Free-free and recombination radiation from massive star-forming regions

The contribution of free-free and recombination radiation to the IR emission from regions of massive star formation is investigated. A method for calculating Gaunt factors for free-free emission from a hydrogenic non-relativistic gas is employed and the results are compared with approximation formula from the literature. While the cooling of ionized star forming regions due to free-free and free-bound transitions emerges in the MIR, which is dominated by dust emission, the overall cooling is mainly provided by line emission of heavier elements. Only in metal-free and therefore dust-free H ii regions can freefree and free-bound radiation become dominant. We investigate the global energy balance of those regions and determine temperatures of the ionized gas, which are in general 2 to 8 times larger than found in present day star forming regions of massive stars.