Resonance scattering of Lyman alpha from interstellar hydrogen

Abstract

We derive a set of new line-of-sight (LOS) integrals and phase functions for the resonance scattering of unpolarized Lyman alpha light in the optically thin medium limit. To explicitly show the approximations done, and to indicate the range of validity of previously published formulas, we start by treating the general theory of resonance scattering. For broad band Lyalpha and a hot gas the resulting LOS integral can be evaluated analytically. This is the case for Lyalpha light from the sun scattering off neutral interstellar hydrogen. The ground state populations of the hydrogen atom will effect the value of the scattered intensity. Two population distributions are considered: a thermal equilibrium population and a population established by the atom-light interaction.