Interstellar matter near the Pleiades. II. CH+ formation.

CH⁺ can form near stars in the Pleiades via the endothermic reaction of C⁺ with H₂ in photoelectrically heated gas flowing through the cluster. This conclusion follows from either of two models, one in which gas and dust remain well mixed, the other in which radiation pressure separates dust from gas in a layer well outside the H₂ dissociation front near each star. Other common features of the two models include the following. (1) The observed insensitivity of N(CH⁺) to spectral type constrains the effective wavelength for photoelectron emission from the dust. (2) Predicted CH⁺ line widths increase toward later spectral types. (3) The A0 V star HD 23512, which lies in or behind the molecular cloud, shows exceptional Ca depletion. The models differ in their ability to account for the bimodal distribution of N(CH⁺) with E(B - V). This seems to be a natural consequence of emergence of stars from the molecular gas if radiation pressure creates a dust layer, but not if gas and dust remain well mixed.