Astronomy and Astrophysics, volume 468, 391-403 (2007/6-3)
X-ray emission from the young brown dwarfs of the Taurus Molecular Cloud.
GROSSO N., BRIGGS K.R., GUEDEL M., GUIEU S., FRANCIOSINI E., PALLA F., DOUGADOS C., MONIN J.-L., MENARD F., BOUVIER J., AUDARD M. and TELLESCHI A.
Abstract (from CDS):
We report the X-ray properties of young (∼3Myr) bona fide brown dwarfs of the Taurus Molecular Cloud (TMC). 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. 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*≤10L☉) and brown dwarfs detected in X-rays, which is consistent with a mean X-ray fractional luminosity <log(LX/L*)≥-3.5 ±0.4; for the XEST brown dwarfs, the median of log(LX/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. 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.
stars: low-mass, brown dwarfs - X-rays: stars - ISM: individual objects: Taurus molecular cloud
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