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Issue A&A
Volume 404, Number 1, June II 2003
Page(s) 171 - 185
Section Stellar clusters and associations
DOI http://dx.doi.org/10.1051/0004-6361:20030407



A&A 404, 171-185 (2003)
DOI: 10.1051/0004-6361:20030407

The $\sigma$ Orionis substellar population

VLT/FORS spectroscopy and 2MASS photometry
D. Barrado y Navascués1, V. J. S. Béjar2, R. Mundt3, E. L. Martín4, R. Rebolo2, 5, M. R. Zapatero Osorio1 and C. A. L. Bailer-Jones3

1  Laboratorio de Astrofísica Espacial y Física Fundamental, INTA, PO Box 50727, 28080 Madrid, Spain
2  Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
3  Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
4  Institute of Astronomy. University of Hawaii at Manoa. 2680 Woodlawn Drive, Honolulu, HI 96822, USA
5  Consejo Superior de Investigaciones Científicas, CSIC, Spain

(Received 27 November 2002 / Accepted 12 March 2003 )

Abstract
VLT/FORS spectroscopy and 2MASS near-infrared photometry, together with previously known data, have been used to establish the membership and the properties of a sample of low-mass candidate members of the $\sigma$ Orionis cluster with masses spanning from 1 $M_{\odot}$ down to about 0.013 $M_{\odot}$ (i.e., deuterium-burning mass limit). We have observed K-band infrared excess and remarkably intense H $\alpha$ emission in various cluster members, which, in addition to the previously detected forbidden emision lines and the presence of Li I in absorption at 6708 Å, have allowed us to tentatively classify $\sigma$ Orionis members as classical or weak-line T Tauri stars and substellar analogs. Variability of the H $\alpha$ line has been investigated and detected in some objects. Based on the K-band infrared excesses and the intensity of H $\alpha$ emission, we estimate that the minimum disk frequency of the $\sigma$ Orionis low-mass population is in the range 5-12%.


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

Offprint request: D. Barrado y Navascués, barrado@laeff.esa.es

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