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 LX = 1.3 × 1028 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 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.
Astronomy and Astrophysics
- Pub Date:
- October 2008
- Galaxy: open clusters and associations: individual: NGC 752;
- stars: coronae;
- stars: rotation;
- X-rays: stars;
- Accepted for publication in A&