J. Christopher Howk

MODELING THE WARM IONIZED INTERSTELLAR MEDIUM AND ITS IMPACT ON ELEMENTAL ABUNDANCE STUDIES

We present model calculations of ionization fractions for elements in the warm (T D 104 K), low- density photoionized interstellar medium (WIM) of the Milky Way. We model the WIM as a com- bination of overlapping low-excitation H II regions having Our adopted standard n(H)/n(H) Z 0.8. model incorporates an intrinsic elemental abundance pattern similar to that found for warm neutral clouds in the Galaxy and includes the eUects of interstellar dust grains. The radiation —eld is character- ized by an ionizing spectrum of a star with K and an ionization parameter log (q) B (4.0. T eff B 35,000 The emergent emission-line strengths are in agreement with the observed ratios of (S II)/Ha ,( NII)/Ha, (S II)/(N II), (O I)/Ha ,( OIII)/Ha, and He I/Ha in the Galactic WIM. Although the forbidden emission- line intensities depend strongly on the input model parameters, the ionization fractions of the 20 ele- ments studied in this work are robust over a wide range of physical conditions considered in the models. These ionization fractions have direct relevance to absorption-line determinations of the elemental abun- dances in the warm neutral and ionized gases in the Milky Way and other late-type galaxies. We demonstrate a method for estimating the WIM contributions to the observed column densities of singly and doubly ionized atoms used to derive abundances in the warm neutral gas. We apply this approach to study the gas-phase abundances of the warm interstellar clouds toward the halo star HD 93521. Subject headings: ISM: abundancesISM: atomsH II regionsradiative transfer

Ionization Properties and Elemental Abundances in Damped Lyα Systems

We analyze extant data of Al+2, Al+, and other low ions with the aim of studying the ionization properties of damped Lyα systems (DLAs) from the analysis of the ratio R(Al+2/Al+) ≡ N(Al+2)/N(Al+). We find the good correlations log N(Al+)-log N(Si+) and log N(Al+)-log N(Fe+) that we use to indirectly estimate N(Al+) from N(Si+) and/or N(Fe+) measurements. In this way, we determine the ratio R(Al+2/Al+) for a sample of 20 DLAs. Contrary to common belief, the ratio can attain relatively high values (up to 0.6), suggesting that the gas of the intermediate ionization state plays an important role in DLAs. On the other hand, the lack of any trend between abundance ratios, such as Si/H and Si/Fe and R(Al+2/Al+) indicates that abundances are not severely influenced by ionization effects. We find a log R(Al+2/Al+)-log N(H0) anticorrelation that we use in conjunction with idealized photoionization equilibrium calculations to constrain the ionization properties and to predict ionization corrections in DLAs. We consider two possible origins for the species of the low- and intermediate-ionization state: (1) neutral regions devoid of Al+2 and/or (2) partially ionized, Al+2-bearing regions. The log R(Al+2/Al+)-log N(H0) anticorrelation can be naturally explained in terms of a two-region model with a soft, stellar-type ionizing radiation field. We present abundance ionization corrections for 14 elements of astrophysical interest derived with different types of ionizing spectra. For most of these elements, the corrections are generally below measurements errors, which is contrary to the predictions of recent models presented in the literature. We briefly discuss the potential effects of inaccuracies in the Al recombination rates used in the photoionization calculations.

A Search for Extraplanar Dust in Nearby Edge-on Spirals*

We present high-resolution (06 to ~ 10) BV images of 12 edge-on spiral galaxies observed with the WIYN 3.5 m telescope. These images were obtained to search for extraplanar (|z| > 0.4 kpc) absorbing dust structures similar to those previously found in NGC 891 (Howk & Savage). Many of these galaxies have been previously searched for diffuse ionized gas at high z. Our imaged galaxies include a sample of seven massive L*-like spiral galaxies within D 25 Mpc that have inclinations i 87? from the plane of the sky. We find that five of these seven systems show extraplanar dust, visible as highly structured absorbing clouds against the background stellar light of the galaxies. These dust structures lie at heights |z| 0.4 kpc, which should be above most of the thin disk molecular material in these galaxies. The more prominent structures are estimated to have associated gas masses 105 M?; the implied potential energies are 1052 ergs. All of the galaxies in our sample that show detectable H? emission at large z also show extraplanar dust structures. None of those galaxies for which extraplanar H? searches were negative show evidence for extensive high-z dust. The existence of extraplanar dust is a common property of massive spiral galaxies. We discuss several mechanisms for shaping the observed dust features. We emphasize in this discussion the possibility that these dusty clouds represent the dense phase of a multiphase medium at high z in spiral galaxies. In a few cases interactions with close galaxy companions could be responsible for the high-z dust, either through dynamical stripping or triggered star formation. We can rule out warps as the source of the observed high-z dust. Flaring gas layers seem an unlikely source of the observed material but cannot be ruled out at this time, except for those features that clearly connect to energetic processes in the disk. The correlation between high-z dust and extraplanar H? emission may simply suggest that both trace the high-z interstellar medium in its various forms (or phases), the existence of which may ultimately be driven by vigorous star formation in the underlying disk. The absorption produced by high-z dust and associated gas in spiral galaxies must be accounted for when studying extraplanar emission from spiral galaxies over much of the electromagnetic spectrum.

Abundances and Physical Conditions in the Warm Neutral Medium toward μ Columbae

We present ultraviolet interstellar absorption-line measurements for the sightline toward the O9.5 V star μ Columbae (l = 2373, b = -271; d ≈ 400 pc, z ≈ 180 pc; n ≈ 0.06 cm-3) obtained with the Goddard High Resolution Spectrograph (GHRS) on board the Hubble Space Telescope. These archival data represent the most complete GHRS interstellar absorption-line measurements for any line of sight toward an early-type star. The 3.5 km s-1 resolution of the instrument allows us to accurately derive the gas-phase column densities of many important ionic species in the diffuse warm neutral medium, including accounting for saturation effects in the data and for contamination from ionized gas along this sightline. For the low-velocity material (-20 vLSR +15 km s-1), we use the apparent column density method to derive column densities. For the individual absorbing components at vLSR ≈ -28.8, +20.1, +31.0, and +41.2 km s-1, we apply component fitting techniques to derive column densities and b-values. We have also used observations of interstellar Lyα absorption taken with the GHRS intermediate resolution gratings to accurately derive the H I column density along this sightline. The resulting interstellar column density, log N(H I) = 19.86 ± 0.015, is in agreement with other determinations but is significantly more precise. The low-velocity material shows gas-phase abundance patterns similar to the warm cloud (cloud A) toward the disk star ζ Ophiuchi, while the component at vLSR ≈ +20.1 km s-1 shows gas-phase abundances similar to those found in warm halo clouds. We find that the velocity-integrated gas-phase abundances of Zn, P, and S relative to H along this sightline are indistinguishable from solar system abundances. We discuss the implications of our gas-phase abundance measurements for the composition of interstellar dust grains. We find a dust-phase abundance d = 2.7-3.3 in the low-velocity gas; therefore the dust cannot be composed solely of common silicate grains, but must also include oxides or pure iron grains. The low-velocity material along this sightline is characterized by T ≈ 6000-7000 K with ne ≈ 0.3 cm-3, derived from the ionization equilibrium of Mg and Ca. The relative ionic column density ratios of the intermediate-velocity components at vLSR = +31.0 and +41.2 km s-1 show the imprint both of elemental incorporation into grains and (photo)ionization. These clouds have low total hydrogen column densities [log N(H) ~ 17.4-17.7], and our component fitting b-values constrain the temperature in the highest velocity component to be T = 4000 ± 700 K. The electron density of this cloud is ne ≈ 0.6 cm-3, derived from the 2P1/2 to 2P3/2 fine structure excitation of C II. The components at vLSR ≈ -30 and -48 km s-1 along this sightline likely trace shocked gas with very low hydrogen column densities. The vLSR ≈ -30 km s-1 component is detected in a few strong low-ionization lines, while both are easily detected in Si III. The relative column densities of the -30 km s-1 suggest that the gas is collisionally ionized at moderate temperatures (T ≈ 25,000 K). This is consistent with the measured b-values of this component, though nonthermal motions likely contribute significantly to the observed breadths.

Interstellar Deuterium, Nitrogen, and Oxygen Abundances toward GD 246, WD 2331–475, HZ 21, and Lanning 23: Results from the FUSE Mission*

The interstellar abundances of D i ,N i, and O i in the local ISM are studied using high-resolution spectra of four hot white dwarfs. The spectra of GD 246, WD 2331� 475, HZ 21, and Lan 23 were obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) in the wavelength range 905–1187 A ˚ . The line of sight to GD 246 probes the Local Interstellar Cloud and at least one other H i cloud inside the Local Bubble, which contains most of the gas seen along this line of sight. The column densities of Hi ,C ii*, Sii, and Siii are measured using archival Hubble Space Telescope STIS echelle-mode observations. The H i column density is deter... . .. . .. . .. . .. ... ...

The UCSD HIRES/Keck I Damped Lyα Abundance Database.* III. An Empirical Study of Photoionization in the Damped Lyα System toward GB 1759+7539

We investigate the ionization state of the damped Lyα system at z = 2.62 toward GB 1759+7539 through an analysis of ionic ratios sensitive to photoionization: Ar0/S+, Fe++/Fe+, N+/N0, and Al++/Al+. Approximately half of the metals arise in a mostly neutral velocity component with H I/H > 0.9, based on Fe++/Fe+ < 0.013. In contrast, the remaining half exhibit Fe++/Fe+ ≈ 0.3, indicative of a partially ionized medium with H I/H ≈ 0.5. These conclusions are supported by the observed N+/N0, Al++/Al+, and Ar0/Si+ ratios. We assess ionization corrections for the observed column densities through photoionization models derived from the CLOUDY software package. In the neutral gas, the ionization corrections are negligible, except for Ar0. However, for the partially ionized gas, element abundance ratios differ from the ionic ratios by 0.1-0.3 dex for (Si+, S+, Ni+, Al+)/Fe+ ratios and more for (N0, Ar0)/Fe+. Independent of the shape of the photoionizing spectrum and assumptions of the number of ionization phases, these ionization corrections have minimal impact (0.1 dex) on the total metallicity inferred for this damped Lyα system. Measurements of the relative elemental abundances of the partially ionized gas, however, have a greater than ≈0.15 dex uncertainty, which hides the effects of nucleosynthesis and differential dust depletion. We caution the reader that this damped system is unusual for a number of reasons (e.g., a very low Ar0/S+ ratio), and we believe its ionization properties are special but not unique. Nevertheless, it clearly shows the value of examining photoionization diagnostics such as Fe++/Fe+ in a larger sample of damped systems.

FUSE Observations of Atomic Abundances and Molecular Hydrogen in the Leading Arm of the Magellanic Stream

We present Far-Ultraviolet Spectroscopic Explorer observations of the atomic and molecular absorption in high-velocity cloud HVC 287.5]22.5]240, which lies in front of the ultraviolet-bright nucleus of the Seyfert 1 galaxy NGC 3783. We detect N I ,N II ,S iII, and Fe II and set limits on the amount of H 2 , absorption due to P III ,A rI, and Fe III. We extend the earlier metallicity and dust-depletion measurements made by Lu and collaborators by examining the relative gas-phase abundances of Si, P, S, and Fe. Corrections to the derived gas-phase abundances due to ionized gas in the HVC are small ([15%). The HVC has metallicity 0.2¨0.4 solar, similar to that of the Small Magellanic Cloud. The relative abundance pattern for the elements studied resembles that of warm gas in the Small Magellanic Cloud (SMC), which supports the idea that this HVC is part of the tidally stripped leading arm of the Magellanic Stream. The abundance pattern implies that the HVC contains dust grains that have been processed signi—cantly; it is likely that the grain mantles have been modi—ed or stripped back to expose the grain cores. We have identi—ed more than 30 lines of arising in the HVC from rotational levels J 0t o H 2 J 3. Synthetic spectra and a curve of growth —tted to these lines with b 12 km s~1 indicate that and A two-component temlog [N(H 2 )] 16.80 ^ 0.10 f H2 2N(H 2 )/[N(H I) ] 2N(H 2 )] 1.6 ] 10~3. perature distribution is necessary to explain the observed populations of the rotational levels. We H 2 —nd K, and K, indicating that the conditions in the molecular gas are more T 01 133 ~2137 T 23 241 ~1720 similar to those found for diUuse molecular clouds in the Galactic halo than to those for molecular clouds in the Galactic disk. From an analysis of the J 2 and J 3 populations, we —nd an absorption rate (at 1000 of times the average value in the solar neighborhood. The presence of molecuAe ) b uv 0.1 lar gas in the HVC requires that either the formed in situ or that molecules formed within the SMC H 2 survived tidal stripping. We favor the latter possibility because of the long formation time (D108 yr) H 2

Stagnation and Infall of Dense Clumps in the Stellar Wind of τ Scorpii

Observations of the B0.2 V star q Scorpii have revealed unusual stellar wind characteristics: redshifted absorption in the far-ultraviolet O VI resonance doublet up to D)250 km s~1 and extremely hard X-ray emission implying gas at temperatures in excess of 107 K. We describe a phenomenological model to explain these properties. We assume the wind of q Sco consists of two components: ambient gas in which denser clumps are embedded. The clumps are optically thick in the UV resonance lines primarily responsible for accelerating the ambient wind. The reduced acceleration causes the clumps to slow and even infall, all the while being con—ned by the ram pressure of the out—owing ambient wind. We calcu- late detailed trajectories of the clumps in the ambient stellar wind, accounting for a line radiation driving force and the momentum deposited by the ambient wind in the form of drag. We show that these clumps will fall back toward the star with velocities of several hundred km s~1 for a broad range of initial conditions. The velocities of the clumps relative to the ambient stellar wind can approach 2000 km s~1, producing X-rayemitting plasmas with temperatures in excess of (1¨6) ) 107 K in bow shocks at their leading edge. The infalling material explains the peculiar redshifted absorption wings seen in the O VI doublet. Of order 103 clumps with individual masses g are needed to explain the m c D 1019¨1020 observed X-ray luminosity and also to explain the strength of the O VI absorption lines. These values correspond to a mass-loss rate in clumps of to 10~8 yr~1, a small fraction of the total M0 c D 10~9 M _ mass-loss rate yr~1). We discuss the position of q Sco in the H-R diagram, conclud- (M0 D 3 ) 10~8 M _ ing that q Sco is in a crucial position on the main sequence. Hotter stars near the spectral type of q Sco have too powerful winds for clumps to fall back to the stars, and cooler stars have too low mass-loss rates to produce observable eUects. The model developed here can be generally applied to line-driven out—ows with clumps or density irregularities. Subject headings: stars: early-typestars: individual (q Scorpii) ¨ stars: winds, out—ows ¨ ultraviolet: starsX-rays: stars

Empirical Verification of the Fe II Oscillator Strengths in the FUSE Bandpass

We report empirical determinations of atomic oscillator strengths, or f-values, for 11 ground-state transitions of Fe II in the wavelength range We use ultraviolet absorption-line obser- 1050 ( j ( 1150 Ae . vations of interstellar material toward stars in the Galaxy and the Magellanic Clouds taken with Coper- nicus, the Goddard High Resolution Spectrograph on board the Hubble Space Telescope, and the Far-Ultraviolet Spectroscopic Explorer. We derive absolute oscillator strengths by a combination of the apparent optical depth, component-—tting, and curve-of-growth —tting techniques. Our derived oscillator strengths are generally in excellent agreement with recent theoretical calculations by Raassen & Uylings using the orthogonal operator technique. However, we identify three of the 11 transitions studied here whose f-values seem to be incompatible with these calculations by as much as a factor of 2. We suggest revisions to these f-values based on our analysis. Subject headings: atomic dataISM: abundancesISM: atomsultraviolet: ISM

Ionized Gas in Damped Lyα Systems and Its Effects on Elemental Abundance Studies

Recent high-resolution observations of metal absorption lines in high-redshift damped Ly? systems have shown that Al III, a tracer of moderately ionized gas, very often has a velocity structure indistinguishable from that of low-ionization gas. Regions of ionized and neutral hydrogen in these systems are likely cospatial. The higher ionization Si IV and C IV absorption shows a much weaker or nonexistent correlation with the low-ionization material, implying that the regions traced by Al III are photoionized by a soft (stellar) spectrum, by a hard (power-law) spectrum with a very low ionization parameter, or a combination of both. We discuss the ionization of the damped Ly? systems and use photoionization equilibrium models to make quantitative estimates of its effects on abundance studies in these systems. We show that ionization effects may be large enough to account for the observed dispersion in absolute metal abundances in damped Ly? systems, causing systematically higher abundances in lower column density systems. The observed Si+/Fe+ and Zn+/Cr+ ratios may systematically overestimate the intrinsic Si/Fe and Zn/Cr ratios, respectively, if ionized gas is present in these systems, thereby mimicking the effects of ?-element enrichment or dust depletion.