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

Tseliakhovich, Dmitriy and Hirata, Christopher M. and Heng, Kevin (2012) Excitation and charge transfer in H-H+ collisions at 5–80 keV and application to astrophysical shocks. California Institute of Technology , Pasadena, CA. (Submitted)

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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^(−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 ~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 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 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^(−1)) shocks are impinging upon the ambient interstellar medium.

Item Type:Report or Paper (Discussion Paper)
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Alternate Title:Excitation and charge transfer in hydrogen-proton collisions at 5--80 keV and application to astrophysical shocks
Additional Information:We are grateful to Avi Loeb for his help with getting the time on the Odyssey cluster supported by the FAS Science Division Research Computing Group at Harvard University. We are also grateful to Mark Scheel for helping to get the time on the SHC cluster at Caltech. D.T. and C.H. are supported by the U.S. Department of Energy (DE-FG03-92-ER40701) and the National Science Foundation (AST-0807337). C.H. is supported by the David and Lucile Packard Foundation. K.H. is supported by the Zwicky Prize Fellowship of the Institute for Astronomy of ETH Zürich.
Funding AgencyGrant Number
Department of Energy (DOE)DE-FG03-92-ER40701
David and Lucile Packard FoundationUNSPECIFIED
Institute for Astronomy of ETH ZürichUNSPECIFIED
Subject Keywords:atomic processes – ISM: lines and bands - High Energy Astrophysical Phenomena (astro-ph.HE) - Atomic Physics (physics.atom-ph)
Record Number:CaltechAUTHORS:20120206-095805024
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:29145
Deposited By: Tony Diaz
Deposited On:24 Apr 2012 21:40
Last Modified:26 Dec 2012 14:47

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