A team of astronomers from the United Kingdom and Germany has spotted a superflare coming from an L-dwarf star approximately 250 light-years away that’s estimated to be 10 times more powerful than any such event emitted by the Sun.
This is a superflare on an L-dwarf star. Image credit: Mark Garlick / University of Warwick.
L-dwarf stars are among the lowest mass objects that could still be considered to be a star, lying in the transition region between stars and brown dwarfs.
These stars are also very cool compared to the more common main sequence stars, such as red dwarfs, and emit radiation mostly in the infrared, which may affect their ability to support the creation of life.
An L-dwarf called ULAS J224940.13-011236.9 is only a tenth of the radius of the Sun, almost the same size as Jupiter.
It was too faint for most telescopes to observe until University of Warwick astronomer James Jackman and colleagues spotted the massive stellar explosion in its chromosphere in an optical survey of the surrounding stars.
Using the Next Generation Transit Survey (NGTS) facility at ESO’s Paranal Observatory, with additional data from the Two Micron All Sky Survey (2MASS) and NASA’s Wide-field Infrared Survey Explorer (WISE), they observed the brightness of the star over 146 nights.
The superflare occurred on August 13, 2017, and gave off energy equivalent to 80 billion megatons of TNT, ten times as much energy as the Carrington event in 1859, the highest energy event observed on the Sun.
It is one of the largest flares ever seen on an L-dwarf star, making the star appear 10,000 times brighter than normal.
“We knew from other surveys that this kind of star was there and we knew from previous work that these kinds of stars can show incredible flares,” James said.
“However, the quiescent star was too faint for our telescopes to see normally — we wouldn’t receive enough light for the star to appear above the background from the sky. Only when it flared did it become bright enough for us to detect it with our telescopes.”
“It is amazing that such a puny star can produce such a powerful explosion,” said Professor Peter Wheatley, also from the University of Warwick.
“This discovery is going to force us to think again about how small stars can store energy in magnetic fields.”
“We are now searching giant flares from other tiny stars and push the limits on our understanding of stellar activity.”
The findings were published in the Monthly Notices of the Royal Astronomical Society: Letters.
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James A.G. Jackman et al. 2019. Detection of a giant white-light flare on an L2.5 dwarf with the Next Generation Transit Survey. MNRASL 485 (1): L136-L140; doi: 10.1093/mnrasl/slz039
