Characterizing X‐ray activity cycles of young solar‐like stars with solar observations

Abstract

AbstractThroughout an activity cycle, magnetic structures rise to the stellar surface, evolve, and decay. Tracing their evolution on a stellar corona allows us to characterize the X‐ray cycles. However, directly mapping magnetic structures is feasible only for the Sun, while such structures are spatially unresolved with present‐day X‐ray instruments on stellar coronae. We present here a new method, implemented by us, that indirectly reproduces the stellar X‐ray spectrum and its variability with solar magnetic structures. The technique converts solar corona observations into a format identical to that of stellar X‐ray observations and, specifically, spectra of the X‐ray satellite XMM‐Newton. From matching these synthetic spectra with those observed for a star of interest, a fractional surface coverage with solar magnetic structures can be associated to each X‐ray observation. We apply this method to two young solar‐like stars: Eri ( Myr), the youngest star to display a coronal cycle ( yr), and Kepler 63 ( Myr), for which the X‐ray monitoring did not reveal a cyclic variability. We found that even during the cycle minimum a large portion of Eri's corona is covered with active structures. Therefore, there is little space for additional magnetic regions during the maximum, explaining the small observed cycle amplitude () in terms of the X‐ray luminosity. Kepler 63 displays an even higher coverage with magnetic structure than the corona of Eri. This supports the hypothesis that for stars younger than Myr the X‐ray cycles are inhibited by a massive presence of coronal regions.