Line emission from the molecules HCO + and NH3 in confined regions is frequently found in clumps almost directly ahead of Herbig—Haro (HH) emission knots. The optical knots exhibit high excitation and high velocities; however, the molecular emission is more characteristic of conditions in the quiescent dense cores that precede the formation of the star. Given that the HH objects are caused by the interaction of a jet with the ambient medium, it is likely that the molecular clumps are irradiated by a photon flux that both heats it and alters its chemical state. Here, we argue that the appearance of both molecules is a consequence of the removal of ice grain mantles by heating combined with the photochemistry that follows.
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Taylor et al. (1996) studied this question.