Laboratoire d'Astrophysique, Observatoire de Grenoble (LAOG), BP 53, 38041 Grenoble Cedex 9, France e-mail: Estelle.Moraux@obs.ujf-grenoble.fr
2 Spitzer Science Center, Caltech M/S 220-6, 1200 East California Boulevard, Pasadena, CA 91125, USA
3 Laboratorio de Astrofisica Espacial y Fisica Fundamental (LAEFF-INTA), Apdo. 50727, 28080 Madrid, Spain
4 Canada-France-Hawaii Telescope Corporation, 65-1238 Mamalahoa Highway, Kamuela, HI 96743, USA
5 Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France
Accepted: 7 June 2007
Aims.We performed a deep wide field optical survey of the young (~ Myr) open cluster Blanco 1 to study its low mass population well down into the brown dwarf regime and estimate its mass function over the whole cluster mass range.
Methods.The survey covers 2.3 square degrees in the I and z-bands down to with the CFH12K camera. Considering two different cluster ages (100 and 150 Myr), we selected cluster member candidates on the basis of their location in the () CMD relative to the isochrones, and estimated the contamination by foreground late-type field dwarfs using statistical arguments, infrared photometry and low-resolution optical spectroscopy.
Results.We find that our survey should contain about 57% of the cluster members in the mass range, including 30–40 brown dwarfs. The candidate's radial distribution presents evidence that mass segregation has already occured in the cluster. We took it into account to estimate the cluster mass function across the stellar/substellar boundary. We find that, between and , the cluster mass distribution does not depend much on its exact age, and is well represented by a single power-law, with an index . Over the whole mass domain, from 0.03 to 3 , the mass function is better fitted by a log-normal function with and .
Conclusions.Comparison between the Blanco 1 mass function, other young open clusters' MF, and the galactic disc MF suggests that the IMF, from the substellar domain to the higher mass part, does not depend much on initial conditions. We discuss the implications of this result on theories developed to date to explain the origin of the mass distribution.
Key words: stars: low-mass, brown dwarfs / stars: luminosity function, mass function / Galaxy: open clusters and associations: individual: Blanco 1
Based on observations obtained at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique of France, and the University of Hawaii. Based on observations collected at the European Southern Observatory, Chile (ESO programmes 68.C-0233 and 71.C-0446). Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration.
© ESO, 2007