Radial pulsations of stars on the stage of the final helium flash

Yu. A. Fadeyev

doi:10.1134/S1063773719100037

paper

Radial pulsations of stars on the stage of the final helium flash

arXiv:1909.04500 · doi:10.1134/S1063773719100037

Abstract

Stellar evolution calculations to the stage of the cooling white dwarf were done for population~I stars with masses on the main sequence $1 M_{⊙} \leq M_{0} \leq 1.5 M_{⊙}$ . The final helium flash LTP is shown to occur in post--AGB stars with initial masses $1.3 M_{⊙} \leq M_{0} \leq 1.32 M_{⊙}$ for the overshooting parameter $f = 0.016$ . In the case of more effective overshooting ( $f = 0.018$ ) the final helium flash occurs at initial masses $1.28 M_{⊙} \leq M_{0} \leq 1.3 M_{⊙}$ . Fivefold variations of the parameter responsible for the mass loss rate during the post--AGB stage do not affect occurrence of the final helium flash but lead to perceptible changes of the evolutionary time. Selected models of two evolutionary sequences with initial mass $M_{0} = 1.3 M_{⊙}$ computed with overshooting parameters $f = 0.016$ and $f = 0.018$ were used as initial conditions in solution of the equations of hydrodynamics describing radial oscillations of stars on the stage of the final helium flash at effective temperatures less than $1 0^{4}$ K. The maximum pulsation period $Π = 117$ day determined for the evolutionary sequence $M_{0} = 1.3 M_{⊙}$ , $f = 0.016$ is in a good agreement with observational estimates of the period of FG Sge. The mass, the radius and the effective temperature of the star are $M = 0.565 M_{⊙}$ , $R = 126 R_{⊙}$ and $T_{eff} = 4445$ K, respectively. At the same time the average period change rate of FG Sge from 1960 to 1990 is nearly three time larger than its theoretical estimate.

9 pages, 1 table, 7 figures, accepted for publication in Astronomy Letters

Radial pulsations of stars on the stage of the final helium flash

Abstract

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