AAS 195th Meeting, January 2000
Session 84. Submillimeter Wave Astronomy Satellite
Display, Friday, January 14, 2000, 9:20am-6:30pm, Grand Hall

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[84.03] Implications of SWAS Observations for Water Abundance and Velocity Structure in Molecular Clouds

M.L.N. Ashby, E.A. Bergin, R. Plume (CfA), J.M. Carpenter (CalTech), G.J Melnick, J.R. Stauffer, S.C. Kleiner, B.M. Patten, V. Tolls, Z. Wang, Y.F. Zhang (CfA), P.F. Goldsmith (Cornell U., NAIC), M. Harwit (Cornell U.), N.R. Erickson, J.E. Howe, R.L. Snell (UMass, Amherst), D.A. Neufeld (JHU), D.G. Koch (NASA ARC), R. Schieder, G. Winnewisser (Univ. Köln), G. Chin (NASA GSFC)

Now a year into its mission, the Submillimeter Wave Astronomy Satellite (SWAS) has conducted observations of the 556.9~GHz 11,0arrow10,1 ground-state rotational transition of ortho-{\rm H}2\,16{\rm O} towards numerous molecular clouds and other sources. The fact that the upper state energy of this transition is only 27~K above the ground state, and that it has a relatively high critical density (~\times108 cm-3) makes the 556.9~GHz H2O line an effective tracer of the cold, dense gas in cloud cores. The H2O line profiles obtained by SWAS therefore provide an excellent opportunity to understand the velocity structure and H2O abundance in these environments.

We have developed a Monte Carlo radiative transfer model to help interpret SWAS line profiles and better understand the physical conditions in molecular clouds. The Monte Carlo technique accounts globally for the mechanisms of photon absorption and reemission, unlike simpler (but computationally faster) models like the LVG approximation. Such an approach is particularly useful for the SWAS 556.9~GHz line since the line optical depths are large. We compare model spectra to a variety of SWAS-measured H2O line profiles to show that, together with assumptions about the fractional abundance of H2O, variations in the ratio of large-scale bulk motions to small-scale turbulent motions can affect the spectra in ways that may account for the range of observed profiles. As an illustration of the power of this approach, we present a detailed study of the S140 star forming region.

The SWAS team gratefully acknowledges NASA contract NAS5-30702.

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