2018ApJ...856...96G

We have combined emission from the 158 µm fine structure transition of C⁺ observed with the GREAT and upGREAT instruments on SOFIA with 21 cm absorption spectra and visual extinction to characterize the diffuse interstellar clouds found along the lines of sight. The weak [C II] emission is consistent in velocity and line width with the strongest H I component produced by the cold neutral medium. The H I column density and kinetic temperature are known from the 21 cm data and, assuming a fractional abundance of ionized carbon, we calculate the volume density and thermal pressure of each source, which vary considerably, with 27 cm^–3≤ n(H⁰)≤ 210 cm^–3 considering only the atomic hydrogen along the lines of sight to be responsible for the C⁺, while 13 cm^–3≤ n(H⁰+H₂)≤ 190 cm^–3 including the hydrogen in both forms. The thermal pressure varies widely with 1970 cm^–3 K ≤ P_th/k≤ 10,440 cm^–3 K for H⁰ alone and 750 cm^–3 K <= P_th/k <= 9360 cm^–3 K including both H⁰ and H₂. The molecular hydrogen fraction varies between 0.10 and 0.67. Photoelectric heating is the dominant heating source, supplemented by a moderately enhanced cosmic ray ionization rate, constrained by the relatively low 45 K to 73 K gas temperatures of the clouds. The resulting thermal balance for the two lower-density clouds is satisfactory, but for the two higher-density clouds, the combined heating rate is insufficient to balance the observed C⁺ cooling.