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Session 21 - Massive Star Formation & Ultracompact HII Regions.
Oral session, Monday, January 13
Harbour B,

[21.03] A Submillimeter Imaging Survey of Ultracompact HII Regions

T. R. Hunter (Caltech/CfA)

This research explores the process of massive star formation in the Galaxy through submillimeter continuum and spectral line observations of ultracompact HII (UCHII) regions. First, I briefly describe the optical design and sensitivity of the Submillimeter High Angular Resolution Camera (SHARC), a 24-pixel linear bolometer array camera for broadband imaging at 350 and 450\mum at the Caltech Submillimeter Observatory (CSO). Second, I present 10'' to 12'' resolution SHARC images of 350 and 450\mum continuum emission from a sample of 17 UCHII regions with different radio morphologies. Although the dust emission typically peaks at or near the UCHII region, additional sources are often present, sometimes coincident with the position of water masers. New submillimeter sources have been discovered near G34.3+0.2, G75.77+0.34 and Mon R2. The combination of submillimeter, millimeter and IRAS far-infrared flux densities forms the basis of greybody models of the spectral energy distributions. The average dust temperature is 40 K and the average grain emissivity index (\beta) is 2.00. Using a radiative transfer program that solves for the dust temperature versus radius, the distribution of dust around UCHII regions is modeled with a power-law spherical density profile to match the observed radial flux density profiles. A correlation between the dust luminosity-to-mass ratio and the dust temperature indicates that the more centrally-condensed sources exhibit higher star formation rates. Third, I present 20'' to 30'' resolution CO maps which reveal bipolar outflows from many of these UCHII regions. The outflow mechanical luminosities and mass ejection rates follow the scaling relations with bolometric luminosity established for less luminous pre-main sequence stars. Some regions show evidence of separate, overlapping outflows from young stellar clusters, most notably G45.12+0.13.

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Program listing for Monday