The Puzzling Story of Flare Inactive Ultra Fast Rotating M dwarfs. I. Exploring their Magnetic Fields

TL;DR

This study investigates the magnetic fields of ultra-fast rotating M dwarfs with low flaring activity, revealing that magnetic field strength alone does not explain their activity levels, indicating a more complex rotation-activity relationship.

Contribution

It provides spectropolarimetric measurements of magnetic fields in UFR M dwarfs and analyzes their flare activity and variability over multiple years, challenging simple activity-rotation models.

Findings

50% of stars have detectable magnetic fields of 1-2 kG

No significant difference in flare rate between observation cycles

Magnetic field strength may not determine flaring activity

Abstract

Stars which are rapidly rotating are expected to show high levels of activity according to the activity-rotation relation. However, previous TESS studies have found Ultra Fast Rotating (UFR) M dwarfs with periods less than one day displaying low levels of flaring activity. As a result, in this study, we utilise VLT/FORS2 spectropolarimetric data of ten M dwarf UFR stars between spectral types $\sim$M2 - M6 all with $P_{\rm rot} <$ 1, to detect the presence of a magnetic field. We divide our sample into rotation period bins of equal size, with one star having many more flares in the TESS lightcurve than the other. We also provide an analysis of the long-term variability within our sample using TESS lightcurves taken during Cycles 1 and 3 (up to three years apart). We identify 605 flares from our sample which have energies between 2.0$\times$10$^{31}$ and 5.4$\times$10$^{34}$ erg. Although we find no significance difference in the flare rate between the Cycles, two of our targets display changes in their lightcurve morphology, potentially caused by a difference in the spot distribution. Overall, we find five stars (50%) in our sample have a detectable magnetic field with strengths $\sim$1 - 2 kG. Of these five, four were the more flare active stars within the period bins with one being the less flare active star. It would appear the magnetic field strength may not be the answer to the lack of flaring activity and supersaturation or magnetic field configuration may play a role. However, it is clear the relationship between rotation and activity is more complex than a steady decrease over time.