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

We present HCO + ( J =3→2), H 13 CO + ( J =3→2), CS ( J =5→4), and 13 CO ( 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 H 13 CO + ( J =3→2) and CS ( J =5→4) lines are marginally detected. Comparison between NH 3 , 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 NH 3 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.