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

点击翻译按钮获取中文摘要

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. **