DAVs: Red Edge and Outbursts

Abstract

As established by photometric surveys, white dwarfs with hydrogen atmospheres and surface gravity, log(g) ≈ 8.0 pulsate as they cool across the temperature range of 12,500 K ≳ T_(eff) ≳ 10,800 K. Known as DAVs or ZZ Ceti stars, their oscillations are attributed to gravity modes excited by convective driving. Overstability requires convective driving to exceed radiative damping. Previous works have demonstrated that ω ≳ max(τ_c^( −1), L_(ℓ,b)) is a necessary and sufficient condition for overstability. Here τ_c and L ℓ,b are the effective thermal timescale and Lamb frequency at the base of the surface convection zone. Below the observational red edge, L(ℓ,b) » τ_c^( −1), so overstable modes all have ωτ_c » 1. Consequently, their photometric amplitudes are reduced by that large factor rendering them difficult to detect. Although proposed previously, the condition ω ≳ L_(ℓ,b) has not been clearly interpreted. We show that modes with ω < L_(ℓ,b) suffer enhanced radiative damping that exceeds convective driving rendering them damped. A quasi-adiabatic analysis is adequate to account for this enhancement. Although this approximation is only marginally valid at the red edge, it becomes increasingly accurate toward both higher and lower T_(eff). Recently, Kepler discovered a number of cool DAVs that exhibit sporadic flux outbursts. Typical outbursts last several hours, are separated by days, and release ~10^(33) – 10^(34) erg. We attribute outbursts to limit cycles arising from sufficiently resonant 3-mode couplings between overstable parent modes and pairs of radiatively damped daughter modes. Limit cycles account for the durations and energies of outbursts and their prevalence near the red edge of the DAV instability strip.