Flash Mixing on the White Dwarf Cooling Curve: Spectroscopic
  Confirmation in NGC 2808
  [Abridged] We present new HST FUV spectroscopy of 24 hot evolved stars in
NGC2808, a massive globular cluster with a large population of "blue-hook"
(BHk) stars. The BHk stars are found in UV color-magnitude diagrams of the most
massive globular clusters, where they fall at luminosities immediately below
the hot end of the horizontal branch (HB), in a region of the HR diagram
unexplained by canonical stellar evolution theory. Using new evolutionary and
atmospheric models, we have shown that these subluminous HB stars are very
likely the progeny of stars that undergo extensive internal mixing during a
late He-core flash on the white dwarf cooling curve. This flash mixing leads to
hotter temperatures and an enormous enhancement of the surface He and C; these
hotter temperatures, together with the decrease in H opacity shortward of the
Lyman limit, make the BHk stars brighter in the EUV while appearing subluminous
in the UV and optical. Our FUV spectroscopy demonstrates that, relative to
normal HB stars at the same color, the BHk stars of NGC2808 are hotter and
greatly enhanced in He and C, thus providing unambiguous evidence of flash
mixing. Although the C abundance in the BHk stars is orders of magnitude larger
than that in the normal HB stars, the C abundance in both the BHk and normal HB
stars appears to be affected by gravitational settling. The variations seen in
Si and the Fe-peak elements also indicate that atmospheric diffusion is at play
in our sample, with all of our hot subdwarfs at 25,000 to 50,000 K exhibiting
large enhancements of the Fe-peak elements. The hottest subdwarfs in our BHk
sample may be pulsators, given that they fall in the temperature range of
newly-discovered pulsating subdwarfs in omega Cen. In addition to the normal
hot HB and BHk stars, we obtain spectra of 5 blue HB stars, a post-HB star, and
3 unclassified stars with unusually blue UV colors.
