Observational Constraints on Red and Blue Helium Burning Sequences
  We derive the optical luminosity, colors, and ratios of the blue and red
helium burning (HeB) stellar populations from archival Hubble Space Telescope
observations of nineteen starburst dwarf galaxies and compare them with
theoretical isochrones from Padova stellar evolution models across
metallicities from Z=0.001 to 0.009. We find that the observational data and
the theoretical isochrones for both blue and red HeB populations overlap in
optical luminosities and colors and the observed and predicted blue to red HeB
ratios agree for stars older than 50 Myr over the time bins studied. These
findings confirm the usefulness of applying isochrones to interpret
observations of HeB populations. However, there are significant differences,
especially for the red HeB population. Specifically we find: (1) offsets in
color between the observations and theoretical isochrones of order 0.15 mag
(0.5 mag) for the blue (red) HeB populations brighter than M_V ~ -4 mag, which
cannot be solely due to differential extinction; (2) blue HeB stars fainter
than M_V ~ -3 mag are bluer than predicted; (3) the slope of the red HeB
sequence is shallower than predicted by a factor of ~3; and (4) the models
overpredict the ratio of the most luminous blue to red HeB stars corresponding
to ages <50 Myr. Additionally, we find that for the more metal-rich galaxies in
our sample (Z> 0.5 Zsolar) the red HeB stars overlap with the red giant branch
stars in the color magnitude diagrams, thus reducing their usefulness as
indicators of star formation for ages >100 Myr.
