White Dwarf Stars Under the Influence of Planets

White dwarf stars represent the final evolutionary state for ~97% of the stars in the Universe, including our own Sun. In the century since their discovery they have provided insight into fundamental physics, stellar evolution and interactions, and the ages of Galactic stellar populations. Observations across the past decade also suggest that 25-50% of white dwarfs host remnant planetary systems, as evidenced by the presence of recently accreted metals in their atmospheres. A small fraction of these polluted white dwarfs host gas and dust disks consisting of debris from disrupted exoplanetesimals, and some show evidence for ongoing electromagnetic interaction with a planetary remnant. These discoveries provide new possibilities to enhance our understanding of the elemental history of our Galaxy, the nature of exoplanetary systems, and the influence of planets on stellar evolution. A research team at the University of North Carolina at Chapel Hill will conduct a spectroscopic survey of the coolest metal-polluted (types DZ or DAZ) white dwarfs from the Gaia map of the Milky Way Galaxy. They will use their results to address a well-known conflict between lithium abundances as inferred from main-sequence stars and the expectations of Big Bang nucleosynthesis. The work will find new white dwarfs that can address this 'Cosmological Lithium Problem' and explore early Galactic element abundances via ancient analogs to Solar System meteorites. In collaboration with their education and outreach partner Morehead Planetarium and Science Center (MPSC), the investigators will bring the excitement of exoplanetary science to the public by developing a challenge for the 'Launch Lab,' MPSC's new interactive tinkering space that lets visitors explore problems drawn from current research.

The Galaxy's elemental history is currently measured primarily from the photospheres of main-sequence stars, but this can be especially unreliable for lithium, which is consumed at relatively low temperatures. In the Solar System, planets, asteroids, and especially meteorites offer an alternate means of inferring the primordial composition of the solar nebula. This method has not hitherto been applicable outside our Solar System, but polluted white dwarfs offer a path to extend it to other Galactic systems and epochs. The researchers will also study DZs and DAZs that are actively accreting planetary debris via observable disks, which typically show Doppler-broadened calcium emission associated with the precession of an elliptical gas disk. The project includes further discovery and monitoring of debris disks to help understand the formation, evolution, and dynamics of relic exoplanetary systems. Finally, the researchers will study white dwarfs with evidence for electromagnetic interaction with a remnant planet, and explore the effects of planets and mergers on the evolution of white dwarfs.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.