Pulsations of intermediate–mass stars on the asymptotic giant branch

Abstract

Evolutionary tracks from the zero age main sequence to the asymptotic giant branch were computed for stars with initial masses 2 M_⊙ ≤ M_ZAMS ≤ 5 M_⊙ and metallicity Z = 0.02. Some models of evolutionary sequences were used as initial conditions for equations of radiation hydrodynamics and turbulent convection describing radial stellar pulsations. The early asymptotic giant branch stars are shown to pulsate in the fundamental mode with periods 30 day ≲ Π ≲ 400day. The rate of period change gradually increases as the star evolves but is too small to be detected (Π̇/Π < 10⁻⁵ yr⁻¹). Pulsation properties of thermally pulsing AGB stars are investigated on time intervals comprising 17 thermal pulses for evolutionary sequences with initial masses M_ZAMS = 2 M_⊙ and 3 M_⊙ and 6 thermal pulses for M_ZAMS = 4 M_⊙ and 5 M_⊙. Stars with initial masses M_ZAMS ≤ 3 M_⊙ pulsate either in the fundamental mode or in the first overtone, whereas more massive red giants (M_ZAMS ≥ 4 M_⊙) pulsate in the fundamental mode with periods Π ≲ 10³ day. Most rapid pulsation period change with rate −0.02 yr⁻¹ ≲ Π̇/Π ≲ −0.01 yr⁻¹ occurs during decrease of the surface luminosity after the maximum of the luminosity in the helium shell source. The rate of subsequent increase of the period is Π̇/Π ≲ 5 × 10⁻³ yr⁻¹.