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Flash Mixing on the White-Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808We present an ultraviolet color-magnitude diagram (CMD) spanning the hot horizontal branch (HB), blue straggler, and white dwarf populations of the globular cluster NGC 2808. These data, obtained with the Space Telescope Imaging Spectrograph (STIS), demonstrate that NGC 2808 harbors a significant population of hot subluminous HB stars, an anomaly only previously reported for the globular cluster omega Cen. Our theoretical modeling indicates that the location of these subluminous stars in the CMD, as well as the high temperature gap along the HB of NGC 2808, can be explained if these stars underwent a late helium-core flash while descending the white dwarf cooling curve. We show that the convective zone produced by such a late helium flash will penetrate into the hydrogen envelope, thereby mixing hydrogen into the hot helium-burning interior, where it is rapidly consumed. This phenomenon is analogous to the "born again" scenario for producing hydrogen-deficient stars following a late helium-shell flash. The flash mixing of the envelope greatly enhances the envelope helium and carbon abundances that, in turn, leads to a discontinuous increase in the HB effective temperatures. We argue that the hot HB gap is associated with this theoretically predicted dichotomy in the HB properties. Moreover, the changes in the emergent spectral energy distribution caused by these abundance changes are primarily responsible for explaining the hot subluminous HB stars. Although further evidence is needed to confirm that a late helium-core flash can account for the subluminous HB stars and the hot HB gap, we demonstrate that an understanding of these stars requires the use of appropriate theoretical models for their evolution, atmospheres, and spectra.
Document ID
20010069578
Acquisition Source
Goddard Space Flight Center
Document Type
Preprint (Draft being sent to journal)
Authors
Brown, Thomas M.
(National Optical Astronomy Observatories Tucson, AZ United States)
Sweigart, Allen V.
(NASA Goddard Space Flight Center Greenbelt, MD United States)
Lanz, Thierry
(Maryland Univ. MD United States)
Landsman, Wayne B.
(Raytheon Information Technology and Scientific Services United States)
Hubeny, Ivan
(National Optical Astronomy Observatories Tucson, AZ United States)
Fisher, Richard R.
Date Acquired
August 20, 2013
Publication Date
January 1, 2001
Subject Category
Astrophysics
Funding Number(s)
CONTRACT_GRANT: NRA-99-01-ATP-039
CONTRACT_GRANT: NAS5-6499
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.
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