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

TL;DR

This paper provides observational evidence supporting the theory that blue hook stars in globular clusters are formed through flash mixing during late helium-core flashes, resulting in hotter, helium- and carbon-enhanced atmospheres.

Contribution

The study combines new theoretical models with recent HST observations to confirm flash mixing as the origin of blue hook stars in globular clusters.

Findings

UV color-magnitude diagrams show blue hook star populations

Far-UV spectroscopy reveals helium and carbon enhancement

Flash mixing is linked to high initial helium abundance

Abstract

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).