* Astronomy

Title: Spitzer-IRAC GLIMPSE of high mass protostellar objects. I Infrared point sources and nebulae
Authors: M. S. N. Kumar, J. M. C. Grave

The GLIMPSE archive was used to obtain 3.6--8.0micron, point source photometry and images for 381 massive protostellar candidates lying in the Galactic mid-plane. 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. 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--8.0mum bands display large values of \alpha=2--5. A colour-magnitude analogue plot was used to identify 79 infrared counterparts to the HMPOs. Compact nebulae are found in 75% of the detected sources with morphologies that can be well described by core-halo, cometary, shell-like and bipolar geometries similar to those observed in ultra-compact HII regions. The IRAC band SEDs of the IR counterparts of HMPOs are best described to represent YSOs with a mass range of 8--20\msun in their Class I stages when compared with 2D radiative transfer models. They also suggest that the high \alpha 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 likely reflect the underlying physical structure of the dense cores and are found to imitate the morphologies of known UCHII regions. Our results favour models of continuing accretion involving both molecular and ionised accretion components to build the most massive stars rather than purely molecular rapid accretion flows.

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