Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Origin of Hot Subluminous Horizontal-Branch Stars in Omega Centauri
  and NGC 2808   Hot subluminous stars lying up to 0.7 mag below the extreme horizontal branch
(EHB) are found in the UV color-magnitude diagrams of omega Cen and NGC 2808.
Such stars are unexplained by canonical HB theory. In order to explore the
origin of these subluminous stars, we evolved a set of low-mass stars from the
main sequence through the helium-core flash to the HB for a wide range in the
mass loss along the red-giant branch (RGB). Stars with the largest mass loss
evolve off the RGB to high effective temperatures before igniting helium in
their cores. Our results indicate that the subluminous EHB stars, as well as
the gap within the EHB of NGC 2808, can be explained if these stars undergo a
late helium-core flash on the white-dwarf cooling curve. Under these conditions
the flash convection will penetrate into the stellar envelope, thereby mixing
most, if not all, of the envelope hydrogen into the hot helium- burning
interior. This phenomenon is analogous to the "born-again" scenario for
producing hydrogen-deficient stars during a very late helium-shell flash.
"Flash mixing" greatly enhances the envelope helium and carbon abundances and,
as a result, leads to an abrupt increase in the HB effective temperature. We
argue that the EHB gap in NGC 2808 is caused by this theoretically predicted
dichotomy in the HB morphology. Using new helium- and carbon-rich stellar
atmospheres, we show that the flash-mixed stars have the same reduced UV flux
as the subluminous EHB stars. Moreover, we demonstrate that models without
flash mixing lie, at most, ~0.1 mag below the EHB and hence fail to explain the
observations. Flash mixing may also provide a new evolutionary channel for
producing the high gravity, He-rich sdO and sdB stars.

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The categories of this paper are: ao-sci
35
The categories of this paper are: mtrl-th
26
The categories of this paper are: bayes-an
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The categories of this paper are: adap-org
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The categories of this paper are: alg-geom
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The categories of this paper are: solv-int
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The categories of this paper are: dg-ga
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The categories of this paper are: q-bio
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The categories of this paper are: math
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The categories of this paper are: nucl-th
34
The categories of this paper are: chem-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: econ
36
The categories of this paper are: cmp-lg
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The categories of this paper are: hep-th
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The categories of this paper are: math-ph