Radiative cooling II: effects of density and metallicity

Abstract

This work follows Lykins et al. discussion of classic plasma cooling function at low density and solar metallicity. Here, we focus on how the cooling function changes over a wide range of density (nH <1012 cm−3) and metallicity (Z < 30 Z⊙). We find that high densities enhance the ionization of elements such as hydrogen and helium until they reach local thermodynamic equilibrium. By charge transfer, the metallicity changes the ionization of hydrogen when it is partially ionized. We describe the total cooling function as a sum of four parts: those due to H&He, the heavy elements, electron–electron bremsstrahlung and grains. For the first three parts, we provide a low-density limit cooling function, a density dependence function, and a metallicity-dependent function. These functions are given with numerical tables and analytical fit functions. We discuss grain cooling only in the interstellar medium case. We then obtain a total cooling function that depends on density, metallicity and temperature. As expected, collisional de-excitation suppresses the heavy elements cooling. Finally, we provide a function giving the electron fraction, which can be used to convert the cooling function into a cooling rate.