David M. Meyer

The Definitive Abundance of Interstellar Oxygen

Using the Goddard High Resolution Spectrograph (GHRS) onboard the Hubble Space Telescope, we have obtained high signal-to-noise (S/N) ratio echelle observations of the weak interstellar O I λ1356 absorption toward the stars γ Cas, Per, δ Ori, Ori, 15 Mon, τ CMa, and γ Ara. In combination with previous GHRS measurements in six other sight lines (ζ Per, ξ Per, λ Ori, < Ori, κ Ori, and ζ Oph), these new observations yield a mean interstellar gas-phase oxygen abundance (per 106 H atoms) of 106 O/H = 319 ± 14. The largest deviation from the mean is less than 18%, and there are no statistically significant variations in the measured O abundances from sight line to sight line and no evidence of density-dependent oxygen depletion from the gas phase. Assuming various mixtures of silicates and oxides, the abundance of interstellar oxygen tied up in dust grains is unlikely to surpass 106 O/H ≈ 180. Consequently, the GHRS observations imply that the total abundance of interstellar oxygen (gas plus grains) is homogeneous in the vicinity of the Sun and about two-thirds of the solar value of 106 O/H = 741 ± 130. This oxygen deficit is consistent with that observed in nearby B stars and similar to that recently found for interstellar krypton with GHRS. Possible explanations for this deficit include: (1) early solar system enrichment by a local supernova, (2) a recent infall of metal-poor gas in the local Milky Way, or (3) an outward diffusion of the Sun from a smaller Galactocentric distance.

Interstellar abundance standards revisited

We evaluate the stellar abundances often used to represent the total (gas plus dust) composition of the interstellar medium. Published abundances for B stars, young later type (F and G) stars, and the Sun are compared to the modeled dust-phase and measured gas-phase compositions of the interstellar medium. This study uses abundances for the five most populous elements in dust grains—C, O, Mg, Si, and Fe—and the cosmically abundant element, N. We find that B stars have metal abundances that are too low to be considered valid representations of the interstellar medium. The commonly invoked interstellar standard that is two-thirds of the solar composition is also rejected by recent observations. Young (≤2 Gyr) F and G disk stars and the Sun, however, cannot be ruled out as reliable proxies for the total interstellar composition. If their abundances are valid representations of the interstellar medium, then the apparent underabundance of carbon with respect to that required by dust models, i.e., the carbon crisis, is substantially eased. Subject headings: dust, extinction — ISM: abundances — ultraviolet: ISM

Quasar Absorbing Galaxies at z 1: Deep Imaging and Spectroscopy in the Field of 3C 336

We present very deep WFPC2 images and FOS spectroscopy from the Hubble Space Telescope (HST) together with numerous supporting ground-based observations of the field of the quasar 3C 336 (zem = 0.927). The observations are designed to investigate the nature of galaxies producing metal-line absorption systems in the spectrum of the QSO. Along a single line of sight, we find at least six metal-line absorption systems (of which three are newly discovered) ranging in redshift from 0.317 to 0.892. Through an extensive program of optical and IR imaging, QSO spectroscopy, and faint galaxy spectroscopy, we have identified five of the six metal-line absorption systems with luminous (L -->K≥0.1L -->*K) galaxies. These have morphologies ranging from very late-type spiral to S0, and they exhibit a wide range of inclination and position angles with respect to the QSO sight line. The only unidentified absorber, despite our intensive search, is a damped Lyman-α system at zabs = 0.656. Analysis of the absorption spectrum suggests that the metal abundances ([Fe/H] = -1.2) in this system are similar to those in damped systems at z ~ 2 and to the two other damped systems for which abundances have been determined at z *K) near the QSO sight line that eludes detection despite our deep HST and high-resolution ground-based near-IR images. We have found no examples of intrinsically faint galaxies (L 0.1LK) within 50 h-1 kpc that do not produce detectable metal lines (of Mg II λλ2796, 2803 and/or C IV λλ1548, 1550) in the QSO spectrum. All of these results generally support the inferences we have previously reached from a larger survey for absorption-selected galaxies at z 1. There are several other galaxies with redshifts near that of 3C 336, suggesting that the QSO is situated in an overdense region, perhaps a galaxy cluster. Previously published reports of a cluster around 3C 336 were largely misled by the presence of many foreground galaxies seen in projection near the QSO. It is possible that a reported measurement of weak shear gravitational lensing in this field may be produced by the QSO cluster itself, since there appear to be no other groups or clusters in the foreground. We find no evidence for a normal, bright QSO host galaxy, although there are several faint objects very close to the quasar and at similar redshift that might either be companions or part of a disorganized QSO host.

The abundance of interstellar nitrogen

Using the Hubble Space Telescope Goddard High Resolution Spectrograph (GHRS), we have obtained high S/N echelle observations of the weak interstellar N I λλ1160, 1161 absorption doublet toward the stars γ Cas, λ Ori, ι Ori, κ Ori, δ Sco, and κ Sco. In combination with a previous GHRS measurement of N I toward ζ Oph, these new observations yield a mean interstellar gas-phase nitrogen abundance (per 106 H atoms) of 106 N/H = 75 ± 4 (±1 σ). There are no statistically significant variations in the measured N abundances from sight line to sight line and no evidence of density-dependent nitrogen depletion from the gas phase. Since N is not expected to be depleted much into dust grains in these diffuse sight lines, its gas-phase abundance should reflect the total interstellar abundance. Consequently, the GHRS observations imply that the abundance of interstellar nitrogen (gas plus grains) in the local Milky Way is about 80% of the solar system value of 106 N/H = 93 ± 16. Although this interstellar abundance deficit is somewhat less than that recently found for oxygen and krypton with GHRS, the solar N abundance and the N I oscillator strengths are too uncertain to rule out definitively either a solar ISM N abundance or a solar ISM N abundance similar to that of O and Kr.

Interstellar Carbon in Translucent Sight Lines

We report interstellar C II column densities or upper limits determined from weak absorption of the 2325.4029 ? intersystem transition observed in six translucent sight lines (AV 1) with the Space Telescope Imaging Spectrograph (STIS). The sight lines sample a wide range of interstellar characteristics, including total to selective extinction, RV = 2.6-5.1; average hydrogen density along the sight line, n(H) = 3-14 cm-3; and fraction of H in molecular form, 0-~40%. Four of the sight lines, those toward HD 37021, HD 37061, HD 147888, and HD 207198, have interstellar gas-phase abundances that are consistent with the diffuse sight line ratio of 161 ? 17 carbon atoms in the gas per million hydrogen nuclei. We note that while it has a gas-phase carbon abundance that is consistent with the other sight lines, a large fraction of the C II toward HD 37061 is in an excited state. The sight line toward HD 152590 has a measured interstellar gas-phase carbon abundance that is well above the diffuse sight line average; the column density of C in this sight line may be overestimated because of noise structure in the data. Toward HD 27778 we find a 3 ? abundance upper limit of ?108 C atoms in the gas per million H, a substantially enhanced depletion of C as compared to the diffuse sight line value. The interstellar characteristics toward HD 27778 are otherwise not extreme among the sample, except for an unusually large abundance of CO molecules in the gas.

The Homogeneity of Interstellar Oxygen in the Galactic Disk

We present an analysis of high-resolution Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) observations of Kr I λ1236 absorption in seven sight lines that probe a variety of interstellar environments. In combination with krypton and hydrogen column densities derived from current and archival STIS and Far-Ultraviolet Spectroscopic Explorer data, the number of sight lines with reliable Kr/H ISM abundance ratios has been increased by 50% to 26—including paths that sample a range of nearly 5 orders of magnitude in f(H2) and over 2 orders of magnitude in nH, and extend up to 4.8 kpc in length. For sight lines contained entirely within the local spiral arm (the Orion spur), the spread of Kr/H ratios about the mean of [N(Kr)/N(H)]ISM = -9.02 ± 0.02 is remarkably tight (0.06 dex), less than the typical data-point uncertainty. Intriguingly, the only two sight lines that extend through neighboring structures, in particular gas associated with the Carina/Sagittarius arm, exhibit relatively large, near-solar krypton abundances ([N(Kr)/N(H)]combined = -8.75). Although these deviations are only measured at the 2 σ level, they suggest the possibility that krypton abundances beyond the Orion spur may differ from the local value.

Carbon in the Diffuse Interstellar Medium

We have obtained spectra of the interstellar intersystem C II] λ2325 line toward the star τ Canis Majoris. The absorption spectra were obtained with the echelle mode (3.5 km s-1 resolution) of the Goddard High Resolution Spectrograph aboard the Hubble Space Telescope and have a co-added signal-to-noise ratio of approximately 850. The C II] line has a measured equivalent width Wλ = 0.21 ± 0.07 mA, which corresponds to a column density of 7.6 ± 2.5 × 1016 cm-2. Of the six interstellar lines of sight that have reliably measured (>2 σ) carbon abundances, the τ CMa sight line has the lowest fractional H2 abundance, f(H2), by over 3 orders of magnitude. Although this suggests that the physical conditions in the interstellar gas toward τ CMa are different from the other sight lines, the measured gas-phase C/H ratio is the same: 106 C/H = 135 ± 46 for τ CMa versus 106 C/H = 140 ± 20 for the others (Cardelli et al.). The constant interstellar gas-phase C/H over a wide range of f(H2) suggests that even under very different conditions, no carbon is being exchanged between the gas and dust phases of the interstellar medium. It also supports, and extends to a larger distance, the suggestion by Cardelli et al. that the intrinsic (gas-phase plus dust-phase) interstellar C/H ratio in the vicinity of the Sun is constant and below the solar C/H value.

Discovery of interstellar NH

The discovery of interstellar NH is reported in the diffuse clouds toward Zeta Per and HD 27778 from high-resolution high S/N spectra of the NH A 3Pi-X 3Sigma (0,0) absorption band near 3358 A. These observations represent the first detection of this molecule anywhere in the interstellar medium. Measured equivalent widths of 0.37 {plus minus} 0.08 and 1.1 {plus minus} 0.3 mA for the NH R1(0) lines toward Zeta Per and HD 27778 correspond to respective NH column densities of 9.0 {times} 10 to the 11th and 2.7 {times} 10 to the 12th/sq cm. N(NH) in the Zeta Per cloud is close to previous upper limits and much greater than that predicted by current models of the gas-phase nitrogen chemistry. The incorporation of a more rapid rate for the reaction N(+) + H2 yields NH(+) + H in these models may provide an effective gas-phase route to explain the NH abundance. A contribution involving the direct or indirect production of NH from grain surface reactions is also possible. In any case, these NH measurements have additional implications for a better understanding of the observed CN abundances in diffuse interstellar clouds. 43 refs.

The homogeneity of interstellar elemental abundances in the Galactic disk

We present interstellar elemental abundance measurements derived from Space Telescope Imaging Spectrograph echelle observations of 47 sight lines extending up to 6.5 kpc through the Galactic disk. These paths probe a variety of interstellar environments, covering ranges of nearly 4 orders of magnitude in molecular hydrogen fraction f(H2) and more than 2 in mean hydrogen sight-line density nH. Coupling the current data with Goddard High Resolution Spectrograph data from 17 additional sight lines and the corresponding Far Ultraviolet Spectroscopic Explorer and Copernicus observations of H2 absorption features, we explore magnesium, phosphorus, manganese, nickel, copper, and germanium gas-phase abundance variations as a function of nH: density-dependent depletion is noted for each element, consistent with a smooth transition between two abundance plateaus identified with warm and cold neutral interstellar medium depletion levels. The observed scatter with respect to an analytic description of these transitions implies that total elemental abundances are homogeneous on length scales of hundreds of parsecs, to the limits of abundance measurement uncertainty. The probable upper limit we determine for intrinsic variability at any nH is 0.04 dex, aside from an apparent 0.10 dex deficit in copper (and oxygen) abundances within 800 pc of the Sun. Magnesium dust abundances are shown to scale with the amount of silicon in dust, and in combination with a similar relationship between iron and silicon, these data appear to favor the young F and G star values of Sofia & Meyer as an elemental abundance standard for the Galaxy.

Small-Scale Interstellar Medium Structure: The Remarkable Sight Line toward μ Crucis

We present very high resolution (Δv ≈ 0.4 km s-1) observations of the interstellar Na I D and Ca II K lines toward the binary μ Cru, which consists of two B stars separated by 388 at a distance of 170 pc. These observations reveal line strength variations in four velocity components, indicative of interstellar medium structure on scales less than the projected binary separation of 6600 AU (0.03 pc). The components exhibiting the greatest Na I column density variations have the narrowest line widths and the largest N(Na I)/N(Ca II) ratios. The most visually striking of these variations involves a Na I component seen at a heliocentric velocity of -8.6 km s-1 toward μ1 Cru but not at all toward μ2 Cru. This component has an appreciable column density [N(Na I) = 7 × 1010 cm-2], and it clearly exhibits hyperfine splitting with a Na I line width (b = 0.4 km s-1) indicative of a cloud temperature below 220 K. It is likely that the μ Cru variations are sampling a widespread pattern of small-scale structure in the cold diffuse interstellar gas that could easily have been missed in previous observations.