Astronomy & Astrophysics

Issue		A&A Volume 470, Number 3, August II 2007
Page(s)		903 - 918
Section		Galactic structure, stellar clusters, and populations
DOI		10.1051/0004-6361:20066993

A&A 470, 903-918 (2007)
DOI: 10.1051/0004-6361:20066993

The substellar mass function in $\sigma$ Orionis

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

J. A. Caballero^{1, 2}, V. J. S. Béjar³, R. Rebolo^{1, 4}, J. Eislöffel⁵, M. R. Zapatero Osorio^{1, 6}, R. Mundt², D. Barrado y Navascués⁶, G. Bihain^{1, 4}, C. A. L. Bailer-Jones², T. Forveille^{7, 8}, and E. L. Martín^{1, 9}

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 $\sigma$ Orionis open cluster (3 $\pm$ 2 Ma, d = $360^{\rm +70}_{\rm -60}$ 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 $HK_{\rm s}$ - and Spitzer 3.6-8.0 $\mu$ m-bands. Using colour-magnitude diagrams, we have selected 49 candidate cluster members in the magnitude interval 16.1 mag < I < 23.0 mag.
Results.Accounting for flux excesses at 8.0 $\mu$ m 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 $(\Delta N / \Delta {M} \approx a ^{-\alpha})$ in the mass interval 0.11 $M_\odot$ < M < 0.006 $M_\odot$ is $\alpha$ = +0.6 $\pm$ 0.2. Any mass limit to formation via opacity-limited fragmentation must lie below 0.006 $M_\odot$ . The frequency of $\sigma$ Orionis brown dwarfs with circumsubstellar discs is 47 $\pm$ 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.

Key words: stars: low mass, brown dwarfs -- Galaxy: open clusters and associations: individual: $\sigma$ Orionis -- stars: planetary systems: protoplanetary discs

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The substellar mass function in Orionis

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

The substellar mass function in $\sigma$ Orionis