Limits on Mode Coherence in Pulsating DA White Dwarfs Due to a Non-static Convection Zone. (arXiv:2001.05048v1 [astro-ph.SR])

Limits on Mode Coherence in Pulsating DA White Dwarfs Due to a Non-static Convection Zone. (arXiv:2001.05048v1 [astro-ph.SR])
<a href="http://arxiv.org/find/astro-ph/1/au:+Montgomery_M/0/1/0/all/0/1">M. H. Montgomery</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hermes_J/0/1/0/all/0/1">J. J. Hermes</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Winget_D/0/1/0/all/0/1">D. E. Winget</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Dunlap_B/0/1/0/all/0/1">B. H. Dunlap</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Bell_K/0/1/0/all/0/1">K. J. Bell</a>

The standard theory of pulsations deals with the frequencies and growth rates
of infinitesimal perturbations in a stellar model. Modes which 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 finite amplitude pulsations. Models
predict that the mass of convection zones in pulsating hydrogen-atmosphere
(DAV) white dwarfs is very sensitive to temperature (i.e., $M_{rm CZ} propto
T_{rm eff}^{-90}$), leading to the possibility that even low-amplitude
pulsators may experience significant nonlinear effects. In particular, the
outer turning point of finite-amplitude g-mode 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.

The standard theory of pulsations deals with the frequencies and growth rates
of infinitesimal perturbations in a stellar model. Modes which 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 finite amplitude pulsations. Models
predict that the mass of convection zones in pulsating hydrogen-atmosphere
(DAV) white dwarfs is very sensitive to temperature (i.e., $M_{rm CZ} propto
T_{rm eff}^{-90}$), leading to the possibility that even low-amplitude
pulsators may experience significant nonlinear effects. In particular, the
outer turning point of finite-amplitude g-mode 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.

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