The substellar mass function in σ Orionis^*,**

II. Optical, near-infrared and IRAC/Spitzer photometry of young cluster brown dwarfs and planetary-mass objects

¹ Instituto de Astrofísica de Canarias, Avenida Vía Láctea, 38200 La Laguna, Tenerife, Islas Canarias, Spain
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: caballero@mpia.de
³ Proyecto Gran Telescopio Canarias, Instituto de Astrofísica de Canarias
⁴ Consejo Superior de Investigaciones Científicas, Spain
⁵ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
⁶ LAEFF-INTA, P.O. Box 50727, 28080, Madrid, Spain
⁷ Canada-France-Hawaii Telescope Corporation, 65-1238 Mamalahoa Highway, Kamuela, HI96743, Hawai'i, USA
⁸ Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 9, France
⁹ University of Central Florida, Dept. of Physics, PO Box 162385, Orlando, FL 32816-2385, USA

Received: 21 December 2006
Accepted: 12 April 2007

Abstract

Aims.We investigate the mass function in the substellar domain down to a few Jupiter masses in the young σ Orionis open cluster (3±2 Ma, d =  pc).

Methods.We have performed a deep IJ-band search, covering an area of 790 arcmin² close to the cluster centre. This survey was complemented with an infrared follow-up in the HK_s- and Spitzer 3.6–8.0 μm-bands. Using colour–magnitude diagrams, we have selected 49 candidate cluster members in the magnitude interval 16.1 mag 23.0 mag.

Results.Accounting for flux excesses at 8.0 μm and previously known spectral features of youth, we identify 30 objects as bona fide cluster members. Four are first identified from our optical-near infrared data. Eleven have most probable masses below the deuterium burning limit which we therefore classify as candidate planetary-mass objects. The slope of the substellar mass spectrum in the mass interval 0.11  0.006  is α = +0.6 ± 0.2. Any mass limit to formation via opacity-limited fragmentation must lie below 0.006 . The frequency of σ Orionis brown dwarfs with circumsubstellar discs is 47±9 %.

Conclusions.The continuity in the mass function and in the frequency of discs suggests that very low-mass stars and substellar objects, even below the deuterium-burning mass limit, share the same formation mechanism.