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Article Dans Une Revue Astronomy and Astrophysics - A&A Année : 2007

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

N. Grosso
K. R. Briggs
  • Fonction : Auteur
M. Güdel
  • Fonction : Auteur
E. Franciosini
F. Palla
  • Fonction : Auteur
C. Dougados
F. Ménard
M. Audard
  • Fonction : Auteur
A. Telleschi
  • Fonction : Auteur

Résumé

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* ≤ 10 L_ȯ) and brown dwarfs detected in X-rays, which is consistent with a mean X-ray fractional luminosity < log(L_X/L_*)> =-3.5 ± 0.4; for the XEST brown dwarfs, the median of log(L_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(Hα). 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. Appendices A, B, C, are only available in electronic form at http://www.aanda.org

Dates et versions

hal-00398362 , version 1 (24-06-2009)

Identifiants

Citer

N. Grosso, K. R. Briggs, M. Güdel, S. Guieu, E. Franciosini, et al.. X-ray emission from the young brown dwarfs of the Taurus molecular cloud. Astronomy and Astrophysics - A&A, 2007, 468, pp.391. ⟨10.1051/0004-6361:20065559⟩. ⟨hal-00398362⟩
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