DAVs: Red edge and Outbursts

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.

accepted to ApJ

References in corpus (7)

• Modules for Experiments in Stellar Astrophysics (MESA)
• White Dwarf Rotation as a Function of Mass and a Dichotomy of Mode Linewidths: Kepler Observations of 27 Pulsating DA White Dwarfs Through K2 Campaign 8
• Calibration of the Mixing-Length Theory for Convective White Dwarf Envelopes
• KIC 4552982: Outbursts and Asteroseismology from the Longest Pseudo-Continuous Light Curve of a ZZ Ceti
• A second case of outbursts in a pulsating white dwarf observed by Kepler
• Outbursts in two new cool pulsating DA white dwarfs
• The First Six Outbursting Cool DA White Dwarf Pulsators

Cited by in corpus (7)

• Pulsating white dwarfs: new insights
• Current challenges in the physics of white dwarf stars
• Low-frequency variability in massive stars: Core generation or surface phenomenon?
• Limits on Mode Coherence in Pulsating DA White Dwarfs Due to a Non-static Convection Zone
• Tidal Asteroseismology: Possible Evidence of Non-linear Mode Coupling in an Equilibrium State in Kepler Eclipsing Binary KIC 3230227
• Flares hunting in hot subdwarf and white dwarf stars from Cycles 1-5 of TESS photometry
• White-dwarf asteroseismology: an update