On the nature of the molecular condensation downstream from HH 80 North

We present HCO+ (J = 3 --> 2), (HCO+)-C-13 (J = 3 --> 2), CS (J = 5 --> 4), and (CO)-C-13 (J = 2 --> 1) Caltech Submillimeter Observatory observations toward the dense quiescent clump downstream from HH 80 North. This condensation is one in a class of sources that show anomalous molecular abundances, probably produced by UV radiation from a nearby shock. The HCO+ emission properties-location, size, center velocity, line widths, and excitation temperature-are similar to those of the ammonia emission, suggesting that it arises from the same region. The (HCO+)-C-13 (J = 3 --> 2) and CS (J = 5 --> 4) lines are marginally detected. Comparison between NH3, HCO+, and CS implies an ammonia enhancement relative to CS of at least I order of magnitude, whereas HCO+ shows only a weak enhancement relative to CS. These results are compatible with the radiative shock-induced chemistry models (Taylor & Williams; Wolfire & Konigl) when we take into account the extinction. Our results suggest that the dense clump detected in NH3 and HCO+ is illuminated by HH 80 North, making this region a good laboratory for studying the chemical processes in a quiescent clump radiatively excited by shocks.