DETECTION OF QUIESCENT EXTREME-ULTRAVIOLET EMISSION FROM THE VERY LOW MASS DWARF VAN BIESBROECK 8: EVIDENCE FOR A TURBULENT FIELD DYNAMO

摘要

We report the detection of quiescent EUV emission from the very low mass dwarf VB 8 by the Extreme Ultraviolet Explorer (EUVE) in the Lexan/B band (65-190 A). We interpret this emission in terms of a hot coronal plasma and combine this information with previous X-ray detections to estimate the quiescent plasma temperature and emission measure. The combined observations made by Einstein, ROSAT, and EUVE between 1979 and 1994 are consistent with a quiescent coronal plasma temperature of (2-6) x 10~6 degrees and indicate the same emission measure to within a factor of about 2. The non-flaring corona of VB 8 then appears relatively constant over timescales of more than 10 yr. Our results are consistent with the picture of a turbulently driven or distributive dynamo for VB 8, rather than with a large-scale field dynamo which appears to dominate the solar corona. Evidence from X-ray and optical data concerning the long-term coronal variability of the more active stars of higher mass also points toward the idea that active late-type stars in general are dominated by a turbulent dynamo.