Evolved Massive Stars at Low-metallicity II. Red Supergiant Stars in the
  Small Magellanic Cloud
  We present the most comprehensive RSG sample for the SMC up to now, including
1,239 RSG candidates. The initial sample is derived based on a source catalog
for the SMC with conservative ranking. Additional spectroscopic RSGs are
retrieved from the literature, as well as RSG candidates selected from the
inspection of CMDs. We estimate that there are in total $\sim$ 1,800 or more
RSGs in the SMC. We purify the sample by studying the infrared CMDs and the
variability of the objects, though there is still an ambiguity between AGBs and
RSGs. There are much less RSGs candidates ($\sim$4\%) showing PAH emission
features compared to the Milky Way and LMC ($\sim$15\%). The MIR variability of
RSG sample increases with luminosity. We separate the RSG sample into two
subsamples ("risky" and "safe") and identify one M5e AGB star in the "risky"
subsample. Most of the targets with large variability are also the bright ones
with large MLR. Some targets show excessive dust emission, which may be related
to previous episodic mass loss events. We also roughly estimate the total gas
and dust budget produced by entire RSG population as $\rm
\sim1.9^{+2.4}_{-1.1}\times10^{-6}~M_{\odot}/yr$ in the most conservative case.
Based on the MIST models, we derive a linear relation between $T_{\rm eff}$ and
observed $\rm J-K_S$ color with reddening correction for the RSG sample. By
using a constant bolometric correction and this relation, the Geneva
evolutionary model is compared with our RSG sample, showing a good agreement
and a lower initial mass limit of $\sim$7 $\rm M_\odot $ for the RSG
population. Finally, we compare the RSG sample in the SMC and the LMC. Despite
the incompleteness of LMC sample in the faint end, the result indicates that
the LMC sample always shows redder color (except for the $\rm IRAC1-IRAC2$ and
$\rm WISE1-WISE2$ colors due to CO absorption) and larger variability than the
SMC sample.
