1999A&A...351..657G

Interstellar C₂ absorption line observations towards the southern OB associations NGC 2439, Vela OB1, and Cen OB1 are used to infer gaskinetic temperatures T_kin and densities n_c towards lines of sight with previously determined large CH and CH⁺ column densities. Towards NGC 2439, the material is characterised by temperatures of T_kin=75-85K and densities exceeding n_c>1000cm^–3, and a fractional C₂ abundance of about x(C₂)=1.5x10^–8. Temperatures and densities inferred towards two stars in Vela OB1 are T_kin=65-85K and n_c≥600cm^–3. C₂ fractional abundances in the cloud complex obscuring Vela OB1 are about half the value found in NGC 2439. For the lines of sight towards Cen OB1, values of T_kin=70-95K and fractional abundances of x(C₂)≃5x10^–9 are found, and towards HD 114213, T_kin=25K and n_c=400±100cm^–3. The observations demonstrate that C₂ resides in cool gas at temperature of T_kin<100K. The C₂ column densities are correlated with those of CH, which is expected from theoretical models. This suggests that CH is efficiently formed in the cool material as well, together with C₂. This finding does not support expectations from recent MHD and vortex models of CH⁺ formation, which stipulate that neutrals such as CH are abundantly produced in hot regions of many 100K temperature.