Thomas Preibisch, Mark J. McCaughrean,
Nicolas Grosso, Eric D. Feigelson,
Ettore Flaccomio,
Konstantin Getman,
Lynne A. Hillenbrand,
Gwendolyn Meeus,
Giusi Micela,
Salvatore Sciortino,
Beate Stelzer
X-ray emission from young brown dwarfs in the
Orion Nebula Cluster
Astrophysical Journal Supplements, COUP Special
Issue, 160, 582-593 (2005)
Abstract.
We use the sensitive X-ray data from the Chandra Orion Ultradeep
Project (COUP) to study the X-ray properties of 34
spectroscopically-identified brown dwarfs with near-infrared spectral
types between M6 and M9 in the core of the Orion Nebula Cluster. Nine
of the 34 objects are clearly detected as X-ray sources. The apparently
low detection rate is in many cases related to the
substantial extinction of these brown dwarfs; considering only the BDs
with A_V <= 5 mag, nearly half of the
objects (7 out of 16) are detected in X-rays.
Our 10-day long X-ray lightcurves of these objects exhibit strong
variability, including numerous flares. While one of the objects was
only detected during a short flare, a statistical analysis of the
lightcurves provides evidence for continuous (`quiescent') emission in
addition to flares for all other objects. Of these, the ~M9 brown dwarf
COUP 1255 = HC 212 is one of the coolest known objects with a clear
detection of quiescent X-ray emission. The X-ray properties (spectra,
fractional X-ray luminosities, flare rates) of these young brown dwarfs
are similar to those of the low-mass stars in the ONC, and thus there
is no evidence for changes in the magnetic activity around the
stellar/substellar boundary, which lies at ~M6 for ONC sources. Since
the X-ray properties of the young brown dwarfs are also similar to
those of M6-M9 field stars, the key to the magnetic activity in very
cool objects seems to be the effective temperature, which determines
the degree
of ionization in the atmosphere.
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