Photoelectric heating and [C II] cooling of high galactic latitude translucent clouds

The (P-2(3/2) --> P-2(1/2)) transition of singly ionized carbon, [C II], is the primary coolant of diffuse interstellar gas. We describe observations of [C II] emission toward nine high Galactic latitude translucent molecular clouds, made with the long-wavelength spectrometer on board the Infrared Space Observatory. To understand the role of dust grains in processing the interstellar radiation field (ISRF) and heating the gas, we compare the [C II] integrated intensity with the far-infrared (far-IR) integrated surface brightness for the 101 sampled lines of sight. We find that [C II] is linearly correlated with far-IR, and the average ratio is equal to that measured with the COBE satellite for all high-latitude Milky Way gas. There is a significant decrease that was not detected with COBE in [C II] emissivity at high values of far-IR. Our sample splits naturally into two populations depending on the 60 mum/100 mum surface brightness ratio, or color: warm positions with 60/100 > 0.16 and cold positions with 60/100 < 0.16. A transition from sources with warm to those with cold 60/100 colors coincides approximately with the transition from constant to decreasing [C II] emissivity. We model the [C II] and far-IR emission under conditions of thermal equilibrium, using the simplifying assumptions that, in all regions heated by the ISRF, the most important source of gas heating is the photoelectric effect on grains and the most important source of gas cooling is [C II] emission. The model matches the data well, provided the ISRF incident flux is χ(0) ≈ 1.6 ( in units of the nominal value near the Sun), and the photoelectric heating efficiency is ε ≈ 4.3%. There are no statistically significant differences in the derived values of χ(0) and ε for warm and cold sources. The observed variations in the [C II] emissivity and the 60/100 colors can be understood entirely in terms of the attenuation and softening of the ISRF by translucent clouds, not changes in dust properties.