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

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Interstellar CH [FORMULA] 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


Optical absorption line observations of interstellar CH [FORMULA] 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 [FORMULA] = 0.5-4.5 mag. The derived CH [FORMULA] and CH velocities agree within the measurement errors. Towards a particular association, the CH [FORMULA] column density N (CH [FORMULA]) 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 [FORMULA]. It is found that N (CH [FORMULA]) is correlated to N (CH), which indicates that N (CH [FORMULA]) is correlated to the optical depth of a cloud. The correlations are difficult to reconcile with scenarios where the sites of CH [FORMULA] formation are constrained to the surface of molecular clouds. The observations support ideas which involve turbulence as a major CH [FORMULA] production mechanism. In particular, the results are in agreement with expectations from a scenario where the CH [FORMULA] formation proceeds in cool gas via a fraction of fast, non-Maxwellian H2 or C [FORMULA], 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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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998