A new model shows that tidal forces in short-period binary star systems can raise the surface temperature to 10–30 thousand Kelvin and roughly double the radius of the white dwarf – so that mass transfer begins with cycles up to three times longer than previously thought; example: J1539+5027 with a cycle of 6.9 minutes.
Artist's impression of J1539+5027, a binary white dwarf system with a period of 6.9 minutes, consisting of a tidally heated white dwarf (in yellow) and its more dense companion (in blue). It is about to begin mass transfer. Credit: KyotoU / Lucy McNeill
Ancient binary star systems may contain stars that are hotter than scientists previously thought
White dwarfs are the dense, compressed remnants left behind when stars exhaust their nuclear fuel, a process that will one day occur in our Sun. These stellar remnants are called degenerate stars because their internal physics defies normal expectations: as they add mass, their size actually decreases.
Many white dwarfs exist in pairs, forming binary systems, where two stars orbit each other. Most of these systems are ancient in galactic terms and have cooled over time to surface temperatures of around 4,000 Kelvin.
Yet astronomers have recently identified a remarkable group of short-period binary systems in which the stars complete an orbit in less than an hour. Surprisingly, these white dwarfs appear to be about twice as large as models predict, with much higher surface temperatures, between 10,000 and 30,000 Kelvin.
Investigating the role of tidal heating
This inspired a team of researchers led by Lucy Olivia McNeill to explore tidal theory and use it to predict the temperature rise of white dwarfs in short-period binary orbits. Tidal forces often distort celestial bodies in binary orbits, determining the evolution of their orbits.
“Tidal heating has had some success in explaining the temperatures of hot Jupiter-type exoplanets and their orbital properties with their host star. So we wondered: to what extent can tidal heating explain the temperatures of white dwarfs in short-period binary orbits?” asks McNeil.
The researchers have built a theoretical framework that explains the temperature rise of white dwarfs in short-period binary systems. This framework is completely general, and allows us to predict the evolution of temperatures in the past and future, as well as the evolution of the orbits of white dwarfs in binary star systems.
The results revealed that tidal forces can have a strong influence on the evolution of such white dwarfs. In particular, the gravity of a small white dwarf affects the internal heating of its larger but less massive companion, causing it to expand and raise the surface temperature to at least 10,000 degrees Kelvin.
Inflated stars and extended orbital interactions
Because of this swelling, the team predicts that white dwarfs will typically be twice as large as theory predicts when the interaction, or mass transfer, between them begins. As a result, the interaction in short-period white dwarf binary systems could begin with rotation cycles three times longer than previously expected.
“We expected tidal heating to raise the temperatures of these white dwarfs, but we were surprised to see how much smaller the orbital period is in the oldest white dwarfs when the roc lobes come into contact,” says McNeil.
White dwarfs in binary systems with such small orbital periods will eventually interact and emit gravitational waves, which are thought to cause astronomical phenomena such as Type Ia supernovae and cataclysmic variables.
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