Limits on Mode Coherence in Pulsating DA White Dwarfs Due to a Nonstatic Convection Zone
Abstract
The standard theory of pulsations deals with the frequencies and growth rates of infinitesimal perturbations in a stellar model. Modes that are calculated to be linearly driven should increase their amplitudes exponentially with time; the fact that nearly constant amplitudes are usually observed is evidence that nonlinear mechanisms inhibit the growth of finiteamplitude pulsations. Models predict that the mass of convection zones in pulsating hydrogenatmosphere (DAV) white dwarfs is very sensitive to temperature (I.e., ${M}_{\mathrm{CZ}}\propto {T}_{\mathrm{eff}}^{90}$ ), leading to the possibility that even lowamplitude pulsators may experience significant nonlinear effects. In particular, the outer turning point of finiteamplitude gmode pulsations can vary with the local surface temperature, producing a reflected wave that is out of phase with what is required for a standing wave. This can lead to a lack of coherence of the mode and a reduction in its global amplitude. In this paper we show that (1) whether a mode is calculated to propagate to the base of the convection zone is an accurate predictor of its width in the Fourier spectrum, (2) the phase shifts produced by reflection from the outer turning point are large enough to produce significant damping, and (3) amplitudes and periods are predicted to increase from the blue edge to the middle of the instability strip, and subsequently decrease as the red edge is approached. This amplitude decrease is in agreement with the observational data while the period decrease has not yet been systematically studied.
 Publication:

The Astrophysical Journal
 Pub Date:
 February 2020
 DOI:
 10.3847/15384357/ab6a0e
 arXiv:
 arXiv:2001.05048
 Bibcode:
 2020ApJ...890...11M
 Keywords:

 White dwarf stars;
 DA stars;
 Computational methods;
 Stellar oscillations;
 Analytical mathematics;
 Stellar interiors;
 1799;
 348;
 1965;
 1617;
 38;
 1606;
 Astrophysics  Solar and Stellar Astrophysics
 EPrint:
 11 pages, 11 figures, accepted for publication in ApJ