Stefan I. B. Cartledge

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.

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.

Space Telescope Imaging Spectrograph Observations of Interstellar Oxygen and Krypton in Translucent Clouds

We have obtained high-resolution Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) observations of O I λ1356 and Kr I λ1236 absorption in 11 sight lines characterized by high extinction, large H I column densities, and/or long path lengths. Previous Goddard High Resolution Spectrograph (GHRS) measurements of these weak features in seven relatively nearby diffuse clouds have shown no evidence for density-dependent depletion of either oxygen or krypton and have yielded a weighted mean gas-phase abundance ratio of log[N(O)/N(Kr)]GHRS = 5.56 ± 0.04. Our STIS measurements yield a lower weighted mean of log[N(O)/N(Kr)]STIS = 5.48; the difference is due primarily to several translucent sight lines in the STIS data set that diverge from the GHRS value. These translucent cloud sight lines pass near dense, star-forming regions, notably the ρ Oph, Orion, and Taurus molecular clouds. Since Kr, as a noble gas, should not be depleted much into grains, these cases suggest a trend toward the enhanced oxygen depletion predicted for denser ISM clouds.

FUSE MEASUREMENTS OF FAR-ULTRAVIOLET EXTINCTION. II. MAGELLANIC CLOUD SIGHT LINES

We present an extinction analysis of nine reddened/comparison star pairs in the Large and Small Magellanic Clouds (LMC and SMC) based on Far-Ultraviolet Spectroscopic Explorer (FUSE) FUV observations. To date, just two LMC sight lines have probed dust grain composition and size distributions in the Magellanic Clouds using spectral data for wavelengths as short as 950 A. We supplement these two with data from four regions distinguished by their IR through UV extinction curves and grouped as LMCAvg, LMC2, SMC bar, and SMC wing. Despite the distinct characters of extinction in the Clouds and Milky Way, our results are generally analogous to those found for Galactic curves—namely, that the FUSE portions of each extinction curve are described reasonably well by Fitzpatrick & Massa curves fitted only to longer wavelength data and lack any dramatic new extinction features, and any deviations from the Cardelli, Clayton, & Mathis (CCM) formalism continue into FUV wavelengths. A maximum entropy method analysis of all of these curves suggests that LMCAvg and SMC wing sight lines, whose extinction parameters more closely resemble those for Galactic paths, require more silicon and/or carbon in dust than current abundance measurements would indicate are available. The requirements for LMC2 and SMC bar sight lines do not fully tax the available reservoirs, in part because large grains contribute less to the extinction in these directions. An intermediate product of this extinction analysis is the measurement of new H2 abundances in the Magellanic Clouds. Collectively considering Cloud sight lines that possess significant H2 column densities, E(B - V)/N(H ) ratios are reduced by significant factors relative to the Galactic mean, whereas the corresponding E(B - V)/N(H2) values more closely resemble their Galactic counterpart. These trends reflect the fact that among these sight lines f(H2) values are lower than those common in the Milky Way for paths with similar degrees of reddening.

FUSE Measurements of Far-Ultraviolet Extinction. I. Galactic Sight Lines*

We present extinction curves that include data down to far-ultraviolet wavelengths (FUV; 1050-1200 ?) for nine Galactic sight lines. The FUV extinction was measured using data from the Far Ultraviolet Spectroscopic Explorer. The sight lines were chosen for their unusual extinction properties in the infrared through the ultraviolet; that they probe a wide range of dust environments is evidenced by the large spread in their measured ratios of total to selective extinction, RV = 2.43-3.81. We find that extrapolation of the Fitzpatrick & Massa relationship from the ultraviolet appears to be a good predictor of the FUV extinction behavior. We find that predictions of the FUV extinction based on the Cardelli, Clayton, & Mathis (CCM) dependence on RV give mixed results. For the seven extinction curves well represented by CCM in the infrared through ultraviolet (x < 8 ?m-1), the FUV extinction is well predicted in three sight lines, overpredicted in two sight lines, and underpredicted in two sight lines. A maximum entropy method analysis using a simple three-component grain model shows that seven of the nine sight lines in the study require a larger fraction of grain materials to be in dust when FUV extinction is included in the models. Most of the added grain material is in the form of small (radii 200 ?) grains.

FUSE MEASUREMENTS OF FAR-ULTRAVIOLET EXTINCTION. III. THE DEPENDENCE ON R(V) AND DISCRETE FEATURE LIMITS FROM 75 GALACTIC SIGHTLINES

We present a sample of 75 extinction curves derived from Far Ultraviolet Spectroscopic Explorer (FUSE) far-ultraviolet spectra supplemented by existing International Ultraviolet Explorer (IUE) spectra. The extinction curves were created using the standard pair method based on a new set of dereddened FUSE+IUE comparison stars. Molecular hydrogen absorption features were removed using individualized H2 models for each sightline. The general shape of the FUSE extinction (8.4 μm–1 9.6 μm–1 on features with a resolution of 104 (~0.1 A width).

Interstellar Iron and Silicon Depletions in Translucent Sight Lines

We report interstellar Fe II and Si II column densities toward six translucent sight lines (AV 1) observed with the Space Telescope Imaging Spectrograph (STIS). The abundances were determined from the absorption of Si II] at 2335 A, and several weak Fe transitions including the first reported detections of the λ2234 line. We derive an empirical f-value for the Fe II λ2234 transition of log(fλ) = -1.54 ± 0.05. The observed sight lines sample a variety of extinction characteristics as indicated by their RV values, which range from 2.6 to 5.8. The dust-phase abundances of both Si and Fe are positively correlated with the small-grain population (effective radii smaller than a few hundred μm) toward the targets. The physical conditions along the sight lines suggest that this relationship may be due to differences in the survival of small particles in some interstellar environments. The chemical composition of the small grains could either resemble dust mantles or be silicate-rich.

FUSE Far Ultraviolet Dust Extinction Curves

Far‐ultraviolet dust extinction curves for 75 sightlines have been created using Far‐Ultraviolet Spectroscopic Explorer (FUSE) observations. This represents the largest sample of far‐UV extinction curves ever measured. The FUSE archive was searched for all observations of stars with existing good IUE or STIS ultraviolet extinction curves. The H2 absorptions in the FUSE observations were modeled and removed to allow for dust extinction curves to be created. These far‐UV extinction curves probe small dust grains. The 75 curves allows for the R(V) dependent relationship (ala CCM89) to put on a firm basis in the far‐UV for the first time. For the full details of this work and further analysis, see Gordon et al. (2009, in prep.).