Superflares: Their origins and connection with planets

Our Sun is a common star which shows from time to time variations in its radiation and plasma environment. These variations are usually caused by dynamics of the solar outer layers and the magnetic field. Therefore, the Sun shows outbreaks of radiation, so-called flares, and also plasma, so- called Coronal Mass Ejections (CMEs). The outbreaks interact with our Earths magnetic field and cause aurorae, and may even damage satellites and power stations leading to power outages. Only recently it has been found that other stars similar to the Sun show so-called superflares, which are much more energetic than the flares we know nowadays from the Sun. But also the Sun had a superflare, which was the first flare ever detected on the Sun in the middle of the 19th century. There may be severe consequences for life on planets orbiting stars harbouring such superflares, especially as recent studies revealed solar-type stars which show superflares every day. In the present project we try to better understand these superflares and their influence on planets. To realize this project goal we need, apart from photometric observations (i.e. images), also spectroscopic observations which give information on the energy distribution of the light obtained by the stars. By doing so we are able to determine parameters of superflares, such as e.g. temperature and energy. By involving modelling we will also investigate from where the superflare emission originates, either from the so-called footpoints which are anchored in the solar surface, or from flaring loops which are standing higher above the solar surface. We also address the origin of superflares, i.e. is a superflare solely a stellar phenomenon or do close-in orbiting planets trigger superflares. We intend to observe superflare stars with and without planets to answer this question. To see whether atmospheres of planets orbiting superflare stars, which are frequently hit by superflares, are different from stars harbouring no superflares, we will do transmission spectroscopy, which is the investigation of starlight falling through the atmosphere of the planet to obtain its composition. The observations of target stars in the northern hemisphere will be obtained from the 2m Schmidt telescope located in Ondrejov, Czech Republic, belonging to the Astronomical Institute of the Czech Academy of Sciences. For targets in the southern hemisphere we will use the newly refurbished ESO 1.5m telescope located in La Silla at the border of the Atacama desert in Chile, hosted by the PlatoSpec consortium (a consortium for ground-based support for exoplanetary space missions). By combining expertise from both, the Czech (Astronomical Institute, Czech Academy of Sciences, Czech Republic) and the Austrian (Institute of Physics, IGAM, University of Graz, Austria) side within this bilateral project we expect to solve the questions posed above on superflares on solar-like stars and their influence on planets orbiting them.

Superflares are outbreaks of radiation (so-called flares) which are far more energetic as the strongest flares on the present-day Sun. Even the well known Carrington event, the first ever witnessed solar flare, did not reach such energies. In the last years hundreds of Superflares were detected on solar-like stars in TESS and Kepler data. The generation of Superflares is still a subject of discussion in literature, either they are generated solely by the magnetic field of the star or they might be triggered by close-in orbiting more massive companions. Furthermore, Superflares are also relevant because they have an impact on young exo-planetary systems. The goal of the project was to provide a contribution to answer these open questions. Therefore we collected data on one of the first telescopes of the European Southern Observatory, the ESO1.52m telescope hosted by the PLATOSpec consortium, together with the spectrograph Pucheros+. The target stars were stars which showed the largest flare rates, determined from TESS data, among others those were AU Microscopii, CC Eridani, and LQ Hydrae. All of these stars showed Superflares during their observations at the ESO1.52m telescope, and therefore it became possible for the first time to analyse Superflare-spectra from various Superflares from stars of different type. Also normal Flares were detected and a comparison to Superflares could be made, if there is a difference between Flare- and Superflare-spectra. What we have found is that the more energetic the flare the more spectral lines are affected. From that we concluded that the generation mechanism of Superflares is similar to the one of flares, and therefore their generation mechanism is powered solely by the stellar magnetic field. This does not say that even more energetic Superflares, which we have not detected, might not have a different generation mechanism, and this does also not rule out the the possibility that single Superflares may be triggered by close-in orbiting companions. Moreover we have investigated to what degree Flares and Superflares are related to star spots. We have found a relation for solar like G-type stars, a less distinct relation for the cooler K-type stars, and no relation for the even cooler M-type stars. On the other hand, we have found distinct relations for single stars of different types. This shows, that not the number of star spots leads to more Flares and Superflares, but single spots or spot groups which have the characteristics to produce those.