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 brown dwarfs 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 light curves 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 light curves 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 Orion Nebula Cluster, and thus there is no evidence for changes in the magnetic activity around the stellar/substellar boundary, which lies at ∼M6 for Orion Nebula Cluster 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.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science