ATOMIC AND MOLECULAR CARBON AS A TRACER OF TRANSLUCENT CLOUDS

Journal Article · Fri Jan 01 04:00:00 EST 2010 · Astrophysical Journal

DOI:https://doi.org/10.1088/0004-637X/708/1/334· OSTI ID:21392489

Burgh, Eric B; France, Kevin ^[1]; Jenkins, Edward B ^[2]

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1. Center for Astrophysics and Space Astronomy, University of Colorado-Boulder, 593 UCB, Boulder, CO 80309 (United States)
2. Princeton University Observatory, Peyton Hall, Ivy Lane, Princeton, NJ 08544 (United States)

Using archival, high-resolution far-ultraviolet Hubble Space Telescope/Space Telescope Imaging Spectrograph spectra of 34 Galactic O and B stars, we measure C I column densities and compare them with measurements from the literature of CO and H{sub 2} with regard to understanding the presence of translucent clouds along the line of sight. We find that the CO/H{sub 2} and CO/C I ratios provide good discriminators for the presence of translucent material, and both increase as a function of molecular fraction, f{sup N} = 2N(H{sub 2})/N(H). We suggest that sightlines with values below CO/H{sub 2} approx10{sup -6} and CO/C I approx1 contain mostly diffuse molecular clouds, while those with values above sample clouds in the transition region between diffuse and dark. These discriminating values are also consistent with the change in slope of the CO versus H{sub 2} correlation near the column density at which CO shielding becomes important, as evidenced by the change in photochemistry regime studied by Sheffer et al. Based on the lack of correlation of the presence of translucent material with traditional measures of extinction, we recommend defining 'translucent clouds' based on the molecular content rather than line-of-sight extinction properties.

• OSTI ID: 21392489
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 708; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
ABUNDANCE
CARBON
CARBON COMPOUNDS
CARBON MONOXIDE
CARBON OXIDES
CHALCOGENIDES
CHEMISTRY
DISCRIMINATORS
ELECTROMAGNETIC RADIATION
ELECTRONIC CIRCUITS
ELEMENTS
FAR ULTRAVIOLET RADIATION
HYDROGEN
NONMETALS
OXIDES
OXYGEN COMPOUNDS
PHOTOCHEMISTRY
RADIATIONS
SPECTRA
STARS
TELESCOPES
ULTRAVIOLET RADIATION