III] (λ5007), Hα, and [S II] (λλ6717, 6731) We include low-resolution (25'') global maps and smaller fields at 6'' resolution from observations using the Prime Focus Corrector on the 08 m telescope at McDonald Observatory Despite its shell-like appearance, the Cygnus Loop is not a current example of a Sedov-Taylor blast wave Rather, the optical emission traces interactions of the supernova blast wave with clumps of gas The surrounding interstellar medium forms the walls of a cavity through which the blast wave now propagates, including a nearly complete shell in which nonradiative filaments are detected We identify nonradiative shocks around half the perimeter of the Cygnus Loop, and they trace a circle of radius R = 14 (19 pc) in the spherical cavity walls The Cygnus Loop blast wave is not breaking out of a dense cloud but is instead running into confining walls Modification of the shock velocity and gas temperature due to interaction of the blast wave with the surrounding medium introduces errors in estimates of the age of this supernova remnant The optical emission of radiative shocks arises only where the blast wave encounters inhomogeneities in the ambient medium; it is not a consequence of gradual evolution to a global radiative phase Distance measurements that rely on this uniform blast wave evolution are uncertain, but the radiative shocks can be used as distance indicators because of the spherical symmetry of the surrounding medium The interstellar medium dominates not only the appearance of the Cygnus Loop but also the continued evolution of the blast wave If this is a typical example of a supernova remnant, then global models of the interstellar medium must account for such significant blast wave deceleration

A New Optical Sample of Supernova Remnants in M33

Abstract: We present a new and larger sample of supernova remnants in the nearby spiral galaxy M33. The sample is based upon CCD interference filter observations obtained with the Kitt Peak 4 m telescope and spectroscopic observations obtained with the Multiple Mirror Telescope. Using optical emission-line ratios, supplemented by a radio continuum map of M33 (Duric et al.; Gordon et al.), we have identified 98 supernova remnant (SNR) candidates, of which 53 were previously unknown. We have obtained spectra of 27 SNR candidates, bringing the total number of M33 SNRs for which spectra are available to 72. All the spectra show the characteristic signature of shock-heated gas, which leads us to believe that the rest of the candidates are also supernova remnants. The large sample provides a useful database to investigate the global properties of SNRs. In this paper, we present a new cumulative number-diameter [N(

FG Sagittae: A newborn R coronae borealis star?

Abstract: We have monitored FG Sge's spectroscopic changes since the time just prior to its dramatic fading in 1992 August. The most significant qualitative changes in the spectrum include large variations in the strength of the C2 molecular bands and the gradual appearance of broad blueshifted high-velocity (~200 km s-1 relative to the photosphere) absorption components of the Na I D lines. During the deep minima of 1994 May and 1996 June, an emission-line spectrum temporarily appeared superimposed on a weak continuum; in addition to the previously reported nebular emission lines, the spectra displayed the C2 and rare earth element lines in emission. Much of the behavior exhibited by FG Sge since 1992 resembles that seen in R CrB stars, including the photometric behavior, the evolution of the Na I D line profiles, variations of the C2 band strengths, and the appearance of narrow emission lines. The results of our abundance analysis (using model atmospheres with a solar He/H ratio) indicate that the carbon abundance is currently greater than that determined by Langer et al., who reported on the dramatic increase in the abundances of the rare earth elements in FG Sge. We derive higher relative abundances of the rare earths ([Me/Fe] ~ 3) than either Langer et al. or Kipper & Kipper, which we attribute to some enhancement of these elements since ~1970. We confirm previous claims that the relative scandium abundance is high ([Sc/Fe] ~ 1) in FG Sge and suggest that it is the result of neutron captures by the light elements leading up to 45Sc. The Hα profile of FG Sge is very similar to that of V854 Cen, a R CrB star deficient in H by 2-3 dex. This is the first evidence pointing toward H-deficiency in the atmosphere of FG Sge, which further strengthens its link with the R CrB class. Additional study is required before we can say definitively whether or not its atmosphere is H deficient.

A High-Dispersion Spectroscopic Survey of the Hot White Dwarfs: The IUE NEWSIPS SWP Echelle Data Set

Abstract: This paper summarizes the results obtained from a comprehensive analysis of all of the SWP echelle spectra of the white dwarf stars contained in the IUE Final Archive. A total of 209 NEWSIPS spectra of 55 degenerate stars of various types have been systematically reduced and analyzed. These include, in addition to conventional white dwarfs, several examples of the hot planetary nebula central stars such as NGC 246, which represent the initial stages of He-rich degenerate evolution. A representative summary of the stellar, circumstellar, and interstellar features found to be present in these spectra is presented. For 33 of the 55 stars, multiple spectra of sufficient quality exist so that co-added spectra with improved signal-to-noise ratio can be constructed. Much previously unrecognized detail and many new features are evident in these data. In addition, it was found necessary to apply several corrections to the NEWSIPS extracted spectra. These corrections, involving the wavelength scale and flux uncertainty vector, are described.

Identification of a Complete Sample of Northern ROSAT All-Sky Survey X-Ray Sources. III. The Catalog

Abstract: We present a catalog of optical identifications of a representative sample of northern (δ > -9°) ROSAT All-Sky Survey (RASS) sources. A full identification has been carried out for a count-rate- and area-limited complete RASS subsample comprising 674 sources. All sources are within six study areas outside the Galactic plane (| b | > 196), one area being near the north Galactic pole and one near the north ecliptic pole.