## New Model Explains Unexpected Heat and Orbital Behavior of White Dwarf Binaries
White dwarfs are the dense remnants left behind after a star stops producing energy through nuclear fusion, a stage our own sun will reach far in the future. These stellar remnants behave in unusual ways,since their internal structure makes them shrink as they gain mass,which is why they are known as degenerate stars.
Recent observations have revealed a puzzling phenomenon in some white dwarf binary systems – pairs of white dwarfs orbiting each other in very close proximity. These systems, typically ancient and cooled to around 4,000 Kelvin, are exhibiting unexpectedly high temperatures (10,000-30,000 Kelvin) and are roughly twice the size predicted by current models. Many complete an orbit in less than an hour, further deviating from established expectations.
Tidal Heating as a Key Factor
To explain this discrepancy, a team led by Lucy Olivia McNeill of Kyoto university investigated the role of tidal forces. Just as tides on Earth are caused by the gravitational pull of the Moon,close orbits between stars generate critically important tidal forces that can distort and heat the stars involved.
“We wondered: to what extent can tidal heating explain the temperatures of white dwarfs in short period binaries?” asks McNeill,drawing a parallel to the successful application of tidal heating models to explain the behavior of “Hot Jupiters” orbiting their stars.
The team developed a new theoretical model to estimate the amount of heating experienced by white dwarfs in these tight binary systems. This model is designed to predict both temperature evolution and future orbital changes.
Reshaping Our understanding of White Dwarf Evolution
the analysis revealed that tidal interactions are a significant driver of white dwarf evolution. the gravitational pull between the stars generates internal heat within the larger, less massive white dwarf, causing it to expand and reach temperatures of at least 10,000 Kelvin.
This expansion has critically important implications for when the stars begin to exchange material - a process called mass transfer. The researchers believe white dwarfs are likely to be twice the predicted size at the onset of mass transfer, meaning these binary systems may begin interacting at orbital periods three times longer than previously thought.
“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,” explains McNeill.
Implications for Stellar Explosions and Future Research
These tightly orbiting white dwarf systems are potential precursors to dramatic cosmic events like type Ia supernovae and cataclysmic variables,which emit gravitational radiation as they interact. Understanding their evolution is thus crucial.
The team plans to apply their model to binary systems composed of carbon-oxygen white dwarfs, aiming to better understand the pathways leading to type Ia supernovae and to determine if realistic temperature predictions support the “double degenerate” (merger) scenario for these explosions.