X-ray activity cycles in young solar-like stars

MARTINA COFFARO, SBA-IAAT — January 17, 2022

Probing the magnetic activity in solar-like stars is under an intense debate in the astrophysical community and still not well understood. While it is well known that 60 % of solar-like stars show magnetic activity in the chromosphere, probing the coronal X-ray counterparts is still challenging.
The XMM-Newton satellite has detected coronal cycles in five old solar-like stars (ages of few Gyr) with long X-ray cycle periods (8-12 yr). More recently, two young solar-like stars (with ages of 400-600 Myr and cycles lasting up to 3 yr) were added to this sample, ϵ Eridani and ι Horologii, defining at which age and at which activity level X-ray cycles set in.
An intriguing characteristic is that both ϵ Eridani and ι Horologii exhibit higher X-ray luminosity  and  shorter cycle amplitudes (i.e. the variation of the X-ray luminosity throughout their coronal cycle), differently from the old solar-like stars. The explanation lays in the high coverage fraction of magnetic structures, that rise on the corona and evolve during the activity cycle, pre-venting significant variations of the X-ray luminosity throughout the cycle. In this sense, direct evidences were found for ϵ Eridani, for which a new method to describe the evolution of its coronal cycle in terms of solar magnetic structures was applied and it is presented in this talk.
The method overcomes the difficulty of not being able to spatially resolve magnetic structures on the coronae of solar-like stars. This study shows that, during the X-ray cycle, the magnetic structures cover from 76 % to 88 % of ϵ Eridani’s corona. Most likely such feature is expected in all young solar-like stars. As matter of fact, this  method  can  be  applied  to  any  solar-like stars, and preliminary results have been obtained for another interesting target, Kepler 63. This latter star is even younger than ϵ Eridani (200 Myr) and it shows a photospheric cycle lasting 1.27 yr. Thus, its age and its short activity cycle make Kepler 63 a perfect target to better understand magnetic cycles. The preliminary results, obtained from a short X-ray campaign that was recently carried out for Kepler 63, are also presented within this talk.