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Session 50 - Molecular Outflows, Stellar Jets, and Herbig-Haro Objects.
Display session, Tuesday, January 14
Metropolitan Ballroom,

[50.01] A Systematic Study of SiO as an Outflow Tracer in High Mass Star Forming Regions

J. M. Acord, E. Churchwell (U. Wisconsin-Madison), C. M. Walmsley (Oss. Ast. di Arcetri)

Study of molecular outflows towards high mass star forming regions has long been ``neglected'' in comparison with the wealth of data on outflows from nearby low mass star forming regions and low mass YSOs. We have observed three rotational transitions of SiO in its ground vibrational state at 1.3, 2, and 3mm with the Pico Veleta 30-m MRT towards 75 ultracompact HII regions. SiO is an outflow tracer, enhanced in regions with outflow activity, due to sputtering or destruction of dust grain mantles in the outflow. It also appears to be an optically thin tracer which is present only as outflow material and not as ambient gas. Our previous study of the outflow towards G5.89-0.39 showed that all of the SiO emission was associated with the outflow, confined to two peaks bracketing the continuum source. Use of CO as an outflow tracer is problematic because of self-absorption and contamination due to ambient gas at systemic velocities.

Over 50% of the sources surveyed were detected in at least one SiO line, and over 40% were detected in all three SiO transitions. The SiO lines are systematically broader than other molecular lines in our survey (H^13CO^+, SO, DCN). Analysis of the lines shows that they are optically thin, with total column densities of order 10^13 cm^-2 and rotation temperatures from 10 to 20 K. We see no variation in SiO line width or strength with galactocentric distance or excitation temperature.

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