Periodic Reporting for period 4 - WD3D (Evolution of white dwarfs with 3D model atmospheres)
Reporting period: 2020-12-01 to 2021-11-30
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. This action has designed 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 radiative transfer have been performed to tackle current issues with widely used crude 1D model atmospheres. This has directly led to more accurate masses and cooling ages for white dwarfs. The ambitious goal realised for this project has been 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 was timely since the improved theoretical tools have been essential to analyse the Gaia satellite data from the European Space Agency, which has increased the number of known white dwarfs by a factor of ten. Gaia data was supplemented by other surveys and dedicated follow-up observations to create a complete volume of white dwarfs within 130 light years. We have used this observational sample and our new theoretical framework to extract an unprecedented wealth of information from white dwarfs. The long term goal is to derive 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 have identified 350,000 white dwarfs among the 1.5 billion objects released by the Gaia mission in April 2018. Our published white dwarf catalogue derived from this enormous sample forms the base of our involvement in the forthcoming large spectroscopic surveys (4MOST, WAVE, DESI, SDSS-V) over the next decade, and has become a standard reference in the research area with hundreds of citations. Along the way we have also quantified data quality and the accuracy of current white dwarf models.
More than 300 simulations of white dwarf atmospheres 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 350,000 white dwarfs detected from Gaia. We have derived precise and accurate white dwarf parameters (mass, temperature, age) by combining our models, Gaia data, and external independent spectroscopic observations. This has resulted in many unexpected discoveries, including the first direct evidence of the core crystallisation of white dwarfs, a chemical phase transition of the carbon-oxygen interior from liquid to solid that was predicted 50 years ago but never observed. This has set the stage for a number of external follow-up publications on the evolution of white dwarfs and dense plasma physics. We have also discovered the closest double white dwarf merger product, an important system to quantify how stars merge and create supernovae explosions visible at distant cosmic scales.
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. This behaviour is due to the previously unexplored concept of convective overshoot in deep white dwarf layers, a phenomenon also important to understand nuclear burning, pulsations and chemical mixing in Sun-like stars. This opens new avenues of research following this action.
More than 300 simulations of white dwarf atmospheres 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 350,000 white dwarfs detected from Gaia. We have derived precise and accurate white dwarf parameters (mass, temperature, age) by combining our models, Gaia data, and external independent spectroscopic observations. This has resulted in many unexpected discoveries, including the first direct evidence of the core crystallisation of white dwarfs, a chemical phase transition of the carbon-oxygen interior from liquid to solid that was predicted 50 years ago but never observed. This has set the stage for a number of external follow-up publications on the evolution of white dwarfs and dense plasma physics. We have also discovered the closest double white dwarf merger product, an important system to quantify how stars merge and create supernovae explosions visible at distant cosmic scales.
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. This behaviour is due to the previously unexplored concept of convective overshoot in deep white dwarf layers, a phenomenon also important to understand nuclear burning, pulsations and chemical mixing in Sun-like stars. This opens new avenues of research following this action.
Building on the success of many years of improving the realism of white dwarf models, the major outcome of the project is a new theoretical framework based on 3D model atmospheres for practically all white dwarf studies. These atmospheric models have also been 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. This has allowed us to better understand the fate of planetary systems and the bulk composition of exoplanets, going beyond the state-of-the-art by discovering the prime evidence for rocky planetary crust formation from the presence of lithium spectral lines in cool white dwarfs. We have used our Gaia white dwarf catalogue to study for the star formation history in the disk and halo of the Milky Way, providing a benchmark for galactic evolution models. We have found strong evidence of a peak in stellar formation in the last 3 Gyr in our region of the Milky Way. Finally, we have worked to clarify and 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.