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Title:
Molecular and atomic fractionation effects in the NGC 1977 molecular cloud.
Authors:
Minchin, N. R.; White, G. J.
Publication:
Astronomy and Astrophysics, v.302, p.L25 (A&A Homepage)
Publication Date:
10/1995
Origin:
CDS; KNUDSEN
Astronomy Keywords:
STARS EARLY-TYPE, INTERSTELLAR MEDIUM: HII REGIONS, JETS AND OUTFLOWS
Bibliographic Code:
1995A&A...302L..25M

Abstract

The photon-dominated region (PDR) associated with the NGC 1977 molecular cloud has been observed at high resolution in the ^13^CO and C^18^O J=2-1 and the [CI] ^3^P_1_-^3^P_0_ lines. The N(^13^CO)/N(C^18^O) ratio has been plotted against visual extinction (Anu_) and fits a power law relation. The highest values, as expected, occur for observed positions with the lowest derived extinction, with N(^13^CO)/N(C^18^O) exceeding the terrestrial value (5.5) for Anu_<=60 magnitudes. In the outermost parts of the cloud (Anu_<=20 magnitudes) the N(^13^CO)/N(C^18^O) ratio is largest, up to 20. Comparison with similar observations of the S140 and Orion Bright Bar regions implies that higher incident UV field leads to increased fractionation effects. The N(CI)/N(CO) ratio has been plotted against visual extinction, as derived from N(C^18^O), for the range Anu_=20-100 magnitudes and also fits a power law relation. N(CI)/N(CO) increases with decreasing extinction from ~0.03-0.04 for Anu_~100 mags. to >=0.1 for Anu_~20 mags., corresponding to positions near the edge of the HII region/molecular cloud interface. Comparison with identical observations of the S140 and Orion Bright Bar regions implies that, unlike N(^13^CO)/N(C^18^O), the behaviour of N(CI)/N(CO) is insensitive to incident UV field over this range of extinction.

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