Thermal Properties of Interstellar Gas Heated by Cosmic Rays

Abstract

We present detailed calculations of thermal properties of predominantly neutral interstellar gas heated by low-energy cosmic rays. The heating rate does not depend on the form of the energy spectrum of the cosmic rays, but only on an integral over the spectrum. This integral is proportional to the ioniza~ tion rate ~-. it is shown that two beating processes are important: (i) heating of the free electrons of the gas by direct interaction ~ ith cosmic ra~ s anu i ~ he4tlr~ cau ed }~ elect or 1ibera~'~d b~ osrrnc ra~ particles, which electrons transfer part of their energy to the free electrons of the gas. ii the electron density is low (less than 0.02 na), the second process dominates; otherwise both processes must be con- sidered. Calculations of thermal equilibrium show that the kinetic pressure in the gas has a maximum at an atomic density of about 0.2 cm3. This leads to a model in which the interstellar gas is represented by two thermally stable phases in pressure equilibrium-a rarefied one at about 1()~ ° K and a dense one at about 1000 K. Finally, we discuss the influence on equilibrium temperatures of lowering abundances by accretion of trace elements on grains. Evidence is presented for underabundance of all trace eIemen~s observed in the interstellar gas