High-resolution Formula observations of M17SW - III. Analysis of density and velocity structure

Abstract

In an earlier paper, we presented an analysis of the small-scale structure of the M17SW molecular cloud based on observations of the optically thick|${\rm HCO^+}$| and |${\rm HCN} \ J=3\rightarrow2$| transitions. In this paper we present observations of the |${\rm C^{17}O} \ J=3\rightarrow2$| transition. This transition is optically thin, is observed with a slightly smaller beam, and suffers less from confusion by cool gas; as a result it should be a better tracer of the kinematics of the warm molecular gas in the region. We analyse the structure using the same techniques as in Paper I, and find that our earlier results are confirmed by these new data. In particular, we identify 35 individual clumps, with masses ranging from |$10-1000 \ {\rm M}_{\odot}$|⁠. The clump mass spectrum has a slope of |$-1.9\pm0.2$|⁠, which is identical to our earlier value. Autocorrelation and structure function analysis of the velocity centroid map confirms the presence of small-scale, nearly isotropic motion with a characteristic scale of |$\approx 0.2 \ {\rm pc}$|⁠; the variation with scale of the turbulent pressure and energy dissipation rate are similar to those derived for the |${\rm HCO}^+$| and HCN data. The Gaussian-clump decomposition is broadly similar to that for |${\rm C^{18}O} \ J=2\rightarrow1$| by Stutzki & Güsten, within the area common to both maps, but differs greatly from that for |${\rm HCO}^+$| and HCN. Thus, although clump fitting is a useful formal description of structure, it is only an approximate description of real (non-Gaussian) molecular cloud components.