Astronomy and Astrophysics – Astronomy
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008aas...212.0505s&link_type=abstract
American Astronomical Society, AAS Meeting #212, #5.05; Bulletin of the American Astronomical Society, Vol. 40, p.195
Astronomy and Astrophysics
Astronomy
Scientific paper
We analyzed archival HST/STIS and FUSE data to extract column densities of 12CO, 13CO, and H2 along all diffuse molecular sight lines that show evidence of CO absorption. High-resolution spectra of CH+, CH, and CN acquired at McDonald Observatory and ESO expanded the database of results on these species as an aid in interpreting the UV measurements. The average value of the 12-to-13 carbon ratio in the solar neighborhood is known to be 70 ± 7 from radio observations of CO emission and from optical determinations of the 12CH+/13CH+ ratio. We find that most (17 of 25) sight lines have 12CO/13CO ratios consistent with 12/13 = 70, with the rest showing significant fractionation. Of the latter, 5 directions have lower 12CO/13CO ratios, which can be attributed to charge exchange between 12CO and 13C+, while 3 sight lines have higher ratios due to selective photodissociation effected by self shielding of 12CO. A comparison of 12CO and H2 column densities shows the dependence of the CO/H2 ratio on physical conditions, notably total gas density. We find that the CN/CH+ ratio is better than CN/CH as a tracer of total gas density, owing to the preferential formation of CH+ and CN in lower- or higher-density clouds, respectively. We use both analytical CN chemistry and numerical modeling with CLOUDY to infer that many of our sight lines belong to a low-density regime of diffuse molecular gas, where non-equilibrium chemistry is needed for correctly predicting CH+ and CO abundances.
Abel Nicholas Paul
Federman Steven Robert
Gredel Roland
Lambert David L.
Rogers Mathew
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