White dwarfs, the dense remnants of exhausted stars, are usually ancient, dim, and cool. Yet some binary systems containing them defy expectations, glowing tens of thousands of degrees hotter than theory predicts. A research team led by Lucy Olivia McNeill at Kyoto University has now shown that tidal heating could be the cause of this unexpected brightness.
White dwarfs typically cool to a by Riko Seibo
Tokyo, Japan (SPX) Oct 15, 2025
White dwarfs, the dense remnants of exhausted stars, are usually ancient, dim, and cool. Yet some binary systems containing them defy expectations, glowing tens of thousands of degrees hotter than theory predicts. A research team led by Lucy Olivia McNeill at Kyoto University has now shown that tidal heating could be the cause of this unexpected brightness.
White dwarfs typically cool to about 4,000 Kelvin after billions of years, but those in short-period binaries - where two stars orbit faster than once per hour - can reach surface temperatures between 10,000 and 30,000 Kelvin. These systems also appear nearly twice as large as standard models suggest, prompting scientists to explore the role of tidal interactions in their evolution.
"Tidal heating has had some success in explaining temperatures of Hot Jupiters and their orbital properties with their host stars. So we wondered: to what extent can tidal heating explain the temperatures of white dwarfs in short period binaries?" asks McNeill.
The Kyoto team built a comprehensive theoretical model linking tidal deformation to heating and orbital evolution. Their calculations showed that the gravitational pull of a smaller white dwarf can warm and inflate its companion, driving its temperature up and altering its orbital period.
According to the study, tidal effects can make white dwarfs nearly twice as large as predicted when they start transferring mass, allowing them to begin interacting at orbital periods up to three times longer than previously expected. "We expected tidal heating would increase the temperatures of these white dwarfs, but we were surprised to see how much the orbital period reduces for the oldest white dwarfs when their Roche lobes come into contact," McNeill noted.
Such interactions ultimately lead to gravitational wave emission and may set the stage for dramatic events like type Ia supernovae and cataclysmic variables. The researchers now plan to extend their framework to carbon-oxygen white dwarfs to better understand potential supernova progenitors and assess whether the so-called double degenerate merger scenario aligns with observed temperatures.
Research Report:Tidal heating in detached double white dwarf binaries
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