MESA models show residual hydrogen envelope mass sets effective temperature on the horizontal branch, with maximum values of 0.05-0.30 solar masses to avoid later thermally pulsing AGB evolution, plus explanations for blue hook stars and puffed-up pre-HB configurations.
HST Observations of New Horizontal Branch Structures in the Globular Cluster omega Centauri
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abstract
The globular cluster omega Centauri contains the largest known population of very hot horizontal branch (HB) stars. We have used the Hubble Space Telescope to obtain a far-UV/optical color-magnitude diagram of three fields in omega Cen. We find that over 30% of the HB objects are ``extreme'' HB or hot post-HB stars. The hot HB stars are not concentrated toward the cluster center, which argues against a dynamical origin for them. A wide gap in the color distribution of the hot HB stars appears to correspond to gaps found earlier in several other clusters. This suggests a common mechanism, probably related to giant branch mass loss. The diagram contains a significant population of hot sub-HB stars, which we interpret as the ``blue-hook'' objects predicted by D'Cruz et al. (1996a). These are produced by late He-flashes in stars which have undergone unusually large giant branch mass loss. omega Cen has a well-known spread of metal abundance, and our observations are consistent with a giant branch mass loss efficiency which increases with metallicity.
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Shaping the horizontal branch: The role of envelope mass in the evolution of stripped core-helium-burning stars
MESA models show residual hydrogen envelope mass sets effective temperature on the horizontal branch, with maximum values of 0.05-0.30 solar masses to avoid later thermally pulsing AGB evolution, plus explanations for blue hook stars and puffed-up pre-HB configurations.