Researchers have discovered that tidal forces in tightly orbiting binary white dwarfs generate significant internal heat, causing the stars to expand and reach unexpectedly high temperatures. This phenomenon challenges previous models and could explain the early onset of cosmic interactions like supernovae. The findings come from a study led by Kyoto University.
White dwarfs, the compact remnants of stars that have exhausted their nuclear fuel, typically cool over time in binary systems where they orbit a companion. However, observations of short-period binaries—those completing orbits in under an hour—reveal anomalies: these white dwarfs are about twice their predicted size and maintain surface temperatures between 10,000 and 30,000 degrees Kelvin, far hotter than the usual 4,000 degrees for aged remnants.
A team led by Lucy Olivia McNeill at Kyoto University explored tidal heating as a potential cause. Tidal forces from a companion star distort and heat the white dwarf's interior, leading to expansion. Their theoretical model predicts that this heating pushes temperatures to at least 10,000 degrees Kelvin and alters orbital dynamics.
As McNeill noted, "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?"
The model indicates that mass transfer between the stars may begin at orbital periods three times longer than previously estimated, due to the inflated sizes when Roche lobes contact. "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 added.
These insights have implications for understanding type Ia supernovae and cataclysmic variables, which arise from merging white dwarfs emitting gravitational waves. Future work will focus on carbon-oxygen white dwarfs to assess merger scenarios for explosions.
The study, co-authored with Ryosuke Hirai, appears in The Astrophysical Journal (2025, volume 992, issue 1).