Minchin, Nigel R.; White, Glenn J. and Ward-Thompson, Derek
13CO and C18O observations of S140: delineation of the outflow structure, a study of fractionation effects and comparison with CI observations.
Astronomy and Astrophysics, 301
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The outflow and photon-dominated region (PDR) associated with the S140 complex have been observed at high resolution (~14 arcsec) in the 13CO and C18O J=3→2 lines. The C18O map confirms earlier C17O J=3→2line 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 C17O 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(13CO)/N(C18O) ratio has been plotted against extinction and fits the power law relation N(13CO)/N(C18O)=21Av-0.35. The highest values, as expected, occur for observed positions towards the PDR, with N(13CO)/N(C18O) exceeding the terrestrial value (5.5) for Av ≤40 magnitudes. In the outermost parts of the cloud (Av ≤10 magnitudes) the N(13CO)/N(C18O) ratio is largest, up to 20. The increased fractionation may be due to higher photoionization of the optically thinner isotope, C18O. There is a close correlation between N(CI)/N(CO) and visual extinction over a wide extinction range (Av=3-100 mags.). The best fit power law is N(CI)/N(CO)=4.2Av0.9. For positions toward the outflow (Av~50-100) N(CI)/N(CO) ~0.1(0.07-0.12). N(CI)/N(CO) increases with decreasing extinction to ~1 for Av≤5 mags., corresponding to positions near the edge of the cloud. A detailed comparison of antenna temperatures and linewidths for the 13CO, C18O and CI lines is presented. The 13CO and C18O 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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