H++Hscattering and ambipolar diffusion heating

We report new and highly accurate quantum mechanical calculations of the astrophysically interesting scattering of H+ by H atoms. The effects of the quantum indistinguishability of the two protons are treated consistently throughout, including the calculation of the momentum transfer cross section where elastic scattering and charge transfer cannot be separated at low energies. We are able to resolve the numerous oscillations in the energy variation of the angle-integrated cross sections. With decreasing energy below 1 eV, the oscillations grow in amplitude until a smooth, approximately 1/nu(2) dependence on velocity is reached below 10(-5) eV. The 1/nu behavior, traditionally associated with a constant Langevin rate coefficient, is never realized down to the lowest energy calculated (10(-10) eV). We use the momentum transfer cross section to calculate accurately the transport rate coefficient that characterizes the drag force between ionic and neutral fluids and the strength of ambipolar diffusion heating for temperatures and ion-neutral drift speeds relevant for astrophysical applications. An early fit of this rate coefficient by Draine is sustained except at low energies. The application of these results to the role of ambipolar diffusion in heating jets from young stellar objects is discussed.