LINKING STELLAR CORONAL ACTIVITY AND ROTATION AT 500 MYR: A DEEP CHANDRA OBSERVATION OF M37

Empirical calibrations of the stellar age-rotation–activity relation (ARAR) rely on observations of the co-eval populations of stars in open clusters. We used the Chandra X-ray Observatory to study M37, a 500-Myr-old open cluster that has been extensively surveyed for rotation periods (). M37 was observed almost continuously for five days, for a total of 440.5 ks, to measure stellar X-ray luminosities (), a proxy for coronal activity, across a wide range of masses. The cluster’s membership catalog was revisited to calculate updated membership probabilities from photometric data and each star’s distance to the cluster center. The result is a comprehensive sample of 1699 M37 members: 426 with , 278 with X-ray detections, and 76 with both. We calculate Rossby numbers, , where τ is the convective turnover time, and ratios of the X-ray-to-bolometric luminosity, , to minimize mass dependencies in our characterization of the rotation-coronal activity relation at 500 Myr. We find that fast rotators, for which , show saturated levels of activity, with log(). For , activity is unsaturated and follows a power law of the form , where β = . This is the largest sample available for analyzing the dependence of coronal emission on rotation for a single-aged population, covering stellar masses in the range 0.4–1.3 , in the range 0.4–12.8 days, and in the range . Our results make M37 a new benchmark open cluster for calibrating the ARAR at ages of Myr.