Enza Magaudda (IAAT) Tübingen, January 29, 2024

The activity of the Sun and solar-like stars is driven by a dynamo mechanism, according to which the combination of differential rotation and convective motions of the outer atmospheric envelope continuously regenerates the magnetic field that manifests itself in the form of powerful optical, UV, and X-ray radiation. M-L dwarfs are also known to be magnetically active, but the physical mechanism is poorly understood. Studying their X-ray emission and its variability with rotation and stellar parameters allows to constrain the dynamo mechanism that powers the magnetic field and causes activity in the atmosphere. Specifically, magnetically active stars release energy that often causes a sudden increase of luminosity in their light curve. These not periodic and short-term variability events are defined as flares, and when emitted they influence the aspect of the outermost atmospheric layer, that is the Corona. Moreover, cool dwarfs with spectral type later than M7 display a remarkable radio emission that is tied to auroral electron precipitation typical of giant planets.
In this talk, I present our attempt on constraining the magnetic dynamo of M dwarfs by studying the mass-dependent activity-rotation relation for the largest and most uniform sample of M dwarfs with observations taken with XMM-Newton, Chandra, eROSITA, K2 and TESS combined with X-ray and rotation data from the literature. Then, I will show the relation between the X-ray and radio luminosity of ultracool dwarfs and how much their emission deviates from the typical seen in solar-like or young stars.
Finally, I will focus on the study of the X-ray variability of M dwarfs revealed when comparing data from different X-ray instruments, and the investigation of how the coronal emission changes in presence of X-ray flares using data of the very well-known active M dwarf, Proxima Centauri, as an example.