THE PHOTOELECTRIC HEATING MECHANISM FOR VERY SMALL GRAPHITIC GRAINS AND POLYCYCLIC AROMATIC-HYDROCARBONS

We have theoretically modeled the gas heating associated with the phot oelectric ejection of electrons from a size distribution of interstell ar carbon grains which extends into the molecular domain. We have cons idered a wide range of physical conditions for the interstellar gas (1 < G0 < 10(5), with G0 being the intensity of the incident far-UV fiel d in units of the Habing interstellar radiation field; 2.5 x 10(-3) < n(e) < 75 cm-3, with n(e) being the electron density; 10 < T < 10,000 K, with T being the gas temperature). The results show that about half of the heating is due to grains less than 1500 C atoms (< 15 angstrom ). The other half originates in somewhat larger grains (1500-4.5 x 10( 5) C atoms; 15 < a < 100 angstrom). While grains larger than this do a bsorb about half of the available far-UV photons, they do not contribu te appreciably to the gas heating. This strong dependence of gas heati ng on size results from the decrease in yield and from the increased g rain charge (hence larger Coulomb losses) with increasing grain size. We have determined the net photoelectric heating rate and evaluated a simple analytical expression for the heating efficiency, dependent onl y on G0, T, and n(e). This expression is accurate to 3% over the whole parameter range and is valid up to gas temperatures of 10(4) K, at wh ich point the dominant gas-dust heat exchange mechanism becomes the re combination of electrons with grains rather than photoelectric ejectio n. The calculated heating efficiency for neutral grains is in good agr eement with that derived from observations of the diffuse interstellar clouds. Our results also agree well with the FIRAS observations on CO BE. Finally, our photoelectric heating efficiency is compared to previ ous studies.