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  1–10⁴  K  . For kinetic temperatures   T _K? 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? 5 K)  H–H⁺ collisions become several thousand times more efficient than H–H and even more important than H–e^? collisions.