The neutral atomic phases of the interstellar medium
M. G. Wolfire, D. Hollenbach, C. F. McKee, A. G. G. M. Tielens, E. L. O. Bakes;
ApJ, 1995, 443, 152
ABSTRACT:We calculate the thermal equilibrium gas temperature of the diffuse interstellar medium.
Our
method incorporates a new photoelectric heating rate from small grains and
polycyclic aromatic hydrocarbons (PAHs) that accounts for a size distribution of
particles extending from 100 to 3 A radius.
We also include a detailed treatment
of the ionization rates and heating due to the soft X-ray background and
due to cosmic rays.
Phase diagrams (thermal pressure P versus hydrogen
density n) are presented for gas that is illuminated by local interstellar
far-ultraviolet (FUV) and X-ray radiation fields.
A stable two-phase medium is
produced with thermal pressure in the range P/k approximately = to 10(exp 3-4)
K/cc.
We demonstrate that photoelectric heating from PAHs dominates in the
warm neutral phase (WNM) and cold neutral phase (CNM).
If the C II (158
micrometers cooling per hydrogen nucleus in the solar neighborhood represents an
average value for the Galaxy, we predict L(sub CII) approximately = to 7 x 10(exp
7) solar luminosities from the CNM in the Galaxy, comparable to that
observed by the Cosmic Background Explorer (COBE).
We discuss the dependence
of the results on absorbing column density, gas phase abundances, dust
abundances and metallicity, FUV field, and the X-ray radiation field.
These
results will be useful in modeling the multiphase structure of high-velocity
clouds in the halo, the interstellar matter (ISM) at other galactocentric
radii, and the ISM in external galaxies and galactic nuclei.
KEYWORDS: atomic energy levels, gas heating, gas temperature, interstellar matter, ion production rates, thermodynamic equilibrium, abundance, cosmic background explorer satellite, cosmic x rays, metallicity, particle size distribution, phase diagrams, photoelectric effect, polycyclic aromatic hydrocarbons, radiation distribution
PERSOKEY:h_i, general ism, ,
CODE: wolfire95