2008A&A...488..409M

The Alpha Persei open cluster (Melotte 20) is one of the nearby clusters (d<500pc) for which a long-term systematic radial-velocity monitoring of the solar-type stars has not been published so far. 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. 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 9.5<V<13.85 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 -1.39±0.17km/s based on 18 stars within 2σ. We found a systematic difference of 3.67±1.48km/s between observed- and astrometric radial velocities. 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.