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The origin and pulsations of extreme helium stars†

Published online by Cambridge University Press:  18 February 2014

C. Simon Jeffery
C. Simon Jeffery*
Affiliation:
Armagh Observatory, College Hill, Armagh BT61 9DG, Northern Ireland, UK
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Abstract

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Stars consume hydrogen in their interiors but, generally speaking, their surfaces continue to contain some 70% hydrogen (by mass) throughout their lives. Nevertheless, many types of star can be found with hydrogen-deficient surfaces, in some cases with as little as one hydrogen atom in 10 000. Amongst these, the luminous B- and A-type extreme helium stars are genuinely rare; only ~15 are known within a very substantial volume of the Galaxy.

Evidence from surface composition suggests a connection to the cooler R CrB variables and some of the hotter helium-rich subdwarf O stars. Arguments currently favour an origin in the merger of two white dwarfs; thus there are also connections with AM CVn variables and Type Ia supernovae. Pulsations in many extreme helium stars provide an opportune window into their interiors. These pulsations have unusual properties, some being “strange” modes, and others being driven by Z-bump opacities. They have the potential to deliver distance-independent masses and to provide a unique view of pulsation physics.

We review the evolutionary origin and pulsations of these stars, and introduce recent progress and continuing challenges.

Keywords

stars: early-typestars: chemically-peculiarstars: supergiantsstars: white dwarfsstars: evolutionstars: oscillationsstars: variable: otherstars: individual: V652 HerFQ Aqr
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 9 , Symposium S301: Precision Asteroseismology , August 2013 , pp. 297 - 304
Copyright
Copyright © International Astronomical Union 2014 

Footnotes

†

The full version of this paper, including additional sections on the asteroarchæology and origin of EHes, is available online at ArXiv: 1311.1635.

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