Astronomers have detected an outburst from what they are terming a “vampire star” in data accidentally collected by the Kepler Space Telescope. The event is one of the rarest types of nova – nothing like as dramatic as a supernova, but still a window on stellar behavior.
Novas occur when a white dwarf and another star orbit very close to each other. The white dwarf's density gives it a gravitational field sufficient to draw material away from the other star.
Dr Ryan Ridden-Harper of the Australian National University told IFLScience his discovery is an unusual nova, known as a WZ Sge dwarf nova. Instead of a proper star, the gas is being drained from a brown dwarf – an object that falls somewhere between a large planet and a star. Brown dwarfs are too small to initiate hydrogen fusion.
Brown dwarfs may be puny compared to stars, but their hydrogen is as good as anyone's and when the accretion disk of stolen material around the white dwarf becomes sufficiently dense, a fusion reaction is triggered, releasing a major burst of light. The European Southern Observatory refers to white dwarfs that come back from the dead by feeding on companions as vampire stars. It remains to be seen if the name sticks.
Kepler's mission was to look for stars that dip in brightness as planets pass across their face, but its capacity to track changes in brightness with a precision far beyond ground-based telescopes extends to stars close to them in the sky. Ridden-Harper told IFLScience others made algorithms to filter out any increases in brightness from stars that happen to be in the same field of view as a Kepler target. He, on the other hand, has been searching the Kepler archives for stellar outbursts, and hit paydirt.
“The incredible data from Kepler reveals a 30-day period during which the dwarf nova rapidly became 1,600 times brighter before dimming quickly and gradually returning to its normal brightness,” Ridden-Harper said in a statement. “The disk reached up to 11,700º Celsius at the peak of the super-outburst.” The slow initial brightening, followed by a phenomenal acceleration, was like nothing previously seen before.
The unfortunate companion is much too faint to be seen, but in Monthly Notices of the Royal Astronomical Society, Ridden-Harper calculates its mass from the orbital period of the white dwarf, confirming its brown dwarf status.
At its peak, Ridden-Harper told IFLScience, the white dwarf reached 13th magnitude – visible in large amateur telescopes. No one on Earth saw it however, because the system was hidden behind the Sun at the time. Kepler only witnessed it because the satellite was located so far from the Earth that the Sun was in a different part of the sky from its perspective.
Since his discovery, Ridden-Harper has used a large ground-based telescope to confirm the now quiescent white dwarf's location.