EXTREME HELIUM STARS C.S. JEFFERY

EXTREME HELIUM STARS: PULSATION AND EVOLUTION

C. SIMON JEFFERY

Abstract:

Extreme helium stars are highly evolved luminous stellar remnants. Their exotic surface abundances point to previous evolution through the white dwarf sequence, followed by re-ignition due to a late helium shell flash or to a binary merger. The existence of pulsations in many helium stars, due to strange-mode instabilities or the Z-bump $\kappa$-mechanism, provide a range of diagnostics including radii from Baade's method and contraction rates from period changes. I review the basic observational and theoretical properties of extreme helium star pulsations and show how these have been used to constrain evolutionary models, with particular reference to the cases of V652 Her and BX Cir.