Membership, binarity, and rotation of red dwarfs in the nearby open cluster Coma Berenices (Melotte 111) ^*,**

¹ Laboratoire d'Astrophysique, École polytechnique fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland e-mail: Jean-Claude.Mermilliod@epfl.ch
² Observatoire de Genève, 1290 Sauverny, Switzerland

Received: 22 May 2008
Accepted: 22 August 2008

Abstract

Context. Although several attempts have been made to identify solar-type members on the main sequence of the nearby open cluster Coma Berenices (Mel 111), the population of the lower main sequence is still poorly known.

Aims. We observed 46 new candidates to search for new members and monitored known spectroscopic-binary members to determine orbital parameters.

Methods. We obtained a total of 903 radial-velocity measurements of 69 solar-type stars in the field of Mel 111 with the CORAVEL spectrovelocimeter over 20 years.

Results. Among the 35 stars from Trumpler's list, 23 are members according to their radial velocities and photometry. We were able to confirm the membership of only 8 stars among the 46 candidates observed. Six double-lined and ten single-lined spectroscopic binaries were discovered. Six only are members and an orbit was determined for them and for 4 double-lined non-members. The binary frequency is 22% (7/32). The cluster mean radial velocity is +0.01 ± 0.08 km s^-1 based on 28 members.

Conclusions. The lower main sequence of the Coma Berenices open cluster is still rather poorly populated. The cluster size may be much larger that usually accepted. Accordingly extensive programmes to determine precise proper motions, radial velocities and photometry should be undertaken to identify faint cluster members outside the cluster central area. If a significant population of faint members cannot be identified, Coma Ber could be a prominent example of dynamical evolution leading to star evaporation.