Physical parameters of red supergiants in dwarf irregular galaxies in
  the Local Group
  Increasing the statistics of evolved massive stars in the Local Group enables
investigating their evolution at different metallicities. During the late
stages of stellar evolution, the physics of some phenomena, such as episodic
and systematic mass loss, are not well constrained. For example, the physical
properties of red supergiants (RSGs) in different metallicity regimes remain
poorly understood. Thus, we initiated a systematic study of RSGs in dwarf
irregular galaxies (dIrrs) in the Local Group. The target selection is based on
3.6 $\mu$m and 4.5 $\mu$m photometry from archival Spitzer Space Telescope
images of nearby galaxies. We selected 46 targets in the dIrrs IC 10, IC 1613,
Sextans B, and the Wolf-Lundmark-Melotte (WLM) galaxy that we observed with the
GTC-OSIRIS and VLT-FORS2 instruments. We used several photometric techniques
together with a spectral energy distribution analysis to derive the
luminosities and effective temperatures of known and newly discovered RSGs. We
identified and spectroscopically confirmed 4 new RSGs, 5 previously known RSGs,
and 5 massive asymptotic giant branch (AGB) stars. We added known objects from
previous observations. In total, we present spectral classification and
fundamental physical parameters of 25 late-type massive stars in the following
dIrrs: Sextans A, Sextans B, IC 10, IC 1613, Pegasus, Phoenix, and WLM. This
includes 17 RSGs and 8 AGB stars that have been identified here and previously.
Based on our observational results and PARSEC evolutionary models, we draw the
following conclusions: (i) a trend to higher minimum effective temperatures at
lower metallicities and (ii) the maximum luminosity of RSGs appears to be
constant at $log$($L/L$$_{\odot}$) $\approx$ $5.5$, independent of the
metallicity of the host environment (up to $\mathrm{[Fe/H]}$ $\approx$ $-1$
dex).
