Tm. Brown et al., Flash mixing on the white dwarf cooling curve: Understanding hot horizontal branch anomalies in NGC 2808, ASTROPHYS J, 562(1), 2001, pp. 368-393
We present an ultraviolet color-magnitude diagram (CMD) spanning the hot ho
rizontal branch (HB), blue straggler, and white dwarf populations of the gl
obular cluster NGC 2808. These data were obtained with the far-UV and near-
UV cameras on the Space Telescope Imaging Spectrograph (STIS). Although pre
vious optical CMDs of NGC 2808 show a high-temperature gap within the hot H
B population, no such gap is evident in our UV CMD. Instead, we find a popu
lation of hot subluminous HB stars, an anomaly only previously reported for
the globular cluster omega Cen. Our theoretical modeling indicates that th
e location of these subluminous stars in the UV CMD, as well as the high-te
mperature gap along the HB in optical CMDs, can be explained if these stars
underwent a late helium-core flash while descending the white dwarf coolin
g curve. We show that the convection zone produced by such a late helium fl
ash will penetrate into the hydrogen envelope, thereby mixing hydrogen into
the hot helium-burning interior, where it is rapidly consumed. This phenom
enon is analogous to the "born again" scenario for producing hydrogen-defic
ient stars following a late helium-shell flash. The flash mixing of the env
elope greatly enhances the envelope helium and carbon abundances, and leads
, in turn, to a discontinuous increase in the HB effective temperatures at
the transition between canonical and flash-mixed stars. We argue that the h
ot HB gap is associated with this theoretically predicted dichotomy in the
HB properties. Moreover, the changes in the emergent spectral energy distri
bution caused by these abundance changes are primarily responsible for expl
aining the hot subluminous HB stars. Although further evidence is needed to
confirm that a late helium-core flash can account for the subluminous HB s
tars and the hot HB gap, we demonstrate that an understanding of these star
s requires the use of appropriate theoretical models for their evolution, a
tmospheres, and spectra.