  High precision photometry of solar-like members of the open cluster M67 with
Kepler/K2 data has recently revealed enhanced activity for stars with a large
Rossby number, which is the ratio of rotation period to the convective turnover
time. Contrary to the well established behavior for shorter rotation periods
and smaller Rossby numbers, the chromospheric activity of the more slowly
rotating stars of M67 was found to increase with increasing Rossby number. Such
behavior has never been reported before, although it was theoretically
predicted to emerge as a consequence of antisolar differential rotation (DR)
for stars with Rossby numbers larger than that of the Sun, because in those
models the absolute value of the DR was found to exceed that for solar-like DR.
Using gyrochronological relations and an approximate age of 4 Gyr for the
members of M67, we compare with computed rotation rates using just the B-V
color. The resulting rotation--activity relation is found to be compatible with
that obtained by employing the measured rotation rate. This provides additional
support for the unconventional enhancement of activity at comparatively low
rotation rates and the possible presence of antisolar differential rotation.
