In Hot Subdwarf Stars and Related Objects ASP Conference Series, Vol. 392, Proceedings of the conference held 23-27 July, 2007, at Otto-Friedrich-Universität, Bamberg, Germany. Edited by Ulrich Heber, C. Simon Jeffery, and Ralf Napiwotzki., p.3
astro-ph/0808.0254

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THE CURRENT UNDERSTANDING ON THE UV UPTURN

Sukyoung K. Yi


Yonsei University, Department of Astronomy, Seoul 120-749, Korea


Abstract: The unexpected high bump in the UV part of the spectrum found in nearby giant elliptical galaxies, a.k.a. the UV upturn, has been a subject of debate. A remarkable progress has been made lately from the observational side, mainly involving space telescopes. The GALEX UV telescope has been obtaining thousands of giant ellipticals in the nearby universe, while HST is resolving local galaxies into stars and star clusters. An important clue has also been found regarding the origin of hot HB stars, and perhaps of sdB stars. That is, extreme amounts of helium are suspected to be the origin of the extended HB and even to the UV upturn phenomenon. A flurry of studies are pursuing the physics behind it. All this makes me optimistic that the origin of the UV upturn will be revealed in the next few years. I review some of the most notable progress and remaining issues.


Table of Contents

INTRODUCTION

PREVIOUS OBSERVATIONS

THEORY
Metal-poor HB hypothesis
Metal-rich HB hypothesis
Metal-poor or metal-rich HB?

ISSUES
alpha-enhancement
EHB stars in star clusters
Binaries
Other issues

GALEX OBSERVATIONS

NEW ISSUE: ENHANCED-HELIUM POPULATION

NEW ISSUE: UV-BRIGHT GLOBULAR CLUSTERS IN M87

CONCLUSIONS

REFERENCES

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