Theoretical expressions for evolutionary period changes in nonradially pulsating stars.
Abstract
The twotimevariable approach is used to study the equations governing linear pulsations, radial or nonradial, of a star evolving on its HelmholtzKelvin time scale. It is assumed that the isentropic approximation of the period of the pulsation mode considered is much shorter than the star's HelmholtzKelvin time scale. An integral expression for the rate of evolutionary change of the isentropic approximation of the pulsation period is derived and is shown to be determined by the isentropic approximation of the eigenfunctions and by the rates of change of three properties of the mass layers inside the evolving star: the radial distance to the center, the isentropic sound velocity, and the density. A decrease or an increase of the isentropic sound velocity in a mass layer contributes to an increase or a decrease, respectively, of the period. A tentative approximation for the rates of evolutionary change of the isentropic approximations of periods of highorder nonradial gmodes is derived under the restriction that the region of the star that predominates in determining these periods is not located close to the surface.
 Publication:

Astronomy and Astrophysics
 Pub Date:
 November 1987
 Bibcode:
 1987A&A...186..170B
 Keywords:

 Radial Distribution;
 Stellar Evolution;
 Stellar Oscillations;
 Variable Stars;
 Vibration Mode;
 Density Distribution;
 Isentropic Processes;
 KelvinHelmholtz Instability;
 Stellar Mass;
 Astrophysics