White dwarf chows down on brown dwarf companion in feeding frenzy

An artist’s impression of a dwarf nova system in which a white dwarf is stripping material away from a brown dwarf in a close orbit. Image: NASA and L. Hustak/STSci:

A previously unnoticed dwarf nova found in archived data from NASA’s Kepler space telescope underwent a super outburst that was caught by chance, brightening by a factor of 1,600 in less than a day.

The system in question is made up of a compact white dwarf, the burned-out, Earth-size core of a star like the Sun, and a brown dwarf companion, a “failed” star 10 to 80 times more massive than Jupiter but too small to support nuclear fusion. The brown dwarf orbits its partner every 83 minutes at a distance of only 400,000 kilometres (250,000 miles).

At that close distance, the white dwarf’s powerful gravity strips material away from the brown dwarf, forming an accretion disc that spirals inward in a cosmic maelstrom. Theories suggest a super outburst like the one observed by Kepler may be caused when the accretion disc reaches a tipping point.

As material builds up in the disc, the outer edge can end up in gravitational resonance with the brown dwarf, possibly triggering a thermal instability. The disc’s temperature, normally between 2,700 and 5,300 degrees Celsius (5,000 to 10,000 degrees Fahrenheit), can shoot up to more than 11,000 C (21,000 F) at the peak of a super outburst.

Only about 100 such systems are known, and years to decades can go by between outbursts.

Kepler was built to search for exoplanets by monitoring slight changes in a star’s brightness as a planet crosses in front, or transits, as viewed from Earth. Kepler caught the dwarf nova’s outburst from start to finish.

But it was not seen right away. The event remained buried in the Kepler archive until a team led by Ryan Ridden-Harper of the Space Telescope Science Institute in Baltimore and the Australian National University stumbled across it.

“In a sense, we discovered this system accidentally,” said said Ridden-Harper. “We weren’t specifically looking for a super-outburst. We were looking for any sort of transient.”

While the rapid brightening of such systems is predicted by theory, what causes the observed slow rise at the beginning of the event remains a mystery.

“These dwarf nova systems have been studied for decades, so spotting something new is pretty tricky,” said Ridden-Harper. “We see accretion disks all over, from newly forming stars to supermassive black holes, so it’s important to understand them.”