Young T-dwarf candidates in IC 348^*

¹ Laboratoire d'Astrophysique, Observatoire de Grenoble, CNRS, Université Joseph Fourier, BP 53, 38041 Grenoble Cedex 9, France e-mail: aburgess@obs.ujf-grenoble.fr
² Institut d'Astrophysique de Paris, 98bis Bd Arago, 75014 Paris, France
³ Canada-France-Hawaii Telescope Corporation, 65-1238 Mamalahoa Highway, Kamuela, HI 96743, USA
⁴ Instituto de Astrofísica de Canarias, C/ Vía Láctea s/n, 38205 – La Laguna, Tenerife, Spain

Received: 7 May 2009
Accepted: 21 August 2009

Abstract

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 CH₄off and CH₄on 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 μm methane absorption bands.

Results. We report three faint T-dwarf candidates with CH₄on-CH₄off colours >0.4 mag. Extinction was estimated for each candidate and lies in the range A_V ~ 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 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 ~ 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.