The Rotation–activity Correlations in K and M Dwarfs. II. New Constraints on the Dynamo Mechanisms in Late-K and M Dwarfs before and at the Transition to Complete Convection^∗

Abstract

We study the rotation-activity correlations (RACs) in a sample stars from spectral type dK4 to dM4. We study RACs using chromospheric data and coronal data. We study the Ca ii line surface fluxes-P/ sin i RACs. We fit the RACs with linear homoscedastic and heteroscedastic regression models. We find that these RACs differ substantially from one spectral sub-type to another. For dM3 and dM4 stars, we find that the RACs cannot be described by a simple model, but instead that there may exist two distinct RAC behaviors for the low activity and the high activity stellar sub-samples respectively. Although these results are preliminary and will need confirmation, the data suggest that these distinct RACs may be associated with different dynamo regimes.We also study R'HK as a function of the Rossby number R0. We find that: (i) For dK4 stars, we confirm R'HK as a function of R0 agrees well with previous results for F-G-K stars. (ii) In dK6, dM2, dM3 and dM4 stars, we find that, at a given R0, the values of R'HK lie a factor of 3, 10, 20 and 90 respectively below the F-G-K RAC. Our results suggest a significant decrease in the efficiency of the dynamo mechanism(s) as regards chromospheric heating before and at dM3, i.e. before and at the TTCC.We also show that the ratio of coronal heating to chromospheric heating LX /LHK increases by a factor of 100 between dK4 and dM4 stars.

ROTATION–ACTIVITY CORRELATIONS IN K AND M DWARFS. I. STELLAR PARAMETERS AND COMPILATIONS OF v sin i AND P/sin i FOR A LARGE SAMPLE OF LATE-K AND M DWARFS*

“…These factors reinforce our decision to study homogeneous samples where the stars are confined to a narrow range of T eff (e.g., Papers XIV, XVI, XVII, this study). Not only do RACs vary with the spectral type (e.g., Stepien 1989Stepien , 1993Stepien , 1994Houdebine et al 2016), but also the distributions of rotation periods vary from one spectral subtype to another (HM and this paper).…”

“…One can see in this table that most stars are dK5 or dK6 stars, with a few dK4 and dK7 stars. In view of this relatively large range of spectral types, we decided to restrict our sample to the range dK5.7 to dK7.3 in order to obtain a better-defined RAC (Houdebine et al 2016). This restricted sample is centered on the spectral subtype dK5.9.…”

“…The instrumental stability makes the spectra well suited for the measurement of rotation in M dwarfs (see HM). We included in our sample the SOPHIE observations obtained in the high-efficiency (HE) mode for the purpose of measuring the Ca II and H α line equivalent widths (Houdebine et al 2016). For further details of the spectrographs, see HM.…”

“…Analogously, the brightness-limited bias in our late-K dwarf sample is expected to have only minor effects in the RAC. The bias may contribute somewhat to the density of sampling at different parts of the RAC, but this is not expected to cause significant discrepancies as regards the overall RAC (Houdebine et al 2016). …”

The Rotation of M Dwarfs Observed by the Apache Point Galactic Evolution Experiment

“…It has long been known that very low-mass stars tend to exhibit more rapid rotation as compared to solar-type stars, but exactly how and where along the main sequence the transition from gradual spin down to long-lived rotation occurs is not well understood (e.g., Houdebine et al 2017).…”