Chromospheric and Coronal Activity in the 500myr Old Open Cluster M37: Evidence for Coronal Stripping?

We present the results of a spectroscopic survey to characterize chromospheric activity, as measured by a H emission, in low-mass members of the 500Myr old open cluster M37. Combining our new measurements of a H luminosities ( a LH ) with previously cataloged stellar properties, we identify saturated and unsaturated regimes in the dependence of the a LH -to-bolometric luminosity ratio, a L L H bol, on the Rossby number Ro. All rotators with Ro smaller than 0.03±0.01 converge to an activity level of ( ) =  ́ a L L 1.27 0.02 10 H bol 4. This saturation threshold ( =  R 0.03 0.01 o,sat ) 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 a L L H bol(Ro) following a power-law of index b =  0.51 0.02, slightly shallower than that found for a combined ≈650Myr 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, LX), we confirm that chromospheric activity decays at a much slower rate than coronal activity with increasing Ro. While a comparison of a LH and LX for M37 members with measurements of both reveals a nearly 1:1 relation, removing the mass-dependencies by comparing instead a L L H bol and L L X bol does not provide clear evidence for such a relation. Finally, we find that Ro,sat is smaller for our chromospheric than for our coronal indicator of activity ( =  R 0.03 0.01 o,sat versus 0.09±0.01). We interpret this as possible evidence for coronal stripping.