High-resolution observations of interstellar CH and CH+ towards the Scorpius OB1 association

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Abstract
High-resolution observations (⁠|$\lambda/\Delta\lambda = 10^5$|⁠) of interstellar CH and CH+ towards nine stars in the Sco OB1 association are presented. Several velocity components were observed towards most stars and these are interpreted to arise in foreground diffuse molecular clouds. Three distinct types of absorption component were found: components present in both CH and CH+, components present in CH+ only, and components present in CH only. Possible origins for these three classes of component are discussed. Although shock models are generally thought to be necessary for the production of CH+ in interstellar clouds, it was found that, for those components observed in both species, the very small velocity differences observed between them (⁠|$\lesssim 0.9\enspace \text {km s}^{-1}$|⁠) correspond to shock velocities that are too small for significant CH+ production. However, on the basis of published shock models, it is suggested that the components observed in CH+ alone may arise in shocked low-density clouds with only a small fractional abundance of molecular hydrogen.

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