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Issue A&A
Volume 436, Number 3, June IV 2005
Page(s) 853 - 865
Section Galactic structure, stellar clusters and populations
DOI http://dx.doi.org/10.1051/0004-6361:20042048



A&A 436, 853-865 (2005)
DOI: 10.1051/0004-6361:20042048

A near-infrared survey for new low-mass members in $\alpha$ Per

N. Lodieu1, 2, M. J. McCaughrean2, 3, D. Barrado y Navascués4, J. Bouvier5 and J. R. Stauffer6

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 ( $K^{\prime}$-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 ($R_{\rm c}$ and $I_{\rm c}$) obtained with the KPNO/MOSA detector and presented in Stauffer et al. (1999) with the $K^{\prime}$ 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 $M_{\odot}$ are added to the ~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 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.


Key words: stars: low-mass, brown dwarfs -- techniques: photometric -- techniques: spectroscopic -- infrared: stars -- stars: Hertzsprung-Russell (HR) and C-M diagrams -- Galaxy: open clusters and associations: individual: $\alpha$ Persei

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