RACE-OC project: rotation and variability in the open cluster M 11 (NGC 6705)

Abstract

Rotation and magnetic activity are intimately linked in main-sequence stars of G or later spectral types. The presence and level of magnetic activity depend on stellar rotation, and rotation itself is strongly influenced by the strength and topology of the magnetic fields. Open clusters represent especially useful targets for investigating the connection among rotation, activity, and age. Over time, stellar activity and rotation evolve, providing us with a promising diagnostic tool to determine the age of the field stars.

Aims. The open cluster M11 has been studied as a part of the RACE-OC project (Rotation and ACtivity Evolution in Open Clusters), which aims to explore the evolution of rotation and magnetic activity in the late-type members of open clusters with different ages.

Methods. Photometric observations of the open cluster M 11 were carried out in June 2004 using the LOAO 1 m telescope. The rotation periods of the cluster members were determined by Fourier analysis of photometric data time series. We investigated the relations between the surface activity, characterized by the light curve amplitude, and rotation.

Results. We have discovered a total of 75 periodic variables in the M 11 FoV, of which 38 are candidate cluster members. Specifically, among cluster members we discovered 6 early-type periodic variables, 2 eclipsing binaries, and 30 bona-fide single periodic late-type variables. Considering the rotation periods of 16 G-type members of the almost coeval 200-Myr M 34 cluster, we could determine the rotation period distribution from a more numerous sample of 46 single G stars at an age of about 200-230 Myr and determine a median rotation period of P = 4.8 d.

Conclusions. A comparison with the younger M 35 cluster (~150 Myr) and with the older M37 cluster (~550 Myr) shows that G stars rotate more slowly than younger M 35 stars and more rapidly than older M 37 stars. The measured variation in the median rotation period is consistent with the scenario of rotational braking of main-sequence spotted stars as they age. Finally, G-type M11 members have a level of photospheric magnetic activity, as measured by light curve amplitude, comparable to what is observed in the younger 110-Myr Pleiades stars of similar mass and rotation.