Horizontal Branch Stars and the Ultraviolet Universe
Extremely hot horizontal branch (HB) stars and their progeny are widely considered to be responsible for the “ultraviolet upturn” (or UVX) phenomenon observed in elliptical galaxies and the bulges of spirals. Yet, the precise evolutionary channels that lead to the production of these stars remain the source of much debate. In this review, we discuss two key physical ingredients that are required in order for reliable quantitative models of the UV output of stellar populations to be computed, namely, the mass loss rates of red giant branch stars and the helium enrichment “law” at high metallicities. In particular, the recent evidence pointing towards a strong enhancement in the abundances of the α-elements in the Galactic bulge (compared to the disk), and also the available indications of a similar overabundance in (massive) elliptical galaxies, strongly suggest that the helium abundance Y may be higher in ellipticals and bulges than it is in spiral disks by an amount that may reach up to 0.15 at [Fe/H] ∼ +0.5. If so, this would strongly favor the production of hot HB stars at high metallicity in galactic spheroids. We also discuss the existence of mass loss recipes beyond the commonly adopted Reimers “law” that are not only more consistent with the available empirical data, but also much more favorable to the production of extended HB stars at high metallicity. Finally, we discuss new empirical evidence that suggests that different evolutionary channels may be responsible for the production of EHB stars in the field and in clusters.
KeywordsMass Loss Rate Globular Cluster Stellar Population Elliptical Galaxy Horizontal Branch
Unable to display preview. Download preview PDF.
- Caloi, V., & D’Antona, F. 2007, A&A, 463, 949Google Scholar
- Catelan, M. 2007a, Graduate School in Astronomy: XI Special Courses at the National Observatory of Rio de Janeiro (XI CCE), 930, 39Google Scholar
- Catelan, M. 2007b, in Horizontal Branch Stars: Observations, Theory, and Implications for the Formation of the Galaxy, in press (astro-ph/0507464)Google Scholar
- Catelan, M., et al. 2008, A Search for EHB Pulsators in the Globular Cluster NGC 6752. In: Hot Subdwarf Stars and Related Objects, ed U. Heber, H. Drechsel (ASP, San Francisco), in preparation Google Scholar
- Matteucci, F. 2007, Chemical Evolution Models of Ellipticals and Bulges. In: The Metal Rich Universe, in press, astro-ph/0610832Google Scholar
- Moehler, S. 2004, Horizontal branch A- and B-type stars in globular clusters. In: The A-Star Puzzle, IAU Symposium 224, ed by J. Zverko, J. Ziznovský, S. J. Adelman, W. W. Weiss (IAU, San Francisco), pp 395–402Google Scholar
- Moni Bidin, C., Moehler, S., Piotto, G., Momany, Y., Recio-Blanco, A., & Méndez, R. A. 2007a, The lack of binaries among hot horizontal branch stars: M80 and NGC 5986, in press (astro-ph/0606035)Google Scholar
- Moni Bidin, C., Catelan, M., & Altmann, M. 2007b, in preparation Google Scholar
- Reimers, D. 1975a, Circumstellar envelopes and mass loss of red giant stars. In: Problems in Stellar Atmospheres and Envelopes, ed by B. Baschek, W. H. Kegel, G. Traving (Springer, Berlin) pp 229–256Google Scholar
- Reimers, D. 1975b, Mem. Soc. R. Liège 6 Sér., 8, 369Google Scholar
- Shapley, H. 1915, Contrib. Mt. Wilson Obs., 116Google Scholar
- Valcarce, A. A. C., & Catelan, M. 2007, A&A, submitted Google Scholar