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

Kenyon, M.J.; Jeffries, R.D.; Naylor, Tim; Oliveira, J.M.; Maxted, P.F.L.

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


M.J. Kenyon

Tim Naylor

P.F.L. Maxted


Intermediate resolution (R=7000) spectroscopy is presented for 76 photometrically selected very low mass (0.04<M<0.3M_{sun}) candidate members of the Sigma Orionis cluster. More than two thirds appear to be genuine cluster members on the basis of Li I absorption, weak Na I features and radial velocities. Photometric selection alone therefore appears to be very effective in identifying cluster members in this mass range. Only 6 objects appear to be certain non-members, however a substantial subset of 13 have ambiguous or contradictory indications of membership and lack Li absorption. Four candidate binary cluster members are identified. Consideration of sampling and precision leads us to conclude that either the fraction of very low mass stars and brown dwarfs in small separation (a<1au) 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, but is significant even when only Li-rich candidate members are considered. Broadened H alpha emission, indicative of circum(sub)stellar accretion discs is found in 5 or 6 of the candidate cluster members, 3 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 cluster using H alpha emission as a diagnostic, but much lower than found for very low mass stars and brown dwarfs of younger clusters. The timescale for accretion rates to drop to less than 10^{-11} M_sun/yr is hence less than the age of the Sigma Ori cluster (3 to 7 Myr) for most low-mass objects (abridged).

Journal Article Type Article
Acceptance Date Sep 21, 2004
Online Publication Date Jan 1, 2005
Publication Date 2005-01
Publicly Available Date May 26, 2023
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Publisher Oxford University Press
Peer Reviewed Peer Reviewed
Volume 356
Issue 1
Pages 89 -106
Keywords techniques, radial velocities, spectroscopic, stars, low-mass, brown dwarfs, pre-main-sequence, open clusters and associations, s Orionis
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