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Blue hook stars are a class of subluminous extreme horizontal branch starsthat were discovered in UV images of the massive globular clusters omega Cenand NGC 2808. These stars occupy a region of the HR diagram that is unexplainedby canonical stellar evolution theory. Using new theoretical evolutionary andatmospheric models, we have shown that the blue hook stars are very likely theprogeny of stars that undergo extensive internal mixing during a latehelium-core flash on the white dwarf cooling curve. This "flash mixing"produces hotter-than-normal EHB stars with atmospheres significantly enhancedin helium and carbon. The larger bolometric correction, combined with thedecrease in hydrogen opacity, makes these stars appear subluminous in theoptical and UV. Flash mixing is more likely to occur in stars born with a highhelium abundance, due to their lower mass at the main sequence turnoff. Forthis reason, the phenomenon is more common in those massive globular clustersthat show evidence for secondary populations enhanced in helium. However, ahigh helium abundance does not, by itself, explain the presence of blue hookstars in massive globular clusters. Here, we present new observational evidencefor flash mixing, using recent HST observations. These include UVcolor-magnitude diagrams of six massive globular clusters and far-UVspectroscopy of hot subdwarfs in one of these clusters (NGC 2808).

New Observational Evidence of Flash Mixing on the White Dwarf Cooling Curve