Young T-Dwarf Candidates in IC 348

Context. The determination of the lower-end of the initial mass function (IMF) provides strong constraints on star formation theories. Aims. We report here on a search for isolated planetary-mass objects in the 3 Myr-old star-forming region IC 348. Methods. Deep, narrowband CH(4)off and CH(4)on images were obtained with CFHT/WIRCam over 0.11 sq.deg. in the central part of IC 348 to identify young T-dwarfs from their 1.6 mu m methane absorption bands. Results. We report three faint T-dwarf candidates with CH(4)on-CH(4)off colours >0.4 mag. Extinction was estimated for each candidate and lies in the range AV similar to 5- 12 mag. Comparisons with T-dwarf spectral models, and colour/colour and colour/magnitude diagrams, reject two of the three candidates because of their extreme z' - J blueness. The one remaining object is not thought to be a foreground field dwarf because of a number density argument and also its strong extinction A(V) similar to 12 mag, or thought to be a background field T-dwarf which would be expected to be much fainter. Models and diagrams give this object a preliminary T6 spectral type. Conclusions. With a few Jupiter masses, the young T-dwarf candidate reported here is potentially amongst the youngest, lowest mass objects detected in a star-forming region so far. Its frequency is consistent with the extrapolation of current lognormal IMF estimates down to the planetary mass domain.