The lower mass function of the young open cluster Blanco 1:: from 30 MJup to 3 M ⊙

Aims. We performed a deep wide field optical survey of the young (similar to 100-150 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 I similar or equal to z similar or equal to 24 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 ( I, I -z) 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 0.03-0.6 M-circle dot 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 0.03 M-circle dot and 0.6 M-circle dot, the cluster mass distribution does not depend much on its exact age, and is well represented by a single power-law, with an index a = 0.69 +/- 0.15. Over the whole mass domain, from 0.03 M-circle dot to 3 M-circle dot, the mass function is better fitted by a log-normal function with m(0) = 0.36 +/- 0.07 M-circle dot and sigma = 0.58 +/- 0.06. 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.