You can be thankful that we orbit a placid, main sequence, yellow dwarf star. Astronomers recently spied a massive superflare on a diminutive star, a powerful, radiation spewing event that you wouldn’t want to witness up close.
The ‘star’ was ULAS J224940.13-011236.9, an L-type sub-stellar brown dwarf near the Aquarius-Pisces border. The cumbersome, phone number-style name comes from the UKIDSS Large Area Survey (ULAS) study hunting for dwarf stars, plus the object’s position in the sky in right ascension and declination. Located 248 light-years distant, ULAS J2249-0112 (for short) weighs in at just around 15 Jupiter masses, with a radius about a 1/10th that of our Sun; any tinier, and it wouldn’t even rank as a sub-stellar brown dwarf.
The action began on the night of August 13, 2017, as the Next Generation Transit Survey (NGTS) was scouring the sky for exoplanets. Based at the Paranal Observatory complex in the Atacama desert, NGTS is a wide-field survey with 12 telescopes, imaging a 96 square degree swath of sky once every 13 seconds on the hunt for transiting exoplanets. While these sorts of transit events feature tiny changes in brightness, what ULAS J2249-0112 produced was anything but. The faint +24.5th magnitude dwarf briefly flared up over 10 magnitudes in brightness for 9.5 minutes, reaching a peak magnitude of +14. That’s a change of brightness of 10,000-fold.
“NGTS has tens to hundreds of thousands of stars in its field of view at any one time, which gives us the same amount of light curves,” James Jackman (Warwick University) told Universe Today. “So, along with searching for planets in this data we can search for other astrophysical events, such as stellar flares.”
This brilliant white light flare was over 10 times brighter and more powerful than anything witnessed on our Sun. The Great Carrington superflare of 1859, for example, unleashed a powerful flare that set telegraph offices aflame and sent colorful auroral displays as far south as the Caribbean. The 2017 exoflare would have registered as an X-100 class event, were it to have occurred on our Sun.
“As the star is so faint, we could only see it when it was flaring,” says Jackman. “So, most of our light curve sits at a count rate of zero. Then when the flare occurs, it suddenly spiked up!”
The study was published in the April 2019 Monthly Notices of the Royal Astronomical Society: Letters.
This event shows that even tiny L-dwarfs can pack a big punch. Though larger, tempestuous red dwarfs are well known producers of flares, a flare on a smaller L-type brown dwarf is rare. The 2017 event was only the sixth such event observed from a L-dwarf, and the second captured from the ground. Of these, the 2017 event was the most powerful such event observed thus far.
“Flares are produced through reconnection events in the magnetic fields of stars,” says Jackman. “The energy released is provided by the magnetic field, so a stronger field gives high energy flares. M stars in particular can have very strong magnetic fields, which results in high energy flares. We’ve observed that after a point as we go to smaller stars, they become less active. This corresponds with the magnetic field getting weaker, producing fewer high energy flares. The presence of a large flare on our incredibly small star is a bit puzzling, as it suggests that these tiny stars can hold vast amounts of energy in their magnetic fields after all.”
The NGTS team continues to scour the data, looking for more superflares. The Transiting Exoplanet Survey Satellite (TESS) may also prove to be a treasure trove of such events, as it carries out its all-sky survey for nearby transiting exoplanets.
“We’re currently running a dedicated survey to search for M and L dwarf flares in the NGTS dataset,” says Jackman. “Other groups are also targeting nearby bright stars to try and get information not just on flares themselves, but how they may be related to the quiescent behavior as well (e.g. starspots). It’s a really exciting time to be in the field.”
And of course, such a powerful superflare would be deadly to life as we know it. When it comes to life on planets orbiting red or brown dwarfs, the safest places are on the far hemisphere of a tidally locked world, or perhaps in a subsurface ocean, either of which would be protected from life-sterilizing radiation. On the plus side, such stars are miserly, taking trillions of years to burn through the fusion cycle. (longer than the current age of the Universe) giving potential life on a planet orbiting a red or brown dwarf lots of time to evolve.
Though brown dwarfs cannot sustain traditional hydrogen fusion via the proton-proton chain of stellar nucleosynthesis, they can derive energy from some of the very first steps in the process via deuterium and lithium fusion.
And while we’re witnessing such a massive superflare on a faraway star, our own star the Sun has been anything but active, as we approach another profound solar minimum between solar cycle #24 and #25 in late 2019 and 2020.
Be thankful we aren’t subjected to such a punishing superflare like those emitted by smaller dwarf stars… it might just be why we evolved here in the first place.
Did you know: though they’re the most common type of star in the Universe, not one red dwarf is visible to the naked eye? Check out our list of red dwarf stars for backyard scopes.
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