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Title:
OH Observations of HI Self-Absorption (HISA) Clouds
Authors:
Kavars, D. W.; Dickey, J. M.; Skillman, E. D.
Affiliation:
AA(University of Minnesota), AB(University of Tasmania), AC(University of Minnesota)
Publication:
American Astronomical Society Meeting 206, #54.03; Bulletin of the American Astronomical Society, Vol. 37, p.506
Publication Date:
05/2005
Origin:
AAS
Bibliographic Code:
2005AAS...206.5403K

Abstract

We present observations of all four transitions (1612, 1665, 1667, and 1720 MHz) of the OH molecule using the Green Bank Telescope with lines of sight towards HI self-absorption (HISA) clouds found in the VLA Galactic Plane Survey (VGPS). All lines of sight are near the galactic plane (b=0 deg.) and within the galactic longitude range of l=55-67 deg. We compare our OH spectra with the HISA line profiles and existing 12CO data. Recent studies show a number of HISA clouds associated with molecular gas, as probed by CO emission, but also a number of clouds that show no association with CO emission. It has been proposed that HISA clouds are in a molecular/atomic transition and the fraction of molecular gas depends on the transition stage of the cloud. Although CO is the most abundant tracer of molecular gas it is not the best tracer in HISA clouds where typical number densities are below the critical density for CO excitation. The OH molecule will emit at lower densities than that required for CO excitation and hence may show that HISA with no CO association may be associated with other molecular tracers. This will further our understanding of the role that HISA plays in the ISM. This work was supported by NSF grant AST-0307603 to the University of Minnesota.
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