2017ApJ...834..176N

We present the results of a spectroscopic survey to characterize chromospheric activity, as measured by Hαemission, in low-mass members of the 500 Myr old open cluster M37. Combining our new measurements of Hαluminosities (L_Hα) with previously cataloged stellar properties, we identify saturated and unsaturated regimes in the dependence of the L_Hα-to-bolometric luminosity ratio, L_Hα/L_bol, on the Rossby number R_o. All rotators with R_o smaller than 0.03 ± 0.01 converge to an activity level of L_Hα/L_bol=(1.27±0.02)×10^–4. This saturation threshold (R_o,sat=0.03±0.01) is statistically smaller than that found in most studies of the rotation-activity relation. In the unsaturated regime, slower rotators have lower levels of chromospheric activity, with L_Hα/L_bol(R_o) following a power-law of index β=-0.51±0.02, slightly shallower than that found for a combined ≃650 Myr old sample of Hyades and Praesepe stars. By comparing this unsaturated behavior to that previously found for coronal activity in M37 (as measured via the X-ray luminosity, L_X), we confirm that chromospheric activity decays at a much slower rate than coronal activity with increasing R_o. While a comparison of L_Hα and L_X for M37 members with measurements of both reveals a nearly 1:1 relation, removing the mass-dependencies by comparing instead L_Hα/L_bol and L_X/L_bol does not provide clear evidence for such a relation. Finally, we find that R_o,sat is smaller for our chromospheric than for our coronal indicator of activity (R_o,sat=0.03±0.01 versus 0.09 ± 0.01). We interpret this as possible evidence for coronal stripping.