The substellar mass function in the central region of the open cluster Praesepe from deep LBT observations⋆
Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
2 Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
3 Visiting Astronomer at the Department of Physics and Astronomy, State University of New York, Stony Brook, NY 11794-3800, USA
4 Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey, RH5 6NT, UK
5 Centro de Astrobiología (CSIC-INTA), Departamento de Astrofísica, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
Received: 16 August 2010
Accepted: 2 May 2011
Context. Studies of the mass function (MF) of open clusters of different ages allow us to probe the efficiency with which brown dwarfs evaporate from clusters to populate the field. Surveys of older clusters (age ≳ 100 Myr) are not affected so severely by several problems encountered in young clusters, such as intra-cluster extinction and large uncertainties in brown dwarf models.
Aims. We present the results of a deep photometric survey to study the MF of the central region of the old open cluster Praesepe (age 590 Myr, distance 190 pc), down to the substellar regime.
Methods. We performed an optical (riz and Y-band) photometric survey of Praesepe using the Large Binocular Telescope Camera covering an area of 0.59 deg2 in the cluster centre from i ~ 19.0 mag (~100 MJup) down to a 5σ detection limit at i ~ 25.6 mag (~40 MJup). The survey is approximately 95% complete at i = 23.8 mag and z = 22.0 mag (~55 MJup).
Results. We identify 59 cluster member candidates, of which 37 are substellar, by comparing with the predictions of a dusty atmosphere model. The MF of those candidates rises from the substellar boundary until ~67 MJup and then declines. This is quite different from the form inferred for other open clusters older than 50 Myr, but seems to be similar to those found in very young open clusters, the MFs of which peak at ~10 MJup. Either Praesepe really does have a different MF from other clusters or they had similar initial MFs but a different dynamical evolution. Since most of the candidates are faint, we lack astrometric or spectroscopic follow-ups to test their memberships. However, the contaminations by field dwarfs, galaxies, or giants are found to have little effect on the shape of MF and therefore the MF of “real” cluster members should have similar characteristics.
Key words: brown dwarfs / stars: low-mass / stars: luminosity function, mass function / open clusters and associations: individual: Praesepe
Tables 2 and 3 are available in electronic form at http://www.aanda.org
© ESO, 2011