2023ApJ...949...51I

An αΩ dynamo, combining shear and cyclonic convection in the tachocline, is believed to generate the solar cycle. However, this model cannot explain cycles in fast rotators (with minimal shear) or in fully convective stars (no tachocline); an analysis of these stars could therefore provide key insights into how these cycles work. We reexamine ASAS data for 15 M dwarfs, 11 of which are presumed fully convective; the addition of newer ASAS-SN data confirms cycles in roughly 12 of them, while presenting new or revised rotation periods for 5 stars. The amplitudes and periods of these cycles follow ${A}_{\mathrm{cyc}}\propto {P}_{\mathrm{cyc}}^{0.94\pm 0.11}$, with P_cyc/P_rot ∝ Ro^{–1.02±0.06} (where Ro is the Rossby number), very similar to P_cyc/P_rot ∝ Ro^{–0.81±0.17} that we find for 40 previously studied FGK stars, although P_cyc/P_rot and α are a factor of ∼20 smaller in the M stars. The very different P_cyc/P_rot-Ro relation seen here compared to previous work suggests that two types of dynamo, with opposite Ro dependences, operate in cool stars. Initially, a (likely α² or α²Ω) dynamo operates throughout the convective zone in mid- to late-M and fast-rotating FGK stars, but once magnetic breaking decouples the core and convective envelope, a tachocline αΩ dynamo begins and eventually dominates in older FGK stars. A change in α in the tachocline dynamo generates the fundamentally different P_cyc/P_rot-Ro relation.