Flash Mixing on the White Dwarf Cooling Curve: Understanding Hot Horizontal Branch Anomalies in NGC 2808

Abstract

We present an ultraviolet color-magnitude diagram (CMD) spanning the hot horizontal branch (HB), blue straggler, and white dwarf populations of the globular cluster NGC 2808. These data were obtained with the far-UV and near-UV cameras on the Space Telescope Imaging Spectrograph (STIS). Although previous optical CMDs of NGC 2808 show a high-temperature gap within the hot HB population, no such gap is evident in our UV CMD. Instead, we Ðnd a population of hot subluminous HB stars, an anomaly only previously reported for the globular cluster u Cen. Our theoretical modeling indicates that the location of these subluminous stars in the UV CMD, as well as the high-temperature gap along the HB in optical CMDs, can be explained if these stars underwent a late helium-core Ñash while descending the white dwarf cooling curve. We show that the convection zone produced by such a late helium Ñash will penetrate into the hydrogen envelope, thereby mixing hydrogen into the hot helium-burning interior, where it is rapidly consumed. This phenomenon is analogous to the "" born again ÏÏ scenario for producing hydrogen-deÐcient stars following a late helium-shell Ñash. The Ñash mixing of the envelope greatly enhances the envelope helium and carbon abundances, and leads, in turn, to a discontinuous increase in the HB e †ective temperatures at the transition between canonical and Ñash-mixed stars. We argue that the hot HB gap is associated with this theoretically predicted dichotomy in the HB properties. Moreover, the changes in the emergent spectral energy distribution caused by these abundance changes are primarily responsible for explaining the hot subluminous HB stars. Although further evidence is needed to conÐrm that a late helium-core Ñash can account for the subluminous HB stars and the hot HB gap, we demonstrate that an understanding of these stars requires the use of appropriate theoretical models for their evolution, atmospheres, and spectra.

Effects of tidally enhanced stellar wind on the horizontal branch morphology of globular clusters

“…Stars that undergo an early hot flash are located on the blue end of the canonical EHB of the H-R diagram (e.g., see Fig. 9 in Brown et al 2001). The second type is called late hot flash, for which the helium flash occurs on a WD cooling curve (after the primary star experiences a huge mass loss on the RGB).…”

“…The second type is called late hot flash, for which the helium flash occurs on a WD cooling curve (after the primary star experiences a huge mass loss on the RGB). In this type, the helium flash can change the chemical composition of its envelope by enhancing the helium and carbon abundance through helium-flash mixing (Iben 1976;Sweigart 1997;Brown et al 2001). Therefore, the late hot flashers have significantly higher effective temperatures on the HB, and they are fainter than the canonical EHB stars in the CMD.…”

“…The late-hot-flash model has been used to explain the existence of blue-hook stars found in several massive GCs (Whitney et al 1998;D'Cruz et al 2000;Brown et al 2001). In our calculations, however, we did not consider the helium-flash mixing process.…”

“…3.1, which correspond to a higher effective temperature than the canonical EHB stars. Brown et al (2001) have suggested that these stars undergo a late hot helium flash while descending along one of the WD cooling curves. The helium and carbon abundance in the envelope of these stars are enhanced by the helium flash mixing, which is not considered in our model calculations (see discussion in Sect.…”

Horizontal Branch Stars and the Ultraviolet Universe

“…In fact, even beyond the EHB proper there may still be "life" on the HB phase: as recently discussed by several authors, the RGB progenitors of HB stars may (somehow) lose so much mass prior to arriving on the ZAHB that they may miss the helium "flash" at the RGB tip altogether, but still ignite helium during the white dwarf cooling curve (e.g., [8,13]). Such "late flashers" are predicted to be even hotter than EHB stars, and have quite anomalous surface abundances compared to EHB stars (see, e.g., [60,61,49]).…”