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 A, 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%.
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