Excitation and charge transfer in H-H + collisions at 5-80keV and application to astrophysical shocks

Abstract

In astrophysical regimes where the collisional excitation of hydrogen atoms is relevant, the cross-sections for the interactions of hydrogen atoms with electrons and protons are necessary for calculating line profiles and intensities. In particular, at relative velocities exceeding ~1000kms -1, collisional excitation by protons dominates over that by electrons. Surprisingly, the H-H + cross-sections at these velocities do not exist for atomic levels of n≥ 4, forcing researchers to utilize extrapolation via inaccurate scaling laws. In this study, we present a faster and improved algorithm for computing cross-sections for the H-H + collisional system, including excitation and charge transfer to the n≥ 2 levels of the hydrogen atom. We develop a code named bdscx which directly solves the Schrödinger equation with variable (but non-adaptive) resolution and utilizes a hybrid spatial-Fourier grid. Our novel hybrid grid reduces the number of grid points needed from ~4000n 6 (for a 'brute force', Cartesian grid) to ~2000n 4 and speeds up the computation by a factor of ~50 for calculations going up to n= 4. We present (l, m)-resolved results for charge transfer and excitation final states for n= 2-4 and for projectile energies of 5-80keV, as well as fitting functions for the cross-sections. The ability to accurately compute H-H + cross-sections to n= 4 allows us to calculate the Balmer decrement, the ratio of Hα to Hβ line intensities. We find that the Balmer decrement starts to increase beyond its largely constant value of 2-3 below 10keV, reaching values of 4-5 at 5keV, thus complicating its use as a diagnostic of dust extinction when fast (~1000kms -1) shocks are impinging upon the ambient interstellar medium. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.