X-ray Emission from Brown Dwarfs

During the last few years, X-ray emission has been detected from numerous brown dwarfs. Most of the X-ray detected brown dwarfs are very young objects with ages of at most 107 years, and all are still relatively warm, with late M spectral types. Their typical fractional X-ray luminosities are (LX/Lbol) ∼ 10—4 — 10—3, i.e. very similar to the values observed for active very-low mass stars. Their X-ray lightcurves show low-level variability, but in most cases no large flares; this implies that the young brown dwarfs are able to produce quiescent X-ray emission, not only occasional flares. An analysis of the Chandra X-ray spectra of several brown dwarfs yields surprisingly low plasma temperatures between 3 MK and 10 MK for some of the M8-9 dwarfs and indicates a decline in plasma temperature with decreasing effective temperature (or increasing age). The lack of X-ray detections for dwarfs cooler than spectral type M9 is consistent with the strong drop of activity observed in Hα at spectral types around M9. The observed X-ray emission from the young brown dwarfs with late M spectral type can be understood as a consequence of the fact that these objects are still warm enough to maintain partially ionized atmospheres which are capable of sustaining electrical currents. In the cooler, essentially neutral atmospheres of the older L and T dwarfs such currents are probably shut off, preventing the buildup of magnetic free energy and the support for magnetically heated chromospheres and coronae.