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.
Spectroscopic Analyses of the "Blue Hook" Stars in omega Centauri: A Test of the Late Hot Flasher Scenario
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abstract
omega Cen contains the largest population of very hot horizontal branch (HB) stars known in a globular cluster. Recent UV observations (Whitney et al. 1998; D'Cruz et al. 2000) show a significant population of hot stars below the zero-age horizontal branch (``blue hook'' stars), which cannot be explained by canonical stellar evolution. Stars which suffer unusually large mass loss on the red giant branch and thus experience the helium core flash while descending the white dwarf cooling curve could populate this region. Theory predicts that these ``late hot flashers'' should show higher temperatures than the hottest canonical HB stars and should have helium- and carbon-rich atmospheres. We obtained and analysed medium resolution spectra of a sample of blue hook stars to derive their atmospheric parameters. The blue hook stars are indeed both hotter (Teff > 35,000K) and more helium-rich than classical extreme HB stars. In addition we find indications for a large enhancement of the carbon abundance relative to the cluster abundance.
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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.