Astronomers using NASA’s Transiting Exoplanet Survey Satellite (TESS) have studied large, long-duration flares in white light on red dwarfs, a class of stars that have a lower temperature and mass than our Sun.
Red dwarfs flare actively and expel particles that can alter and evaporate the atmospheres of planets in their orbit. Image credit: Leibniz Institute for Astrophysics Potsdam / J. Fohlmeister.
Flares are magnetic explosions on the surface of stars that expel intense electromagnetic radiation into space.
Large flares like superflares emit a cascade of energetic particles that can hit exoplanets orbiting the flaring star, and in the process alter or even evaporate planetary atmospheres.
“We’ve known these are big flares, much larger than we see on our own Sun,” said Dr. James Davenport, an astronomer at the University of Washington.
“Now we see superflares occur at high latitudes, near the ‘poles’ of the star, which means that the bursts of radiation are not directed toward the paths of orbiting exoplanets.”
Using TESS data, Dr. Davenport and colleagues white-light flares on fast-rotating red dwarfs.
These types of flares last long enough that their brightness, as observed by TESS, varies as they rotate in and out of view on the stellar surface.
“Since we can’t see the surfaces of these stars, determining the latitudes of things like hot flares and cool spots has traditionally been between difficult and impossible,” Dr. Davenport said.
“This work combines clever data modeling with the uniquely precise data that comes from missions like TESS, and finds something remarkable.”
The astronomers found rotating flares by processing the light curves of more than 3,000 red dwarfs by TESS.
Among these stars, they found four with flares large enough for their new method.
They used the precise shape of each star’s light curve to infer the latitude of the flaring region, and found that all four flares occurred above approximately 55 degrees latitude, which is much closer to the pole than flares and spots on the surface of our Sun, which usually occur below 30 degrees latitude.
They also showed that their detection method was not biased toward a particular stellar latitude.
These findings, even with only four flares, are significant: if flares were spread equally across the stellar surface, the chances of finding four flares in a row at such high latitudes would be about 1-in-1,000.
This has implications for models of the magnetic fields of stars and for the habitability of exoplanets that orbit them.
“We discovered that extremely large flares are launched from near the poles of red dwarf stars, rather than from their equator, as is typically the case on the Sun,” said Ekaterina Ilin, a doctoral student at the Leibniz Institute for Astrophysics Potsdam and the Institute for Physics and Astronomy at the University of Potsdam.
“Exoplanets that orbit in the same plane as the equator of the star, like the planets in our own Solar System, could therefore be largely protected from such superflares, as these are directed upwards or downwards out of the exoplanet system.”
“This could improve the prospects for the habitability of exoplanets around small host stars, which would otherwise be much more endangered by the energetic radiation and particles associated with flares compared to planets in the Solar System.”
The detection of these flares is further evidence that strong and dynamic concentrations of stellar magnetic fields, which can manifest themselves as dark spots and flares, form close to the rotational poles of fast-rotating stars.
The existence of such ‘polar spots’ has long been suspected from indirect reconstruction techniques like Doppler imaging of stellar surfaces, but has not been detected directly so far.
“Nature is telling us something important about how these little, typically young stars produce magnetic fields that are much stronger than our Sun,” Dr. Davenport said.
“That has huge implications for how we think about the planets that orbit them.”
The team’s paper was published in the Monthly Notices of the Royal Astronomical Society.
Ekaterina Ilin et al. Giant white-light flares on fully convective stars occur at high latitudes. MNRAS, published online August 5, 2021; doi: 10.1093/mnras/stab2159