CO emission from fragmentary molecular clouds: a model applied to observations of M17 SW

We present observations of the J = 3–2 and J = 1–0 transitions of CO towards M17 SW. The results indicate that M17 SW is composed of separate fragments which are optically thick in 12CO but which do not fill the beam at velocities away from the line centre. We have developed a simple model for emission from an isothermal cloud composed of randomly moving fragments which are in LTE. By comparing model profiles with our observed profiles in 12CO and 13CO, we find that typical fragments in M17 SW have τ ≃ 2 in the 1–0 transition of 13CO. This result is combined with dynamical arguments to derive estimates of the characteristic size, mass, and column density of the fragments in M17 SW.