Ekaterina Ilin

Flares are giant explosions that occur on the surface of stars. We can observed them in great detail on the Sun. If they are sufficiently intense, their radiation can stir up Earth’s magnetic field, disturbing satellite communication and dragging on their orbits. But what about other stars? In fact, small, red stars have strong magnetic fields that cause flares far more intense than anything we have ever seen on the Sun. Their habitable-zone planets reside in orbits very close to the star, so that repeated flare radiation could potentially strip the planet’s entire atmosphere. On the Sun, flares occur in a belt around the equator. Any flare at the right time will radiate close to the ecliptic plane, where Earth’s orbit resides. But if the same flare occurred closer to the poles, an orbiting planet will receive much less radiation, because most emission will be directed out of the plane.

Naturally, we want to find out: Where do giant flares occur on small (spectral type >M5) stars, of which there are so many? In general, this is hard to tell, because we cannot resolve the stellar disk like we resolve the Solar disk.
The stars we studied rotate rapidly with rotation periods <12h (Sun: 25 days). They released gigantic flares that were visible throughout multiple rotation periods.

The model we developed uses this information to locate the flaring region. It convolves the empirical flare shape (Davenport et al. 2014) with the geometric effect of stellar rotation, and return observable light curves for all possible combinations of stellar axis inclination and latitude.
One of the examples shows the flare light curve of an M7 dwarf (blackbody temperature ~2650 K). The rotating star is seen nearly equator-on, and we derived that the flare occurred approximately at 82 deg latitude.

For the four flares we studied on late-type (M5, M6, M7 and L1) ultrafast spinning dwarfs, we found that they occurred much closer to the poles (>45°) than typical flares on the Sun (<30°), and were more energetic than the Carrington event, the famous solar flare in 1859 (Carrington 1859, Cliver & Dietrich 2013). Surface magnetic fields on fully convective stars have so far been mostly studied using spectroscopy, and spectropolarimetry of atomic and molecular lines. Only a handful of magnetic maps exist for these spectral types.

While our technique relies on serendipitous observations of large flares in optical light curves, we expect that the Transiting Exoplanet Survey Satellite mission (Ricker et al. 2015) alone will collect 1-5 such events per year of observations, which will allow us to consolidate or revise the trend that megaflares occur at high latitudes in fully convective ultrafast rotators.

Multi-period flares put important constraints on dynamo models for fully convective stars, and the loci bear noteworthy implications for the impact of flaring activity on exoplanets in their orbits.

Contributing authors:

Ekaterina Ilin (1,2,*), Katja Poppenhäger (1,2), Sarah J. Schmidt (1), Silva Järvinen (1), Mahmoud Oshagh (3,4)

(1) Leibniz Institute for Astrophysics Potsdam (AIP)
(2) University of Potsdam
(3) Instituto de Astrofísica de Canarias (IAC)
(4) Departamento de Astrofísica, Universidad de La Laguna (ULL)
*email: eilin@aip.de