Throwing Icebergs at White Dwarfs

Where do the metals come from that pollute the atmospheres of many white dwarfs? Close-in asteroids may not be the only culprits! A new study shows that distant planet-size and icy objects could share some of the blame.

Pollution Problems

white dwarf debris

Artist’s impression of rocky debris lying close around a white dwarf star. [NASA/ESA/STScI/G. Bacon]

When a low- to intermediate-mass star reaches the end of its life, its outer layers are blown off, leaving behind its compact core. The strong gravity of this white dwarf causes elements heavier than hydrogen and helium to rapidly sink to its center in a process known as sedimentation, leaving an atmosphere that should be free of metallic elements.

Therefore it’s perhaps surprising that roughly 25–50% of all white dwarfs are observed to have atmospheric pollution by heavy elements. The short timescales for sedimentation suggest that these elements were added to the white dwarf recently — but how did they get there?

Bringing Ice Inward

In the generally accepted theory, pre-existing rocky bodies or an orbiting asteroid belt survive the star’s evolution, later accreting onto the final white dwarf. But this scenario doesn’t explain a few observations that suggest white dwarfs might be accreting larger planetary-size bodies and bodies with ices and volatile materials.

exciting eccentricity

Dynamical evolution of a Neptune-like planet (a) and a Kuiper belt analog object (b) in wide binary star systems. Both have large eccentricity excitations during the white dwarf phase. [Stephan et al. 2017]

How might you get large or icy objects — which would begin on very wide orbits — close enough to a white dwarf to become disrupted and accrete? Led by Alexander Stephan, a team of scientists at UCLA now suggest that the key is for the white dwarf to be in a binary system.

Influence of a Companion

In the authors’ model, the white-dwarf progenitor is orbited by both a distant stellar companion (a common occurrence) and a number of large potential polluters, which could have masses between that of a large asteroid up to several times the mass of Jupiter. These potential polluters have very wide orbits that allow them to maintain ice and volatile materials.

At the end of the progenitor’s lifetime it loses a significant amount of mass, causing the orbits of the surviving objects in the system to expand. After this stage, the stellar companion gravitationally perturbs the potential polluters onto extremely eccentric orbits, bringing these massive and long-period objects close enough accrete onto what is now the white dwarf.

The Need for Observations

orbital parameters

The likelihood distributions for orbital parameters of the systems that result in white dwarfs polluted by Neptune-like planets and Kuiper-belt-analog objects. The black arrows mark the parameters for one of the few observed systems, WD 1425+540, for comparison. [Stephan et al. 2017]

By running large Monte Carlo simulations, Stephan and collaborators demonstrate that this scenario can successfully produce accretion of both Neptune-like planets and Kuiper-belt-analog objects. Their simulation results indicate that ~1% of all white dwarfs should accrete Neptune-like planets, and ~7.5% of all white dwarfs should accrete Kuiper-belt-analog objects.

While these fractions are broadly consistent with observations, it’s hard to say with certainty whether this model is correct, as observations are scant. Only ~200 polluted white dwarfs have been observed, and of these, only ~15 have had detailed abundance measurements made. Next steps for understanding white-dwarf pollution certainly must include gathering more observations of polluted white dwarfs and establishing the statistics of what is polluting them.

Citation

Alexander P. Stephan et al 2017 ApJL 844 L16. doi:10.3847/2041-8213/aa7cf3

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