X-Ray Emission from Brown Dwarfs
(Invited Review Talk)
International Astronomical Union. Symposium no. 219,
Stars as Suns: Activity, Evolution and Planets,
21-25 July, 2003, Sydney, Australia
Proceedings to be published by PASP (2003)
During the last few years,
X-ray emission has been detected from several
brown dwarfs with the
ROSAT, Chandra, and XXM-Newton X-ray observatories.
Most of the X-ray detected brown dwarfs are very young objects
with ages of at most 10^7 years, and all are still relatively warm,
with late M spectral types.
Their typical fractional X-ray luminosities are
(L_X/L_bol) ~ 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
Halpha 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 shut off, preventing
the buildup of magnetic free energy and the
support for magnetically heated chromospheres