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$\alpha$ emission, in low-mass members of the 500-Myr-old open cluster M37. Combining our new measurements of H$\alpha$ luminosities ($L_{H\alpha}$) with previously cataloged stellar properties, we identify saturated and unsaturated regimes in the dependence of the $L_{H\alpha}$-to-bolometric-luminosity ratio, $L_{H\alpha}/L_{bol}$, on the Rossby number $R_o$. All rotators with Ro smaller than 0.03$\pm$0.01 converge to an activity level of $L_{H\alpha}/L_{bol}$ = (1.27$\pm$0.02) x 10$^{-4}$. This saturation threshold ($R_{o,sat}$ = 0.03$\pm$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 $L_{H\alpha}/L_{bol}$($R_o$) following a power-law of index $\beta$ = -0.51$\pm$0.02, slightly shallower than the one found for a combined $\approx$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 $L_{H\alpha}$ and $L_X$ for M37 members with measurements of both reveals a nearly 1:1 relation, removing the mass-dependencies by comparing instead $L_{H\alpha}/L_{bol}$ 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$\pm$0.01 versus 0.09$\pm$0.01). We interpret this as possible evidence for coronal stripping.