White dwarf star in the process of solidifying. (University of Warwick/Mark Garlick)
White dwarf star in the process of solidifying. (University of Warwick/Mark Garlick)

The first direct evidence of crystallized white dwarf stars has been discovered by an international team of researchers that includes two former astronomers from the University of North Carolina at Chapel Hill. Predicted half a century ago, the discovery of these stars will be published in the January 10 edition of the journal Nature.

Observations have revealed that these stars have a core of solid carbon and oxygen due to a phase transition during their lifecycle, similar to water turning into ice. This phase transition slows their cooling in multiple ways, making them potentially billions of years older than previously thought.

The discovery, led by Pier-Emmanuel Tremblay of the U.K.’s University of Warwick, is largely based on observations taken with the European Space Agency’s Gaia satellite.

Bart Dunlap, a postdoctoral fellow with UT Austin’s Wootton Center for Astrophysical Plasma Properties, along with JJ Hermes, made the discovery independently of the Warwick team while working together at UNC-Chapel Hill and later joined forces with Tremblay. Hermes is now an assistant professor at Boston University.

Almost all stars end up as white dwarfs, and some of them are among the oldest stars in the universe. They are useful to astronomers because their predictable cooling rate allows them to be used as cosmic clocks to estimate the ages of groups of stars. They are the leftover cores of red giant stars, after these huge stars have died and shed their outer layers. They are then constantly cooling as they release their stored-up heat over billions of years.

The Gaia satellite has enabled the selection of a sample of white dwarfs with precise luminosities and colors that is significantly larger and more complete than any previous survey. For the study, the team selected 15,000 white dwarfs within about 300 light-years of Earth.

White dwarfs get fainter and redder as they cool, which leads to a predictable distribution of white dwarfs in a plot of brightness versus color. The astronomers identified a pile-up in this plot, an excess in the number of stars at specific colors and luminosities. When compared with evolutionary models of white dwarfs, the pile-up strongly coincides with the phase in their development in which latent heat is predicted to be released in large amounts, resulting in a slowdown of their cooling process. It is estimated that in some cases these stars have slowed their aging by as much as 2 billion years.

“More than 50 years ago, Hugh Van Horn, an astronomer at the University of Rochester, predicted that we should see a crystallization sequence because of a slowdown in cooling when white dwarfs crystallize, but at the time, the data weren’t good enough to check this prediction,” Dunlap said. “Gaia finally made it possible to see what he predicted, and it really pops out in the data.”

The inclusion of the UNC researchers in the work led by Tremblay is a testament to international cooperation in science. Dunlap and Hermes came to the same independent interpretation of the crystallization sequence from Gaia data while working together at UNC-Chapel Hill and presented the findings at a biannual conference on white dwarf stars last summer at UT Austin. Instead of racing to compete, the teams joined forces in order to add different expertise and perspectives to the analysis.

“This is the clearest confirmation that white dwarf stars form crystal cores of oxygen and carbon, but our models still have a lot of room to improve to match the observations,” said Hermes. “No lab on Earth can recreate the conditions at the centers of these stars. The best way to advance our knowledge of these extreme conditions is to keep looking up at the stars with exquisite space telescopes like Gaia.”

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