Given the rarity of young O star candidates, compact HII regions embedded in dense molecular cores continue to serve as potential sites to peer into the details of high-mass star formation. To uncover the ionizing sources of the most luminous and com
pact HII regions embedded in the RCW106 and RCW122 giant molecular clouds, known to be relatively nearby (2-4 kpc) and isolated, thus providing an opportunity to examine spatial scales of a few hundred to a thousand AU in size. High spatial resolution (0.3), mid-infrared spectra (R=350), including the fine structure lines [ArIII] and [NeII], were obtained for four luminous compact HII regions, embedded inside the dense cores within the RCW106 and RCW122 molecular cloud complexes. At this resolution, these targets reveal point-like sources surrounded by nebulosity of different morphologies, uncovering details at spatial dimensions of <1000AU. The point-like sources display [ArIII] and [NeII] lines - the ratios of which are used to estimate the temperature of the embedded sources. The derived temperatures are indicative of mid-late O type objects for all the sources with [ArIII] emission. Previously known characteristics of these targets from the literature, including evidence of disk or accretion suggest that the identified sources may grow more to become early-type O stars by the end of the star formation process.
We aim to estimate and analyse the physical properties of the infrared counterparts of HMPOs by comparing their spectral energy distributions (SED) with those predicted by radiative transfer accretion models of YSOs. The SED of 68 IRCs are extended b
eyond the GLIMPSE photometry to the possible limits, from the near-infrared to the millimetre wavelengths by using the 2MASS, GLIMPSE version 2.0 catalogs, MSX, IRAS and some single dish (and interferometric) (sub)mm data. An online SED fitting tool that uses 2D radiative transfer accretion models of YSOs is employed to fit the observed SED to obtain various physical parameters. The SED of IRCs were fitted by models of massive protostars with a range of masses between 5-42 Msun and ages between 10^3 and 10^6 years. The median mass and age are 10 Msun and 10^4 yrs. The envelopes are large with a mean size of ~ 0.2-0.3 pc and show a distribution that is very similar to the distribution of the sizes of 8 micron nebulae discussed in Paper I. The estimated envelope accretion rates are high with a mean value of 10^(-3) Msun/yr and show a power law dependence to mass with an exponent of 2, suggesting spherical accretion at those scales. Disks are found to exist in most of the sources with a mean mass of 10^(-1.4+-0.7) Msun. The observed infrared-millimetre SED of the infrared counterparts of HMPOs are successfully explained with an YSO accretion model. The modelled sources mostly represent proto-B stars although some of them could become O stars in future. We demonstrate that many of these results may represent a realistic picture of massive star formation, despite some of the results which may be an effect of the assumptions within the models.
The Spitzer-GLIMPSE point source catalog and images have been used to study a sample of 381 massive protostellar candidates. IRAC-Point source photometry was used to analyse colours, magnitudes and spectral indicies of the infrared counterparts (IRCs
) of massive protostellar objects and a bonafide sample of 50 point sources was obtained. Spectral energy distributions (SEDs) of these 50 sources was extended to the near-infrared and millimeter range by using 2MASS and millimeter data from the literature. An online SED fitter tool based on Monte-Carlo radiative transfer of an accretion model involving star,disk and envelope was used to fit the SEDs of the 50 sources. The IRCs to massive protostellar objects are found to successfully imitate the SEDs of evolutionary phases similar to low mass star formation. Envelope accretion, rather than disk accretion is found to be dominant in building the most massive stars. Unresolved centimeter continuum emission is associated with 27 IRCs classified as massive protostars suggesting that ionised accretion flows may play an important role along with the molecular component. The morphology of the infrared nebulae surrounding the IRCs have an unusual resemblance to the morphologies of ultra-compact HII regions suggesting that these infrared nebulae are possible precursors to the UCHII regions.
