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 10th 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.

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.

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.

“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.”

Just as liquid water releases extra energy when it changes into ice — this energy is known as latent heat — the dense plasmas in the interiors of white dwarfs were predicted to release enough energy to noticeably slow their trek toward cool, faint stellar embers.

“All white dwarfs will crystallize at some point in their evolution, although more massive white dwarfs go through the process sooner,” said Tremblay, who led the study. “This means that billions of white dwarfs in our galaxy have already completed the process and are essentially metallic crystal spheres in the sky.”

This includes our own sun, which will become a crystal white dwarf in about 10 billion years.

Crystallization is the process of a material becoming a solid state in which its atoms form an ordered structure. Under the extreme pressures in white dwarf cores, atoms are packed so densely that their electrons become unbound, leaving a conducting electron gas governed by quantum physics, and positively charged nuclei in a fluid form. When the core cools to about 10 million degrees, the dense carbon oxygen plasma is cool enough that the fluid begins to solidify, forming a crystalline core at its heart.

“These results are really telling us a lot about the amount of pent up energy these stars can release while cooling off,” said Dr. Dunlap. “If you put a glass of water in the freezer, it will start losing heat to its cold surroundings, so its temperature will steadily drop. But this cooling actually stops once the water starts to freeze. While the liquid is turning into ice crystals, it releases extra energy, so its temperature hangs out around the freezing point until it’s turned into ice. Then it starts getting colder again. The white dwarfs do essentially the same thing, just at more extreme temperatures and densities.”

Not all transitions from a liquid to a solid result in crystals. Sometimes, the resulting solid is made up of an amorphous jumble of molecules or atoms, and these transitions do not release latent heat. When molten glass cools off, for example, its molecules freeze into place without becoming a structured crystal, so no extra heat is released. The new observations confirm that white dwarfs release extra heat at the temperatures where they are predicted to do so from crystallizing carbon and oxygen. However, the observations indicate that there is even more extra energy being released than can be explained by this latent heat.

“There are lots of ways energy can be released. When a rock falls and hits your toe, or when water at a hydroelectric plant falls over a dam, both of these are releasing gravitational energy,” explains Dr. Dunlap. “Models show that when a white dwarf is crystallizing, it starts in the center, and the solid mixture ends up more oxygen-rich than the liquid mixture. Basically, some of the oxygen sinks. Since oxygen is more massive than carbon, this releases gravitational energy. Including this effect in the models matches the data much better. But the data also suggest there’s even more energy being released, so there’s still work to do.”

Many models widely used to determine the ages of white dwarf stars do not include this effect of gravitational settling, so researchers will have to revise current estimates.

The inclusion of the UNC researchers in the work led by Dr. 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 Dr. 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.”

Fortunately, the revolutionary Gaia survey mission continues to collect data and improve its measurement precision, constructing a three-dimensional map of more than 1.3 billion stars. A third data release from Gaia is expected in 2021.

                                        A white dwarf star is in the process of solidifying in this artist’s rendering. Scientists have found the first direct evidence that white dwarf stars crystallize as they cool.

                                                                                                        PHOTO CREDITED TO UNIVERSITY OF WARWICK/MARK GARLICK


More information:  Core crystallization and pile-up in the cooling sequence of evolving white dwarfs, Tremblay et al. Nature (2019). DOI: 10.1038/s41586-018-0791-x   <>

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