High-resolution HCO+ and HCN observations of M17SW: Clumps, turbulence and cloud support

Abstract

The M17SW molecular cloud core has been mapped with a linear resolution of 0.2 pc in the J =3 → 2 lines of HCO+ and HCN, using the JCMT. The HCO+ and HCN maps are very similar in large-scale structure, with both molecules tracing the clumpy nature of the cloud core, but a detailed comparison reveals an anticorrelation of these clumps indicative of the different excitation of the two molecules. Analysis of the intensity distributions, using power spectrum and autocorrelation function techniques, indicates the presence of structure in both lines, on all scales down to the resolution limit of the telescope. The ACF of the velocity centroid fluctuations closely reproduces the ACF of turbulent gas motions in the cloud and reveals a correlation length ≤0.25 pc. The turbulent pressure and maximum energy dissipation rate are derived as functions of scalesize, and the relationship is markedly different from that assumed in standard turbulent cascade models. A Gaussian decomposition of the HCO+ and HCN emission identifies 32 and 19 clumps respectively. After deconvolving the beam, they range in size from 0.1 to about 0.5 pc, with characteristic linewidths of 2-4 kms-1, and are consistent with the results obtained from the Fourier analysis. Clump masses range from a few to a few thousand solar masses and, for both data sets, are consistent with a power-law mass spectrum of the form dN/dM ∞ M-19. The HCN clumps are marginally smaller and less massive than the HCO+ clumps, as expected for the different excitation parameters of these species. The clumps have a volume filling factor of about 0.3, molecular hydrogen densities between 1011-1012.5 molecule m-3, with a clump-interclump contrast of at least 10, and are in approximate virial equilibrium. Larson's empirical relation between mass and size is observed, but may be severely affected by observational selection effects. There appears to be no correlation between size and linewidth.