Research

My research is mostly about the fate of planetary systems, particularly around white dwarfs.

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.

Some of our work is reported in this news article: Study of material surrounding distant stars shows Earth’s ingredients ‘pretty normal’

We are hosting an IAU symposium White Dwarfs as probes of fundamental physics and tracers of planetary, stellar & galactic evolution in Hawaii in 2019. Please visit this website for details.

Chemical 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, 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.

Sometimes, there are unexpected discoveries. So far, we have only found one object with a composition similar to Kuiper Belt Object in the solar system. The object has been accreted on WD 1425+540 and it is very enriched in carbon, nitrogen and water (from Xu et al. 2017, ApJL, 836, L7). Or you can read about it here: Hubble finds big brother of Halley’s Comet ripped apart by white dwarf

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.