Star formation at very low metallicity. I. Chemistry and cooling at low densities

We present a simplified chemical and thermal model designed to allow computationally efficient study of the thermal evolution of metal-poor gas within large numerical simulations. Our main simplification is the neglect of the molecular chemistry of the heavy elements. The only molecular chemistry retained within the model is the formation and destruction of molecular hydrogen. Despite this major simplification, the model allows for accurate treatment of the thermal evolution of the gas within a large volume of parameter space. It is valid for temperatures 50 < T < 10, 000 K and metallicities 0 < Z < 0.1 Z(circle dot). In gas with a metallicity Z = 0: 1 Z(circle dot), and in the absence of an incident ultraviolet radiation field, it is valid for hydrogen number densities n(H) less than or similar to 500/tchar cm(-3), where t(char) is the size in megayears of the characteristic physical timescale of interest in the problem. If Z << 0.1 Z(circle dot), or if a strong ultraviolet radiation field is present, then the model remains accurate up to significantly higher densities. We also discuss some possible applications of this model.