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Issue A&A
Volume 477, Number 3, January III 2008
Page(s) 895 - 900
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20078600



A&A 477, 895-900 (2008)
DOI: 10.1051/0004-6361:20078600

New constraints on the membership of the T dwarf S Ori 70 in the $\sigma$ Orionis cluster

M. R. Zapatero Osorio1, V. J. S. Béjar1, G. Bihain1, 2, E. L. Martín1, 3, R. Rebolo1, 2, I. Villó-Pérez4, A. Díaz-Sánchez5, A. Pérez Garrido5, J. A. Caballero6, T. Henning6, R. Mundt6, D. Barrado y Navascués7, and C. A. L. Bailer-Jones6

1  Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenerife, Spain
    e-mail: mosorio@iac.es
2  Consejo Superior de Investigaciones Científicas, Spain
3  University of Central Florida, Department of Physics, PO Box 162385, Orlando, FL 32816, USA
4  Departamento de Electrónica, Universidad Politécnica de Cartagena, 30202 Cartagena, Spain
5  Departamento de Física Aplicada, Universidad Politécnica de Cartagena, 30202 Cartagena, Spain
6  Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany


7  LAEFF-INTA, PO 50727, 28080 Madrid, Spain

(Received 2 September 2007 / Accepted 10 October 2007 )

Abstract
Aims.The nature of S Ori 70 (S Ori J053810.1-023626), a faint mid-T type object found towards the direction of the young $\sigma$ Orionis cluster, is still under debate. We intend to find out whether it is a field brown dwarf or a 3-Myr old planetary-mass member of the cluster.
Methods.We report on near-infrared ${\it JHK}_{\rm s}$ and mid-infrared [3.6] and [4.5] IRAC/Spitzer photometry recently obtained for S Ori 70. The new near-infrared images (taken 3.82 yr after the discovery data) allowed us to derive the first proper motion measurement for this object.
Results.The colors $(H-K_{\rm s})$, $(J-K_{\rm s})$ and $K_{\rm s}$ - [3.6] appear discrepant when compared to T4-T7 dwarfs in the field. This behavior could be ascribed either to a low-gravity atmosphere or to an atmosphere with a metallicity that is significantly different than solar. The small proper motion of S Ori 70 (11.0 $\pm$ 5.9 mas yr-1) indicates that this object is farther away than expected if it were a single field T dwarf lying in the foreground of the $\sigma$ Orionis cluster. Our measurement is consistent with the proper motion of the cluster within 1.5$\sigma$ the astrometric uncertainty.
Conclusions.Taking into account both S Ori 70's proper motion and the new near- and mid-infrared colors, a low-gravity atmosphere remains as the most likely explanation for our observations. This supports S Ori 70's membership in $\sigma$ Orionis, with an estimated mass in the interval 2-7  $M_{\rm Jup}$, in agreement with our previous derivation.


Key words: stars: low mass, brown dwarfs -- stars: pre-main-sequence -- open clusters and associations: individual: $\sigma$ Orionis



© ESO 2008

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