Anchoring the Chromospheric Activity - Rotation Relation: A Study of the Pleiades and Group X

We present a study of the rotation and the chromospheric H α activity of low-mass stars in the about 120 Myr-old Pleiades and the about 400 Myr-old Group X open clusters. We combine published and new rotation periods (P rot) from TESS and ZTF, and use LAMOST, SDSS, and MDM spectra to measure H α equivalent widths (EWs). While overall the Group X distributions of P rot and EWs are as expected, detailed comparisons to the Pleiades and to the 700-Myr-old benchmark Praesepe produce some surprises. For F through mid K stars, the Group X color - period sequence lies between those of the Pleiades and of Praesepe, as expected. But the P rot for its late K and early M dwarfs and the Praesepe slow-rotator sequence overlap, indicating that the 700 Myr-old stars have not spun down significantly despite being nearly twice as old as their Group X counterparts. In the Rossby number-L Hα/L bol plane, rapid rotators in Group X are saturated, as is seen in older clusters, whereas the rapid rotators in the Pleiades show a weak but measurable dependence of L Hα/L bol on rotation. This implies that the chromospheric activity - rotation relation attains its mature form between 120 and 400 Myr. Dividing the sample by color reveals that K, M0, and M1 dwarfs are no longer saturated by about 120 Myr, M2 and M3 dwarfs become unsaturated between about 400 and about 700 Myr, and later M dwarfs remain largely saturated through field ages even as their saturated L Hα/L bol levels decline, indicating that chromospheric evolution is not determined by rotation period alone.