My research interests include the fate of planetary systems, paritcularly around white dwarfs and the compositions of extrasolar planets.

White dwarfs are the final stage of stellar evolution and most stars will end up as a white dwarf. Recent studies show that at least some fraction of extrasolar asteroids and planets can survive to the star’s white dwarf phase. Occasionally, an asteroid can be perturbed into the white dwarf’s tidal radius and get disrupted. This process often creates a hot dust disk very close to the white dwarf. Measuring the atmospheric compositions of externally-polluted white dwarfs can provide unique information about the composition of those extraterrestrial material.

Sometimes, there is unexpected discovery… one of them is highlighted in this recent news article: Hubble finds big brother of Halley’s Comet ripped apart by white dwarf

Compositions of Extrasolar Rocky Material

Because a white dwarf’s atmosphere is very clean (pure H or He), even a small amount of heavy elements will be easily detectable. I have been using high-resolution spectrographs on Keck, the Very Large Telescope, Magellan and the Hubble Space Telescope to observe these heavily polluted white dwarfs and understand the composition of its accreting material.

This is a summary of what we have found so far (from Xu et al. 2014, ApJ, 783, 79). To zeroth order, all the extrasolar rocky objects very much resemble bulk Earth with four dominating elements, O, Fe, Si and Mg.

Circumstellar Environment around White Dwarfs

About 4% white dwarfs display excess infrared radiation from a dust disk. These disks can be modeled by a geometric thin and optically thick dust disk within the white dwarf’s tidal radius. Those are very hot (~1000 K) dust very close (0.001 AU) to the white dwarf! It comes from the planetary debris.

A recent discovery of the drop in the infrared flux of a dusty white dwarf (from Xu & Jura 2014, ApJ, 792, L39). In less than 300 days, the infrared luminosity of the dust disk (I-1, I-2 and W-1, W-2) dropped by 30% and it remained like that afterwards. Up to this date, it is still a puzzle what caused this rapid change.