X-ray emission from young brown dwarfs in the Orion Nebula Cluster

Astrophysics, abstract
astro-ph/0506049

From: Thomas Preibisch [view email]

Date: Thu, 2 Jun 2005 08:40:21 GMT   (195kb)

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
$A_V \leq 5$ 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 $\sim$ 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 $\sim$ 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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