^13^CO and C^18^O observations of S140: delineation. of the outflow structure, a study of fractionation effects and comparison with CI observations

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Title:		^13^CO and C^18^O observations of S140: delineation. of the outflow structure, a study of fractionation effects and comparison with CI observations
Authors:		Minchin, N. R.; White, G. J.; Ward-Thompson, D.
Publication:		Astronomy and Astrophysics, v.301, p.894 (A&A Homepage)
Publication Date:		09/1995
Origin:		CDS; KNUDSEN
Astronomy Keywords:		ISM: S 140, ISM: CLOUDS, ISM: ATOMS, ISM: MOLECULES, JETS AND OUTFLOWS, RADIO LINES: ISM
Bibliographic Code:		1995A&A...301..894M

Abstract

The outflow and photon-dominated region (PDR) associated with the S140 complex have been observed at high resolution (~14 arcsec) in the ^13^CO and C^18^O J=3->2 lines. The C^18^O map confirms earlier C^17^O J=3->2 line observations (Minchin et al. 1994) that show an `arc' of emission observed to the south of the peak, and also reveals a similar (and more prominent) arc feature to the east, a region not covered by the C^17^O map. This is a particularly fine example of the classic `tuning fork' morphology, where emission at the ambient cloud velocity is tracing the outflow cavity wall of the blueshifted lobe. The N(^13^CO)/N(C^18^O) ratio has been plotted against extinction and fits the power law relation N(^13^CO)/N(C^18^O)=21A_v_^-0.35^. The highest values, as expected, occur for observed positions towards the PDR, with N(^13^CO)/N(C^18^O) exceeding the terrestrial value (5.5) for A_v_=<40 magnitudes. In the outermost parts of the cloud (A_v_=<10 magnitudes) the N(^13^CO)/N(C^18^O) ratio is largest, up to 20. The increased fractionation may be due to higher photoionization of the optically thinner isotope, C^18^O. There is a close correlation between N(CI)/N(CO) and visual extinction over a wide extinction range (A_v_=3-100 mags.). The best fit power law is N(CI)/N(CO)=4.2A_v_^-0.9^. For positions toward the outflow (A_v_~50-100) N(CI)/N(CO) ~0.1(0.07-0.12). N(CI)/N(CO) increases with decreasing extinction to ~1 for A_v_=<5 mags., corresponding to positions near the edge of the cloud. A detailed comparison of antenna temperatures and linewidths for the ^13^CO, C^18^O and CI lines is presented. The ^13^CO and C^18^O antenna temperatures and linewidths are closely correlated, and imply the emission, for both isotopes, emanates from gas that is in LTE and is well mixed. The CI emission from the PDR implies that here the atomic carbon is in LTE, but occupies a different volume of gas than the isotopic CO. Towards the outflow the CI linewidths are systematically broadened relative to those for the isotopic CO lines. This is interpreted as evidence that atomic carbon is produced by the effect of shocks on the chemical and physical processes at the interface between a stellar wind and the outflow cavity wall.

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