Spin Exchange Rates in Proton-Hydrogen Collisions

Details Spin Exchange Rates in Proton-Hydrogen Collisions Spin Exchange Rates in Proton-Hydrogen Collisions

2007-02-19

arxiv.org/abs/astro-ph/0702487v2

Mon.Not.Roy.Astron.Soc.379:130-134,2007

Astronomy and Astrophysics

Astrophysics

submitted to MNRAS, 5 pages, 2 figures, typos corrected

Scientific paper

10.1111/j.1365-2966.2007.11921.x

The spin temperature of neutral hydrogen, which determines the optical depth and brightness of the 21 cm line, is determined by the competition between radiative and collisional processes. Here we examine the role of proton-hydrogen collisions in setting the spin temperature. We use recent fully quantum mechanical calculations of the relevant cross sections, which allow us to present accurate results over the entire physically relevant temperature range 1-10,000 K. For kinetic temperatures T_K>100 K, the proton-hydrogen rate coefficient exceeds that for hydrogen-hydrogen collisions by about a factor of two. However, at low temperatures (T_K < 5 K) H-p collisions become several thousand times more efficient than H-H and even more important than H-e^- collisions.

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