C+ in the Interstellar Medium: Collisional Excitation by H2 Revisited
Résumé
C+ is a critical constituent of many regions of the interstellar medium, as it can be a major reservoir of carbon and, under a wide range of conditions, the dominant gas coolant. Emission from its 158 μm fine structure line is used to trace the structure of photon-dominated regions in the Milky Way and is often employed as a measure of the star formation rate in external galaxies. Under most conditions, the emission from the single [C II] line is proportional to the collisional excitation rate coefficient. We here used improved calculations of the deexcitation rate of [C II] by collisions with H2 to calculate more accurate expressions for interstellar C+ fine structure emission, its critical density, and its cooling rate. The collision rates in the new quantum calculation are ~25% larger than those previously available, and narrow the difference between rates for excitation by atomic and molecular hydrogen. This results in [C II] excitation being quasi-independent of the molecular fraction and thus dependent only on the total hydrogen particle density. A convenient expression for the cooling rate at temperatures between 20 K and 400 K, assuming an LTE H2 ortho to para ratio is Λ = (11.5 + 4.0 e^{-100 {K}/T^kin}) e^{-91.25 {K}/T^kin} n (C^{+}) n(H_2)× 10^{-24} erg cm^{-3} s^{-1}. The present work should allow more accurate and convenient analysis of the [C II] line emission and its cooling.