Dust–Gas Scaling Relations and OH Abundance in the Galactic ISM (original) (raw)
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Interstellar OH + , H 2 O + and H 3 O + along the sight-line to G10.6–0.4
Astronomy and Astrophysics, 2010
We report the detection of absorption lines by the reactive ions OH + , H 2 O + and H 3 O + along the line of sight to the submillimeter continuum source G10.6−0.4 (W31C). We used the Herschel HIFI instrument in dual beam switch mode to observe the ground state rotational transitions of OH + at 971 GHz, H 2 O + at 1115 and 607 GHz, and H 3 O + at 984 GHz. The resultant spectra show deep absorption over a broad velocity range that originates in the interstellar matter along the line of sight to G10.6−0.4 as well as in the molecular gas directly associated with that source. The OH + spectrum reaches saturation over most velocities corresponding to the foreground gas, while the opacity of the H 2 O + lines remains lower than 1 in the same velocity range, and the H 3 O + line shows only weak absorption. For Local Standard of Rest (LSR) velocities between 7 and 50 kms −1 we estimate total column densities of N(OH + ) ≥ 2.5 × 10 14 cm −2 , N(H 2 O + ) ∼ 6 × 10 13 cm −2 and N(H 3 O + ) ∼ 4.0 × 10 13 cm −2 . These detections confirm the role of O + and OH + in initiating the oxygen chemistry in diffuse molecular gas and strengthen our understanding of the gas phase production of water. The high ratio of the OH + by the H 2 O + column density implies that these species predominantly trace low-density gas with a small fraction of hydrogen in molecular form.
Oxygen Gas‐Phase Abundance Revisited
The Astrophysical Journal, 2003
We present new measurements of the interstellar gas-phase oxygen abundance along the sight lines towards 19 early-type galactic stars at an average distance of 2.6 kpc. We derive O I column densities from HST/STIS observations of the weak 1355Å intersystem transition. We derive total hydrogen column densities [N(H I)+2N(H 2 )] using HST/STIS observations of Lyman-α and FUSE observations of molecular hydrogen. The molecular hydrogen content of these sight lines ranges from f(H 2 ) = 2N(H 2 )/[N(H I)+2N(H 2 )] = 0.03 to 0.47. The average H tot /E B−V of 6.3×10 21 cm −2 mag −1 with a standard deviation of 15% is consistent with previous surveys. The mean oxygen abundance along these sight lines, which probe a wide range of galactic environments in the distant ISM, is 10 6 (O/H) gas = 408 ± 13 (1 σ in the mean). We see no evidence for decreasing gas-phase oxygen abundance with increasing molecular hydrogen fraction and the relative constancy of (O/H) gas suggests that the component of dust containing the oxygen is not readily destroyed. We estimate that, if 60% of the dust grains are resilient against destruction by shocks, the distant interstellar total oxygen abundance can be reconciliated with the solar value derived from the most recent measurements of 10 6 (O/H) gas⊙ = 517 ± 58 (1 σ). We note that the smaller oxygen abundances derived for the interstellar gas within 500 pc or from nearby B star surveys are consistent with a local elemental deficit.
Abundances and Depletions of Interstellar Oxygen
The Astrophysical Journal, 2005
We report on the abundance of interstellar neutral oxygen (OI) for 26 sightlines, using data from the Far Ultraviolet Spectroscopic Explorer (FUSE), the International Spectroscopic Explorer (IUE), and the Hubble Space Telescope (HST). OI column densities are derived by measuring the equivalent widths of several ultraviolet absorption lines, and subsequently fitting those to a curve of growth. We consider both our general sample of 26 sightlines and a more restrictive sample of 10 sightlines that utilize HST data for a measurement of the weak 1355Å line of oxygen, and are thus better constrained due to our sampling of all three sections of the curve of growth. The column densities of our HST sample show ratios of O/H that agree with the current best solar value if dust is considered, with the possible exception of one sightline (HD 37903). We note some very limited evidence in the HST sample for trends of increasing depletion with respect to R V and f (H 2 ), but the trends are not conclusive. Unlike a recent result from Cartledge et al. , we do not see evidence for increasing depletion with respect to n H , but our HST sample contains only two points more dense than the critical density determined in that paper. The column densities of our more general sample show some scatter in O/H, but most agree with the solar value to within errors. We discuss these results in the context of establishing the best method for determining interstellar abundances, the unresolved question of the best value for O/H in the interstellar medium (ISM), the O/H ratios observed in Galactic stars, and the depletion of gas-phase oxygen onto dust grains. Subject headings: ISM: abundances -ISM: depletions -ultraviolet: ISM Keenan et al. (1985) explored 26 sightlines-including one sightline from York et al. Cartledge et al. (2001) examined 11 sightlines using the Space Telescope Imaging Spectrograph (STIS) onboard HST to observe the 1355Å line. This study used two methods: profile fitting, where velocity information from other atomic species is incorporated into the fit of the oxygen line; and the apparent optical depth (AOD) method, where it is as-
The Astrophysical Journal, 2015
We modelled recent observations of UV absorption of HD and H 2 in the Milky Way and toward damped/sub-damped Lyman alpha systems at z=0.18 and z > 1.7. N(HD)/N(H 2) ratios reflect the separate self-shieldings of HD and H 2 and the coupling introduced by deuteration chemistry. Locally, observations are explained by diffuse molecular gas with 16 cm −3 n(H) 128 cm −3 if the cosmic-ray ionization rate per H-nucleus ζ H = 2×10 −16 s −1 as inferred from H 3 + and OH +. The dominant influence on N(HD)/N(H 2) is the cosmic-ray ionization rate with a much weaker downward dependence on n(H) at Solar metallicity, but dust-extinction can drive N(HD) higher as with N(H 2). At z > 1.7, N(HD) is comparable to the Galaxy but with 10x smaller N(H 2) and somewhat smaller N(H 2)/N(H I). Comparison of our Galaxy and the Magellanic Clouds shows that smaller H 2 /H is expected at sub-Solar metallicity and we show by modelling that HD/H 2 increases with density at low metallicity, opposite to the Milky Way. Observations of HD would be explained with higher n(H) at low metallicity but high-z systems have high HD/H 2 at metallicity 0.04 Z 2 Solar. In parallel we trace dust-extinction and self-shielding effects. The abrupt H 2 transition to H 2 /H ≈ 1-10% occurs mostly from self-shielding although it is assisted by extinction for n(H) 16 cm −3. Interior H 2 fractions are substantially increased by dust extinction below 32 cm −3. At smaller n(H), ζ H , small increases in H 2 triggered by dust extinction can trigger abrupt increases in N(HD).
Molecular Hydrogen and its Proxies HCO+ and CO in the Diffuse Interstellar Medium
The Astrophysical Journal
There is a robust polyatomic chemistry in diffuse, partially molecular interstellar gas that is readily accessible in absorption at radio/millimeter/submillimeter wavelengths. Accurate column densities are derived owing to the weak internal excitation, so relative molecular abundances are well known with respect to each other but not with respect to H2. Here we consider the use of proxies for hydrogen column densities N(H2) and N(H) = N(H i)+2N(H2) based on measurements of HCO+ absorption and CO emission and absorption, and we compare these with results obtained by others when observing H i, H2 and CO toward stars and active galactic nuclei. We consider the use of HCO+ as a proxy for H2 and show that the assumption of a relative abundance N(H2) = N(HCO+)/3 × 10−9 gives the same view of the atomic–molecular hydrogen transition that is seen in UV absorption toward stars. CO on the other hand shows differences between the radio and optical regimes because emission is always detected wh...
A High Resolution Study of the HIH2 Transition across the Perseus Molecular Cloud
2011
To investigate the fundamental principles of H2 formation in a giant molecular cloud (GMC), we derive the HI and H2 surface density (Sigma_HI and Sigma_H2) images of the Perseus molecular cloud on sub-pc scales (~0.4 pc). We use the far-infrared data from the Improved Reprocessing of the IRAS Survey and the V-band extinction image provided by the COMPLETE Survey to estimate the dust column density image of Perseus. In combination with the HI data from the Galactic Arecibo L-band Feed Array HI Survey and an estimate of the local dust-to-gas ratio, we then derive the Sigma_H2 distribution across Perseus. We find a relatively uniform Sigma_HI ~ 6-8 Msun pc^-2 for both dark and star-forming regions, suggesting a minimum HI surface density required to shield H2 against photodissociation. As a result, a remarkably tight and consistent relation is found between Sigma_H2/Sigma_HI and Sigma_HI+Sigma_H2. The transition between the HI- and H2-dominated regions occurs at N(HI)+2N(H2) ~ (8-14) x 10^20 cm^-2. Our findings are consistent with predictions for H2 formation in equilibrium, suggesting that turbulence may not be of primary importance for H2 formation. However, the importance of a warm neutral medium for H2 shielding, an internal radiation field, and the timescale of H2 formation still remain as open questions. We also compare H2 and CO distributions and estimate the fraction of "CO-dark" gas, f_DG ~ 0.3. While significant spatial variations of f_DG are found, we do not find a clear correlation with the mean V-band extinction.
The relation between column densities of interstellar OH and CH molecules
Monthly Notices of the Royal Astronomical Society, 2010
We present a new, close relation between column densities of OH and CH molecules based on 16 translucent sightlines (six of them new) and confirm the theoretical oscillator strengths of the OH A-X transitions at 3078 and 3082 Å (0.001 05, 0.000 648) and CH B-X transitions at 3886 and 3890 Å (0.003 20, 0.002 10), respectively. We also report no difference between observed and previously modelled abundances of the OH molecule.
The Homogeneity of Interstellar Oxygen in the Galactic Disk
The Astrophysical Journal, 2004
We present an analysis of high resolution HST Space Telescope Imaging Spectrograph (STIS) observations of O I λ1356 and H I Lyman-α absorption in 36 sight lines that probe a variety of Galactic disk environments and include paths that range over nearly 4 orders of magnitude in f (H 2 ), over 2 orders of magnitude in n H , and that extend up to 6.5 kpc in length. Since the majority of these sight lines have also been observed by the Far-Ultraviolet Spectroscopic Explorer (FUSE), we have undertaken the study of gas-phase O/H abundance ratio homogeneity using the current sample and previously published Goddard High-Resolution Spectrograph (GHRS) results. Two distinct trends are identified in the 56 sight line sample: an apparent decrease in gas-phase oxygen abundance with increasing mean sight line density ( n H ) and a gap between the mean O/H ratio for sight lines shorter and longer than about 800 pc. The first effect is a smooth transition between two depletion levels associated with large mean density intervals; it is centered near n H = 1.5cm −3 and is similar to trends evident in gas-phase abundances of other elements. Paths less dense than the central value exhibit a mean O/H ratio of log 10 (O/H) = -3.41±0.01 (or 390±10 ppm),
Ju n 20 04 The Homogeneity of Interstellar Oxygen in the Galactic Disk 1
2004
We present an analysis of high resolution HST Space Telescope Imaging Spectrograph (STIS) observations of O I λ1356 and H I Lyman-α absorption in 36 sight lines that probe a variety of Galactic disk environments and include paths that range over nearly 4 orders of magnitude in f(H2), over 2 orders of magnitude in 〈nH〉, and that extend up to 6.5 kpc in length. Since the majority of these sight lines have also been observed by the Far-Ultraviolet Spectroscopic Explorer (FUSE), we have undertaken the study of gas-phase O/H abundance ratio homogeneity using the current sample and previously published Goddard HighResolution Spectrograph (GHRS) results. Two distinct trends are identified in the 56 sight line sample: an apparent decrease in gas-phase oxygen abundance with increasing mean sight line density (〈nH〉) and a gap between the mean O/H ratio for sight lines shorter and longer than about 800 pc. The first effect is a smooth transition between two depletion levels associated with lar...