The σ Orionis substellar population ^*,**

VLT/FORS spectroscopy and 2MASS photometry

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

Corresponding author: D. Barrado y Navascués, barrado@laeff.esa.es

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 σ Orionis cluster with masses spanning from 1  down to about 0.013  (i.e., deuterium-burning mass limit). We have observed K-band infrared excess and remarkably intense Hα 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 σ Orionis members as classical or weak-line T Tauri stars and substellar analogs. Variability of the Hα line has been investigated and detected in some objects. Based on the K-band infrared excesses and the intensity of Hα emission, we estimate that the minimum disk frequency of the σ Orionis low-mass population is in the range 5–12%.