Contributed Talk - Splinter Magnetic
Thursday, 24 September 2020, 17:47 (virtual room H)
The case of Epsilon Eridani: reproducing its X-ray activity cycle with solar magnetic structures
M. Coffaro , B. Stelzer [1,2] and S. Orlando 
Institut fuer Astronomie und Astrophysik Tuebingen; INAF - Osservatorio Astronomico di Palermo
The solar-stellar connection refers to the often cited but poorly studied analogy between late-type stars and the Sun, especially in terms of their outer atmospheres that are driven by the magnetic field. We have developed a novel technique which allows us to reproduce the variability of the X-ray emission of active stars in terms of time-variations of solar-like magnetic structures in the corona. I show the application of this method to the young solar-like star Epsilon Eridani and its 3-yr X-ray activity cycle detected by us for the first time in a dedicated XMM-Newton long-term monitoring campaign. On the Sun, the coronal magnetic structures were spatially and temporally resolved and traced throughout its 11-yr cycle with the X-ray satellite Yohkoh. However, the corona of other stars can not be spatially resolved with present-day X-ray instruments. We, therefore, use an indirect approach in which we construct the coronal emission measure distributions of Epsilon Eridani as a composite of three types of emission measure distributions observed on the Sun: active regions (ARs), cores of active regions (COs) and flares (FLs). From a comparison between these pseudo-solar EMDs and the observations of Epsilon Eridani, we are able to associate to each state of the X-ray activity cycle of Epsilon Eridani the percentage of ARs, COs and FLs in the corona of the star. One main finding is that the average flare EMD of Epsilon Eridani is cooler than that of the Sun. We explain this as the flares lasting longer, suggesting a weaker radiative cooling of the corona which is supported by the low metallicity of Epsilon Eridani. The observed amplitude of the X-ray luminosity in the cycle of Epsilon Eridani is much smaller than on the Sun. Our analysis provides a physical explanation for this: the simulated EMDs indicate that in all phases of the X-ray cycle a large portion of the corona of Epsilon Eridani is covered by active structures. Therefore, there is little space for adding more magnetic structures in the cycle maximum. With its age of ~400 Myr Epsilon Eridani is a proxy for our Sun during its youth. In the future, we will apply our method to other stars probing how the solar-stellar connection is influenced by the evolutionary state of the stars.