Astron. Astrophys. 320, 929-944 (1997)

Interstellar CH in southern OB associations

Roland Gredel

European Southern Observatory, Casilla 19001, Santiago 19, Chile (rgredel@eso.org)

Received 6 June 1996 / Accepted 1 October 1996

Abstract

Optical absorption line observations of interstellar CH and CH are presented towards the southern OB associations CMa OB1, NGC 2439, Vela OB1, NGC 4755, and Cen OB1. A total of 5-11 stars per association were observed, with visual extinctions ranging from = 0.5-4.5 mag. The derived CH and CH velocities agree within the measurement errors. Towards a particular association, the CH column density N (CH ) is correlated to the visual extinction of the background star. These findings weaken the possibility that magnetic shocks are the general mechanism that is required to produce interstellar CH . It is found that N (CH ) is correlated to N (CH), which indicates that N (CH ) is correlated to the optical depth of a cloud. The correlations are difficult to reconcile with scenarios where the sites of CH formation are constrained to the surface of molecular clouds. The observations support ideas which involve turbulence as a major CH production mechanism. In particular, the results are in agreement with expectations from a scenario where the CH formation proceeds in cool gas via a fraction of fast, non-Maxwellian H₂ or C , created by the dissipation of interstellar turbulence.

Key words: ISM: abundances ISM: clouds ISM: molecules open clusters and associations

Send offprint requests to: R. Gredel

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Contents

• 1. Introduction
• 2. Observations and reduction
• 3. Results
  • 3.1. CMa OB1
  • 3.2. The association surrounding NGC 2439
  • 3.3. Vela OB1
  • 3.4. NGC 4755 and Cen OB1
• 4. Discussion
  • 4.1. The CH - CH velocity shift and shock formation of CH
  • 4.2. Linear regressions in CH - , CH - , CH - CH
  • 4.3. Statistical significance of the correlations
  • 4.4. CH formation scenarios
    • 4.4.1. Photon-dominated regions
    • 4.4.2. Turbulent boundary layers
    • 4.4.3. Intermittent dissipation zones of interstellar turbulence
    • 4.4.4. Non-Maxwellian velocity distributions
• 5. Conclusions
• Acknowledgements
• References

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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