Asteroseismology, Cosmochronology, Planetology, and Stellar Evolution at the Bottom of the HR Diagram

这项提议是关于一项研究计划，部分目的是进一步利用白矮星的独特性质。作为绝大多数恒星演化的最终产物，白矮星具有恒星生命中至关重要的过去事件的特征：质量损失、对流混合阶段、角动量损失和重新分配、热核燃烧。它们也是理想的宇宙实验室，在极端的压力和温度条件下测试我们的物质理论。白矮星也可以用作宇宙计时器和银河系不同组成部分(盘状、晕状、疏散状和球状星团)的距离指示器。最近，在几颗白矮星的表面发现了行星碎片，这导致了令人兴奋的 以一种独特而新颖的方式确定这些古老行星的“整体”组成的可能性。 为了破译白矮星中包含的信息，白矮星在许多方面都像恒星进化的“化石”一样，研究人员在 基于模型大气理论、恒星演化理论和恒星脉动理论的强大工具的现场处理。在后者中 在这种情况下，大自然选择与我们合作，因为至少有6种不同类型的脉动白矮星-每一种对应于 不同的进化阶段--现在已知，包括最近在2012年和2013年发现的两种新类型。除其他外，星震学可以用来确定白矮星的内部化学分层(核心成分和包层化学 分层)，这是冷却计算中的一个基本输入。因此，白矮星地震可能被用来“校准”，所谓的内部化学结构，这是充分利用这些恒星的宇宙时间潜力所必需的一步。 这一提议还涉及另一类进化和致密的恒星，即热亚矮星，其起源仍在争论中。它们在恒星演化的宏伟计划中构成了一种“灰色地带”，因此，它们的存在对那些对恒星演化理论感兴趣的人来说是一个挑战。众所周知，热矮星是前红巨星，它们的命运是成为低质量的白矮星，但它们的形成过程细节仍不确定。已经提出了一些相互竞争的形成情景，其中一些涉及双星演化(共同包络演化、罗氏叶溢出、合并)，另一些涉及单星演化(在红巨星阶段具有特殊的质量损失率)。最近有人提出，围绕“孤立的”热矮星(即不是双星系统的一部分)运行的行星可能会提供必要的“冲击力”，以驱逐红巨星相中的额外质量。在炽热的亚矮星周围探测到了行星。 这项提议的长期目标之一是区分已经提出的关于热矮星的各种形成情景。要做到这一点，最好的办法是推导出这些恒星的经验质量分布，它带有实际形成过程的特征。不幸的是，通过标准方法得到的这些恒星的可靠质量估计还很少。例如，对于热次矮星，光谱质量是出了名的不确定。然而，强大的星震学方法再次派上用场，因为大自然为我们提供了不少于4种不同类型的脉动热矮星(第一种类型发现于1997年，最新一种发现于2011年)。事实上，脉动的热矮星已被证明是目前存在的最好的小行星地震试验台，可以准确地确定这些恒星的质量以及许多其他性质。

This proposal is about a research program aimed, in part, at further exploiting the unique properties of white dwarf stars. As the final products of stellar evolution for the vast majority of stars, white dwarfs bear the signature of past events of fundamental importance in stellar life: mass loss, phases of convective mixing, loss and redistribution of angular momentum, thermonuclear burning. They are also ideal cosmic laboratories for testing our theories of matter under extreme conditions of pressure and temperature. White dwarfs may also be used as cosmochronometers and distance indicators for the different components of the Galaxy (disk, halo, open and globular clusters). Recently, planetary debris have been identified at the surface of several white dwarfs, which has led to the exciting possibility of determining in a unique and novel way the *bulk* composition of these ancient planets. To decipher the information contained in white dwarfs, which, in many ways, behave as "fossils" of stellar evolution, researchers in the field dispose of powerful tools based on model atmosphere theory, stellar evolution theory, and stellar pulsation theory. In the latter case, Nature has chosen to cooperate with us since no less than 6 distinct types of pulsating white dwarfs -- each corresponding to a different evolutionary phase -- are now known, including the recent discoveries of two new types in 2012 and 2013. Asteroseismology can be used to determine, among other things, the internal chemical layering of white dwarfs (core composition and envelope chemical stratification), which is an essential input in cooling calculations. Hence, white dwarf asteroseismology may be used to "calibrate", so to speak, the internal chemical structure, a step necessary for exploiting fully the cosmochronological potential of these stars. This proposal is also concerned with another category of evolved and compact stars, the hot subdwarfs, whose origin is still being debated. They constitute a sort of "gray zone" in the grand scheme of stellar evolution and, as such, their very existence represents a challenge for those interested in stellar evolution theory. It is known that hot subdwarfs are former red giant stars and that their destiny is to become low-mass white dwarfs, but the details of their formation process are still uncertain. A number of competing formation scenarios have been proposed, some involving binary evolution (common envelope evolution, Roche lobe overflow, mergers), others single star evolution (with peculiar mass loss rates in the red giant phase). It has been proposed recently that planets orbiting around "isolated" hot subdwarfs (i.e., not part of binary systems) may provide the necessary "kick" needed to expel extra mass in the red giant phase. And planets have been detected around hot subdwarfs. One of the long-term goals of this proposal is to discriminate between the various formation scenarios that have been proposed for hot subdwarfs. This can best be done by deriving the empirical mass distribution of these stars, which bears the signature of the actual formation process. Unfortunately, reliable mass estimates for these stars coming from the standard methods are far and few. For instance, spectroscopic masses are notoriously uncertain for hot subdwarfs. However, the powerful method of asteroseismology can again come to the rescue as Nature has provided us with no less than 4 different types of pulsating hot subdwarfs (first type discovered in 1997, and the newest one in 2011). In fact, the pulsating hot subdwarfs have turned out to be the best asteroseismological tesbeds that currently exist, and masses, along with many other properties, can be determined both accurately and precisely for these stars.