The abundance of atomic carbon near the ionization fronts in M17 and S140

Creators: Keene, Jocelyn; Blake, Geoffrey A.; Phillips, T. G.; Huggins, P. J.; Beichman, C. A.

Abstract

We have observed the 492 GHz ground-state line of atomic carbon in the edge-on ionization fronts in M17 and S140. We find that, contrary to expectation, the C_I emission peaks farther into the molecular cloud from the ionization front than does the CO. In fact the peak C_I abundance in M17 occurs more than 60 mag of visual extinction into the cloud from the ionization front. Calculations of the ratio of C_I to CO column densities yield values of 0.1-0.2. These observations do not support chemical models which predict that neutral atomic carbon should be found only near the edges of molecular clouds. Other models are discussed which may explain the observations.

Additional Information

© 1985 American Astronomical Society. Received 1984 December 10; accepted 1985 June 17. We are grateful to M. Frerking, C. Masson, and E. Sutton for help with the observations. We also thank R. E. Miller of AT&T Bell Labs for providing the SIS junctions used in the receivers of the OVRO 10 m telescopes for the CO observations. We acknowledge helpful discussions with N. Evans, M. Frerking, A. Glassgold, D. Hollenbach, W. Langer, and D. Williams. We thank the crew of the NASA Kuiper Airborne Observatory for their effective support. This work was supported by NASA grant NAG 2-1.

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