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White Dwarfs as Tracers of Stellar, Binary and Planetary Evolution

Final Report Summary - WDTRACER (White Dwarfs as Tracers of Stellar, Binary and Planetary Evolution)

Most stars in the Milky Way, including the Sun, will eventually end their lives as white dwarfs. The study of these burnt-out, Earth-sized, stellar cores provides important insight into the evolution of stars, both single and in binary systems, as well as that of planetary systems. The aims of this ERC project are (1) to produce a large and homogeneous catalogue of white dwarfs, and (2) to use this sample to carry out detailed follow-up observations as well as theoretical studies to improve our understanding of the evolution of stars and planets.

We have used large digital pan-chromatic maps of the sky, including the recently released data from the Gaia space mission, to identify ~250.000 white dwarfs, of which only about ten per cent were previously known. We derived basic physical parameters for these stars, which provides first insight into the galactic population of stellar remnants, further characterisation will be carried out with four forthcoming large spectroscopic surveys over the next decade.

Among the white dwarfs that we have identified, several hundred were observed with the Kepler spacecraft, providing extremely accurate measurements of their brightness changes over periods of 1-3 months. Using these "light curves" we have found many white dwarfs that undergo instabilities, pulsations, in their outer layers, and analysing these oscillations allows us to measure the properties deep inside these stars. We found that a small number of these pulsating white dwarfs show irregular flares, or outbursts, which is a totally new and unexpected phenomenon that still lacks a proper theoretical understanding. Another way to measure very accurately the physical properties of white dwarfs is to discover, and follow-up, those that are in binary stars with a close companion that, on its way around the white dwarf, regularly blocks the light from the stellar remnant. Using very fast (seconds) brightness measurements of these eclipses allowed us to determine the masses and the radii of several white dwarfs with a precision of a few per cent. Such measurements are very important to test theoretical models of the internal structure of these stars, and of aspects of fundamental physics as well.

A small number of white dwarfs that are members of binary stars will eventually explode as a supernova Ia, the cosmic distance beacon used to first identify and now characterise dark energy. Multiple pathways are thought to potentially lead to these thermonuclear explosions, and we explored with detailed observations two of the main channels: white dwarfs accreting from a nearby “normal” star, similar to the Sun, and the merging of two white dwarfs in a close binary system. Only within the last year, a handful of remnants surviving thermonuclear supernovae were identified, and we characterised their properties in detail, approaching the question on the nature of supernova Ia progenitors from the opposite end of their evolution.

In ~5 billion years from now, the Sun will become a white dwarf, opening up the question: what will happen to the solar system at that stage? Theoretical studies show that most of the solar system will survive this phase, only Mercury and Venus (and maybe the Earth) will be destroyed, and eventually Mars, Jupiter, Saturn, and the other planets and planetesimals, asteroids, and comets, will orbit the white dwarf remnant left behind by the Sun. The same prediction is true for many of the known exo-planetary systems. Using large amounts of ultraviolet spectroscopy obtained with the Hubble Space Telescope, we have demonstrated that 25-50% of known white dwarfs still host remnants of planetary systems, and we have measured the chemical composition of exo-asteroids straying too close to, and eventually hitting, those white dwarfs. We found that exo-asteroids are overall very similar to their solar-system analogues, being composed to a large fraction of just four elements: Si, Mg, Fe, and O. In two cases, we could show that those exo-asteroids contained large amounts of water, similar to Ceres, the largest asteroid in the solar system, demonstrating that water-bearing planetesimals that could transport water on to dry, terrestrial planets are relatively common. Perhaps most fascinating, one white dwarf exhibits regular dips in its brightness every ~4.5h stemming from dusty debris orbiting across the line-of-sight, and we have investigated the nature of the disrupted parent body, providing the first detailed insight into the internal structure of an exo-asteroid.