A unified picture of white dwarf planetary systems

What are extra-Solar planets made of? To answer this, we turn to the dead stars known as white dwarfs. Many have heavy elements detected spectroscopically in their atmospheres. These are delivered by asteroids scattered close to the white dwarf that break up and deposit their material in its atmosphere. Understanding this process is the only means we have of determining the bulk elemental compositions of extra-Solar asteroids and planets.

I will study the different stages of this process, using computational N-body and hydrodynamical modelling. I will focus in detail on three major steps:
- Asteroids are first scattered out of their orbits by planets orbiting the white dwarf.
- When they come close to the white dwarf, asteroids experience hypervelocity collisions with each other.
- The debris from the disrupted asteroids forms a disc around the white dwarf, which then accretes onto it so that the material from the asteroid can be seen in the white dwarf's atmosphere.

White dwarfs have been seen to be orbited by discs of dust and gas (sometimes changing with time), and by asteroids in the process of breaking up. My models will reproduce these discs and asteroids, as well as the observations of heavy elements in white dwarf atmospheres. By explaining these diverse observations with the model of asteroids being scattered, breaking up and accreting, I will provide a unified picture of white dwarf planetary systems.

What are extra-Solar planets made of? To answer this question, we turn to the dead stars known as white dwarfs. These have such strong gravitational fields that their atmospheres should be composed entirely of the light elements hydrogen and helium. Heavier elements quickly sink out of sight into the white dwarf's interior. Yet, astonishingly, heavy elements—including elements that make up the Earth, such as silicon, oxygen, and iron—are seen in the atmospheres of over a quarter of white dwarfs observed. How do these materials end up in the white dwarf's atmosphere?

These materials are deposited into the white dwarf by asteroids that are scattered close to the white dwarf by planets. Therefore, we can infer, from the types and amounts of elements we see in the white dwarf's atmosphere, what the composition of the extra-Solar asteroids is. Because planets form from the merging of smaller bodies like asteroids, this also tells us what extra-Solar planets are made of.

The fate of asteroids before their material strikes the white dwarf is violent, and observations of white dwarfs show various stages of the process. Many white dwarfs are surrounded by discs of "dust" (pulverised rock) which probably comes from the break-up of asteroids. Several are surrounded by discs of gas, made up of vapourised rock. Some of these discs change with time, revealing a highly interesting and variable environment. Finally, one white dwarf is orbited by disintegrating asteroids, which cause the star to periodically dim as they pass in front of it.

The outlines of the process leading to the asteroids' material being deposited onto the white dwarf are as follows. First, asteroids are scattered out of their previous orbits by planets, and sent onto orbits taking them within the "Roche limit" of the white dwarf. This is a region where the white dwarf's gravitational field is so strong that asteroids cannot hold themselves together, and the asteroids are pulled apart into fragments. The resulting debris—large fragments, dust and gas—then has its orbits further modified. The orbits shrink and become more circular. Then the debris can be detected as gas and dust discs, and the larger fragments seen to pass in front of the star. Finally, further decay of the orbits results in the material being deposited in the white dwarfs' atmosphere.

Many aspects of this picture, however, need to be studied in detail so that our theoretical understanding can be validated, or refuted, with observational data. I will study the important steps in the process that leads from asteroid destabilisation to the deposition of heavy elements onto the white dwarf.

First, I will simulate the changes to the orbits of planets and asteroids as stars evolve and die. I will calculate the rates at which asteroids are scattered onto orbits that take them within the Roche limit of the white dwarf, and compare these rates to the rates at which material falls onto the white dwarf as inferred from observations. In these simulations, I will account for the many forces that cause the orbits of planets and asteroids to change throughout a star's life: the effects of tides (which, for example, slowly cause the Moon to recede from the Earth), the effects of the star's radiation, and the effects of the mass lost by a star as it dies and becomes a white dwarf.

Second, I will simulate what happens to the asteroids after they are scattered close to the white dwarf. When multiple asteroids are scattered, their orbits will intersect, and this will lead to collisions between them. These collisions result in fragmentation of the asteroids, and also to the loss of lighter elements. I will determine the outcomes of collisions between asteroids close to the white dwarf, which take place at very high velocities (100s of km/s) which have not been studied before.

Finally, I will determine what happens to the debris produced in these collisions. Significantly, I will determine whether the composition of the material deposited on the white dwarf faithfully reflects the composition of the parent asteroids, or whether, for example, light elements are lost, not be be observed. I will also reproduce the observed diversity of gas and dust discs, and large asteroids, seen around white dwarfs, thus providing a unified picture of white dwarf planetary systems.