Discovery tests theory on cooling of white dwarf stars

"We discovered the classical picture of all white dwarfs being dead stars is incomplete," says Simon Blouin, co-principal investigator and Canadian Institute of Theoretical Astrophysics National Fellow at UVic. "For these white dwarfs to stop cooling, they must have some way of generating extra energy. We weren't sure how this was happening, but now we have an explanation for the phenomenon."

Understanding the age and other aspects of white dwarf stars helps scientists reconstruct the formation of the Milky Way Galaxy. Using 2019 Gaia data, Blouin collaborated with Antoine Bédard of the University of Warwick and Institute for Advanced Study researcher Sihao Cheng to make the discovery.

Over 97 per cent of stars in the Milky Way will eventually become white dwarfs. Scientists have long considered these stars to be at the end of their lives. Having depleted their nuclear energy source, they stop producing heat and cool down until the dense plasma in their interiors freezes into a solid state, and the star solidifies from the inside out. This cooling process can take billions of years.

According to the new paper, in some white dwarfs, the dense plasma in the interior does not simply freeze from the inside out. Instead, the solid crystals that are formed upon freezing are less dense than the liquid, and therefore want to float. As the crystals float upwards, they displace the heavier liquid downward. The transport of heavier material toward the centre of the star releases gravitational energy, and this energy is enough to interrupt the star's cooling process for billions of years.

"This is the first time this transport mechanism has been observed in any type of star, which is exciting, as it is not every day we uncover a whole new astrophysical phenomenon," says Bédard, Research Fellow at the University of Warwick.

Why this happens in some stars and not others is uncertain, but Blouin thinks it is likely due to the composition of the star.

"Some white dwarf stars are formed by the merger of two different stars. When these stars collide to form the white dwarf, it changes the composition of the star in a way that can allow the formation of floating crystals," says Blouin.

White dwarfs are routinely used as age indicators: the cooler a white dwarf is, the older it is assumed to be. However, due to the extra delay in cooling found in some white dwarfs, some stars of a given temperature may be billions of years older than previously thought.

"This new discovery will not only require that astronomy textbooks be revised but will also require that astronomers revisit the process they use to determine the age of stellar populations," adds Blouin.

The research is supported by the National Sciences and Engineering Research Council of Canada (NSERC), the Banting Postdoctoral Fellowship program, the European Research Council, and the Canadian Institute for Theoretical Astrophysics (CITA).