CHEMICAL AND PHYSICAL GRADIENTS ALONG THE OMC-1 RIDGE

We present a survey of the distribution of 20 chemical and isotopic mo lecular species along the central ridge of the Orion molecular cloud f rom 6' north to 6' south of BN-KL observed with the QUARRY focal plane array on the FCRAO 14 m telescope, which provides an angular resoluti on of approximate to 50 '' in the 3 mm wavelength region. We use stand ard tools of multivariate analysis for a systematic investigation of t he similarities and differences among the maps of integrated intensiti es of the 32 lines observed. The maps fall in three broad classes: fir st, those strongly peaked toward BN-KL; second, those having rather fl at distributions along the ridge; and third, those with a clear north- south gradient or contrast. We identify six positions or regions where we calculate relative abundances. Line velocities and line widths ind icate that the optically thin lines generally trace the same volume of dense gas, except in the molecular bar, where (CO)-O-18, (CS)-S-34, ( HCO+)-C-13, CN, C2H, SO, and C3H2 have velocities characteristic of th e bar itself, whereas the emission from other detected species is domi nated by the background cloud. The strongest abundance variations in o ur data are the well-known enhancements seen in HCN, CH,OH, HC3N, and SO toward BN-KL and, less strongly, toward the Orion-South outflow 1.3 S'. The principal result of this study is that along the extended quie scent ridge the chemical abundances, within factors of 3-4, exhibit an impressive degree of uniformity. The northern part of the ridge has a chemistry closest to that found in quiescent dense clouds. While temp erature and density are similar around the northern radical-ion peak n ear 3.5N' and in the southern core near 4.2S', some abundances, in par ticular, those of the ions HCO+ and N2H+, are significantly lower towa rd 4.2S'. The areas near 4.25' and the molecular bar itself around (1. 7E', 2.45') stand out with peculiar and similar properties-probably ca used by stronger UV fields penetrating deeper into the clumpy molecula r gas. This leads to higher electron abundances and thereby reduced ab undances of the ions, as well as a lack of complex molecules.