CHROMOSPHERIC AND CORONAL ACTIVITY IN THE 500 MYR OLD OPEN CLUSTER M37: EVIDENCE FOR CORONAL STRIPPING?

Abstract

We present the results of a spectroscopic survey to characterize chromospheric activity, as measured by H α emission, in low-mass members of the 500 Myr old open cluster M37. Combining our new measurements of H α luminosities ( L H α ) with previously cataloged stellar properties, we identify saturated and unsaturated regimes in the dependence of the L H α -to-bolometric luminosity ratio, L H α / 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 H α / 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 H α / 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 H α and L X for M37 members with measurements of both reveals a nearly 1:1 relation, removing the mass-dependencies by comparing instead L H α / 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.