Interstellar C_2 absorption lines towards CH⁺ forming regions

Abstract

Interstellar C_2 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 = 75-85 K and densities exceeding n_c > 1000 C_2m, and a fractional C_2 abundance of about x(C_2) = 1.5 10^-8. Temperatures and densities inferred towards two stars in Vela OB1 are T_kin = 65-85 K and n_c >= 600 C_2m. C_2 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 = 70-95 K and fractional abundances of x(C_2) ~ 5 10^-9 are found, and towards HD 114213, T_kin = 25 K and n_c = 400+/-100 C_2m. The observations demonstrate that C_2 resides in cool gas at temperature of T_kin < 100 K. The C_2 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 C_2. 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.