G. H. Macdonald

A SCUBA imaging survey of ultracompact HII regions; the environments of massive star formation

We present a SCUBA submillimetre (450 and 850 μm) survey of the environment of 105 IRAS point sources, selected from the Wood & Churchwell (1989a) and Kurtz et al. (1994) radio ultracompact (UC) Hii region surveys. We detected a total of 155 sub-mm clumps associated with the IRAS point sources and identified three distinct types of object: ultracompact cm-wave sources that are not associated with any sub-mm emission (sub-mm quiet objects), sub-mm clumps that are associated with ultracompact cm-wave sources (radio-loud clumps); and sub-mm clumps that are not associated with any known ultracompact cm-wave sources (radio-quiet clumps). 90% of the sample of IRAS point sources were found to be associated with strong sub-mm emission. We consider the sub-mm colours, morphologies and distance-scaled fluxes of the sample of sub-mm clumps and show that the sub-mm quiet objects are unlikely to represent embedded UC Hii regions unless they are located at large heliocentric distances. Many of the 2.5 arcmin SCUBA fields contain more than one sub-mm clump, with an average number of companions (the companion clump fraction) of 0.90. The clumps are more strongly clustered than other candidate HMPOs and the mean clump surface density exhibits a broken power-law distribution with a break at 3 pc. We demonstrate that the sub-mm and cm-wave fluxes of the majority of radio-loud clumps are in excellent agreement with the standard model of ultracompact Hii regions. We speculate on the nature of the radio-quiet sub-mm clumps and, whilst we do not yet have sufficient data to conclude that they are in a pre-UC Hii region phase, we argue that their characteristics are suggestive of such a stage.

Structure and Chemistry in the Hot Molecular Core G34.3+0.15

New multifrequency spatial and spectral studies of the hot molecular core associated with the ultracompact HIT region G34.3+0.15 have demonstrated an extremely rich chemistry in this archetypal hot core and revealed differing spatiql structure between certain species which may be a dynamical effect of chemical evolution. The structure of the hot core has been studied with the JCMT in the high excitation J=19-18 and J=18-17 lines of CH3CN and with the Nobeyama Millimetre Array at 4“ arc resolution in the J=6-5 transition. Comparison with a VLA NH3(3,3) map shows a displacement between peak emission in the two chemical species which is consistent with chemical processing on a time scale comparable to the dynamical time scale of ≃105 yrs.

The circumstellar structure of lynds 379 IRS1

12CO J=2-1 maps of L379 IRS1 show a molecular outflow seen almost end-on while C18O J=2-1 emission covers a smaller central region, tracing virially bound material deeper within the cloud. Continuum maps at 450, 800 and 1100pm all trace an identical double peaked arc west of IRS1 and VLA NH3 (1,1) & (2,2) integrated intensity maps reveal the same double-peaked structure. An identical velocity gradient is seen in 12CO, 13CO, C18O and NH3 (1,1) & (2,2) following the arc-like structure of the continuum emission.

Detection of New Ammonia Sources

Ammonia is a favoured molecule for the study of molecular clouds since several important parameters of the cloud can be deduced from simple observations of the J,K=1,1 and 2,2 inversion doublet transitions and the hyperfine structure in the (1,1) line. With the additional knowledge of the kinetic temperature T k from observations of CO, for example, it is possible to compute the excitation temperature of the (1,1) line (T 11 ), the rotational temperature between the (1,1) and (2,2) levels (T 21 ), the molecular hydrogen density n(H 2 ) and ammonia column density N(NH 3 ) (see, for example, Martin and Barrett, 1978).