Astronomers have long suspected that superflares, extreme radiation bursts from stars, can cause lasting damage to the atmospheres 鈥� and thus habitability 鈥� of . A new study Aug. 5 in the Monthly Notices of the Royal Astronomical Society reports that they pose only a limited danger to planetary systems.
鈥淲e’ve known these are big flares, much larger than we see on our own sun,鈥� said co-author , a research assistant professor of astronomy at the 天美影院. 鈥淣ow we see superflares occur at high latitudes, near the 鈥榩oles鈥� of the star, which means that the bursts of radiation are not directed toward the paths of orbiting exoplanets.鈥�
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.
Using optical observations from the Transiting Exoplanet Survey Satellite 鈥� or TESS 鈥� the team, led by astronomers at the Leibniz Institute for Astrophysics Potsdam, studied large superflares on red dwarfs, a class of young, small stars that have a lower temperature and mass than our own sun.
Many exoplanets have been found around these types of stars. A lingering question in exoplanet research has been whether these exoplanets are habitable, since red dwarfs are more active than our sun and flare much more frequently and intensely.
The team developed a method to determine the location on the stars鈥� surface where flares originate. The team achieved this by analyzing so-called 鈥渨hitelight flares鈥� on fast-rotating red dwarf stars. 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.
鈥淪ince we can鈥檛 see the surfaces of these stars, determining the latitudes of things like hot flares and cool spots has traditionally been between difficult and impossible!鈥� said Davenport, who is also the associate director of the Data Intensive Research in Astrophysics and Cosmology 鈥� or 鈥� Institute at the UW. 鈥淭his work combines clever data modeling with the uniquely precise data that comes from missions like TESS, and finds something remarkable.鈥�
The team found rotating flares by processing the light curves of more than 3,000 red dwarf stars by TESS. Among these stars, they found four with flares large enough for their new method. The team used the precise shape of each star鈥檚 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. The team 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.
鈥淲e 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,鈥� said lead author Ekaterina Ilin, a doctoral student at Leibniz. 鈥淓xoplanets 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.鈥�
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 鈥減olar spots鈥� has long been suspected from indirect reconstruction techniques like Doppler Imaging of stellar surfaces, but has not been detected directly so far.
鈥淣ature is telling us something important about how these little, typically young stars produce magnetic fields that are much stronger than our sun,鈥� said Davenport. 鈥淭hat has huge implications for how we think about the planets that orbit them.鈥�
Co-authors are Katja Poppenhaeger, Sarah Schmidt, Silva J盲rvinen, Juli谩n Alvarado-G贸mez and Ilya Ilyin at the Leibniz Institute for Astrophysics Potsdam; Elisabeth Newton at Dartmouth College; Sebastian Pineda at the University of Colorado Boulder; and Mahmoudreza Oshagh of the Canary Islands Astrophysics Institute and the University of La Laguna in Spain. Ilin and Poppenhaeger also have appointments at the University of Potsdam. The research was funded by NASA, the German National Scholarship Foundation, the Leibniz Association and the UW.
For more information, contact Davenport at jrad@uw.edu.
Adapted from a by the Leibniz Institute for Astrophysics Potsdam.