2008A&A...490..113G

While observational evidence shows that most of the decline in a star's X-ray activity occurs between the age of the Hyades (∼ 8x10⁸yr) and that of the Sun, very little is known about the evolution of stellar activity between these ages. To gain information on the typical level of coronal activity at a star's intermediate age, we studied the X-ray emission from stars in the 1.9Gyr old open cluster NGC 752. We analysed a ∼140ks Chandra observation of NGC 752 and a ∼50ks XMM-Newton observation of the same cluster. We detected 262 X-ray sources in the Chandra data and 145 sources in the XMM-Newton observation. Around 90% of the catalogued cluster members within Chandra's field-of-view are detected in the X-ray. The X-ray luminosity of all observed cluster members (28 stars) and of 11 cluster member candidates was derived. Our data indicate that, at an age of 1.9Gyr, the typical X-ray luminosity of the cluster members with M=0.8-1.2M_☉ is L_X=1.3x10²⁸erg/s, so approximately a factor of 6 less intense than that observed in the younger Hyades. Given that L_X is proportional to the square of a star's rotational rate, the median L_X of NGC 752 is consistent, for t>1Gyr, to a decaying rate in rotational velocities v_rot∝t^–α with α∼ 0.75, steeper than the Skumanich relation (α≃0.5) and significantly steeper than observed between the Pleiades and the Hyades (where α<0.3), suggesting that a change in the rotational regimes of the stellar interiors is taking place at t∼1Gyr.