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Astron. Astrophys. 357, 637-650 (2000)

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1. Introduction

Ultracompact HII (UCHII ) regions identify recently-formed massive stars. In many cases, their immediate environment still contains the remnants of the natal molecular cloud, so the conditions under which star formation occurred can be studied. In addition, as massive stars are believed to form in clusters, the environments of UCHII regions are a good place to look for massive protostars.

Submillimetre dust emission is a good tracer of column density in star formation regions as it rarely becomes optically thick. Here we present observations of the distribution of warm dust surrounding five UCHII regions, mapped at submillimetre (submm) wavelengths using the Submillimetre Common-User Bolometer Array (SCUBA) on the James Clerk Maxwell Telescope (JCMT). SCUBA has a [FORMULA] field of view allowing us to quickly map the dust distribution over the few parsecs surrounding the UCHII regions at moderate resolution ([FORMULA] at 450/850µm respectively). With the high sensitivity of SCUBA we can detect dust over several orders of magnitude of column density, and investigate the density distribution of the clouds surrounding the UCHII regions.

Spectral energy distributions (SEDs) play a vital part in any determination of the properties of dust surrounding UCHII regions. The SEDs of these sources, which were first collected by Wood & Churchwell (1989; hereafter WC89) have lacked good measurements in the submm part of the spectrum, between the IRAS 100µm datapoint and the 1300µm results of Chini et al. (1986a,b). The SCUBA fluxes at 450 and 850µm fill this gap, and add additional photometry at 1350 and 2000µm.

The sources (G10.47, G12.21, G13.87, G31.41 and G43.89) were selected for their emission from hot, dense molecular gas, as we expect correspondingly strong dust emission. The sources have all been detected previously in high-excitation ammonia (Cesaroni et al. 1992; Olmi et al. 1993), indicating the presence of hot, dense gas. Our molecular line survey towards these (and other) UCHII regions confirmed that several have hot molecular cores, characterised by extremely high column densities and temperature and a rich chemistry with many emission lines (Hatchell et al. 1998a). Interferometric observations have shown that some sources have compact molecular cores which may be separated from the UCHII regions themselves (Cesaroni et al. 1994b). These sources have not been imaged before in the far IR or submm, except with the low resolution of IRAS, although Hoare et al. (1991) obtained flux measurements and cross scans of G10.47 at 350 and 850µm.

In Sect. 2 we describe the observations and data reduction procedure, and present the results. We discuss the images in Sect. 3. In Sect. 4 we model the spectral energy distributions and radial profiles of the sources using the DUSTY code to solve the radiative transfer problem (Ivezic & Elitzur 1997; Ivezic et al. 1997). Sect. 5 contains a discussion of the modelling results, their implications for the source properties and a comparison with the molecular line data. Sect. 6 contains our conclusions.

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© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000