A Submillimeter Imaging Survey of Ultracompact H ii Regions

This research explores the process of massive star formation in the Galaxy through submillimeter continuum and CO spectral line observations of ultracompact H ii (UC H ii) 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 450 mm at the Caltech Submillimeter Observatory (CSO). Detailed information is included on the reflective off-axis optical design and the instrument control software interface (Hunter, T. R., Benford, D. J., & Serabyn, E. 1996, PASP, 108, 1042). Second, I present 100–120 resolution SHARC images of 350 and 450 mm continuum emission from a sample of 17 UC H ii regions with different radio morphologies. The sample includes G19.6120.23, G29.9620.02, G34.2610.14, G45.1210.13, G45.0710.13, K3-50A, K3-50C, G75.8410.34, G75.7810.34, W3(OH), G138.29511.555, G139.90910.197, Monoceros R2, GGD12-15, S255 FIR2, G192.58420.041, and G240.3110.07. Although the dust emission typically peaks at or near the UC H ii region, additional sources are often present, sometimes coincident with the position of H2O or CH3OH masers. The most notable discovery so far is the proto-B star G34.2410.13MM (Hunter, T. R., Neugebauer, G., Benford, D. J., Matthews, K., Lis, D. C., Serabyn, E., & Phillips, T. G. 1998, ApJ, 493, L97). The combination of submillimeter, millimeter, and IRAS far-infrared flux densities forms the basis of modified graybody models of the spectral energy distributions. The average dust temperature is K, and the average 40 5 10 grain emissivity index (b) is . Using a radiative 2.00 5 0.25 transfer program that solves for the dust temperature as a function of radius, the distribution of dust around UC H ii 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 can be classified into four 2p n(r) ∝ r categories: three regions exhibit (isothermal sphere), four p 5 2 exhibit (dynamical collapse), two exhibit in the p 5 1.5 p 5 2 outer regions and in the inner regions, and six exhibit p 5 1.5 (logatropic). Although these simplified models may not p 5 1 be unique, a good correlation between the dust luminosity-tomass ratio and the temperature indicates that the more centrally condensed sources exhibit higher star formation rates. Third, I present 200–300 resolution CO maps that reveal bipolar outflows from 15 out of 17 UC H ii 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, 20 resolution images obtained with the Owens Valley Millimeter Array reveal multiple outflows emanating from the molecular core containing the UC H ii region G45.1210.13, while simultaneous outflow and infall motion is seen in CS toward the neighboring, less evolved J 5 2 r 1 core containing G45.0710.13 (Hunter, T. R., Phillips, T. G., & Menten, K. M. 1997, ApJ, 478, 283).