An interstellar thermostat: gas temperature regulated by grain charge

Abstract

The temperature of interstellar gas at low optical depths, is determined by the balance of heating by photoejected electrons from grains and cooling by fine structure transitions of ionized Carbon. The heating rate is dependant on the UV radiation and the grain charge. The grain charge is determined by the intensity of the UV radiation, and the increase in grain charge with increasing UV intensity, acts to reduce the heating rate, and regulate the gas temperature with respect to increases in the UV intensity. This thermostat is novel compared to others in the interstellar medium, because it does not depend upon the cooling rate of the gas.