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

We have theoretically modeled the gas heating associated with the photoelectric ejection of electrons from a size distribution of interstellar carbon grains which extends into the molecular domain. We have considered a wide range of physical conditions for the interstellar gas (1 < G0 < 10(5), with G0 being the intensity of the incident far-UV field 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 absorb about half of the available far-UV photons, they do not contribute appreciably to the gas heating. This strong dependence of gas heating on size results from the decrease in yield and from the increased grain 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 only 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 which point the dominant gas-dust heat exchange mechanism becomes the recombination of electrons with grains rather than photoelectric ejection. The calculated heating efficiency for neutral grains is in good agreement with that derived from observations of the diffuse interstellar clouds. Our results also agree well with the FIRAS observations on COBE. Finally, our photoelectric heating efficiency is compared to previous studies.