2024ApJ...962...12N

Low-mass (≲1.2 M_⊙) main-sequence stars lose angular momentum over time, leading to a decrease in their magnetic activity. The details of this rotation-activity relation remain poorly understood, however. Using observations of members of the ≈700 Myr old Praesepe and Hyades open clusters, we aim to characterize the rotation-activity relation for different tracers of activity at this age. To complement published data, we obtained new optical spectra for 250 Praesepe stars, new X-ray detections for 10, and new rotation periods for 28. These numbers for Hyads are 131, 23, and 137, respectively. The latter increases the number of Hyads with periods by 50%. We used these data to measure the fractional Hα and X-ray luminosities, L_Hα/L_bol and L_X/L_bol, and to calculate Rossby numbers R_o. We found that at ≈700 Myr almost all M dwarfs exhibit Hα emission, with binaries having the same overall color-Hα equivalent width distribution as single stars. In the R_o-L_Hα/L_bol plane, unsaturated single stars follow a power law with index β = -5.9 ± 0.8 for R_o > 0.3. In the R_o-L_X/L_bol plane, we see evidence for supersaturation for single stars with R_o ≲ 0.01, following a power law with index ${\beta }_{\sup }={0.5}_{-0.1}^{+0.2}$, supporting the hypothesis that the coronae of these stars are being centrifugally stripped. We found that the critical R_o value at which activity saturates is smaller for L_X/L_bol than for L_Hα/L_bol. Finally, we observed an almost 1:1 relation between L_Hα/L_bol and L_X/L_bol, suggesting that both the corona and the chromosphere experience similar magnetic heating.