  We present a UV CMD spanning the hot horizontal branch (HB), blue straggler,
and white dwarf populations of the globular cluster NGC 2808. These data were
obtained with the Space Telescope Imaging Spectrograph (STIS). Although
previous optical CMDs of NGC 2808 show a high temperature gap within the hot HB
population, no such gap is evident in our UV CMD. Instead, we find a population
of hot subluminous HB stars, an anomaly only previously reported for the
globular cluster omega Cen. Our modeling indicates that the location of these
subluminous stars in the UV CMD, as well as the high temperature gap along the
HB in optical CMDs, can be explained if these stars underwent a late He-core
flash while descending the white dwarf cooling curve. We show that the
convection zone produced by such a late He flash will penetrate into the H
envelope, thereby mixing H into the hot He-burning interior, where it is
rapidly consumed. This phenomenon is analogous to the "born again" scenario for
producing H-deficient stars following a late He-shell flash. The flash mixing
of the envelope greatly enhances the envelope He and C 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 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 He-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.

Astrophysics