Studies of ultracompact HII regions - III. Near-infrared survey of selected regions

A survey towards a selection of 35 methanol maser and/or ultracompact (UC) HII regions, reported in Papers I and IT and by Norris et al,, has been conducted in the near-infrared (NIR). Out of 25 methanol maser sites surveyed, 12 are associated with a NIR counterpart. Out of 18 UC II regions (8 of which overlap with maser emission), 12 are associated with a NIR counterpart. Counterparts can be confidently identified not only by the positional agreements, but also by their unusually red colours, Spectral types for the embedded stars can be unambiguously determined for six sources, all of which imply massive, ionizing stars. One of these infrared sources has methanol maser emission, but no UC HII region. It is possible that the maser emission associated with this source arises from a pre-UC H II phase of massive stellar evolution or it could be that nearly all the ultraviolet photons are absorbed by dust within the UC H II region. We have modelled the spectral energy distributions (SEDs) for some sources and find that a single blackbody can be used to estimate the stellar luminosity, but cannot represent the whole infrared SED. A two-component blackbody model and a radiative transfer model were also used to derive essential parameters of the infrared sources. The radiative transfer model also indicates which infrared sources are relatively young and which are older. Both models show that silicate absorption at 9.7 mu m must be a dominant feature of these SEDs.