PHYSICAL-PROPERTIES OF THE OMC-2 AND OMC-3 CORES FROM CS AND C18O OBSERVATIONS

We have investigated the properties of the OMC-2 and OMC-3 cores in th e Orion giant molecular cloud using high spatial and spectral resoluti on observations of several transitions of the CO-13, C-18O, C-32S and C-34S molecules taken with the SEST telescope. The OMC-2 core consists of one clump (22 M(.)) with a radius of 0.11 pc surrounded by a clust er of 11 discrete infrared sources. The H2 column density and volume d ensity in the center of this clump are 2 10(22) cm-2 and 9 10(5) cm-3 respectively. From a comparison between physical parameters derived fr om C-18O and C-32S observations we conclude that the molecular envelop e around the core has been completely removed by these sources and tha t only the very dense gas is left. OMC-3 shows a more complex elongate d structure in C-18O and CS than OMC-2. The C-32S and C34S maps show t hat the denser region can be separated into at least three sub-cores o f roughly equal sizes (radius almost-equal-to 0.13 pc), with n(H2) = 6 10(5) cm-3, and a mass of 10M(.) (from C32S). The very different mass es obtained for the central core from C-18-O and C-32S (35 and 12M(.)r espectively) indicate that a massive envelope is still present around the very dense sub-cores. We report the first detection of several mol ecular outflows in OMC-3. The presence of an IRAS source and the first detection of these outflows confirm that star formation is going on i n OMC-3. Based on the different physical properties of these regions c ompared with OMC-1, OMC-2 appears to be in an intermediate evolutionar y stage between OMC-1 and OMC-3.