A SURVEY OF THE CHEMICAL-PROPERTIES OF THE M17 AND CEPHEUS-A CLOUD CORES

We present the results of a systematic survey of the chemical properti es of two giant molecular cloud (GMC) cores in M17 and Cepheus A. In a ll, we have mapped the emission from 32 molecular transitions of 13 mo lecules and seven isotopic variants over a 4' x 5' region in each core . Each map includes known sites of massive star formation, as well as the more extended quiescent material. In M17 most molecules have emiss ion peaks away from the H II region/molecular cloud interface, while t wo species, HC3N and CH3C2H, deviate from this structure with sharp ma xima closer to this interface. In Cepheus A the core is influenced by a compact high-velocity molecular outflow and a more extended low-velo city flow. The molecular emission distributions in this source are gen erally quite similar, with most molecules peaking near the center of t he core to the east of the compact H II region HW 2. A few molecules, SO, CH3OH, (HCN)-C-13, and (CO)-O-18, have more extended emission. Onl y two molecules, CO and HCO+, appear to trace the high- and low-veloci ty outflows; all other species are tracing the quiescent core. We have used the results of previous studies of the density and temperature o f the dense gas in the same cloud cores to derive accurate abundances relative to CO for several positions in each core. The principal resul t is that the chemical composition of all the cores we have surveyed ( which include OMC-1 as well as M17 and Cepheus A) show remarkable simi larity, both within a given core and among the cores. This suggests th at the chemical processes are similar in quiescent GMC core material. In M17 the lack of variation of molecular abundances is remarkable bec ause the radiation field and the gas temperature are known to vary app reciably throughout the surveyed region, suggesting that the bulk of t he emission arises from gas that is well shielded from radiation.