5 Conclusion

We have conducted a deep wide field photometric survey of the Pleiades cluster to build a sample of probable cluster members with masses in the range $0.03~M_{\odot}$ to $0.48~M_{\odot}$. We have identified 40 brown dwarfs candidates, of which 29 are new discoveries. Taking into account the radial distribution of cluster members, we derive the cluster mass function accross the stellar-substellar boundary. We find that a single power-law ${\rm d}N/{\rm d}M \propto M^{-\alpha }$ with an index $\alpha =0.60\pm 0.11$ provides a good match to the cluster mass function in the $0.03{-}0.48~M_{\odot}$ range. This new estimate is based on a survey which combines a large radial coverage of the cluster and a realistic assessment of the contamination by field stars. Furthermore, the survey completely covers the $0.03{-}0.48~M_{\odot}$ mass range, so that the result does not rely on the combination of heterogeneous surveys, as has been the case before. We therefore believe this new estimate is reasonably robust. Small changes may be expected when our survey will be followed up with either infrared photometry and/or proper motions.

Over a larger mass domain, covering almost 3 decades in masses from $0.03~M_{\odot}$ to $10~M_{\odot}$, we find that the cluster mass function is better fitted by a log-normal distribution with $\langle M\rangle\simeq0.25~M_{\odot}$ and $\sigma_{\log M}\simeq 0.52$. When unresolved Pleiades binaries are taken into account, the log-normal Pleiades mass function is not unlike the Galactic disk mass function. This suggests that the dynamical evolution of the cluster has had yet little effect on its mass content at an age of 120 Myr. It also suggests that the brown dwarf formation process does not lead to the dynamical evaporation of substellar objects at the time the cluster forms.

Acknowledgements

We thank E. Bertin for allowing us access to PSFex before public release, E. Magnier for his help in the astrometric calibration of the frames, J. Adams and N. Hambly for providing us data in electronic form prior to publication, I. Baraffe for computing specific substellar isochrones for us and K. Luhman for providing his Monte-Carlo software program to estimate the effect of unresolved binaries on the mass function. We also gratefully aknowledge helpful discussions with X. Delfosse on estimating field star contamination from DENIS data, with M. Bate, C. Clarke, E. Delgado and M. Sterzik on the dynamical evolution of young brown dwarfs in clusters, and with G. Chabrier who brought our attention on the Galactic disk mass function and on the effect unresolved binary systems might have on the shape of the observed mass function.