Spin exchange rates in proton-hydrogen collisions

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(4) K. For kinetic temperatures T-K greater than or similar to 100 K, the proton-hydrogen rate coefficient exceeds that for hydrogen-hydrogen collisions by about a factor of 2. However, at low temperatures (T-K less than or similar to 5 K) H-H+ collisions become several thousand times more efficient than H-H and even more important than H-e(-) collisions.