A near-infrared survey for new low-mass members in αPer

We present a near-infrared (K'-band) survey of 0.7 square degree area in the alpha 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 Stau. er et al. ( 1999) with the K' 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 M-circle dot are added to the similar to 400 known alpha Per cluster members. If they are indeed alpha 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 Navascues et al. ( 2002) and from our joint optical/infrared catalogue. These results argue in favour of the optical selection method for this particular cluster.