Some white dwarfs may be older than previously thought

More than 97 percent of the stars in our galaxy end their lives with a whimper—cooling slowly and becoming stellar remnants called white dwarfs. The cooling of white dwarfs follows what is thought to be a predictable pattern, so much so that the temperature of the white dwarf can be used to determine the age of the surrounding stars.However, new findings suggest this model may need to be modified [1]. Predictions by Antoine Bédard of the University of Warwick in the UK and his colleagues suggest that some white dwarfs may undergo a process that “revitalizes” the star, significantly slowing down the cooling process. This change could alter the white dwarf’s calculated age by billions of years.

When a small star (8 times the mass of the Sun or less) runs out of nuclear fuel, it sheds its outer layers to form a planetary nebula. The star’s core then collapses into a white dwarf. White dwarfs produce no heat and radiate remaining energy into space during their existence, cooling and solidifying from the inside out. At least that’s what astrophysicists think.

In 2019, this model was undermined by astronomers analyzing data from the European Space Agency’s Gaia mission.Researchers discover a previously unknown cluster of white dwarf stars in the Milky Way with unusual properties [2]. As stars age, their speed increases relative to those stars due to repeated gravitational interactions with nearby stars.The newly discovered white dwarf is called ask The average speed of branches is much higher than what models suggest based on temperature, a finding that suggests ask-Branched white dwarfs are older than previously thought. Certain processes can slow down cooling.

To figure out why white dwarfs are unusually warm for their age, Beida and his colleagues modeled how any impurities present in these stars would affect cooling timescales. The model suggests that as an impurity-laden white dwarf cools, material begins to crystallize at its center.These crystals contain fewer impurities and are less dense than the surrounding fluid, so they rise toward the white dwarf’s surface [3]. Once the crystals reach a certain height, they melt, displacing the heavier liquid downwards.

The team showed that convection-like motions in the white dwarf fluid caused by the repeated formation, rise, and melting of crystals can reheat the white dwarf because this motion converts gravitational energy into heat. The same energy conversion process occurs when an object is dropped, Bedard said. “If you drop a heavy object from a certain height, its gravitational potential energy is converted into kinetic energy, and when it hits the ground, friction converts the kinetic energy into heat,” he said.

This process may be triggered, for example, when a white dwarf merges with another star, creating a more massive white dwarf. During this merger, the temperature of the white dwarf increases significantly, causing much of the hydrogen and helium near the white dwarf’s surface to burn, Bedard said. These nuclear reactions then produce (among other things) neutron-rich impurities.

“this [heating] “This mechanism makes perfect sense, and it dramatically changes our understanding of white dwarfs,” said Mukremin Kilic, an astrophysicist who studies white dwarfs at the University of Oklahoma in Norman and was not involved in the study. Understanding evolution. This heating mechanism also has potentially significant implications for using white dwarfs as cosmic clocks. Although only a small fraction of white dwarfs are likely to undergo mergers, Bedard said determining which ones are delayed is no small problem. “this ask branch is that the statistical density of stars is too high, but in ask We can’t easily distinguish delayed white dwarfs from normal white dwarfs,” he said. “this [issue] This means astronomers need to be careful when using massive white dwarfs to measure stellar ages. Despite this warning, Bedard said the age error could mean the Milky Way is a billion years older than previously thought.

Elizabeth Fernandez

Elizabeth Fernandez is a freelance science writer in Raleigh, North Carolina.

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