On the Nature of the Molecular Condensation Downstream from HH 80 North

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Abstract
We present HCO+ (J=3→2), H13CO+ (J=3→2), CS (J=5→4), and 13CO (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 H13CO+ (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 1 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 & Königl) 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.