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"Vampire" star discovered in the midst of a feeding frenzy

Spectacular revelations courtesy of Hubble
Spectacular revelations courtesy of Hubble 12:12

It's been two years since NASA's enormously successful planet-hunting Kepler mission ended, but its data is still surfacing new mysteries. Astronomers said Friday that they have identified a previously unknown and unexplained dwarf nova that is feasting on its neighbor. 

According to a NASA press release, the newly-discovered dwarf nova experienced a "super-outburst," during which it brightened by a factor of 1,600 times in less than one day. Astronomers are able to explain why the outburst occurred, but the increase in brightness remains a mystery. 

The system they found includes a white dwarf star with a brown dwarf companion about one-tenth as massive, which orbits the white dwarf every 83 minutes. A white dwarf is the core remnant of an old, dying star with a mass similar to the Sun but the volume of Earth. While a white dwarf is extremely dense, its brown counterpart is too large to be classified as a planet, but too small to be considered a star. 

Much like a vampire, the white dwarf is sucking the essence away from the brown dwarf, which is about the same distance away as the moon is from Earth. The close proximity allows the white dwarf's strong gravity to strip the material from the brown dwarf and create an accretion disk, causing the super-outburst. 

This illustration shows a newly discovered dwarf nova system, in which a white dwarf star is pulling material off a brown dwarf companion. The material collects into an accretion disk until reaching a tipping point, causing it to suddenly increase in brightness. NASA / L. Hustak (STScI)

"Such systems are rare and may go for years or decades between outbursts, making it a challenge to catch one in the act," astronomers said. 

After launching in March 2009, the Kepler space telescope searched for exoplanets by looking for stars that dimmed as planets crossed them. Its design allowed it to spot other astronomical transients — objects that brighten or dim over time. 

It was luck that Kepler was in the right place at the right time when the feeding frenzy occurred, able to capture every detail. It's the only instrument that could have seen it, because the system was too close to the Sun to be seen from Earth's point of view at the time.  

The event remained a secret in Kepler's archival data until a team of astronomers recently discovered it by chance. 

"In a sense, we discovered this system accidentally. We weren't specifically looking for a super-outburst. We were looking for any sort of transient," said Ryan Ridden-Harper, of the Space Telescope Science Institute (STScI) in Baltimore and the Australian National University in Canberra. 

Kepler observed a slow rise in brightness in the beginning, followed by rapid intensification. While the sudden brightness can be explained, the slow start is unusual, Ridden-Harper said. 

"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."

One possible explanation is that the accretion disk reached a tipping point, causing the outburst. As it ate up more material and grew in size, its temperature rose at the peak of the super-outburst. 

This type of dwarf nova system is rare, with only about 100 known examples. They are tough to study because a system may go years or even decades between outbursts. 

The team plans to continue searching through data from both Kepler and another exoplanet hunter, TESS, to search for other transients. 

"The detection of this object raises hopes for detecting even more rare events hidden in Kepler data," said co-author Armin Rest of STScI.

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