Magnetic Cycles in a Dynamo Simulation of Fully Convective M-Star Proxima Centauri

Abstract

The recent discovery of an Earth-like exoplanet around Proxima Centauri has shined a spot light on slowly rotating fully convective M-stars. When such stars rotate rapidly (period 20 days), they are known to generate very high levels of activity that is powered by a magnetic field much stronger than the solar magnetic field. Recent theoretical efforts are beginning to understand the dynamo process that generates such strong magnetic fields. However, the observational and theoretical landscape remains relatively uncharted for fully convective M-stars that rotate slowly. Here, we present an anelastic dynamo simulation designed to mimic some of the physical characteristics of Proxima Centauri, a representative case for slowly rotating fully convective M-stars. The rotating convection spontaneously generates differential rotation in the convection zone that drives coherent magnetic cycles where the axisymmetric magnetic field repeatedly changes polarity at all latitudes as time progress. The typical length of the "activity" cycle in the simulation is about nine years, in good agreement with the recently proposed activity cycle length of about seven years for Proxima Centauri. Comparing our results with earlier work, we hypothesis that the dynamo mechanism undergoes a fundamental change in nature as fully convective stars spin down with age.

Rotation–differential rotation relationships for late‐type single and binary stars from Doppler imaging

“…According to Forgács‐Dajka (), a similar mechanism prevents the differential rotation from penetrating down into the solar radiative interior. Note that the differential rotation is highly quenched in fully convective M‐stars as well, until the rotation is fast enough (Yadav et al ). The model by Kitchatinov & Rüdiger () suggests that ΔΩ/Ω ∝ Ω − n , where n ≈ 1 for G–K dwarfs, that is, ΔΩ/Ω∝ P rot .…”

“…While in rapidly rotating young late‐type (G–K) stars supposedly α 2 Ω‐type dynamos work, in fully convective low‐mass stars or brown dwarfs a pure α 2 ‐type dynamo may operate, wherein the helical turbulence ( α ‐effect) is dominant and the Ω‐effect is basically negligible (Chabrier & Küker ). For the effect of the Rossby number on the dynamo in fully convective M‐dwarfs and the presumed transition from α 2 ‐type dynamos to α Ω‐type dynamos, see Yadav et al ().…”

The Composition and Evolutionary Status of Proxima Centauri

“…In addition to its closeness to the solar system, Proxima has also been observed as an active flare star [26,27], although how it (or, for that matter, any other M-spectral type star) generates and maintains a long-lived and coherent magnetic field is still not fully understood [28,10]. Using Sloan Digital Sky Survey data, however, West et al [29] find that M5 V stars (i.e., ones having a spectral type similar to Proxima) have a 7.0 ± 0.5 Gyr flare activity cut-off age.…”

Probing the magnetosphere of the M8.5 dwarf TVLM 513−46546 by modelling its auroral radio emission. Hint of star exoplanet interaction?

“…Similarly to the Sun-like magnetic cycle, it has been theoretically predicted that also in the case of fully-convective slowly-rotating late-type stars the magnetic-field axis periodically changes its orientation, reverting the north to south magnetic polarity (Yadav et al 2016). Such possible magnetic-polarity reversal, acting also in the case of the fast rotating UCDs, was claimed as the origin of the detection in different observing epochs of auroral pulses with opposite polarisation sense (Route 2016).…”