Interstellar CH+ in southern OB associations

Optical absorption line observations of interstellar CH+ andCHare presented towards the southernOBassociations CMa OB1,NGC 2439,Vela OB1,NGC 4755, andCen OB1.A total of 5–11 stars per associationwere observed, with visual extinctions ranging fromAV = 0.5–4.5 mag. The derived CH+ and CH velocities agree within the measurement errors. Towards a particular association, the CH+ column densityN (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 thatN (CH+) is correlated toN (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.