Throwing icebergs at white dwarfs

White dwarfs pack the mass of the Sun inside a radius that is 1% as small as the Sun’s. Any elements heavier than helium that land on a white dwarf are dragged beneath the surface by the star’s intense gravity. So when astronomers spotted evidence of silicon, iron, and other refractory elements in white dwarf spectra, they concluded that something must be replenishing those elements. Given that the sort of stars that become white dwarfs also host planetary systems, it’s likely that some white dwarfs are consuming their former asteroid belts, which offer a nearby supply of those heavier-than-helium elements.

But a few white dwarf spectra have recently been discovered to exhibit nitrogen and other elements that come from volatile species found on icy planets, such as Neptune, and on icy dwarf planets, such as the ones in the Kuiper belt. Because the orbits of those planetary bodies typically lie far outside a star’s asteroid belt, a question arises: How are the volatiles delivered to the white dwarf?

One plausible solution takes advantage of the fact that more than half of white dwarf progenitors are found in binary systems. Provided a white dwarf’s stellar companion is suitably distant, it can trigger an orbital instability—the Kozai–Lidov mechanism—that can send one of the white dwarf’s outer bodies along with its cargo of volatiles crashing into the white dwarf.

To evaluate the likelihood of that scenario, UCLA’s Alexander Stephan, Smadar Naoz, and Benjamin Zuckerman simulated 4500 binary systems that contained a white dwarf and a normal stellar companion. One set of simulations included a Neptune-like planet around the white dwarf; the other set included a Kuiper belt object around the white dwarf. Each simulation drew its initial orbital parameters randomly from distributions typical of white dwarf binary systems. The simulations implied that 1% of white dwarfs should harbor volatiles delivered by a colliding Neptune and 7.5% by a colliding Kuiper belt object. Together, the results are consistent with the observation that about 10% of white dwarf spectra exhibit evidence of volatile species. They also suggest that white dwarf spectra could harbor insights into the outer planetary systems of some binary systems. (A. P. Stephan, S. Naoz, B. Zuckerman, Astrophys. J. Lett. 844, L16, 2017, doi:10.3847/2041-8213/aa7cf3 .)