An Etymological Dictionary of Astronomy and Astrophysics

A heating process occurring in → diffuse molecular clouds which is believed to be the main heating mechanism in cool → H I regions. Far-ultraviolet (FUV) photons, in the energy range 6 eV «i>hν < 13.6 eV, expel electrons from → interstellar dust grains
and the excess → kinetic energy of the electrons is
converted into gas → thermal energy through → collisions. The high energy limit corresponds to the cut-off in the → far-ultraviolet (FUV) radiation field caused by
the hydrogen absorption (hν = 13.6 eV), while the low energy limit corresponds to the energy needed to free electrons from the grains (hν ~ 6 eV). In the cold neutral medium (T_kin≥ 200 K) photoelectric heating accounts for most of the heating, the → X-ray and → cosmic ray heating rates (→ cosmic-ray ionization) being more than an order of magnitude smaller. In a relatively dense neutral medium (n_H≥ 100 cm^-3), where a significant fraction of carbon is in the neutral form, carbon ionization becomes an important heating source, but it is still not comparable to the photoelectric effect.

The heating rate cannot be directly measured, but it can be estimated through observations of the [C II] line emission, since this is believed to be the main → coolant in regions where the photoelectric heating is dominant (See, e.g., Juvela et al., 2003, arXiv:astro-ph/0302365).

See also: → photoelectric; → heating.