A near-infrared survey for new low-mass members in α Per

¹ University of Leicester, Department of Physics & Astronomy, University Road, Leicester LE1 7RH, UK e-mail: nl41@star.le.ac.uk
² Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany
³ University of Exeter, School of Physics, Stocker Road, Exeter EX4 4QL, UK e-mail: mjm@aip.de, mjm@astro.ex.ac.uk
⁴ Laboratorio de Astrofísica Espacial y Física Fundamental, INTA, PO Box 50727, 28080 Madrid, Spain e-mail: barrado@laeff.esa.es
⁵ Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 09, France e-mail: Jerome.Bouvier@obs.ujf-grenoble.fr
⁶ SIRTF Science Center, California Institute of Technology, Pasadena, CA 91125, USA e-mail: stauffer@ipac.caltech.edu

Received: 22 September 2004
Accepted: 21 February 2005

Abstract

We present a near-infrared (-band) survey of 0.7 square degree area in the α Persei open cluster (age = 90 Myr, distance = 182 pc) carried out with the Omega-Prime camera on the Calar Alto 3.5-m telescope. Combining optical data (R_c and I_c) obtained with the KPNO/MOSA detector and presented in Stauffer et al. (1999) with the observations, a sample of new candidate members has been extracted from the optical-infrared colour–magnitude diagram. The location of these candidates in the colour–colour diagram suggests that two-thirds of them are actually reddened background giants. About 20 new candidate members with masses between 0.3 and 0.04  are added to the ~400 known α Per cluster members. If they are indeed α Per members, four of the new candidates would be brown dwarfs. We discuss the advantages and drawbacks of the near-infrared survey as compared to the optical selection method. We also describe the outcome of optical spectroscopy obtained with the Twin spectrograph on the Calar Alto 3.5-m telescope for about 30 candidates, including selected members from the optical sample presented in Barrado y Navascués et al. (2002) and from our joint optical/infrared catalogue. These results argue in favour of the optical selection method for this particular cluster.