Excitation and charge transfer in hydrogen-proton collisions at 5--80 keV 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 ~1000 km/s, collisional excitation by protons dominates over that by electrons. Surprisingly, the hydrogen-proton 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 hydrogen-proton 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 Schrodinger 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 ~4000 n^6 (for a "brute force", Cartesian grid) to ~2000 n^4 and speeds up the computation by a factor ~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--80 keV, as well as fitting functions for the cross sections. The ability to accurately compute proton-hydrogen cross sections to n = 4 allows us to calculate the Balmer decrement, the ratio of Balmer alpha to Balmer beta line intensities. We find that the Balmer decrement starts to increase beyond its largely constant value of 2--3 below 10 keV, reaching values of 4--5 at 5 keV, thus complicating its use as a diagnostic of dust extinction when fast (~1000$ km/s) shocks are impinging upon the ambient interstellar medium.

16 pages, 16 figures, Accepted by MNRAS

References in corpus (3)

• Broad-line Balmer Decrements in Blue Active Galactic Nuclei
• Balmer-Dominated Shocks Revisited
• Broad Lyman-Alpha Emission from Supernova Remnants in Young Galaxies

Cited by in corpus (9)

• Electron-Ion Temperature Equilibration in Collisionless Shocks: the Supernova Remnant-Solar Wind Connection
• Collisionless shock in a partially ionized medium. II. Balmer emission
• Cooling in a Dissipative Dark Sector
• Integral Field Spectroscopy of Balmer-Dominated Shocks in the Magellanic Cloud Supernova Remnant N103B
• Polarized Balmer Line Emission from Supernova Remnant Shock Waves Efficiently Accelerating Cosmic Rays
• Radiative transfer of hydrogen lines from supernova remnant shock waves: contributions of 2s-state hydrogen atoms
• Discovery of faint double-peak Halpha emission in the halo of low redshift galaxies
• Diagnosing Cosmic Ray Modified Shocks with H α Polarimetry
• Elastic and charge transfer cross sections for low to ultralow $\mathrm{H}(1s)+{\mathrm{H}}^{+}$ collisions. Quantal and semiclassical calculations