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 50Myr 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 0.15mag (0.5mag) for the blue (red) HeB populations brighter than M_V -4mag, which cannot be solely due to differential extinction; (2) blue HeB stars fainter than M_V -3mag 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 ≲ 50Myr. Additionally, we find that for the more metal-rich galaxies in our sample (Z ≳ 0.5 Z _∞), 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 ≳ 100Myr.