Most white dwarf binaries, where two stellar remnants orbit each other, have spent millions of years cooling down to surface temperatures around 4,000 degrees Kelvin. These ancient objects sit quietly in space, slowly radiating away their residual heat. But astronomers have discovered a peculiar class of these binary systems that seems to defy all expectations. These white dwarfs orbit each other faster than once per hour, and instead of being cool and compact, they're far hotter than expected, reaching surface temperatures between 10,000 and 30,000 degrees Kelvin, and twice the size theory predicts they should be.
The pair of white dwarfs have been the subject of a study by a team led by Lucy Olivia McNeill from Kyoto University and they found some of the strange features could be explained by the relentless pull of tides. These are the same types of tidal forces we experience here on Eartha and are responsible for the rising and falling of the oceans. In the case of the white dwarf binaries, the two massive objects orbit close together so they gravitationally tug on each other, deforming their shapes. This constant squeezing and stretching converts orbital energy into heat, warming them from within.
The researchers developed a model to predict how tidal heating affects white dwarfs in tight orbits. Their model can offer a temperature profile both backward into the past and forward into the future, providing a complete forecast of how tidal forces could effect these stellar remnants. What they found was remarkable. In close binary systems, the tidal pull from a smaller, denser white dwarf continuously deforms its larger but less massive companion which can heat up the larger star and inflate it in ways that match observations.
McNeill admits the team expected tidal heating would warm these stars, but even they were surprised by the magnitude of the effect. Eventually, during the lift of binary white dwarfs, they get so close that they begin transferring mass between each other. Because tidally heated white dwarfs are twice as large as standard models predict, they reach this interaction phase much earlier. The orbital period shrinks dramatically for the oldest white dwarfs just as their surfaces come into contact, changing our understanding of when and how these systems interact.
As these binary pairs eventually merge or engage in mass transfer they are likely to emit gravitational waves that future space based detectors should be able to observe. Some may even trigger Type Ia supernovae, the brilliant stellar explosions that we use as distance markers throughout the universe. Looking ahead though, the team plans to apply their model to carbon-oxygen white dwarf binaries, to see whether tidal heating could act as a trigger for Type Ia supernovae. The tides it seems, are indeed changing for white dwarfs.