Seminario: Martina Cuffaro (Un. Tuebingen), 18 Febbraio ore 15

Stanza Google meet: https://meet.google.com/sxz-cctp-tsc

Speaker: Martina Coffaro (Univ. Tuebingen, Germany)

Titolo: X-ray activity cycles in young solar-like stars

Abstract:

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 $\sim 60\%$ 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, $\epsilon$~Eridani and $\iota$~Horologii, defining at which age and at which activity
level X-ray cycles set in.

An intriguing characteristic is that both $\epsilon$~Eridani and $\iota$~Horologii exhibit higher X-ray
luminosity and shorter cycle amplitudes (i.e. the variation of the X-ray luminosity throughout their
coronal cycle), counterposed to 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, preventing
significant variations of the X-ray luminosity throughout the cycle. In this sense, direct evidences were
found for $\epsilon$~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 $\sim 76\%$ to $\sim 88\%$ of $\epsilon$~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 $\epsilon$~Eridani (~200 Myr)
and it shows a photospheric cycle lasting 1.27yr. 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.