Heat Balance State of the Interstellar Gas: Dependences on the Amount of Cooling- and Heating-Agencies

Abstract

The properties of the interstellar gas in which the heating balances the cooling (the heat balance state) are investigated for the temperature range 30 K < T < 108 K taking the abundance of heavy elements and the flux of low energy cosmic rays as parameters. The temperature and the pressure of the gas in a heat balance state are investigated as functions of the density for various values of these parameters, and it is found that the interstellar medium can take the so-called two-phase structure for wide ranges of these parameters, and then in almost all stages of the galactic evolution. The growth time of the thermal instability is investigated and found to be short enough to realize the two-phase structure except for the case of a very small flux of low energy cosmic rays with the ionization rate of hydrogen less than 10-19 sec-1. The gas without heavy elements can also have a two-phase structure if more than 0.1 par cent of hydrogen are in molecular form. This may be significant for the star formation at very early stages of the galactic evolution. The phase transition between the two stable phases driven by variation of the flux of low energy cosmic rays is discussed and it is suggested that this phase transition controls the star formation in a galaxy.