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Session 32 - General ISM.
Oral session, Wednesday, January 07
Georgetown,
We have observed emission in the three lowest rotational transitions of the optically thin species C^18O and the dust continuum emission at three millimeter/submillimeter wavelengths. By employing the proper combination of the intensities of the three lowest rotational transitions, we can obtain the total molecular column density, with relatively little sensitivity to density and temperature variations along the line of sight. We use the line and continuum data to determine column densities of the dust and gas across three Giant Molecular Cloud cores. We find that two of the three sources, M17 and Cepheus A, have the same gas column density to dust opticaldepth ratio, given by log[N(C^18O/\tau(790 \micron)] = 18.8. In the third source, the Orion Molecular Cloud, the gas to dust ratio is typically a factor of three lower. We have good evidence for the correlation of the continuum emission in different bands for the Orion Molecular Cloud, and find the frequency dependence of the optical depth in the densest regions near the embedded sources to be given by \tau \propto \nu^1.9. For positions away from the embedded sources there is a larger scatter in the data points, with a suggestion that the frequency-dependence is steeper, \tau \propto \nu^2.4. This may be an indication of a change in the grain properties between less dense and very dense regions, and is consistent with the results of grain growth. Using standard values for the fractional abundance of C^18O relative to H_2, the mean densities of the cloud cores are 3 - 5 \times 10^4 cm-3. These regions appear to be close to virial equilibrium. The dense gas (revealed by multiple transition studies of tracers such as CS and HC_3N to have n(H_2) \simeq 10^6 cm-3) has a volume filling factor of only a few percent.
