Membership, binarity, and rotation of F-G-K stars in the open cluster Blanco 1 ^*,**

¹ Laboratoire d'Astrophysique, École polytechnique fédérale de Lausanne (EPFL), Observatoire, 1290 Sauverny, Switzerland e-mail: Jean-Claude.Mermilliod@epfl.ch
² Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
³ Department of Physics & Astronomy, Box 1807 Station B, Vanderbilt University, Nashville, TN 37235, USA
⁴ Department of Physics, Fisk University, 1000 17th Avenue North, Nashville, TN 37208, USA
⁵ Observatoire de Genève, 1290 Sauverny, Switzerland

Received: 15 November 2007
Accepted: 25 April 2008

Abstract

Context. The nearby open cluster Blanco 1 is of considerable astrophysical interest for formation and evolution studies of open clusters because it is the third highest Galactic latitude cluster known. It has been observed often, but so far no definitive and comprehensive membership determination is readily available.

Aims. An observing programme was carried out to study the stellar population of Blanco 1, and especially the membership and binary frequency of the F5-K0 dwarfs.

Methods. We obtained radial-velocities with the CORAVEL spectrograph in the field of Blanco 1 for a sample of 148 F-G-K candidate stars in the magnitude range 10 < V < 14. New proper motions and CCD photometric data from two extensive surveys were obtained independently and are used to establish reliable cluster membership assignments in concert with radial-velocity data.

Results. The membership of 68 stars is confirmed on the basis of proper motion, radial velocity, and photometric criteria. Fourteen spectroscopic- and suspected binaries (2 SB2s, 9 SB1s, 3 SB?) have been discovered among the confirmed members. Thirteen additional stars are located above the main sequence or close to the binary ridge, with radial velocities and proper motions supporting their membership. These are probable binaries with wide separations. Nine binaries (7 SB1 and 2 SB2) were detected among the field stars. The spectroscopic binary frequency among members is 20% (14/68); however, the overall binary rate reaches 40% (27/68) if one includes the photometric binaries. The cluster mean heliocentric radial velocity is +5.53 ± 0.11 km s^-1 based on the most reliable 49 members. The  distribution is similar to that of the Pleiades, confirming the age similarities between the two clusters.

Conclusions. This study clearly demonstrates that, in spite of the cluster's high Galactic latitude, three membership criteria – radial velocity, proper motion, and photometry – are necessary for performing a reliable membership selection. Furthermore, even with accurate and extensive data, ambiguous cases still remain.