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

¹ ESA – Research and Science Support Department, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands e-mail: ggiardin@rss.essa.int
² DSFA, Università degli Studi di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
³ ESA – Planning and Community Coordination Office, Science Programme, 8-10 rue Mario Nikis, 75738 Paris Cedex 15, France
⁴ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy

Received: 23 April 2008
Accepted: 19 July 2008

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 (~ 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 is  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 Gyr, to a decaying rate in rotational velocities with , steeper than the Skumanich relation () and significantly steeper than observed between the Pleiades and the Hyades (where ), suggesting that a change in the rotational regimes of the stellar interiors is taking place at  Gyr.