Red Variable Stars in Three M81 Group Dwarf Galaxies
  Archival [3.6] and [4.5] images are used to identify and characterize
variable stars in the Magellanic-type galaxies Holmberg II, NGC 2366, and IC
2574. Using parametric and non-parametric detection methods, 74 confirmed or
suspected long period variables (LPVs) are found. The period distributions of
the LPVs in NGC 2366 and IC 2574 are similar. While the period distribution of
LPVs in Ho II is uncertain due to small number statistics there appears to be a
deficiency of LPVs with periods between 550 and 650 days when compared with NGC
2366 and IC 2574. The LPVs are diffusely distributed on the sky, and do not
follow the underlying light from unresolved stars, as expected if episodes of
star formation within the past few hundred Myr have occurred throughout the
galaxies, including their outer regions. Distances computed for Ho II and NGC
2366 from the period-luminosity relations (PLRs) agree to within ~0.1
magnitudes with those based on the tip of the red giant branch (RGB). Efforts
to estimate an LPV-based distance modulus for IC 2574 are complicated by the
presence of first overtone pulsators among LPVs with periods ~ 600 days,
although the PLR at the long period end is consistent with the distance
estimated from the RGB-tip. In addition to the LPVs, 10 candidate sgB[e] or
luminous blue variables and 2 candidate red supergiant variables are also
identified. Nine candidate sgB[e] stars that do not show evidence of
variability are also identified based on their locations in the color-magnitude
diagram.
