Potassium and lithium on a white dwarf
White dwarfs are dense stellar remnants left when a dying parent star throws off its outer layers. The high gravitational fields should cause heavy elements to rapidly sink below the white dwarf surface. Nevertheless some “polluted” white dwarfs have evidence for those materials on their surface, which is thought to be due to the recent accretion of rocky bodies from a surrounding planetary system. Kaiser et al. report a white dwarf with pollution by potassium and lithium. This observation provides a record of the composition of the accreted rocky bodies and of the Galactic lithium abundance when the planetary system formed, billions of years ago.
Science, this issue p. 168
Tidal disruption and subsequent accretion of planetesimals by white dwarfs can reveal the elemental abundances of rocky bodies in exoplanetary systems. Those abundances provide information on the composition of the nebula from which the systems formed, which is analogous to how meteorite abundances inform our understanding of the early Solar System. We report the detection of lithium, sodium, potassium, and calcium in the atmosphere of the white dwarf Gaia DR2 4353607450860305024, which we ascribe to the accretion of a planetesimal. Using model atmospheres, we determine abundance ratios of these elements, and, with the exception of lithium, they are consistent with meteoritic values in the Solar System. We compare the measured lithium abundance with measurements in old stars and with expectations from Big Bang nucleosynthesis.
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