CS 5-4 observations of OMC1: evidence for external heating of the quiescent gas

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Abstract
We have observed the J = 5–4 and 2–1 transitions of 12C32S and 12C34S in the direction of OMC1 and the results provide strong support for a clumpy (or turbulent) model of the high-velocity doughnut source. By using the high-velocity emission as a quasi-continuum background point source we estimate the excitation temperature of the ridge gas as 15–35 K, less than the kinetic temperature of 70–100 K implied by CO observations but in good agreement with other determinations. We suggest that the ridge clouds are heated on the near side by radiation from M42 and the Trapezium stars, creating a temperature gradient in the cloud. CO emission then arises in a thin skin on the hot face of the cloud. This explains the lack of any obvious self-reversal in the CO lines, and resolves the anomalies in previous determinations of Tex in the ridge gas. The kinetic temperature of the ridge gas, and hence the Jeans mass and cooling function, are therefore similar to those in other warm molecular clouds. A simple model of this system is described which qualitatively reproduces many of the features observed.

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