A Spitzer Space Telescope far-infrared spectral atlas of compact sources
  in the Magellanic Clouds. II. The Small Magellanic Cloud
  We present 52-93 micron spectra, obtained with the Spitzer Space Telescope,
of luminous compact far-IR sources in the SMC. These comprise 9 Young Stellar
Objects (YSOs), the compact HII region N81 and a similar object within N84, and
two red supergiants (RSGs). The spectra of the sources in N81 (of which we also
show the ISO-LWS spectrum between 50-170 micron) and N84 both display strong
[OI] 63-micron and [OIII] 88-micron fine-structure line emission. We attribute
these lines to strong shocks and photo-ionized gas, respectively, in a
``champagne flow'' scenario. The nitrogen content of these two HII regions is
very low, definitely N/O<0.04 but possibly as low as N/O<0.01. Overall, the
oxygen lines and dust continuum are weaker in star-forming objects in the SMC
than in the LMC. We attribute this to the lower metallicity of the SMC compared
to that of the LMC. Whilst the dust mass differs in proportion to metallicity,
the oxygen mass differs less; both observations can be reconciled with higher
densities inside star-forming cloud cores in the SMC than in the LMC. The dust
in the YSOs in the SMC is warmer (37-51 K) than in comparable objects in the
LMC (32-44 K). We attribute this to the reduced shielding and reduced cooling
at the low metallicity of the SMC. On the other hand, the efficiency of the
photo-electric effect to heat the gas is found to be indistinguishable to that
measured in the same manner in the LMC, 0.1-0.3%. This may result from higher
cloud-core densities, or smaller grains, in the SMC. The dust associated with
the two RSGs in our SMC sample is cool, and we argue that it is swept-up
interstellar dust, or formed (or grew) within the bow-shock, rather than dust
produced in these metal-poor RSGs themselves. Strong emission from crystalline
water ice is detected in at least one YSO. (abridged)
