ENERGY DEPOSITION BY X-RAY PHOTOELECTRONS INTO INTERSTELLAR MOLECULAR CLOUDS

In certain astrophysical systems, high-energy photoelectrons will deposit their kinetic energy into molecular gas, exciting numerous molecular and atomic levels. We consider the deposition of energy by 100-10,000 eV electrons into molecular hydrogen and helium gas of cosmic abundances (n(He) = 0.2n(H-2)) and fractional ionization ranging from 10(-12) to 10(-1). In order to account for energy deposition by both discrete and continuous degradation processes, we introduce the loss spectrum, a form of the Spencer-Fano degradation spectrum particularly suited to describing electron energy deposition in a partially ionized gas. We include rotational and vibrational excitations of H-2 as well as electronic excitations and ionizations of H-2 and He0. The calculated excitation and ionization yields drop as the fractional ionization increases. We fitted the dependences of these yields on fractional ionization to a form suitable for swift numerical computation.