Abstract:
M dwarfs are the most common stars in the universe and they show magnetic activity due to a dynamo mechanism not yet understood for very late spectral types that appear to have a fully convective internal structure (transition M3.5). The purpose of this work is the study of the coronal X-ray variability of M dwarfs in order to indirectly investigate the stellar dynamo and how it depends on stellar parameters, such as mass and rotation. To this end, I analyzed the activity-rotation relation, that is known to consist of two different regimes: the saturated regime for fast-rotating stars and the unsaturated regime for slowly rotating stars, with the transition between the two regimes located at a period of ~10 d. The bimodal behavior of this relation not only depends on the rotation but it is also linked to the stellar mass and age. Thus, stellar evolution plays an important role in understanding how magnetic activity changes during the stellar lifetime.
The first paper, Magaudda et al 2020, presents the analysis of a sample of 14 M dwarfs observed with XMM-Newton and Chandra satellites, and with rotation periods determined from Kepler Two-Wheel (K2) Mission light curves. These new data combined with updated and homogenized literature results provide the largest uniform sample of M dwarfs (302 stars) for X-ray activity and rotation studies to date. This allowed a detailed investigation of the activity-mass-age-rotation dependence. From the analysis of the mass-effects I confirmed the decrease of the X-ray level for lower stellar mass in the saturated regime, previously observed for a sample of stars with spectral types from G to M and with masses lower than 1.4 M_sun. With the joint analysis of angular momentum evolution models and the observed activity-rotation relation (L_x-P_rot) studied in my work, I predicted the decay of the X-ray luminosity with stellar age finding a good agreement with the observed L_x of partially convective stars (SpT<M3.5) with known age.
As a contribution to the work of Magaudda et al 2020 regarding the study of the M dwarf GJ 357, I analyzed the X-ray data taken with XMM-Newton, that were used to put constraints on the atmospheric evolution of the star's planet. In particular, with help of the L_x-age dependence for M dwarfs I predicted in Magaudda et al 2020 we placed a lower limit on the age of GJ 357. We studied the evolution of the atmosphere of the planet orbiting around GJ 357 performing a backwards reconstruction of the X-ray radiation emitted by GJ 357. In this way, we found the upper limit of the initial primordial atmospheric mass of the planet, amounting to ~38 M_earth, before evaporating caused by the absorption of the stellar X-ray emission by the planet.
The other two papers, Magaudda et al 2022a,2022b, present the first results from the analysis of new data taken with the ROentgen Survey with an Imaging Telescope Array (eROSITA) on board the Russian Spektrum-Roentgen-Gamma mission (SRG) combined with new rotation periods extracted from the Transiting Exoplanet Survey Satellite (TESS) light curves. I searched for eROSITA and TESS data selecting stars from the \textsc{superblink} proper motion catalog of nearby M dwarfs, enhanced with Gaia-DR2 data. To ensure the correct association between eROSITA X-ray sources and the M~dwarfs I performed a meticulous cross-match procedure the results of which were confirmed with a Bayesian statistical algorithm (NWAY) developed by other members of the eROSITA-DE consortium. With this unprecedented eROSITA- TESS data base for X-ray emitting M dwarfs, combined with my compilation of X-ray and rotation data previously studied, I quantitatively investigated the mass dependence of the saturated regime of the activity-rotation relation. Finally, I investigated the eROSITA capability with respect to M dwarfs. First, I compared the new eROSITA X-ray detections with those from the historical ROSAT satellite. This resulted in a statistical investigation of the long-term X-ray variability of the M dwarf sample presented in Magaudda et al 2022a, from which I concluded that many faint M dwarfs were previously detected by ROSAT only because they happened to be in a higher activity state. Later, in Magaudda et al 2022b, I studied the rotation dependence of the X-ray luminosity in the saturated regime for three different mass bins and I observed a paucity of low mass stars with intermediate rotation periods (~1-10 d), probably caused by fast period evolution. By comparing the eROSITA detections for those stars that have also reliable rotation periods from TESS light curves, I found that eROSITA is sensitive for detecting slower rotating M~dwarfs that are in the unsaturated regime with periods inaccessible to TESS. [untranslated]
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