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

这个提议是关于一个研究项目的，部分目的是进一步探索白矮星的独特特性。对于绝大多数恒星来说，白矮星是恒星演化的最终产物，它带有过去对恒星生命至关重要的事件的特征：质量损失、对流混合阶段、角动量的损失和重新分配、热核燃烧。它们也是在极端压力和温度条件下检验我们的物质理论的理想宇宙实验室。白矮星也可用作宇宙计时器和银河系不同组成部分（星系盘、光晕、开放星团和球状星团）的距离指示器。最近，在几颗白矮星的表面发现了行星碎片，这引起了人们的兴奋

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.