Physics
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........22p&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF MARYLAND COLLEGE PARK, 1995.Source: Dissertation Abstracts International, Volume: 57-03, Section:
Physics
3
Scientific paper
We present high-resolution observations of molecular line and continuum emission toward the massive star-forming complexes Sgr B2 and W33A with the BIMA millimeter array, the Very Large Array and the FCRAO 14m telescope. Several star-forming cores in various evolutionary states are found in these complexes, allowing us to study the chemistry and physical conditions and how these are related to evolutionary state and environment. Similar to previous work, we find that the hot-cores, dense concentrations of star-forming gas heated by embedded newborn stars, have distinctive molecular abundances that derive from chemistry mediated by dust grains. However, we find significant variations in abundances among the hot cores, which can be attributed to both evolutionary and environmental effects. Very large NH_3 column densities are detected toward the northern hot-core in Sgr B2, indicating an unusually young and massive core. Deuterated ammonia is also observed, consistent with an essential role for grain chemistry. The lower (NH_2D) /(NH_3 ) abundance ratio compared to young local hot-cores can be attributed to the higher temperatures prevailing in the galactic center region. Particularly large variations in SO abundance among the hot cores are also observed, which are interpreted as indicating differences in evolutionary state. The important role of grain chemistry is found not only in the hot cores, but also in the low-density envelope of Sgr B2, where SiO abundance is enhanced. The enhancement, which is unusual for diffuse gas, is attributed to the destruction of icy grain mantles by shocks produced by cloud-cloud collisions, which are frequent and at relatively high velocities in the environment of the galactic center. We have also detected outflows associated with the hot cores. The abundances of SO and HCN are enhanced in the Sgr B2 outflows, and the abundances of SiO and CS are enhanced in the W33A outflow. A smaller SO abundance in the W33A outflow than in the Sgr B2 outflows may imply that SO formation depends on temperature since the W33A outflow is significantly colder than Sgr B2 outflows. Outflows are found not only to affect the chemistry of the cloud, but also to change the structure of their parent cloud. We have detected high-velocity HCO+ and H13 CN in Sgr B2, which we interpret as the remnant of material once close to Sgr B2 (M) which has been dispersed by older outflows.
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