Structure of Dense Cores in M17 SW. I. A Multitransition CS and C 34S Study

Abstract

view Abstract Citations (47) References (51) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Structure of Dense Cores in M17 SW. I. A Multitransition CS and C 34S Study Wang, Yangsheng ; Jaffe, Daniel T. ; Evans, Neal J., II ; Hayashi, Masahiko ; Tatematsu, Ken'ichi ; Zhou, Shudong Abstract We present results of a multitransition CS and C34S study of the M17 SW molecular cloud core. Fully sampled maps were obtained in the CS J = 1 → 0, 2 → 1, and 7 → 6 lines and the C34S J = 2 → 1 line with 1 8"-36" resolution. Velocity channel maps reveal the clumpy emission from the dense gas on scales of ∼0.2 pc (20"). The overall agreement in the cloud morphology among maps of different CS and C34S transitions suggests that all CS and C34S lines originate in the same dense gas. Excitation and opacity effects probably cause the modest differences between the maps. We carried out a detailed excitation analysis of the multitransition data. The J = 2 → 1 and J = 7 → 6 transitions of CS, analyzed with a large velocity gradient (LVG) radiative transfer model, produced 250 pixel maps of the volume density and the CS column density over an area of about 1.8 pc × 2.4 pc. Peaks in the CS and the C34S line temperature maps are maxima in column density, but not in density. The density maps shows a fairly uniform, high density (n ∼105.7 cm-3) throughout the cloud core. An independent estimate of the gas densities from analysis of the C34S observations confirms the CS results. Along with other evidence, these results imply a clumpy cloud model in which the CS emission arises from structures smaller than our beam. We compared the observed CS maps with a specific clumpy cloud model with 179 clumps decomposed from the C18O J = 2 → 1 maps (Stutzki & Güsten 1990). Model channel maps of CS were synthesized based on the clump parameters listed in Stutzki & Güsten (1990) and were compared with the observed maps. The gas densities used in the models were derived from the clump column densities (based on the C18O J = 2 → 1 emission) and sizes. Most of the dominant clumps had densities near 105 cm-3. The resulting synthesized map does not reproduce the observed CS J = 7 → 6 emission along the eastern ridge of the core. By assuming a constant gas density for all clumps, we were able to synthesize CS channel maps which reproduce the observed cloud morphology and the line intensities reasonably well. A mean clump density in the models of about 5 × 105 cm-3 (about 5 times higher than the density derived from C18O) matches the observed CS line ratios and a CS/H2 abundance ratio of about 4 × 10-9 fits the observed line intensities of the J = 1 → 0, 2 → 1, and 7 → 6 transitions of CS. The discrepancy between densities derived from C18O and CS can be resolved if the clumps have internal density structure. Either smooth density gradients in clumps with sizes just below our angular resolution or a continuation of high-contrast clumping to still smaller scales could account for the difference. While we cannot rule out either of these pictures, it is noteworthy that the scale of the C18O observations (∼0.15 pc) is the largest for which the density discrepancy can be resolved with smooth density gradients in unresolved clumps. Publication: The Astrophysical Journal Pub Date: December 1993 DOI: 10.1086/173521 Bibcode: 1993ApJ...419..707W Keywords: ISM: H II REGIONS; ISM: INDIVIDUAL MESSIER NUMBER: M17; ISM: MOLECULES; ISM: STRUCTURE; RADIO LINES: ISM full text sources ADS | data products SIMBAD (2)