Evolved Massive Stars at Low-metallicity IV. Using 1.6 $\mu$m "H-bump"
  to identify red supergiant stars: a case study of NGC 6822
  We present a case study of using a novel method to identify red supergiant
(RSG) candidates in NGC 6822, based on their 1.6 $\mu$m "H-bump". We collected
32 bands of photometric data for NGC 6822 ranging from optical to MIR. By using
the theoretical spectra from MARCS, we demonstrate that there is a prominent
difference around 1.6 $\mu$m ("H-bump") between low-surface-gravity (LSG) and
high-surface-gravity (HSG) targets. Taking advantage of this feature, we
identify efficient color-color diagrams (CCDs) of rzH and rzK to separate HSG
and LSG targets from crossmatching of optical and NIR data. Moreover, synthetic
photometry from ATLAS9 also give similar results. Further separating RSG
candidates from the rest of the LSG candidates is done by using semi-empirical
criteria on NIR CMDs and resulted in 323 RSG candidates. Meanwhile, the
simulation of foreground stars from Besan\c{c}on models also indicates that our
selection criteria is largely free from the contamination of Galactic giants.
In addition to the "H-bump" method, we also use the traditional BVR method as a
comparison and/or supplement, by applying a slightly aggressive cut to select
as much as possible RSG candidates (358 targets). Furthermore, the Gaia
astrometric solution is used to constrain the sample, where 181 and 193 targets
were selected from the "H-bump" and BVR method, respectively. The percentages
of selected targets in both methods are similar as $\sim$60\%, indicating the
comparable accuracy of the two methods. In total, there are 234 RSG candidates
after combining targets from both methods with 140 ($\sim$60\%) of them in
common. The final RSG candidates are in the expected locations on the MIR CMDs,
while the spatial distribution is also coincident with the FUV-selected star
formation regions, suggesting the selection is reasonable and reliable.
