New Observational Evidence of Flash Mixing on the White Dwarf Cooling
  Curve
  Blue hook stars are a class of subluminous extreme horizontal branch stars
that were discovered in UV images of the massive globular clusters omega 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).
