Resolving stellar pulsational shock waves in the extreme helium star V652 Her

V652 HER正在以8马赫的冲击波超音速脉动。我们首次提出利用强谱线中的谱线轮廓变化来直接观察这颗恒星中向外传播的冲击波。这将促进我们对脉动冲击波物理学的理解，直接应用于我们最近的脉动激波模型来解释新发现的强磁快速振荡AP(RoAp)恒星的行为。这将使我们能够具体地估计V652 Her上层大气中的能量损失，这对模拟这颗罕见的极端氦恒星中的铁不透明驱动具有重要意义。目前还没有包含能量损失的模型；在斯巴鲁观察的指导下，我们将首次将这种能量损失的物理学引入我们的模型中。我们还将测量最小半径处的压缩。我们将首次获得大气快速向外加速的时间分辨率--在大气进入自由落体之前的15分钟内，加速速度是恒星表面重力的10倍，这是令人震惊的短暂15分钟，而在2.5小时的脉动周期中，大气进入自由落体状态。HDS的高分辨率和宽波长范围还将提供前所未有的高S/N谱，从中我们将得出恒星中CNO周期产生的元素的详细丰度，这是一个罕见的、尚未完全理解的恒星演化阶段--可能是合并的白矮星双星。

V652 Her is pulsating supersonically with a Mach-8 shock wave. We propose to use line profile variations in strong spectral lines to see directly for the first time the outwardly propagating shock waves in this star. This will advance our understanding of the physics of pulsational shock waves, in general, with direct applications to our recent model of pulsational shocks to explain newly discovered behaviour in the strongly magnetic rapidly oscillating Ap (roAp) stars. It will allow us specifically to estimate energy losses in the upper atmosphere of V652 Her, with important implications for modelling the Fe-opacity driving in this rare extreme helium star. At present no models include energy losses; with Subaru observations to guide us, we will introduce for the first time the physics of such energy losses into our models. We will also measure the compression at minimum radius. We will have the first temporal resolution of the rapid outward acceleration of the atmosphere -- acceleration 10 times that of the surface gravity of the star for a stunningly brief 15 minutes before the atmosphere goes into free-fall for the rest of the 2.5-hr pulsation cycle. The high resolution and wide wavelength range of HDS will also give an unprecedented high S/N spectrum from which we will derive detailed abundances of the CNO-cycle produced elements in a star in a rare, and not fully understood, stage of stellar evolution -- a probable merged white dwarf binary.