Hydrogen- and Helium-Shell Flashes and FG Sagittae Phenomenon

Extract

Evolution of an electron-degenerate carbon-oxygen star of 1.08 M₀ was computed assuming that hydrogen-rich gas was accreted at 1.58 × 10⁻⁷ M₀y⁻¹. Such a star mimicks the evolution of FG Sge, if a part of the ejected mass accretes back onto the central star. When a hydrogen envelope of 4 × 10⁻⁶ M₀ was formed, a hydrogen-shell flash began. Succeeding 300 hydrogen-shell flashes were suppressed artificially to compute the growth of helium zone. After 8100 years of accretion, helium-shell flash began. Helium convection zone was found to reach the bottom of the hydrogen envelope, because the entropy barrier was low for the thin envelope. Protons were mixed into the helium zone, which produced neutrons and s-process elements. The mixing triggered also the hydrogen-she11 flash. Hydrogen convection zone reached a mass shell of 10⁻⁸ M₀ measured from the surface. After the helium-shell flash ceased, the hydrogen envelope expanded greatly and a deepening surface convection dredged up the s-process elements. (Paper to be submitted to Publ. Astron. Soc. Japan.)