DAVs: Red edge and Outbursts

TL;DR

This paper explains the red edge of the DAV instability strip and the origin of outbursts in cool DAVs, linking nonlinear mode interactions to observed pulsation behaviors and outburst phenomena.

Contribution

It provides a quantitative model connecting the red edge of the DAV instability strip with nonlinear mode couplings that cause outbursts in cool pulsating white dwarfs.

Findings

The minimal frequency for overstable modes increases sharply near the red edge.

High frequency overstable modes exist below the red edge but are usually undetectable.

Nonlinear mode couplings can produce outbursts observed in cool DAVs.

Abstract

As established by ground based surveys, white dwarfs with hydrogen atmospheres pulsate as they cool across the temperature range, $12500\,\mathrm{K} \gtrsim T_{\mathrm{eff}} \gtrsim 10800\,\mathrm{K}$. Known as DAVs or ZZ Ceti stars, their oscillations are attributed to overstable g-modes excited by convective driving. The effective temperature at the blue edge of the instability strip is slightly lower than that at which a surface convection zone appears. The temperature at the red edge is a two-decade old puzzle. Recently, {\it Kepler} discovered a number of cool DAVs which pulsate at higher frequencies and with much smaller photometric amplitudes than expected based on trends extrapolated from DAVs found by ground based observations. Remarkably, some of them exhibit sporadic outbursts separated by days, each lasting several hours, and releasing $\sim 10^{33}-10^{34}\,\mathrm{erg}$. We provide quantitative explanations for both the red edge and the outbursts. The minimal frequency for overstable modes rises abruptly near the red edge. Although high frequency overstable modes exist below the red edge, their photometric amplitudes are generally too small to be detected by ground based observations. Nevertheless, these overstable parent modes can manifest themselves through nonlinear mode couplings to damped daughter modes which generate limit cycles giving rise to photometric outbursts.