A MOLECULAR LINE STUDY OF NGC-1333 IRAS-4

Molecular line surveys and fully sampled spectral line maps at 1.3 and 0.87 mm are used to examine the physical and chemical characteristics of the extreme Class I sources IRAS 4A and 4B in the L1450/NGC 1333 m olecular cloud complex. A very well collimated, jetlike molecular outf low emanates from IRAS 4A, with a dynamical age of a few thousand year s. Symmetric, clumpy structure along the outflow lobes suggests that t here is considerable variability in the mass-loss rate or wind velocit y even at this young age. Molecular emission lines toward IRAS 4A and 4B are observed to be weak in the velocity range corresponding to quie scent material surrounding the young stellar objects (YSOs). Depletion factors of 10-20 are observed for all molecules, including CO, even f or very conservative mass estimates from the measured millimeter and s ubmillimeter dust continuum. However, abundances scaled with respect t o CO are similar to other dark molecular cloud cores. Such depletions could be mimicked by high dust optical depths or increased grain emiss ivities at the observing frequencies of 230 and 345 GHz, but the milli meter and submillimeter spectral energy distributions suggest that thi s is unlikely over the single-dish size scales of 5000-10,000 AU. Dens e, outflowing gas is found to be kinematically, but not spatially, dis tinct from the quiescent material on these size scales. If CO is used as a chemical standard for the high-velocity gas, we find substantial enhancements in the abundances of several molecules in outflowing mate rial, most notably CS, SiO, and CH3OH. The SiO emission is kinematical ly well displaced from the bulk cloud velocity and likely arises from directly shocked material. As is the case for CO, however, the outflow features from more volatile species are centered near the cloud veloc ity and are often characterized by quite low rotational temperatures. We suggest that grain-grain collisions induced by velocity shear zones surrounding the outflow axes transiently desorb the grain mantles, re sulting in large abundance enhancements of selected species. Similar r esults have recently been obtained in several other low-mass YSOs, whe re the outflowing gas is often both kinematically and spatially distin ct, and are illustrative of the ability of accretion and outflow proce sses to simultaneously modify the composition of the gas and dust surr ounding young stars.