The circumstellar environments of high-mass protostellar objects - II. Dust continuum models

We analyse the dust continuum emission seen towards a sample of candidate high-mass protostellar objects, modelling the cores we recently observed at 850 mu m with a one-dimensional radiative transfer code. Fitting radial slices in a range of directions across sources, we identify a number of objects that have non-spherical density profiles and show that for such sources fitting the azimuthal averaged emission produces erroneous estimates of the source properties. We find the majority of cores can be successfully modelled using envelopes of power-law density structure (where rho proportional to r(-alpha)), finding a mean power-law index of (alpha) over bar = 1.3 +/- 0.4. These envelopes extend considerably further, are more dense, and have a more shallow density profile than those bearing low-mass protostars. The majority of best-fit models have a SED resembling the cold-component dust bodies previously proposed for the sample, implying the short wavelength emission seen towards the HMPOs either originates from a separate hot dust component(s), or involves mechanisms such as accretion disks, stochastic heating and/or optically thin cavities not included in the radiative transfer model. We find evidence of smaller dust-free cavities towards some pre-UCHII sources. The modelling indicates a correlation between a and optical depth, suggesting that the densest cores also tend to have the most strongly peaked power-law density profiles.