Heating of H i Regions by Energetic Particles

The heating of interstellar H Iregions by energetic particles, originally analyzed by Hayakawa, Nishi- mura, and Takayanagi, has been reconsidered. A fourfold increase in the relative abundance of carbon and inclusion of C+ and 0 excitation by H atoms increases the loss rate per cubic centimeter by more than an order of magnitude, decreasing the temperatures appreciably. Consideration of the ionization and excitation produced by the secondary electrons which result from ionizing collisions of energetic particles with H atoms indicates that 3 4 eV of kinetic energy are available for heating the gas for each hydrogen atom ionized. Temperature and ionization levels are computed for various values of ~r, the rate coefficient for ionization of H by energetic particles, including ionization produced by the secondaries. The minimum value of ~ consistent with the cosmic radiation observed at the Earth is 6.8 X 1018 sec'; the corresponding equilibrium temperatures are less than 25° K for ~H greater than 0.1 cm3, much less than the observed temperatures. A much higher value of ~ is obtained if the atoms in Type I supernova shells, which have an energy of 2 MeV per nucleon at a velocity of 20000 km sec', are as- sumed to permeate the Galaxy; the range of such protons in the galactic plane is about 700 Pc. If at most one-third the shell energy of 1O~ ergs is available in this form, a galactic frequency of one Type I supernova per 100 years gives an upper limit for ~c equal to 1.2 X 1015 sec~; the computed tempera- tures range between about 30° and 1200 K as n~ decreases from 10 to 0.1 cm3. These theoretical results are somewhat uncertain, but an increase of temperature with decreasing density seems consistent with the 21-cm data The computed mean electron density in H i regions corresponding to the higher value for ~ is about 0.01 cm3. Deviations from equipartition are generally small, since collisions between elec- trons and positive ions and between positive ions and H atoms couple closely the kinetic temperature of all these components