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Issue A&A
Volume 382, Number 3, February II 2002
Page(s) L22 - L25
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20011778



A&A 382, L22-L25 (2002)
DOI: 10.1051/0004-6361:20011778

Letter

No disks around low-mass stars and brown dwarfs in the young $\sigma$ Orionis cluster?

J. M. Oliveira1, R. D. Jeffries1, M. J. Kenyon1, S. A. Thompson1 and T. Naylor1, 2

1  Department of Physics, Keele University, Staffordshire ST5 5BG, UK
2  School of Physics, University of Exeter, Exeter EX4 4QL, UK

(Received 6 November 2001 / Accepted 13 December 2001 )

Abstract
We report on the analysis of 2MASS near-infrared data of a sample of low-mass stars and brown dwarfs in the $\sigma$ Orionis cluster. Youth and cluster membership have been spectroscopically confirmed using the Li I spectral line. We find little evidence in the $JHK_{\rm s}$ colour-colour diagram for near-infrared excess emission for these cluster members. By comparison with model expectations, at most 2 out of 34 stars show $(H-K_{\rm s})$ colour consistent with a near-infrared excess. This scarcity of near-infrared signatures of circumstellar disks in the lower-mass and substellar regimes of this cluster contrasts with findings in younger clusters, hinting at an age dependence of the disk frequency. Taking into account the apparent cluster age, our result supports the idea of a relatively fast (few Myr) disk dissipation and extends this conclusion to the substellar regime. We also find some evidence that, in this cluster, the disk frequency as measured by the $K_{\rm s}$-band excess may be mass dependent.


Key words: circumstellar matter -- infrared: stars -- stars: pre-main sequence -- stars: low mass, brown dwarfs -- open clusters and associations: individual ( $\sigma$ Orionis)

Offprint request: J. M. Oliveira, joana@astro.keele.ac.uk

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