The polytropic equation of state of interstellar gas clouds

Models are presented for the polytropic equation of state of self-gravitati ng, quiescent interstellar gas clouds. A detailed analysis, including chemi stry, thermal balance, and radiative transfer, is performed for the physica l state of the gas as a function of density, metallicity, velocity held, an d background radiation field. We find that the stiffness of the equation of state strongly depends on all of these physical parameters, and the adiaba tic index varies between similar to 0.2-1.4. The implications for star form ation, in particular at high redshift and in starburst galaxies, and the in itial stellar mass function are discussed.