AAS 201st Meeting, January, 2003
Session 112. The ISM: Clouds and Regions
Poster, Thursday, January 9, 2003, 9:20am-4:00pm, Exhibit Hall AB

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[112.11] A Neutral Hydrogen Self-Absorption Cloud in the SGPS

D. W. Kavars, J. M. Dickey (University of Minnesota), N. M. McClure-Griffiths (Australia Telescope National Facility), B. M. Gaensler (Harvard-Smithsonian Center for Astrophysics), A. J. Green (University of Sydney)

Using data from the Southern Galactic Plane Survey (SGPS) we analyze an HI self-absorption cloud centered on l = 318.0\circ, b = -0.5\circ, and velocity, v = -0.82 km s-1. The cloud was observed with the Australia Telescope Compact Array (ATCA) and the Parkes Radio Telescope, and is at a near kinematic distance of ~ 60 pc with derived dimensions of 0.7 \times 1.6 pc. We apply two different methods to find the optical depth and spin temperature. In both methods we find upper limit spin temperatures ranging from 20 K to 25 K and lower limit optical depths ~ 1. We look into the nature of the HI emission and find that 60-70% originates behind the cloud. We analyze a second cloud at the same velocity centered on l = 319\circ and b = 0.4\circ with an upper limit spin temperature of 20 K and a lower limit optical depth of 1.6. The similarities in spin temperature, optical depth, velocity, and spatial location are evidence the clouds are associated, possibly as one large cloud consisting of smaller clumps of gas. We compare HI absorption with 12CO emission and find a physical association of the HI self-absorption cloud with molecular gas. It is possible that this cloud is in a transition between a mostly atomic and a mostly molecular state. This particular cloud stands out in our initial search of HISA clouds in the SGPS. To develop a better understanding of the cold neutral medium (CNM), a search of the entire SGPS is currently underway to find similar HISA clouds.

The author(s) of this abstract have provided an email address for comments about the abstract: dkavars@astro.umn.edu

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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.