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

By Thomas M. Brown, A. Sweigart, T. Lanz, Wayne B. Landsman, I. Hubeny. 2001

146 citations recorded by the source corpus.

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 find a population of hot subluminous HB stars, an anomaly only previously reported for the globular cluster ω 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 flash while descending the white dwarf cooling curve. We show that the convection zone produced by such a late helium flash 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" scenario for producing hydrogen-deficient stars following a late helium-shell flash. The flash mixing of the envelope greatly enhances the envelope helium and carbon abundances, and leads, in turn, to a discontinuous increase in the HB effective temperatures at the transition between canonical and flash-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 confirm that a late helium-core flash 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.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/bbbe812eb2172c15a538d42518c4b6401caa1ee3

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Astronomy and Astrophysics

1. F. D’Antona
2. Jay Anderson
3. G. Piotto
4. A. Karakas
5. A. Milone
6. L. Bedin
7. R. Gratton
8. A. Renzini

In Instrumentation

1. F. D’Antona
2. Jay Anderson
3. R. Gratton
4. G. Piotto
5. A. Milone
6. L. Bedin
7. A. Renzini
8. A. Karakas

Citations in the corpus

• The fraction of second generation stars in globular clusters from the analysis of the horizontal branch (2008, methodology): “…we try to model better the EBT2 and EBT3 (in the definition by Bedin et al. 2004) blue clumps, which contain the extreme HB (D’Cruz et al. 2000; Brown et al. 2001) and the “blue hook” stars (Moehler et al. 2004), respectively, and compare also simulations based on HST ultraviolet and visual…”
• CONVECTIVE–REACTIVE PROTON–12C COMBUSTION IN SAKURAI'S OBJECT (V4334 SAGITTARII) AND IMPLICATIONS FOR THE EVOLUTION AND YIELDS FROM THE FIRST GENERATIONS OF STARS (2010, background): “…and accreting white dwarfs (Cassisi et al. 1998) that may be the progenitors of SN Ia. Convective-reactive events have been found in post-RGB stellar evolution models and associated with the horizontal branch anomalies in certain globular clusters (Brown et al. 2001; Miller Bertolami et al. 2008).”
• Asteroseismology of old open clusters with Kepler: direct estimate of the integrated red giant branch mass-loss in NGC 6791 and 6819 (2011, background): “The latter may be populated by the less massive stars evolving from the RGB that were still able to ignite a late helium core flash (e.g. Castellani & Castellani 1993; Brown et al. 2001).”
• THE HUBBLE SPACE TELESCOPE UV LEGACY SURVEY OF GALACTIC GLOBULAR CLUSTERS. I. OVERVIEW OF THE PROJECT AND DETECTION OF MULTIPLE STELLAR POPULATIONS (2014, background): “These structures are the outcome of “late heliumcore flash” ignition (D’Cruz et al. 1996, Brown et al. 2001).”
• Evolution and nucleosynthesis of extremely metal-poor and metal-free low- and intermediate-mass stars - I. Stellar yield tables and the CEMPs (2008, background): “Sweigart 1997; Brown et al. 2001; Cassisi et al. 2003).”
• Evolution of Stars and Stellar Populations (2005, background): “which experience this event while cooling along the white dwarf cooling sequence ([22],[45]) as well as in extremely metal-poor stars ([196]).”
• Super-Helium-rich Populations and the Origin of Extreme Horizontal-Branch Stars in Globular Clusters (2005, methodology): “In Figure 4, we have compared HST/STIS UV CMD of extreme HB and blue HB stars in NGC 2808 (Brown et al. 2001) with the models constructed in the same band passes based on the ZAHB loci and post-ZAHB evolutionary tracks of Sweigart (1987).”
• The Dawes Review 2: Nucleosynthesis and Stellar Yields of Low- and Intermediate-Mass Single Stars (2014, background): “Note that PIEs are also seen in the “late hot flasher” scenario (Sweigart 1997; Brown et al. 2001; Cassisi et al. 2003) and the “born-again” or “(very) late thermal pulse” scenario for explaining objects like Sakurai’s Object (Iben et al. 1983; Asplund et al. 1997; Herwig et al. 1999).”
• A Helium Spread among the Main-Sequence Stars in NGC 2808 (2005, background): “As a consequence the group of HB stars fainter than the “normal” ZAHB detected, e.g., in ω Cen and in NGC 2808 with UV observations (D’Cruz et al. 2000; Brown et al. 2001) could perhaps be intepreted as belonging to a sub-population having an helium abundance higher than those populating the…”
• Metallicities on the Double Main Sequence of ω Centauri Imply Large Helium Enhancement (2004, methodology): “Moehler et al. interpret this anomalously high He abundance as the effect of the extra mixing in late He-flash stars (Brown et al. 2001).”
• The origin of subdwarf B stars – II (2003, background): “Brown et al. (2001) pointed out that the envelope composition can be changed dramatically (e.g. from hydrogen-rich to helium-rich) owing to heliumflash-induced mixing between the interior and the envelope.”