Early-stage young stellar objects in the Small Magellanic Cloud

J.M. Oliveira (Keele Univ.), J.Th. van Loon (Keele Univ.), G.C. Sloan (Cornell), M. Sewilo (Johns Hopkins Univ.) K.E. Kraemer (Boston Coll.), P.R. Wood (Australian National Univ.), R. Indebetouw (Univ. of Virginia and NRAO), M.D. Filipovic (Univ. of W. Sydney), E.J. Crawford (Univ. of W. Sydney), G.F. Wong (Univ. of W. Sydney), J.L. Hora (Harvard-Smithsonian CfA), M. Meixner (STScI), T.P. Robitaille (MPIA Heidelberg), B. Shiao (STScI), and J.D. Simon (Carnegie Inst.).

2013, MNRAS, 428, 3001

Full manuscript available locally (PDF) or from astro-ph (1210.5193)

We present new observations of 34 Young Stellar Object (YSO) candidates in the Small Magellanic Cloud (SMC). The photometric selection required sources to be bright at 24 and 70 µm (to exclude evolved stars and galaxies). The anchor of the analysis is a set of Spitzer-IRS spectra, supplemented by groundbased 3-5 µm spectra, Spitzer IRAC and MIPS photometry, near-IR imaging and photometry, optical spectroscopy and radio data. The sources' spectral energy distributions (SEDs) and spectral indices are consistent with embedded YSOs; prominent silicate absorption is observed in the spectra of at least ten sources, silicate emission is observed towards four sources. Polycyclic Aromatic Hydrocarbon (PAH) emission is detected towards all but two sources. Based on band ratios (in particular the strength of the 11.3-µm and the weakness of the 8.6-µm bands) PAH emission towards SMC YSOs is dominated by predominantly small neutral grains. Ice absorption is observed towards fourteen sources in the SMC. The comparison of H₂O and CO₂ ice column densities for SMC, Large Magellanic Cloud (LMC) and Galactic samples suggests that there is a significant H₂O column density threshold for the detection of CO₂ ice. This supports the scenario proposed by Oliveira et al. (2011), where the reduced shielding in metal-poor environments depletes the H₂O column density in the outer regions of the YSO envelopes. No CO ice is detected towards the SMC sources. Emission due to pure-rotational 0-0 transitions of molecular hydrogen is detected towards the majority of SMC sources, allowing us to estimate rotational temperatures and H₂ column densities. All but one source are spectroscopically confirmed as SMC YSOs. Based on the presence of ice absorption, silicate emission or absorption, and PAH emission, the sources are classified and placed in an evolutionary sequence. Of the 33 YSOs identified in the SMC, 30 sources populate different stages of massive stellar evolution. The presence of ice- and/or silicate-absorption features indicates sources in the early embedded stages; as a source evolves, a compact H II region starts to emerge, and at the later stages the source's IR spectrum is completely dominated by PAH and fine-structure emission. The remaining three sources are classified as intermediate-mass YSOs with a thick dusty disc and a tenuous envelope still present. We propose one of the SMC sources is a D-type symbiotic system, based on the presence of Raman, H and He emission lines in the optical spectrum, and silicate emission in the IRS-spectrum. This would be the first dust-rich symbiotic system identified in the SMC.

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Last modified 8 January, 2013. © Gregory C. Sloan and others.