DAV：红色边缘和爆发
DAVs: Red edge and Outbursts
摘要：
根据地面观测的结果，当白矮星在温度范围$12500\,\mathrm{K} \gtrsim T_{\mathrm{eff}} \gtrsim 10800\,\mathrm{K}$内冷却时，其氢大气层会发生脉动。被称为DAVs或ZZ Ceti星，它们的振荡归因于对流驱动激发的超稳定g模式。不稳定带蓝边的有效温度略低于表面对流区出现的温度。红边的温度是一个20年前的谜题。最近，{ \it Kepler}发现了许多冷DAVs，它们以更高的频率脉动，并且比基于地面观测发现的DAVs推断的趋势所预期的光度振幅小得多。值得注意的是，它们中的一些表现出以天为间隔的零星爆发，每次持续几个小时，并释放$\sim 10^{33}-10^{34}\,\mathrm{erg}$。我们提供了定量的解释红边和爆发。过稳模态的最小频率在红边附近突然上升。虽然在红边以下存在高频过稳模，但它们的光度振幅通常太小，地面观测无法探测到。然而，这些超稳定的父模式可以表现自己通过非线性模式耦合阻尼子模式产生极限环引起光度爆发。
英文摘要：
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.
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