Membership and binarity of solar-type dwarfs in the nearby open cluster Alpha Persei (Mel 20) ^*,**

¹ Laboratoire d'Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire, 1290 Sauverny, Switzerland e-mail: jean-claude.mermilliod@obs.unige.ch
² Observatoire de Genève, 1290 Sauverny, Switzerland

Received: 21 April 2008
Accepted: 6 June 2008

Abstract

Context. The Alpha Persei open cluster (Melotte 20) is one of the nearby clusters ( pc) for which a long-term systematic radial-velocity monitoring of the solar-type stars has not been published so far.

Aims. We undertook an observing program in 1982 to study the membership, duplicity, and rotation of the main-sequence stars and to compare the results with those of the other nearby clusters.

Methods. Radial-velocity observations of 60 solar-type stars in Mel 20 were obtained with CORAVEL, and of 44 fainter stars with ELODIE spectrographs, in the magnitude range

Results. The membership of 37 stars from the list compiled by Heckmann is confirmed according to their proper motions, radial velocities, and photometry. We discovered twelve spectroscopic binaries (3 double-lined, 8 single-lined, 1 suspected) among the members and determined an orbit for the double-lined binary He 848. The observed frequency of spectroscopic binaries is therefore 32% (12/37). Four stars are photometric binaries. In addition, the membership of 33 stars with AP numbers is supported by the present radial velocities. Two single-lined and three double-lined binaries were detected in this sample. The cluster mean radial velocity is  km s^-1 based on 18 stars within 2σ. We found a systematic difference of  km s^-1 between observed- and astrometric radial velocities.

Conclusions. The Alpha Persei cluster members define a tight main sequence in the colour-magnitude diagrams. The remaining scatter observed in the colour-magnitude diagrams and in radial velocities may be due to the difficulty of separating the core Alpha Persei stars from the extended comoving stream.