Early-stage young stellar objects in the Small Magellanic Cloud
  We present new observations of 34 YSO candidates in the SMC. The anchor of
the analysis is a set of Spitzer-IRS spectra, supplemented by groundbased 3-5
micron spectra, Spitzer and NIR photometry, optical spectroscopy and radio
data. The sources' 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. PAH emission is
detected towards all but two sources. Based on band ratios (in particular the
strength of the 11.3 micron and the weakness of the 8.6 micron 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, 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 H2
is detected towards the majority of SMC sources, allowing us to estimate
rotational temperatures and column densities. All but one source are
spectroscopically confirmed as SMC YSOs. Of the 33 YSOs identified in the SMC,
30 sources populate different stages of massive stellar evolution. 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 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. (abridged)
