Dust heating Photon absorption versus electron collisions

Aims. We investigate and quantify the effects of the electron collisional heating of dust in a hot gas and compare this with photon heating by the interstellar radiation field. Methods. We compare the rate of energy absorption by dust due to electron collisional and photon heating as a function of the physical conditions of the gas and the ambient radiation field. We calculate the resulting dust spectral energy distributions (SEDs) for different environments. Results. We find that electron collisions and grain charging effects in a hot gas (10(6)-10(7) K) rapidly destroy small carbonaceous particles and result in a minimum particle size of the order of a few nm. The charging due to the emission of secondary electrons is important and leads to high electric potentials, which quickly destroy the small grains by field ion emission. In the case of weak interstellar radiation fields (G(0) similar to 0.1), electron collisional heating can be the dominant heating process and therefore makes an important contribution to the dust thermal emission. Conclusions. Collisions of electrons with dust grains, in a hot gas, lead to important changes in the dust SED, as a result of their high energy input. We find that grain charge effects and accompanying erosion need to be taken into account in the calculation of the dust SED. The power absorbed by the dust as a result of electron collisions in a hot tenuous gas can be larger than that due to photon absorption in the intergalactic medium close to a galaxy where the radiation field is weak (G(0) less than or similar to 0.1).