Thursday 16 September at 2:30pm

Russell Redman - HIA

"MSX Infrared-Dark Clouds Observed with the JCMT - Coldbeds of Star Formation"

ABSTRACT: "We have used the JCMT to observe a selection of MSX Infrared-Dark Clouds (IRDCs). (This is the same satellite featured in a seminar by Martin Cohen on 22 June.) Scan maps were made with SCUBA at 850/450 microns, supplemented with 2000 micron photometry to determine the spectral energy distributions. Heterodyne observations were made of CS and HCO+, both tracers of dense gas. We also hope to have polarimetry of several regions to investigate the role of magnetic fields in the structure of the clouds. "Many of the dust clouds revealed in the SCUBA images have filamentary structures. Sometimes the filaments appear flocculent, with short, partially aligned segments. Bright spots, often unresolved, appear along the filaments. In all cases but one, these bright sources do not appear in the IRAS or MSX point source catalogs. "Observations of CS 5-4, C^{34}S 5-4, and HCO^+ 3-2 were made of the brighter continuum sources dicovered in the SCUBA images. These almost always showed line profiles indicative of outflow/infall. Some of the sources show strong emission at CS (7-6) as well, revealing the presence of warm gas, especially in the outflows. Several sources exhibited strong blue/red asymmetries, usually characteristic of infall (Zhou et al. 1993). "The IRDCs consist of cold, dense dust and gas with temperatures less than 15 K, H_2 number densities >= 10^6 cm^{-3}, and H_2 column densities ranging up to 10^{23} cm^{-2}. From their kinematic distances, the filaments extend in some cases over 20 pc and contain 10^2-10^4 M_solar. These structures resemble the large-scale dust clouds mapped in Orion by Johnstone and Bally (1999), but are even larger. "It is tempting to speculate that the IRDCs may be the long-sought centers of massive star formation in the Galaxy. The filamentary structures with bright spots along them bring to mind early theoretical star formation models involving collapse along magnetic fields."