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Volume 408, Number 2, September III 2003
Page(s) 663 - 673
Section Stellar atmospheres

A&A 408, 663-673 (2003)
DOI: 10.1051/0004-6361:20030987

Photometric variability of a young, low-mass brown dwarf

M. R. Zapatero Osorio1, J. A. Caballero2, V. J. S. Béjar2 and R. Rebolo2, 3

1  LAEFF-INTA, PO Box 50727, 28080 Madrid, Spain
2  Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
3  Consejo Superior de Investigaciones Científicas, Spain

(Received 7 March 2003 / Accepted 16 June 2003 )

We report differential I-band and J-band photometry of S Ori 45, a cool (spectral type M8.5), young (1-8 Myr) brown dwarf of the $\sigma$ Orionis cluster with a likely mass estimated at around 20 times the mass of Jupiter. We detect variability (amplitudes ranging from 34 to 81 mmag) and observe a modulation at a period of 2.5-3.6 h in both optical and near-infrared light curves. The most recent optical data set, however, presents a modulation at the very short period of 46.4 $\pm$ 1.5 min, which remains a mystery. The origin of the 2.5-3.6 h modulation is analyzed in terms of various scenarios: inhomogeneous features (dust clouds or magnetically induced dark spots) co-rotating with the object's surface, and presence of an unseen very low-mass companion that is steadily transferring mass to the primary. Because of the very young age of the object and its persistent strong H $\alpha$ emission, the possible presence of an accreting disk is also discussed. If the period of a few hours is related to rotation, our results suggest that $\sigma$ Orionis low-mass brown dwarfs are rotating faster than more massive cluster brown dwarfs at a rate consistent with their theoretically inferred masses and radii, implying that all of these objects have undergone similar angular momentum evolution.

Key words: stars: low mass, brown dwarfs -- stars: rotation -- stars: pre-main sequence -- stars: formation

Offprint request: M. R. Zapatero Osorio,

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