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

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 × 10⁸ 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 L_X = 1.3 × 10²⁸ erg s^-1, 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 ⪆ 1 Gyr, 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∼ 1 Gyr.

Tables 6 and 7 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/490/113