5. Summary and conclusions
We have derived rotational velocities for a volume limited sample of field M0-M6 dwarfs. We find no measurable rotation in the M0-M3 range. Later than M4 25% of the young disk field stars are rapid rotators. We identify a single old disk star with significant rotation, at spectral type M6. Complementing our data with published rotational velocities shows that at this spectral type rotation becomes common-place in the old kinematic populations (old disk and population II) . These new data extend to lower masses and older ages the well known increase of the spin-down timescale for decreasing masses: measurable rotation is found in G dwarfs at the age of Per (50 Myr), in K dwarfs at the age of the Pleiades (70 Myr), and early M dwarfs in the Hyades (500 Myr). We show that the spin-down timescale is of the order of a few Gyr at spectral type M3-M4, and of the order of 10 Gyr at spectral type M6.
We also show that the well established saturated correlations between rotation and magnetic activity in earlier or younger stars continue in the late field M dwarfs. Saturation occurs for lower rotational velocities but similar rotational periods, roughly consistently with expectations if the Rossby number controls magnetic activity. We find no earlier type equivalent of the fast rotating very late M dwarfs with no measurable chromospheric emission in the Balmer lines. This unexplained phenomenon is therefore only found at spectral types later than M7V.
The present data also imply that rotation is the underlying variable which explains why later type and kinematically younger M dwarfs are more likely to display H emission, as more massive or older stars have had time to spin down to low rotation rates which no longer generate detectable chromospheric emission in the Balmer lines.
Neither the rotational velocity distribution nor the rotation/activity relation show marked features at the spectral type where stars become fully convective. This implies that the transition from a solar-like shell dynamo to magnetic activity driven by another dynamo type (probably turbulent) must occur in stars that retain a sizeable radiative core.
Future work should include rotational period determinations for field M dwarfs, which can be obtained on modest-size telescopes. Since they are free of the orientation uncertainty inherent to v sin i , and can probe much slower rotation, they would define the low mass rotation/activity relations with much reduced observational scatter in the saturated range and confirm it in the unsaturated domain. v sin i measurements for a significantly larger sample would also be very useful, and would better establish the characteristics of the rotation onset at spectral type M3/M4 in the young disk and spectral type M6 in old populations. Finally, Doppler imaging of the fast rotators we have identified can be performed with a 10m-class telescope and would determine the spatial distribution of magnetic activity on the surface of fully convective low mass stars. This crucial information is probably the most direct probe of their magnetic field geometry and would provide a much needed constraint on the mechanism of their dynamo. The generally advocated turbulent field dynamos (Durney et al. 1993, Weiss 1993) probably imply a spatially uniform chromospheric activity, and this is easily tested by a single Doppler image. They would also not sustain cycles analogous to the solar cycle, and at least in principle this can be tested through Doppler imaging monitoring.
© European Southern Observatory (ESO) 1998
Online publication: February 16, 1998