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

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Abstract: AIMS. While observational evidence shows that most of the decline in a star-sX-ray activity occurs between the age of the Hyades ~8 x 10^8 yrs and that ofthe Sun, very little is known about the evolution of stellar activity betweenthese ages. To gain information on the typical level of coronal activity at astar-s intermediate age, we studied the X-ray emission from stars in the 1.9Gyr old open cluster NGC 752. METHODS. We analysed a ~140 ks Chandraobservation 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 theXMM-Newton observation. Around 90% of the catalogued cluster members withinChandra-s field-of-view are detected in the X-ray. The X-ray luminosity of allobserved cluster members 28 stars and of 11 cluster member candidates wasderived. RESULTS. Our data indicate that, at an age of 1.9 Gyr, the typicalX-ray luminosity of the cluster members with M=0.8-1.2 Msun is Lx = 1.3 x 10^28erg s^-1, so approximately a factor of 6 less intense than that observed in theyounger Hyades. Given that Lx is proportional to the square of a star-srotational rate, the median Lx of NGC 752 is consistent, for t > 1 Gyr, to adecaying rate in rotational velocities v rot ~ t^-alpha with alpha ~ 0.75,steeper than the Skumanich relation alpha ~ 0.5 and significantly steeperthan observed between the Pleiades and the Hyades where alpha < 0.3,suggesting that a change in the rotational regimes of the stellar interiors istaking place at t ~ 1 Gyr.

Author: G. Giardino 1, I. Pillitteri 2, F. Favata 3, G. Micela 4 1 ESA - Research and Science Support Department, 2 DSFA, Universita'


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