Kenyon, MJ, Jeffries, RD, Naylor, T, Oliveira, JM and Maxted, PFL (2005) Membership, binarity and accretion among very low-mass stars and brown dwarfs of the Sigma Orionis cluster. Monthly Notices of the Royal Astronomical Society, 356 (1). 89 -106. ISSN 1365-2966

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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).

Item Type: Article
Additional Information: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society © 2005 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
Uncontrolled Keywords: techniques, radial velocities, spectroscopic, stars, low-mass, brown dwarfs, pre-main-sequence, open clusters and associations, s Orionis
Subjects: Q Science > QB Astronomy
Divisions: Faculty of Natural Sciences > School of Physical and Geographical Sciences
Related URLs:
Depositing User: Symplectic
Date Deposited: 20 Aug 2015 14:18
Last Modified: 19 Oct 2017 14:35

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