ENERGY DEPOSITION AND PHOTOELECTRIC-EMISSION FROM THE INTERACTION OF 10 EV TO 1 MEV PHOTONS WITH INTERSTELLAR DUST PARTICLES

This paper presents detailed calculations of the energy deposited by e nergetic photons in spherical, uncharged, interstellar dust particles. The interaction of the photons in the solid creates fast electrons fr om photoionizations, Auger transitions, or Compton scattering, which d eposit a fraction of their energy in the dust. Fluorescent transitions following a K-shell photoionization in iron also contribute to the he ating. The efficiency of the dust heating depends on the initial photo n energy and on the grain size and composition. Calculations are perfo rmed for carbon and silicate particles of radius 50 Angstrom to 1 mu m , irradiated by photons with energies between 10 eV, which is about eq ual to the threshold for the ejection of photoelectrons, and 1 MeV, be yond which pair production dominates the photon interaction in the sol id. Our studies present a consistent treatment of the partitioning of the energy of photons that interact in the dust into an absorbed fract ion and a fraction that is carried away by ejected electrons. The resu lts are presented in tables listing the energy deposited in a dust par ticle, E(dep), as a function of incident photon energy, E(gamma), and plots depicting the energy carried away by the ejected electrons as a function of E(gamma). The results of this work are useful for calculat ing dust temperature fluctuations and equilibrium dust temperatures in astrophysical environments in which the dust is exposed to hard ultra violet and X-ray emission and for calculating the photoelectric heatin g of clouds exposed to similarly hard radiation fields.