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| Paper: |
New Observational Evidence of Flash Mixing on the White Dwarf Cooling Curve |
| Volume: |
452, Fifth Meeting on Hot Subdwarf Stars and Related Objects |
| Page: |
23 |
| Authors: |
Brown, T. M.; Lanz, T.; Sweigart, A. V.; Cracraft, M.; Hubeny, I.; Landsman, W. B. |
| Abstract: |
Blue hook stars are a class of subluminous extreme horizontal branch
stars that were discovered in UV images of the massive globular
clusters ω Cen and NGC 2808. These stars occupy a region of
the HR diagram that is unexplained by canonical stellar evolution
theory. Using new theoretical evolutionary and atmospheric models, we
have shown that the blue hook stars are very likely the progeny of
stars that undergo extensive internal mixing during a late helium-core
flash on the white dwarf cooling curve. This “flash mixing”
produces hotter-than-normal EHB stars with atmospheres significantly
enhanced in helium and carbon. The larger bolometric correction,
combined with the decrease in hydrogen opacity, makes these stars appear
subluminous in the optical and UV. Flash mixing is more likely to
occur in stars born with a high helium abundance, due to their lower
mass at the main sequence turnoff. For this reason, the phenomenon
is more common in those massive globular clusters that show evidence
for secondary populations enhanced in helium. However, a high helium
abundance does not, by itself, explain the presence of blue hook stars
in massive globular clusters. Here, we present new observational
evidence for flash mixing, using recent HST observations. These
include UV color-magnitude diagrams of six massive globular clusters
and far-UV spectroscopy of hot subdwarfs in one of these clusters
(NGC 2808). |
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