Astronomy & Astrophysics

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Issue		A&A Volume 468, Number 2, June III 2007 The XMM-Newton extended survey of the Taurus molecular cloud
Page(s)		391 - 403
DOI		10.1051/0004-6361:20065559

A&A 468, 391-403 (2007)
DOI: 10.1051/0004-6361:20065559

X-ray emission from the young brown dwarfs of the Taurus molecular cloud

N. Grosso¹, K. R. Briggs², M. Güdel², S. Guieu¹, E. Franciosini³, F. Palla⁴, C. Dougados¹, J.-L. Monin^{1, 5}, F. Ménard¹, J. Bouvier¹, M. Audard⁶, and A. Telleschi²

¹ Laboratoire d'Astrophysique de Grenoble, Université Joseph-Fourier, 38041 Grenoble Cedex 9, France
e-mail: Nicolas.Grosso@obs.ujf-grenoble.fr
² Paul Scherrer Institut, 5232 Villigen und Würenlingen, Switzerland
³ INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁴ INAF - Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
⁵ Institut Universitaire de France
⁶ Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, NY 10027, USA

(Received 5 May 2006 / Accepted 5 August 2006 )

Abstract
Aims.We report the X-ray properties of young (~3 Myr) bona fide brown dwarfs of the Taurus Molecular Cloud (TMC).
Methods.The XMM-Newton Extended Survey of the TMC (XEST) is a large program designed to systematically investigate the X-ray properties of young stellar/substellar objects in the TMC. In particular, the area surveyed by 15 XMM-Newton pointings (of which three are archival observations), supplemented with one archival Chandra observation, allows us to study 17 brown dwarfs with M spectral types.
Results.Half of this sample (9 out of 17 brown dwarfs) is detected; 7 brown dwarfs are detected here for the first time in X-rays. We observed a flare from one brown dwarf. We confirm several previous findings on brown dwarf X-ray activity: a log-log relation between X-ray and bolometric luminosity for stars (with $L_* \le 10$ $L_\odot$ ) and brown dwarfs detected in X-rays, which is consistent with a mean X-ray fractional luminosity $\langle \log(L_{\rm X}/L_*)\rangle =-3.5 \pm 0.4$ ; for the XEST brown dwarfs, the median of $\log(L_{\rm X}/L_*)$ (including upper limits) is -4.0; a shallow log-log relation between X-ray fractional luminosity and mass; a log-log relation between X-ray fractional luminosity and effective temperature; a log-log relation between X-ray surface flux and effective temperature. We find no significant log-log correlation between the X-ray fractional luminosity and $EW({\rm H}\alpha)$ . Accreting and nonaccreting brown dwarfs have a similar X-ray fractional luminosity. The median X-ray fractional luminosity of nonaccreting brown dwarfs is about 4 times lower than the mean saturation value for rapidly rotating low-mass field stars. Our TMC brown dwarfs have higher X-ray fractional luminosity than brown dwarfs in the Chandra Orion Ultradeep Project.
Conclusions.The X-ray fractional luminosity declines from low-mass stars to M-type brown dwarfs, and as a sample, the brown dwarfs are less efficient X-ray emitters than low-mass stars. We thus conclude that while the brown dwarf atmospheres observed here are mostly warm enough to sustain coronal activity, a trend is seen that may indicate its gradual decline due to the drop in photospheric ionization degree.

Key words: stars: low-mass, brown dwarfs -- X-rays: stars -- ISM: individual objects: Taurus molecular cloud

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