Atmospheric Pollution of White Dwarf Stars May Record the Lithium Abundance History of the Galaxy

Artist’s conception of the debris disk around a white dwarf. Credit: NASA/JPL-Caltech.

White dwarfs are the remains of main sequence stars that are no longer undergoing nuclear fusion and cool down by thermal radiation over billions of years. In general, the atmospheres of white dwarfs show spectral lines of only the lightest elements, such as hydrogen or helium, because their strong gravitational fields should cause heavy elements to rapidly sink into the white dwarf interiors. Nevertheless, some white dwarfs have heavier elements in their atmospheres, which is thought to be due to the accretion of rocky bodies from a surrounding planetary system. Atmospheric pollution in white dwarf stars offers a unique window into the chemistry of the nebula from which the systems formed by tabulating the abundance of elements untouched by nuclear fusion in the star itself.

A study led by Ben Kaiser at the University of North Carolina, along with colleagues from the Université de Montréal and Los Alamos National Laboratory, used a unique spectrograph for the study of the wavelength region between approximately 3800 to 8400 Å to determine the detailed composition of the atmosphere of a nearby white dwarf. The team studied the white dwarf as part of a survey of ultra-cool objects with the Goodman spectrograph at the Southern Astrophysical Research (SOAR) telescope on Cerro Pachón in Chile and reported the detection of lithium, sodium, potassium, and calcium in the atmosphere of the white dwarf. By comparing with model atmospheres for unpolluted white dwarfs, the team determined abundance ratios of these elements. The ratios are consistent with meteoritic values in our own solar system, with the exception of lithium. Furthermore, this would be the first inferred detection of lithium from an extrasolar rocky body.

The team also compared the measured lithium abundance to measurements in old stars, which show the white dwarf contains relatively more lithium than the old stars. This could be due to lithium being fused in the old stars over time, while rocky bodies polluting the white dwarf retain the same lithium abundance from the nebula from which the star formed. The measurement of lithium from atmospheric pollution of white dwarfs of different ages could then help to track the amount of lithium in our galaxy over time and help to better constrain the amount of lithium formed during the Big Bang. READ MORE