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. Monthly Notices of the Royal Astronomical Society, 422 (3). pp. 2357-2371. ISSN 0035-8711. http://resolver.caltech.edu/CaltechAUTHORS:20120614-103855638
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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 ∼2000n4 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 5–80 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.
|Additional Information:||© 2012 The Authors. Monthly Notices of the Royal Astronomical Society. © 2012 RAS. Accepted 2012 February 21. Received 2012 February 19; in original form 2012 January 23. 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 us to get the time on the SHC cluster at Caltech. DT and CH are supported by the U.S. Department of Energy (DE-FG03-92-ER40701) and the National Science Foundation (AST-0807337). CH is supported by the David and Lucile Packard Foundation. KH is supported by the Zwicky Prize Fellowship of the Institute for Astronomy of ETH Zürich.|
|Subject Keywords:||atomic processes; ISM: lines and bands|
|Official Citation:||Tseliakhovich, D., Hirata, C. M. and Heng, K. (2012), Excitation and charge transfer in H–H+ collisions at 5–80 keV and application to astrophysical shocks. Monthly Notices of the Royal Astronomical Society, 422: 2357–2371. doi: 10.1111/j.1365-2966.2012.20787.x|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Jason Perez|
|Deposited On:||14 Jun 2012 21:15|
|Last Modified:||17 Sep 2012 19:16|
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