Abstract
view Abstract Citations (127) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Detailed observations of the M17 molecular cloud complex. Lada, C. J. Abstract Detailed molecular line observations of CO, CS, H2CO, NH5, and some of their isotopes toward the giant molecular cloud complex near M17 are presented and discussed. Extensive 12CO and 13CO observations are made of two separate clouds characterized by velocities of 20 km s ' and 23 km 1, respectively. The cloud at 20 km 1 is found to be approximately 15' x 30' (10 x 20 pc) in extent and to contain a mass in excess of 3 x 10 M0. Within the 20 km 1 cloud are three bright spots of 12CO emission, two of which are mapped in CS and H2CO emission and shown to be localized high-density fragments ( > 1 0 cm-3) a few parsecs in size. Subsequent identification of infrared sources and H2O maser sources within their boundaries is interpreted as an indication that star formation has recently taken place within those two fragments. Observations of millimeter-wave CO, CS, HCN, H2CO, and SO and centimeter-wave NH emission and H2CO absorption are combined with model cloud calculations in a self-consistent manner to determine the physical and chemical conditions in M17 SW, the largest of the CO bright spots. Integrated intensity ratios of 13CO to C18O are found to be significantly greater than the terrestrial value, suggesting an overabundance of 13CO relative to C18O. Line widths of 12CO and 13CO are found to increase near M17 SW. Careful examination and comparison of 12CO and 13CO line intensities across M17 and within individual profiles show that the entire cloud cannot be in uniform spherical collapse of any form. Collapse, however, could be in progress in a small region around M17 SW, but lack of sufficient angular resolution prohibits direct observational tests of this possibility. It is concluded that line widths of 3 km s ' must be maintained by some mechanism other than systematic collapse or expansion motions. Comparison of the molecular line observations with optical, infrared, and radio continuum data is made. Consideration of all these observations suggests that the 20 km s ' molecular cloud, the M 17 H ii region, and the star cluster NGC 6618 are related in a geometrical evolutionary sequence. Cloud evolution (dissipation) seems to have proceeded from the outer edge of a once more extensive proto-molecular cloud inward to the location of the high-density fragments in the present molecular cloud. The 23 km 1 cloud is found to be about 9' x 15' in extent and to have a 12CO emission peak coincident with the bright northern component of the M 17 H ii region. The mass of this cloud is crudely estimated to be 2 x 10 M0. Subject headings: interstellar: molecules - nebulae: abundances - nebulae: individual - radio sources: general - star clusters - star formation 603 Publication: The Astrophysical Journal Supplement Series Pub Date: October 1976 DOI: 10.1086/190409 Bibcode: 1976ApJS...32..603L full text sources ADS | data products SIMBAD (2)
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