  If magnetic activity in outer stellar atmospheres is due to an interplay
between rotation and subsurface convection, as is generally presumed, then one
would not expect to observe indicators of activity in stars with T_eff > 8300
K. Any X-ray or ultraviolet line emission from hotter stars must be due either
to a different mechanism or to an unresolved, active, binary companion. Due to
their poor spatial resolution, X-ray instruments have been especially
susceptible to source confusion. At wavelengths longward of 1216 Angstroms, the
near ultraviolet spectra of stars hotter than this putative dividing line are
dominated by photospheric continuum. We have used FUSE to obtain spectra of the
subcoronal O VI emission lines, which lie at a wavelength where the
photospheric continuum of the mid- and early-A stars is relatively weak. We
observed 14 stars spanning a range in T_eff from 7720 to 10,000 K. Eleven of
the 14 stars showed O VI emission lines, including 6 of the 8 targets with
T_eff > 8300 K. At face value, this suggests that activity does not fall off
with increasing temperature. However, the emission lines are narrower than
expected from the projected rotational velocities of these rapidly-rotating
stars, suggesting that the emission could come from unresolved late-type
companions. Furthermore, the strength of the O VI emission is consistent with
that expected from an unseen active K or M dwarf binary companinon, and the
high x-ray to far uv luminosity ratios observed indicate that this must be the
case. Our results are therefore consistent with earlier studies that have shown
a rapid drop-off in activity at the radiative/convective boundary expected at
T_eff about 8300 K, in agreement with conventional stellar structure models.
