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Issue A&A
Volume 472, Number 1, September II 2007
Page(s) L9 - L12
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20078116



A&A 472, L9-L12 (2007)
DOI: 10.1051/0004-6361:20078116

Letter

Discs of planetary-mass objects in $\mathsf{\sigma}$ Orionis

M. R. Zapatero Osorio1, J. A. Caballero2, V. J. S. Béjar1, R. Rebolo1, 3, D. Barrado y Navascués4, G. Bihain1, 3, J. Eislöffel5, E. L. Martín1, 6, C. A. L. Bailer-Jones2, R. Mundt2, T. Forveille7, 8, and H. Bouy9, 1

1  Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
    e-mail: mosorio@iac.es
2  Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
3  Consejo Superior de Investigaciones Científicas, Spain
4  LAEFF-INTA, PO Box 50727, 28080 Madrid, Spain
5  Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
6  University of Central Florida, Dept. of Physics, PO Box 162385, Orlando, FL 32816-2385, USA
7  Canada-France-Hawaii Telescope Corporation, 65-1238 Mamalahoa Highway, Kamuela, HI 96743, Hawai'i, USA
8  Laboratoire d'Astrophysique, Observatoire de Grenoble, BP 53, 38041 Grenoble Cedex 9, France
9  Astronomy Department, University of California, Berkeley, CA 94720, USA

(Received 19 June 2007 / Accepted 13 July 2007)

Abstract
Aims.We searched for infrared flux excesses of planetary-mass candidates in the $\sigma$ Orionis cluster (~3 Myr, ~350 pc).
Methods.Using $IJHK_{\rm s}$ data from the literature and the [3.6], [4.5], [5.8], and [8.0] IRAC images of the $\sigma$ Orionis cluster from the Spitzer Space Telescope public archives, we constructed colour-colour diagrams and spectral energy distributions from 0.8 to 8.0 $\mu$m of cluster candidates fainter than J = 18.0 mag, i.e. the planetary-mass borderline for $\sigma$ Orionis.
Results.Infrared flux excesses are detected longward of 5 $\mu$m in seven objects (S Ori 54, 55, 56, 58, 60, S Ori J053956.8-025315 and S Ori J053858.6-025228) with masses estimated in the range 7-14 $M_{\rm Jup}$. Emission at shorter wavelengths (4.5 $\mu$m) in excess of the photosphere is probably observed in S Ori 56 and S Ori J053858.6-025228. The faintest and least massive object, S Ori 60, exhibits flux excess only at 8 $\mu$m. We ascribe these infrared excesses to the presence of circumsubstellar warm discs, providing additional confirmation for the objects' membership of $\sigma$ Orionis. The observed incidence of inner discs around planetary-mass objects is $\ge$50%, which is consistent with the measured inner disc frequency among cluster brown dwarfs and low-mass stars, suggesting that these objects share a common origin. However, there is a trend for the inner disc rate to increase with decreasing mass (from 10 $M_{\odot}$ through the substellar domain), which may be due to a mass-dependent timescale for the dissipation of the interior discs.


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



© ESO 2007


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