The cooling function of HD molecule revisited

We report new calculations of the cooling rate of primordial gas by the HD molecule, taking into account its ro-vibrational structure. The HD cooling function is calculated including radiative and collisional transitions for J\le8 rotational levels, and for the vibrational levels v=0,1,2 and 3. The ro-vibrational level population is calculated from the balance equation assuming steady state. The cooling function is evaluated in the ranges of kinetic temperatures, T_k, from 1e2 to 2e4 K and of number densities, n_H, from 1 to 1e8 cm**-3. We find that the inclusion of collisional ro-vibrational transitions increases significantly the HD cooling efficiency, in particular for high densities and temperatures. For n_H>~1e5 and T_k~1e4 K the cooling function becomes more than an order of magnitude higher than previously reported. We present also the HD cooling rate in the presence of the cosmic microwave radiation field for radiation temperatures of 30, 85 and 275 K (redshifts of 9, 29 and 99). The tabulated cooling functions are available at http://www.cifus.uson.mx/Personal\_Pages/anton/DATA/HD\_cooling/HD\_cool.html. We discuss about the relevance to explore the effects of including our results in models and simulations of galaxy formation, specially in the regime when gas cools down from temperatures above ~3000 K