The HST Key Project galaxies NGC 1326A, NGC 1425 and NGC 4548: New
  variable stars and massive star population
  Studies on the massive star population in galaxies beyond the Local Group are
the key to understand the link between their numbers and modes of star
formation in different environments. We present the analysis of the massive
star population of the galaxies NGC 1326A, NGC 1425 and NGC 4548 using archival
Hubble Space Telescope Wide Field Planetary Camera 2 images in the F555W and
F814W filters. Through high precision point spread function fitting photometry
for all sources in the three fields we identified 7640 candidate blue
supergiants, 2314 candidate yellow supergiants, and 4270 candidate red
supergiants. We provide an estimation the ratio of blue to red supergiants for
each field as a function of galactocentric radius. Using Modules for
Experiments in Stellar Astrophysics (MESA) at solar metallicity, we defined the
luminosity function and estimated the star formation history of each galaxy. We
carried out a variability search in the V and I filters using three variability
indexes: the median absolute deviation, the interquartile range, and the
inverse von-Neumann ratio. This analysis yielded 243 new variable candidates
with absolute magnitudes ranging from Mv= -4 to -10 mag. We classified the
variable stars based on their absolute magnitude and their position on the
color-magnitude diagram using the MESA evolutionary tracks at solar
metallicity. Our analysis yielded 8 candidate variable blue supergiants, 12
candidate variable yellow supergiants, 21 candidate variable red supergiants,
and 4 candidate periodic variables.
