SPIRou reveals unusually strong magnetic fields of slowly rotating M dwarfs

ABSTRACT In this paper, we study six slowly rotating mid-to-late M dwarfs (rotation period $P_{\mathrm{rot}}\approx 40-190\, \mathrm{d}$) by analysing spectropolarimetric data collected with SpectroPolarimetre InfraRouge (SPIRou) at the Canada–France–Hawaii Telescope as part of the SPIRou Legacy Survey from 2019 to 2022. From ≈100–200 least-squares-deconvolved (LSD) profiles of circularly polarized spectra of each star, we confirm the stellar rotation periods of the six M dwarfs and explore their large-scale magnetic field topology and its evolution with time using both the method based on principal component analysis (PCA) proposed recently and Zeeman–Doppler imaging. All M dwarfs show large-scale field variations on the time-scale of their rotation periods, directly seen from the circularly polarized LSD profiles using the PCA method. We detect a magnetic polarity reversal for the fully convective M dwarf GJ 1151, and a possible inversion in progress for Gl 905. The four fully convective M dwarfs of our small sample (Gl 905, GJ 1289, GJ 1151, and GJ 1286) show a larger amount of temporal variations (mainly in field strength and axisymmetry) than the two partly convective ones (Gl 617B and Gl 408). Surprisingly, the six M dwarfs show large-scale field strengths in the range between 20 and 200 G similar to those of M dwarfs rotating significantly faster. Our findings imply that the large-scale fields of very slowly rotating M dwarfs are likely generated through dynamo processes operating in a different regime than those of the faster rotators that have been magnetically characterized so far.