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| Impression of the 6.9 minute double white dwarf binary J1539+5027, composed of a tidally heated white dwarf (yellow) and its more compact companion (blue). It is about to start mass transferring. Image Credit: KyotoU / Lucy McNeill |
White dwarfs are the compact remnants of stars that have stopped nuclear burning, a fate that will eventually befall our sun. These extremely dense objects are degenerate stars because their structure is counterintuitive: the heavier they are, the smaller they are.
White dwarfs often form binary systems, in which two stars orbit one another. The majority of these are ancient even by galactic standards, and have cooled to surface temperatures of about 4,000 degrees Kelvin. However, recent studies have revealed a class of short period binary systems in which the stars orbit each other faster than once per hour. Contrary to theoretical models, these stars are inflated to twice the size as expected due to surface temperatures of 10 to 30 thousand degrees Kelvin.
This inspired a team of researchers, led by Lucy Olivia McNeill of Kyoto University, to investigate the theory of tides and use it to predict the temperature increase of white dwarfs in short period binary orbits. Tidal forces often deform celestial bodies in binary orbits, determining their orbital 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 researchers constructed a theoretical framework accounting for the temperature increase of white dwarfs in short-period binaries. This framework is completely generalized, allowing the prediction of past and future temperature evolution as well as the orbital evolution of white dwarf stars in binary systems.
The results revealed that tidal forces can strongly influence the evolution of such white dwarfs. Specifically, the tidal pull of a small white dwarf affects the internal heating of its larger but less massive companion, causing it to inflate and increasing its surface temperature to at least 10,000 degrees Kelvin.
Due to this inflation, the team predicts that white dwarfs should typically be twice as large as theory predicts when they begin to interact, or mass transfer. Consequently, short period white dwarf binaries can start interacting at orbital periods that are 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," says McNeill.
White dwarfs in binary systems with such short orbital periods will eventually interact and emit gravitational radiation, and are thought to cause astronomical phenomena like type Ia supernovae and cataclysmic variables.
In the future, the team plans to apply their framework to binary systems with carbon-oxygen white dwarfs and potentially learn about type Ia explosion progenitors, paying particular attention to whether or not realistic temperatures favor the so-called double degenerate, or merger scenario.
Title: Tidal Heating in Detached Double White Dwarf Binaries
Authors: Lucy O. McNeill and Ryosuke Hirai
Source/Credit: Kyoto University
Reference Number: asph101425_01