Armagh Observatory Preprint Series No. xxx

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Merged binary white dwarf evolution: rapidly accreting carbon-oxygen white dwarfs and the progeny of extreme helium stars

Authors: H. Saio, C.S.Jeffery

Journal: MNRAS in press

Abstract. We have examined the evolution of merged low-mass double white dwarfs which become luminous helium stars. We have approximated the merging process by the rapid accretion of matter, consisting mostly of helium, onto a carbon-oxygen (CO) white dwarf. After a certain mass is accumulated, a helium shell flash occurs, the radius and luminosity increase and the star becomes a yellow giant. Mass accretion is stopped artificially when the total mass reaches a pre-determined value. When the mass above the helium burning shell becomes small enough, the star evolves blueward almost horizontally in the HR diagram. The theoretical models for the merger of a 0.6 Msolar CO white dwarf with a 0.3 Msolar He white dwarf agree very well with the observed locations of extreme helium stars in the log Teff - log g diagram, with their observed rates of blueward evolution, and with luminosities and masses obtained from their pulsations. Together with predicted merger rates for CO+He white dwarf pairs, the evolutionary time scales are roughly consistent with observed numbers of extreme helium stars. Predicted surface carbon and oxygen abundances can be consistent with observed ones if carbon and oxygen produced in the helium-shell during a previous AGB phase is assumed to exist in the helium zone of the initial CO white dwarfs. These results establish the CO+He white dwarf merger as the best, if not only, viable model for the creation of extreme helium stars and, by association, the majority of R Coronae Borealis stars.

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