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Evolution of white dwarfs with 3D model atmospheres

Periodic Reporting for period 2 - WD3D (Evolution of white dwarfs with 3D model atmospheres)

Reporting period: 2017-12-01 to 2019-05-31

The vast majority of stars will become white dwarfs at the end of the stellar life cycle. These remnants have ended nuclear reactions and can continue to exist owing to the well understood quantum pressure of free electrons. White dwarfs are precise cosmic clocks owing to their predictable cooling rates. They provide one of the most sensitive tests of when our Milky Way was formed. Our team is designing a robust theoretical framework to shed new light on the interior structure of white dwarfs, associate them with their progenitor stars, and enhance their potential as probes of fundamental astrophysical relations. Three dimensional (3D) simulations of white dwarf atmospheres including direct studies of convective flows and radiation transfer are currently performed to tackle current issues with widely used crude 1D model atmospheres. This will directly lead to more accurate masses and cooling ages for white dwarfs. The ambitious goal of the project is to calculate 3D simulations for stellar remnants of all atmospheric chemical compositions and connect these surface calculations to interior structure models, where the gas turns into a liquid and then a solid. The project is timely since the improved theoretical tools are essential to analyse the Gaia satellite data, where the number of known white dwarfs has just increased by a factor of about ten. We are currently using this theoretical framework with Gaia data, supplemented by other surveys and dedicated follow-up observations, to extract an unprecedented wealth of information from white dwarfs. Our team will set the standards for the star formation history and the age of the Milky Way. We are also studying evolved planetary systems in which rocks and debris are currently colliding with their white dwarf hosts, providing direct and unique insight into the bulk chemical composition of exo-terrestrial material.
We have increased the number of known white dwarfs by an order of magnitude. In a major team effort we identified 260.000 white dwarfs among the 1.5 billion objects released by the Gaia mission in April 2018. This first characterisation of this enormous sample forms the base of our involvement in the forthcoming large spectroscopic surveys over the next decade, and will without doubt be a standard reference in the field for years to come.

More than 300 simulations of white dwarfs in three-dimensions were calculated, enhancing by a factor of nearly ten the number of such realistic calculations. Most of our simulations have been focused on helium atmosphere and magnetic white dwarfs, which were modeled in 3D for the first time. Our set of accurate theoretical tools vastly improve the framework to analyse most of the 260.000 white dwarfs detected from Gaia. We have started this task, deriving white dwarf parameters (mass, temperature, age) by combining our models, Gaia data, and external independent spectroscopic observations. This has resulted in unexpected discoveries that will be published next year.

A significant fraction of white dwarfs are hosts to evolved planetary systems, and many of them are currently accreting disrupted minor planets. These systems inform us of the fate of planetary systems and provide accurate insight into the bulk rocky composition of exo-planetary systems that is otherwise inaccessible. We have directly simulated in 3D some of these accretion events, and found that the mass of the accreted material could be orders of magnitude larger than previously thought, suggesting that some white dwarfs could have had a meal of moon-sized planets.
Building on the success of many years of improving the realism of white dwarf models, the major outcome of the project will be a new theoretical framework based on 3D model atmospheres for practically all white dwarf studies. These atmospheric models are currently being connected to white dwarf evolution models. This has enormous potential to strengthen the position of white dwarfs as the most precise cosmic clocks owing to the direct relation between their mass, temperature and age. The next objectives will be to deepen our understanding of the fate of planetary systems and the bulk composition of exoplanets, and use our Gaia white dwarf catalogue to set the standards for the star formation history and initial mass function (2 to 8 solar masses) in the disk and halo of the Milky Way, providing a benchmark for galactic evolution models. We will also tackle the problem of the elusive origin of magnetic white dwarfs. Finally, we will improve the use of white dwarfs as fundamental flux calibrators for measuring the extra-galactic distance scale, with direct implications on Dark Energy and the nature of the Universe.
Interior of white dwarf under crystallisation