The X-ray luminosity of solar-mass stars in the intermediate age open cluster NGC 752

Abstract

Aims. While observational evidence shows that most of the decline in a star’s X-ray activity occurs between the age of the Hyades (∼8 × 10 8 yr) 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 1.9 Gyr old open cluster NGC 752. Methods. We analysed a ∼140 ks Chandra observation of NGC 752 and a ∼50 ks 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. Results. Our data indicate that, at an age of 1.9 Gyr, the typical X-ray luminosity of the cluster members with M = 0.8−1.2 M� is LX = 1.3 × 10 28 erg s −1 , so approximately a factor of 6 less intense than that observed in the younger Hyades. Given that LX is proportional to the square of a star’s rotational rate, the median LX of NGC 752 is consistent, for t > 1 Gyr, to a decaying rate in rotational velocities vrot ∝ 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 ∼ 1G yr.