SIMBAD references

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 A_V=0.5-4.5mag. 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.