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，其部分目的是进一步探索白色矮星的独特性质。作为绝大多数恒星演化的最终产物，白色矮星具有恒星生命中过去重要事件的特征：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测试我们的物质理论的理想宇宙实验室。最近，盖亚数据提供了这些恒星结晶的第一个直接证据。白色矮星也可以作为宇宙计时器和距离指示器，用于银河系的不同组成部分（盘，晕，疏散星团和球状星团）。在过去的十年中，在数十颗白色矮星的表面发现了行星碎片，这使得以一种独特而新颖的方式确定星子的整体组成成为可能。白色矮星在许多方面表现为恒星演化的“化石”，为了破译其中包含的信息，该领域的研究人员可以使用基于模型大气理论、恒星演化理论和恒星脉动理论的强大工具。我提出了几个项目，沿着广阔的途径，一方面是星震学，另一方面是演化，宇宙年代学，和光谱演化的白色矮星。这包括充分利用我们最近在白色矮星地震学方面的建模突破（Giammichele et al. 2018，Nature，554，73）以及来自Kepler，K2和TESS的数据。这也包括光谱演化理论的练习，基于我们最新版本的蒙特利尔演化代码，现在能够处理的问题，如吸积epidodes行星碎片到白色矮星和形成和不同光谱类型的恒星之间的连接，如PG 1159，DO，DB和DQ。此外，还提出了对盖亚发现的结晶特征进行详细研究的建议，这对宇宙年代学有着重要的意义。这一提议还涉及另一类致密恒星，即热亚矮星，其起源仍在争论中。它们在恒星演化理论的宏伟蓝图中构成了一种“灰色地带”。它们中的许多仍然需要被表征，这是最好的方法，结合模型大气理论和光谱观测（理想情况下达到紫外线范围，因为这些都是非常热的恒星）。此外，大自然为我们提供了不少于4种不同类型的脉动热次矮星，这些次矮星已经成为目前存在的最好的星震学试验台之一。质量，沿着与其他结构性质可以确定准确和精确的这些恒星。过去二十年来，我一直对这些物体感兴趣，并根据最近令人困惑的发现和新的空间数据提出了几项研究建议。**

This proposal is about a research program aimed, in part, at further exploring the unique properties of white dwarf stars. As 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. Very recently, Gaia data have provided the first direct evidence of crystallization in these stars. White dwarfs may also be used as cosmochronometers and distance indicators for the different components of the Galaxy (disk, halo, open and globular clusters). In the last decade, planetary debris have been identified at the surface of tens of white dwarfs, which has led to the exciting possibility of determining in a unique and novel way the bulk composition of planetesimals. To decipher the information contained in white dwarfs, which, in many ways, behave as "fossils" of stellar evolution, researchers in the field can use powerful tools based on model atmosphere theory, stellar evolution theory, and stellar pulsation theory. I propose several projects along the broad avenues of asteroseismology on the one hand, and of evolution, cosmochronology, and spectral evolution of white dwarfs on the other. This includes a full exploitation of our recent modeling breakthrough on the front of white dwarf seismology (Giammichele et al. 2018, Nature, 554, 73) in conjunction with data coming from Kepler, K2, and TESS. This also includes exercises in spectral evolution theory based on our most recent version of the Montréal evolutionary code that is now able to handle problems such as accretion epidodes of planetary debris onto white dwarfs and the formation and connection between stars of different spectral types such as PG1159, DO, DB, and DQ. A detailed investigation of the crystallization signature picked up by Gaia is also proposed, with implications for cosmochronology.**This proposal is also concerned with another category of 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 theory. Many of them still needs to be characterized, and this is best done by combining model atmosphere theory with spectroscopic observations (ideally reaching into the UV range since these are very hot stars). In addition, Nature has provided us with no less than 4 different types of pulsating hot subdwarfs, and those have turned out to be among the best asteroseismological testbeds that currently exist. Masses, along with other structural properties can be determined both accurately and precisely for these stars. I have been interested in those objects for the last two decades and propose several studies in the light of recent puzzling discoveries and new space data. **