TY - JOUR
T1 - OH Survey along Sightlines of Galactic Observations of Terahertz C+
AU - Tang, Ningyu
AU - Li, Di
AU - Heiles, Carl
AU - Yue, Nannan
AU - Dawson, J. R.
AU - Goldsmith, Paul F.
AU - Krčo, Marko
AU - McClure-Griffiths, N. M.
AU - Wang, Shen
AU - Zuo, Pei
AU - Pineda, Jorge L.
AU - Wang, Jun Jie
N1 - Publisher Copyright:
© 2017. The American Astronomical Society. All rights reserved..
PY - 2017/4/10
Y1 - 2017/4/10
N2 - We have obtained OH spectra of four transitions in the 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 cm-2 and cm-2, indicating the presence of molecular gas that cannot be traced by H i. Such a linear correlation, albeit weak, is suggestive of 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+ emission. A weak correlation exists between C+ 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.
AB - We have obtained OH spectra of four transitions in the 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 cm-2 and cm-2, indicating the presence of molecular gas that cannot be traced by H i. Such a linear correlation, albeit weak, is suggestive of 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+ emission. A weak correlation exists between C+ 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.
KW - ISM: clouds
KW - ISM: molecules
KW - evolution
UR - http://www.scopus.com/inward/record.url?scp=85018466227&partnerID=8YFLogxK
U2 - 10.3847/1538-4357/aa67e9
DO - 10.3847/1538-4357/aa67e9
M3 - Article
SN - 0004-637X
VL - 839
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 1
M1 - 8
ER -