Membership, binarity and accretion among very low-mass stars and brown dwarfs of the s Orionis cluster

Abstract

Intermediate-resolution (R∼ 7000) spectroscopy is presented for 76 photometrically selected very low-mass (0.04 < M < 0.3 M⊙) candidate members of the young cluster around σ Orionis (σ Ori). More than two-thirds appear to be genuine cluster members on the basis that they exhibit Li i 6708-A absorption, weak Na i 8183/8195 A features and a radial velocity consistent with the cluster mean. Photometric selection alone therefore appears to be very effective in identifying cluster members in this mass range. Only six objects appear to be certain non-members; however, a substantial subset of 13 candidates have ambiguous or contradictory indications of membership and lack Li absorption. Together with an observed spread in the equivalent width of the Li absorption feature in the cooler stars of our sample, this indicates that there may be deficiencies in our understanding of the formation of this line in cool, low-gravity objects. Four candidate binary cluster members are identified. Consideration of sampling and radial velocity measurement precision leads us to conclude that either the fraction of very low-mass stars and brown dwarfs in small separation (a < 1 au) binary systems is larger than in field M-dwarfs, or the distribution of separations is much less skewed towards large separations. This conclusion hinges critically on the correct identification of the small number of binary candidates, although it remains significant even when only the candidate members displaying Li absorption are considered. Broadened Hα emission, indicative of circum(sub)stellar accretion discs is found in five or six of the candidate cluster members, three of which probably have substellar masses. The fraction of accretors (10 ± 5 per cent) is similar to that found in stars of higher mass in the σ Ori cluster using Hα emission as a diagnostic, but much lower than found for very low-mass stars and brown dwarfs of younger clusters. The time-scale for accretion rates to drop to ≲10−11 M⊙ yr−1 is hence less than the age of the σ Ori cluster (3–7 Myr) for most low-mass objects.