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Andrew Swan

I am a postdoc in the Astronomy & Astrophysics group, studying white dwarf planetary systems using optical and infrared telescopes.

Most stars, including the sun, will end their lives by becoming white dwarfs: compact, inert stellar cores that do little but cool down over billions of years. Asteroids and planets continue to orbit them, and some of them will eventually encounter the star (as do sun-grazing comets in the solar system). The intense gravity of a white dwarf raises tides so strong on close-passing objects that they are torn apart, creating a disc of debris around the star. Self-collisions within the debris generate vast quantities of dust, which becomes detectable in the infrared. That debris then gradually rains down onto the star, where it pollutes the stellar atmosphere. As a white dwarf typically shows a pristine surface of only hydrogen or helium, any other elements detected there must come from the debris. Measuring the composition of that material is therefore equivalent to measuring the bulk composition of exoplanetary objects, a feat entirely beyond the reach of other observational techniques, even in the solar system.

I use spectroscopic observations of metal-polluted white dwarfs to infer the compositions of the objects they have swallowed. So far, it appears that rocky exoplanets are made of materials similar to those found in the solar system, but there are variations that may trace the distance from the star that objects formed at. I have developed new analysis techniques, and aim to apply them to the data from large surveys that Warwick are leading, which will include many thousands of white dwarfs.

I am also interested in what happens to exoplanetary objects on their way to being devoured by their stars. Small, cold asteroids are usually invisible to us, but reveal themselves during their fatal close encounters with the star. They are ground down to dust, developing a far greater surface area than the parent object, and can become detectable as the dust is heated by starlight and glows in the infrared. This brief, observable phase of an asteroid's life presents an important opportunity to test its behaviour against theoretical models, and in turn constrain the orbit from which the asteroid originated.

My work has revealed that the population of white dwarfs that host detectable debris discs display significant variation in their infrared emission, on almost all timescales probed so far, from days up to years. The most likely explanation is that the discs are dynamical environments where dust is being continually produced and destroyed. I used the Spitzer space telescope to study a system whose infrared brightness had more than doubled in a spectacular outburst, and I showed that the light curve of the aftermath was consistent with the behaviour of self-colliding debris. I have secured time on the James Webb space telescope to conduct a detailed follow-up of that system, where I will build up an exquisitely detailed light curve to study collisions within the debris on a wide range of timescales. I will also take a sensitive infrared spectrum to learn the composition of the disc material, allowing a comparison between the material orbiting the star and that polluting its surface.

You can find my publications listed on the NASA ADS service.

Write to:

Andrew Swan,
Department of Physics,
University of Warwick,
Coventry CV4 7AL

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