STELLAR ACTIVITY AT THE END OF THE MAIN-SEQUENCE - GHRS OBSERVATIONS OF THE M8 VE STAR VB-10

We present Goddard High Resolution Spectrograph observations of the M8 Ve star VB 10 (= G1 752B), located very near the end of the stellar m ain sequence, and its dM3.5 binary companion G1 752A, These coeval sta rs provide a test bed for studying whether the outer atmospheres of st ars respond to changes in internal structure as stars become fully con vective near mass 0.3 M. (about spectral type M5), where the nature of the stellar magnetic dynamo presumably changes, and near the transiti on from red to brown dwarfs near mass 0.08 M. (about spectral type M9) , when hydrogen burning ceases at the end of the main sequence, We obt ain upper limits for the quiescent emission of VB 10 but observe a tra nsition region spectrum during a large flare, which indicates that som e type of magnetic dynamo must be present. Two indirect lines of evide nce-scaling from the observed X-ray emission and scaling from a time-r esolved flare on AD Leo-suggest that the fraction of the stellar bolom etric luminosity that heats the transition region of VB 10 outside of obvious flares is comparable to, or larger than, that for G1 752A. Thi s suggests an increase in the magnetic heating rates, as measured by L (line)/L(bol) ratios, across the radiative/convective core boundary an d as stars approach the red/brown dwarf boundary. These results provid e new constraints for dynamo models and models of coronal and transiti on-region heating in late-type stars.