2009A&A...498..147G

In a previous work (Paper I, Kumar & Grave, 2007A&A...472..155K) a sample of 380 HMPO targets was studied using the GLIMPSE point source catalog and images. A colour-magnitude analysis of the point sources resulted in the identification of infrared counterparts (IRC) of the (sub)mm cores of HMPO candidates which were considered bona fide targets. 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 beyond 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 5-42M_☉ and ages between 10³ and 10⁶-years. The median mass and age are 10M_☉ and 10⁴yrs. The observed data favours protostars of low effective temperatures (4000-1000K) with correspondingly large effective photospheres (2-200R_☉) for the observed luminosities. The envelopes are large with a mean size of ∼0.2-0.3pc and show a distribution that is very similar to the distribution of the sizes of 8µm nebulae discussed in Paper I. The estimated envelope accretion rates are high with a mean value of 10^–3M_☉/yr and show a power law dependence on 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±0.7M_☉. 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 the future. We demonstrate that many of these results may represent a realistic picture of massive star formation, despite some of the discrepant results which may be an effect of the assumptions within the models.