Interstellar C2 absorption lines towards CH+ forming regions

Interstellar C2 absorption line observations towards the southern OB associations NGC 2439, Vela OB1, and Cen OB1 are used to infer gaskinetic temperatures Tkin and densities nc towards lines of sight with previously determined large CH and CH+ column densities. Towards NGC 2439, the material is characterised by temperatures of Tkin = 75–85 K and densities exceeding nc > 1000 cm−3, and a fractional C2 abundance of about x(C2)= 1.510−8. Temperatures and densities inferred towards two stars in Vela OB1 are Tkin = 65–85 K and nc ≥ 600 cm−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 Tkin = 70– 95 K and fractional abundances of x(C2)≈ 5 10−9 are found, and towards HD 114213, Tkin = 25 K and nc = 400±100 cm−3. The observations demonstrate that C2 resides in cool gas at temperature of Tkin < 100 K. 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 100 K temperature.