EDP Sciences
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Volume 424, Number 3, September IV 2004
Page(s) 857 - 872
Section Galactic structure, stellar clusters and populations
DOI http://dx.doi.org/10.1051/0004-6361:20047048

A&A 424, 857-872 (2004)
DOI: 10.1051/0004-6361:20047048

Photometric variability of young brown dwarfs in the $\mathsf{\sigma}$ Orionis open cluster

J. A. Caballero1, V. J. S. Béjar1, R. Rebolo1, 2 and M. R. Zapatero Osorio3

1  Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
    e-mail: zvezda@ll.iac.es
2  Consejo Superior de Investigaciones Científicas, Spain
3  LAEFF - INTA, PO Box 50727, 28080, Madrid, Spain

(Received 9 January 2004 / Accepted 28 May 2004 )

We have carried out multi-epoch, time-series differential I-band photometry of a large sample of objects in the south-east region of the young (~3 Myr), nearby (~350 pc) $\sigma$ Orionis open cluster. A field of ~1000 arcmin 2 was monitored during four nights over a period of two years. Using this dataset, we have studied the photometric variability of twenty-eight brown dwarf cluster candidates with masses ranging from the stellar-substellar boundary down to the planetary-mass domain. We have found that about 50% of the sample show photometric variability on timescales from less than one hour to several days and years. The amplitudes of the I-band light curves range from less than 0.01 up to ~0.4 mag. A correlation between the near-infrared excess in the $K_{\rm s}$ band, strong H $\alpha$ emission and large-amplitude photometric variation is observed. We briefly discuss how these results may fit the different scenarios proposed to explain the variability of cool and ultracool dwarfs (i.e. magnetic spots, patchy obscuration by dust clouds, surrounding accretion discs and binarity). Additionally, we have determined tentative rotational periods in the range 3 to 40 h for three objects with masses around 60  $M_{\rm Jup}$, and the rotational velocity of 14  $\pm$ 4 km s -1 for one of them.

Key words: stars: low mass, brown dwarfs -- Galaxy: open clusters and associations: individual: $\sigma$ Orionis -- techniques: photometric

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