Cycle times of early M dwarf stars: mean field models versus observations

TL;DR

This study compares mean field models and observations of early M dwarf stars' cycle times, finding that models predict longer cycle durations than observed, challenging the flux transport dynamo explanation.

Contribution

The paper demonstrates that mean field models cannot account for the observed short cycle times of early M dwarf stars, questioning the flux transport dynamo mechanism.

Findings

Models predict longer cycle times than observed.

Observed cycle times are around one and four years.

Flux transport dynamo is unlikely to explain these cycles.

Abstract

Observations of early-type M stars suggest that there are two characteristic cycle times, one of order one year for fast rotators ($P_{\rm rot} < 1$ day) and another of order four years for slower rotators. For a sample of fast-rotating stars, the equator-to-pole differences of the rotation rates up to 0.03 rad d$^{-1}$ are also known from Kepler data. These findings are well-reproduced by mean field models. These models predict amplitudes of the meridional flow, from which the travel time from pole to equator at the base of the convection zone of early-type M stars can be calculated. As these travel times always exceed the observed cycle times, our findings do not support the flux transport dynamo.