A Spitzer Space Telescope survey of massive young stellar objects in the G333.2-0.4 giant molecular cloud

Abstract

The G333 giant molecular cloud contains a few star clusters and Hii regions, plus a number of condensations currently forming stars. We have mapped 13 of these sources with the appearance of young stellar objects (YSOs) with the Spitzer Infrared Spectrograph in the Short-Low, Short-High, and Long-High modules (5-36 μm). We use these spectra plus available photometry and images to characterize the YSOs. The spectral energy distributions (SEDs) of all sources peak between 35 and 110 μm, thereby showing their young age. The objects are divided into two groups: YSOs associated with extended emission in Infrared Array Camera (IRAC) band 2 at 4.5 μm (‘outflow sources’) and YSOs that have extended emission in all IRAC bands peaking at the longest wavelengths (‘red sources’). The two groups of objects have distinctly different spectra: All the YSOs associated with outflows show evidence of massive envelopes surrounding the protostar because the spectra show deep silicate absorption features and absorption by ices at 6.0, 6.8, and 15.2 μm. We identify these YSOs with massive envelopes cool enough to contain ice-coated grains as the ‘bloated’ protostars in the models of Hosokawa et al. All spectral maps show ionized forbidden lines and polycyclic aromatic hydrocarbon emission features. For four of the red sources, these lines are concentrated to the centres of the maps, from which we infer that these YSOs are the source of ionizing photons. Both types of objects show evidence of shocks, with most of the outflow sources showing a line of neutral sulphur in the outflows and two of the red sources showing the more highly excited [Ne iii] and [S iv] lines in outflow regions at some distance from the YSOs. The 4.5-μm emission seen in the IRAC band 2 images of the outflow sources is not due to H2 lines, which are too faint in the 5 – 10 μm wavelength region to be as strong as is needed to account for the IRAC band 2 emission.