Spitzer-IRAC GLIMPSE of high-mass protostellar objects - I. Infrared point sources and nebulae

Aims. We conduct a statistical study of candidate massive protostellar objects in the 3.6-8.0 mu m bands of the Spitzer Space Telescope. Methods. The GLIMPSE archive was used to obtain 3.6-8.0 mu m point source photometry and images for 381 massive protostellar candidates lying in the Galactic midplane. The colours, magnitudes, and spectral indices of sources in each of the 381 target fields were analysed, and compared with the predictions of 2D radiative transfer model simulations. Results. Infrared point sources with intrinsic reddening were found associated with several massive protostars. Although no discernable embedded clusters were found in any targets, multiple sources or associations of reddened young stellar objects were found in many sources, indicating multiplicity at birth. The spectral index (alpha) of these point sources in 3.6-8.0 mu m bands display high values of alpha = 2-5. A colour-magnitude analog plot was used to identify 79 infrared counterparts to the HMPOs that are bright at 8 mu m, centred on millimetre peaks, and that display a values in excess of 2. Compact nebulae are found in 75% of the detected sources with morphologies that can be described well by core-halo, cometary, shell-like, and bipolar geometries similar to those observed in ultra-compact HII regions. Conclusions. The IRAC band spectral energy distributions ( SED) of the infrared counterparts of massive protostellar candidates are best described as representing YSOs with a mass range of 8-20 M-circle dot in their Class I evolutionary stages when compared with 2D radiative transfer models. They also suggest that the high a values represent reprocessed star/star+disk emission that is arising in the dense envelopes. Thus we are witnessing the luminous envelopes around the protostars rather than their photospheres or disks. We argue that the compact infrared nebulae very likely reflect the underlying physical structure of the dense cores and are found to imitate the morphologies of known UCHII regions. The observations are consistent with a scenario where massive protostars have formed inside dense cores and continue to accrete matter. Our results favour models of continuuing accretion involving both molecular and ionised accretion components to build the most massive stars rather than purely molecular, rapid accretion flows.