The circumstellar environments of high-mass protostellar objects - I. Submillimetre continuum emission

We present maps of the 850 pm and 450 pm continuum emission seen towards a sample of 68 high-mass protostellar candidates with luminosities ranging from 10(2.5) L. to similar to10(5) L. Most of these candidate high-mass stars are in the earliest stages of evolution, and have not yet developed an ultra-compact HII region. We observe a variety of continuum emission morphologies, from compact symmetric sources through to multiple cores embedded in long filaments of emission. We find on average there is a 65% probability of an IRAS point-source having a companion detection at submillimetre wavelengths. The ratio of integrated flux to peak flux for our detections shows no strong dependence on distance, suggesting the emission we have observed is primarily from scale-free envelopes with power-law density structures. Assuming a near kinematic distance projection, the clumps we detect vary in mass from similar to1 M. to over 1000 M. with a mean clump mass of 330 M. column density of 9 x 10(23) cm(-2) and diameter of similar to0.6 pc. The high luminosity and low mass of the smallest clumps suggests they are accompanied by a minimal number of stellar companions, while the most massive clumps may be examples of young protogroups and protoclusters. We measure the spectral index of the dust emission (alpha) and the spectral index of the dust grain opacity (beta) towards each object, finding clumps with morphologies suggestive of strong temperature gradients, and of grain growth in their dense inner regions. We find a mean value for beta of 0.9, significantly smaller than observed towards UCHII regions.