Statistical Time-Dependent Model for the Interstellar Gas

Abstract

We present models for temperature and ionization structure of low, uniform-density (n 0.3 cm-3) interstellar gas in a galactic disk which is exposed to soft X-rays from supernova outbursts occurring randomly in space and time. The structure was calculated by computing the time record of temperature and ionization at a given point by Monte Carlo simulation. The calculation yields probability distribution functions for ionized fraction x, temperature T, and their various observable moments. These time-dependent models predict a bimodal temperature distribution of the gas with structure in x, T that agrees with various observations. Cold regions in the low-density gas may have the appearance of clouds in 21-cm absorption. The time-dependent model, in contrast to the steady-state model, predicts large fluctuations in ionization rate and the existence of cold (T 30 K), ionized (x 0.1) regions. Subject headings: atomic processes interstellar matter - supernovae X-rays