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A submillimeter imaging survey of ultracompact HII regions


Hunter, Todd Russell (1997) A submillimeter imaging survey of ultracompact HII regions. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/9k2a-vf26.


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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 describe the design and operation of the Submillimeter High Angular Resolution Camera (SHARC)-a 24-pixel bolometer array camera for broadband continuum imaging at 350 and 450µm at the Caltech Submillimeter Observatory (CSO). Detailed information is included on the reflective off-axis optical design and the instrument control software interface. Second, I present 10" to 12" resolution SHARC images of 350 and 450µm 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 H2O masers. 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 ± 10 K and the average grain emissivity index ([beta]) is 2.00 ± 0.25. 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. By fixing the source boundary at the outer limit of the submillimeter emission, the resulting density profiles n(r) [...] can be classified into four categories: 3 regions exhibit p = 2 (isothermal sphere), 4 exhibit p = 1.5 (dynamical collapse), 2 exhibit p = 2 in the outer regions and p = 1.5 in the inner regions, and 6 exhibit p = 1 (logatropic). Although these simplified models may not be unique, a good correlation between the dust luminosity-to-mass ratio and the 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 15 out of 17 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. However, in contrast to lower luminosity sources, the momentum from stellar radiation pressure is comparable to that required to drive the outflows. Many regions show evidence of separate, overlapping outflows. In a final detailed study, 2" resolution images obtained with the Owens Valley Millimeter Array reveal multiple outflows emanating from the molecular core containing the UCHII region G45.12+0.13, while simultaneous outflow and infall motion is seen toward the neighboring, less-evolved core containing G45.07+0.13.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Astronomy
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Phillips, Thomas G.
Group:Astronomy Department
Thesis Committee:
  • Unknown, Unknown
Defense Date:17 September 1996
Record Number:CaltechETD:etd-05212007-111551
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:1918
Deposited By: Imported from ETD-db
Deposited On:24 May 2007
Last Modified:16 Apr 2021 23:18

Thesis Files

PDF (Hunter_tr_1997.pdf) - Final Version
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