The Blue hook populations of massive globular clusters

We present new Hubble Space Telescope ultraviolet color-magnitude diagrams of five massive Galactic globular clusters: NGC 2419, NGC 6273, NGC 6715, NGC 6388, and NGC 6441. These observations were obtained to investigate the "blue hook" (BH) phenomenon previously observed in UV images of the globular clusters ω Cen and NGC 2808. Blue hook stars are a class of hot (approximately 35,000 K) subluminous horizontal branch stars that occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. By coupling new stellar evolution models to appropriate non-LTE synthetic spectra, we investigate various theoretical explanations for these stars. Specifically, we compare our photometry to canonical models at standard cluster abundances, canonical models with enhanced helium (consistent with cluster self-enrichment at early times), and flash-mixed models formed via a late helium-core flash on the white dwarf cooling curve. We find that flash-mixed models are required to explain the faint luminosity of the BH stars, although neither the canonical models nor the flash-mixed models can explain the range of color observed in such stars, especially those in the most metal-rich clusters. Aside from the variation in the color range, no clear trends emerge in the morphology of the BH population with respect to metallicity.