Dynamics of Rotation in M Dwarfs: Indications for a Change in the Dynamo Regime in Stars at the Onset of Complete Convection

Abstract

We have measured vsin i with high precision for a sample of dM3 stars (86 targets). We detected rotation in 82 stars (73 dM3 stars and nine dM3e stars). We compare our measurements of vsin i for all of the stars in our dM0, dM2, dM3, and dM4 samples to those from other authors. We find a good agreement down to vsin i values of less than 1 km s^-1. The mean of the differences between measurements is only 0.42 km s^-1. We find that the distribution of P/sini for our dM3 stars is different from the distribution of P/sini among our samples of dM2 and dM4 stars. The mean rotation rate for the dM3 stars (excluding dM3e and sdM3 stars) is significantly slower (25.8 days) than for dM2 (14.4 days) and dM4 stars (11.4 days). Analogous behavior also emerges among the faster rotators (dMe stars): we find that a longer rotation period also occurs at spectral subtype dM3e. Our data suggest that, as regards the rotational properties of lower main-sequence stars, spectral subtype dM3 stands out as exhibiting unusually slow rotation compared to that of adjoining subtypes. Our data lead us to suggest that the unusual rotational properties of M3 dwarfs may represent a signature of the transition to complete convection.

Based on observations available at Observatoire de Haute Provence and the European Southern Observatory databases and on Hipparcos parallax measurements.