Core helium flash and the origin of CH and carbon stars

Astronomy and Astrophysics – Astrophysics

Scientific paper

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Abundance, Carbon Stars, Helium, Nuclear Fusion, Stellar Models, Cores, Stellar Envelopes, Stellar Structure, Time Dependence

Scientific paper

Two evolutionary sequences are obtained for a model of a 0.8-solar-mass Population II star with an initial chemical composition of X = 0.7, Y = 0.299, Z = 0.001, whose evolution is followed through an off-center core helium flash. The mass fraction at which helium ignites is considered as a free parameter; helium burning is started at 0.27 solar mass in the first sequence and at 0.40 solar mass in the second. It is found that no carbon produced in helium burning is mixed to the surface in the first case, but the second sequence leads to core penetration by the convective envelope soon after the main flash as well as much carbon mixing into the envelope. The models indicate that the amount of carbon produced during the flash increases with mass fraction at the ignition point. It is suggested that these models may explain the carbon overabundance in CH stars and that at least some R-type carbon stars owe their origin to an off-center core helium flash.

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