OH Survey along Sightlines of Galactic Observations of Terahertz C+

Journal Article · Mon Apr 10 04:00:00 EDT 2017 · Astrophysical Journal

DOI:https://doi.org/10.3847/1538-4357/AA67E9· OSTI ID:22872814

Tang, Ningyu; Li, Di; Yue, Nannan; Krčo, Marko; Wang, Shen; Zuo, Pei; Wang, Jun-Jie ^[1]; Heiles, Carl ^[2]; Dawson, J. R. ^[3]; Goldsmith, Paul F. ^[4]; McClure-Griffiths, N. M.

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1. National Astronomical Observatories, CAS, Beijing 100012 (China)
2. Department of Astronomy, University of California, Berkeley, 601 Campbell Hall 3411, Berkeley, CA 94720-3411 (United States)
3. Department of Physics and Astronomy and MQ Research Centre in Astronomy, Astrophysics and Astrophotonics, Macquarie University, NSW 2109 (Australia)
4. Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109 (United States)

We have obtained OH spectra of four transitions in the {sup 2}Π{sub 3/2} ground state, at 1612, 1665, 1667, and 1720 MHz, toward 51 sightlines that were observed in the Herschel project Galactic Observations of Terahertz C+. The observations cover the longitude range of (32°, 64°) and (189°, 207°) in the northern Galactic plane. All of the diffuse OH emissions conform to the so-called “Sum Rule” of the four brightness temperatures, indicating optically thin emission conditions for OH from diffuse clouds in the Galactic plane. The column densities of the H i “halos” N(H i) surrounding molecular clouds increase monotonically with OH column density, N(OH), until saturating when N(H I)=1.0×10{sup 21} cm{sup −2} and N(OH)⩾4.5×10{sup 15} cm{sup −2}, indicating the presence of molecular gas that cannot be traced by H i. Such a linear correlation, albeit weak, is suggestive of H I halos’ contribution to the UV shielding required for molecular formation. About 18% of OH clouds have no associated CO emission (CO-dark) at a sensitivity of 0.07 K, but are associated with C{sup +} emission. A weak correlation exists between C{sup +} intensity and OH column density for CO-dark molecular clouds. These results imply that OH seems to be a better tracer of molecular gas than CO in diffuse molecular regions.

• OSTI ID: 22872814
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 839; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
BRIGHTNESS
CARBON IONS
CARBON MONOXIDE
CORRELATIONS
COSMIC GASES
DENSITY
EMISSION
GROUND STATES
HYDROXIDES
MOLECULES
SENSITIVITY
SPECTRA
SUM RULES