Joint Astrophysics Seminar

The Connection between Planets and White Dwarf Stars

Gilles Fontaine

Université de Montréal

In the last 15 years or so, evidence has been mounting in favor of the proposition that planets around white dwarfs are probably quite common. The evidence is strong but indirect, and rests on the detection of heavy elements in the atmospheres of many relatively cool white dwarfs. Given the very high efficiency of element sedimentation in white dwarfs, particularly in cooler objects (with no residual stellar winds and with negligible radiative levitation), these stars should show atmospheres with pure H or pure He, and so the source of the heavy elements must necessarily be extrinsic to these stars. Very often, such “polluted” white dwarfs are surrounded by dusty or gaseous disks, which have specific infrared signatures. These short-lived disks (10⁴ to 10⁶ yr) are interpreted as the products of tidally-disrupted planetesimals and/or asteroids that come crashing down onto a white dwarf more or less randomly in time as the result of gravitational perturbations of asteroid orbits due to unseen planets around that white dwarf. The disk material ultimately “rains” onto the white dwarf and pollutes its atmosphere during a few million years typically. Since the settling timescale varies from one heavy element to another, the “instantaneous” chemical composition in the atmosphere of the host white dwarf is not generally that of the primordial material. A current challenge is to decipher the observed abundances of the polluting elements in terms of the original chemical composition of the planetary debris. I will review the basic physics and provide an updated view of the so-called accretion-diffusion model needed to interpret the observed abundances of planetary elements in the atmospheres of white dwarfs. I will show that the potential for using polluted white dwarfs as “laboratories” for inferring the *bulk chemical composition* of planetesimals, asteroids, or comets is immense.