Astronomy & Astrophysics

Free access article

A&A 468, 557-562 (2007)
DOI: 10.1051/0004-6361:20065425

A U-band survey of brown dwarfs in the Taurus molecular cloud with the XMM-Newton optical/UV monitor

N. Grosso¹, M. Audard², J. Bouvier¹, K. R. Briggs³, and M. Güdel³

¹ Laboratoire d'Astrophysique de Grenoble, Université Joseph-Fourier, 38041 Grenoble Cedex 9, France
e-mail: Nicolas.Grosso@obs.ujf-grenoble.fr
² Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, NY 10027, USA
³ Paul Scherrer Institut, 5232 Villigen und Würenlingen, Switzerland

(Received 12 April 2006 / Accepted 16 June 2006)

Abstract
Aims.We aim to characterize the U-band variability of young brown dwarfs in the Taurus Molecular Cloud and discuss its origin.
Methods.We used the XMM-Newton Extended Survey of the Taurus Molecular Cloud, where a sample of 11 young bona fide brown dwarfs (spectral type later than M 6) were observed simultaneously in X-rays with XMM-Newton and in the U-band with the XMM-Newton Optical/UV Monitor (OM).
Results.We obtained upper limits to the U-band emission of 10 brown dwarfs (U $\ge$ 19.6-20.6 mag), whereas 2MASS J04141188+2811535 was detected in the U-band. Remarkably, the magnitude of this brown dwarf increased regularly from U ~ 19.5 mag at the beginning of the observation, peaked 6h later at U ~ 18.4 mag, and then decreased to U ~ 18.65 mag in the next 2 h. The first OM U-band measurement is consistent with the quiescent level observed about one year later thanks to ground follow-up observations. This brown dwarf was not detected in X-rays by XMM-Newton during the OM observation.
Conclusions.We discuss the possible sources of U-band variability for this young brown dwarf, namely a magnetic flare, non-steady accretion onto the substellar surface, and rotational modulation of a hot spot. We conclude that this event is related to accretion from a circumsubstellar disk, where the mass accretion rate was about a factor of 3 higher than during the quiescent level.

Key words: stars: low-mass, brown dwarfs -- stars: individual: 2MASS J04141188+2811535 -- X-rays: stars -- ISM: individual objects: the Taurus Molecular Cloud

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