MOLECULAR LINE EMISSION MODELS OF HERBIG-HARO OBJECTS .2. HCO+ EMISSION

We present time-dependent models of the chemistry and temperature of i nterstellar molecular gas clumps that are exposed to the radiation fro m propagating stellar-jet shocks. The X-ray, EUV, and FUV radiation fr om the shock initiates ion chemistry and also heats the gas in the clu mps. Using representative parameters, we show that, on the shock trans it time between the clumps, the abundances of the ionized molecular sp ecies that are produced in the clumps can exceed the values determined from steady state models by several orders of magnitude. Collisional excitation by the heated gas can lead to measurable line emission from several ionized species; as in previous investigations of X-ray-irrad iated molecular gas, we find that electron impacts contribute signific antly to this process. We apply these results to the interpretation of the HCO+ line emission that has already been detected in several Herb ig-Haro objects (including HH 1-2, HH 34S, and HH 7-11). We demonstrat e that this picture provides a natural explanation of the fact that th e line intensity typically peaks ahead of the associated shock, as wel l as of the reported low line-center velocities and narrow line widths . We tabulate several diagnostic line intensities of HCO+ and other mo lecular species that may be used to infer the physical conditions in t he emitting gas.