Numerical model of fast electron energy deposition in interstellar molecular gas

Abstract

The energy deposition of fast electrons in interstellar molecular gas is considered. We use the rotationally resolved cross sections for electron-impact excitation of H$_2$ molecule that were calculated using the adiabatic-nuclei molecular convergent close-coupling method. The initial electron energy distribution is assumed mono-energetic, and the differential equation for electron energy distribution is solved. We compare calculated energy deposition parameters with the results of similar studies in which the Monte Carlo approach was used. It is shown that about 11 per cent of the initial energy of fast electrons goes into direct ro-vibrational excitation of energy levels of H$_2$ molecule including pure rotational excitation in neutral molecular gas. About 7 per cent of initial electron energy goes into the excitation to $v=1$ vibrational state of H$_2$ molecule, most of this energy eventually converts into emission of transitions at near-infrared wavelengths. For ro-vibrational levels with $v \geq 3$, the electron-impact excitation to electronic states followed by downward radiative transitions to the ground electronic state is the dominant mechanism of excitation. The yields for excitation to vibrational states via radiative cascading from excited electronic states are found to be $1.5-2$ times higher than were obtained in previous studies.