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

TL;DR

This study measures the X-ray luminosity of solar-mass stars in the 1.9 Gyr old cluster NGC 752, revealing a significant decline in activity compared to younger clusters and suggesting a steeper rotational decay rate after 1 Gyr.

Contribution

First detailed X-ray analysis of solar-mass stars at intermediate age (~1.9 Gyr), providing new insights into stellar activity evolution between the Hyades and the Sun.

Findings

X-ray luminosity at 1.9 Gyr is about 6 times lower than in the Hyades.

X-ray luminosity decay rate follows v_rot ~ t^-0.75, steeper than the Skumanich law.

Indicates a change in stellar rotational regimes around 1 Gyr.

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 x 10^8 yrs) 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 Msun is Lx = 1.3 x 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 v_rot ~ t^-alpha with alpha ~ 0.75, steeper than the Skumanich relation (alpha ~ 0.5) and significantly steeper than observed between the Pleiades and the Hyades (where alpha < 0.3), suggesting that a change in the rotational regimes of the stellar interiors is taking place at t ~ 1 Gyr.