Contributed Talk - Splinter Magnetic

Thursday, 24 September 2020, 15:30   (virtual room H)

Relation between stellar rotation and magnetic activity of M dwarfs with K2 long- and short-cadence

St. Raetz(1), B. Stelzer(1,2), M. Damasso(3), and A. Scholz(4)
(1) IAAT (2) INAF - Palermo (3) INAF - Torino (4) SUPA, University of St Andrews

Stellar activity is directly linked to magnetic fields that are believed to be generated and maintained by a dynamo which is driven by differential rotation and convection. Therefore, rotation and stellar activity are intimately connected. Photometric observations with space telescopes provide rotation periods even with low amplitudes as well as a wealth of activity diagnostics. Our previous study of the rotation-activity relation based on photometric activity indicators from long cadence K2 data revealed, that, at a critical rotation period of ~10d, the activity level changes abruptly. This phenomenon represents an open problem within the framework of dynamo theory. We have now extended our work to K2 short cadence data to examine a possible influence of the data sampling on the shape of the rotation-activity relation, in particular with respect to the different sensitivity to the detection of stellar flares. Our sample comprises 56 bright and nearby M dwarfs observed by K2 during campaigns C0-C18 in long and short cadence mode. In my talk I will explain the results of our K2 study with a special focus on stellar flares. I will show how a higher data cadence improves the detection and analysis of such events. The analysis of the short cadence light curves results in a 4.6 times higher flare rate than the long cadence data. We confirm in the short cadence data the abrupt change of the activity level in the rotation-activity relation at a critical period of ~10d. This change is most drastic in the flare duration and the flare frequency for short cadence data. Our flare studies revealed that the highest flare rates are not found among the fastest rotators and that the stars with the highest flare rates do not show the most energetic flares. Finally, we found a ~2 times larger superflares frequency for the fast rotating M stars than for solar-like stars in the same period range.