Ultraviolet Survey Of CO And H-2 In Diffuse Molecular Clouds: The Reflection Of Two Photochemistry Regimes In Abundance Relationships

dc.contributor.utaustinauthorFederman, S. R.en_US
dc.creatorSheffer, Y.en_US
dc.creatorRogers, M.en_US
dc.creatorFederman, S. R.en_US
dc.creatorAbel, N. P.en_US
dc.creatorGredel, R.en_US
dc.creatorLambert, David L.en_US
dc.creatorShaw, G.en_US
dc.date.accessioned2016-04-28T19:34:14Z
dc.date.available2016-04-28T19:34:14Z
dc.date.issued2008-11en
dc.description.abstractWe carried out a comprehensive far-UV survey of (CO)-C-12 and H-2 column densities along diffuse molecular Galactic sight lines. This sample includes new measurements of CO from HST spectra along 62 sight lines and new measurements of H-2 from FUSE data along 58 sight lines. In addition, high-resolution optical data were obtained at the McDonald and European Southern Observatories, yielding new abundances for CH, CH+, and CN along 42 sight lines to aid in interpreting the CO results. These new sight lines were selected according to detectable amounts of CO in their spectra and provide information on both lower density (<= 100 cm(-3)) and higher density diffuse clouds. A plot of log N(CO) versus log N(H-2) shows that two power-law relationships are needed for a good fit of the entire sample, with a break located at log N(CO, cm(-2)) = 14.1 and log N(H-2) = 20.4, corresponding to a change in production route for CO in higher density gas. Similar logarithmic plots among all five diatomic molecules reveal additional examples of dual slopes in the cases of CO versus CH (break at log N 14: 1, 13.0), CH+ versus H-2 (13.1, 20.3), and CH+ versus CO (13.2, 14.1). We employ both analytical and numerical chemical schemes in order to derive details of the molecular environments. In the denser gas, where C-2 and CN molecules also reside, reactions involving C+ and OH are the dominant factor leading to CO formation via equilibrium chemistry. In the low-density gas, where equilibrium chemistry studies have failed to reproduce the abundance of CH+, our numerical analysis shows that nonequilibrium chemistry must be employed for correctly predicting the abundances of both CH+ and CO.en_US
dc.description.departmentMcDonald Observatoryen_US
dc.description.sponsorshipNASA NNG04GD31Gen_US
dc.description.sponsorshipSTScI HST-AR-09921.01-Aen_US
dc.description.sponsorshipNational Science Foundation 0094050, 0607497en_US
dc.identifierdoi:10.15781/T25J7S
dc.identifier.Filename2008_11_ultravioletsurvey.pdfen_US
dc.identifier.citationSheffer, Y., M. Rogers, S. R. Federman, N. P. Abel, R. Gredel, D. L. Lambert, and G. Shaw. "Ultraviolet survey of CO and H2 in diffuse molecular clouds: the reflection of two photochemistry regimes in abundance relationships." The Astrophysical Journal, Vol. 687, No. 2 (Nov., 2008): 1075.en_US
dc.identifier.doi10.1086/591484en_US
dc.identifier.issn0004-637Xen_US
dc.identifier.urihttp://hdl.handle.net/2152/34771
dc.language.isoEnglishen_US
dc.relation.ispartofen_US
dc.relation.ispartofserialAstrophysical Journalen_US
dc.rightsAdministrative deposit of works to Texas ScholarWorks: This works author(s) is or was a University faculty member, student or staff member; this article is already available through open access or the publisher allows a PDF version of the article to be freely posted online. The library makes the deposit as a matter of fair use (for scholarly, educational, and research purposes), and to preserve the work and further secure public access to the works of the University.en_US
dc.rights.restrictionOpenen_US
dc.subjectastrochemistryen_US
dc.subjectism: abundancesen_US
dc.subjectism: moleculesen_US
dc.subjectultraviolet: ismen_US
dc.subjectstar-forming regionsen_US
dc.subjectgalactic plane surveyen_US
dc.subjectinter-stellar cloudsen_US
dc.subjecthigh-resolution observationsen_US
dc.subjectdense interstellar cloudsen_US
dc.subjectcarbon-monoxideen_US
dc.subjectphysical conditionsen_US
dc.subjectoscillator-strengthsen_US
dc.subjectextinctionen_US
dc.subjectcurvesen_US
dc.subjecttranslucent cloudsen_US
dc.subjectastronomy & astrophysicsen_US
dc.titleUltraviolet Survey Of CO And H-2 In Diffuse Molecular Clouds: The Reflection Of Two Photochemistry Regimes In Abundance Relationshipsen_US
dc.typeReviewen_US
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