O VI Observations of the Onset of Convection Zones in Main-Sequence A Stars^*

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 Lyα, the near-ultraviolet spectra of stars hotter than this putative dividing line are dominated by the photospheric continuum. We have used the Far-Ultraviolet Spectroscopic Explorer (FUSE) to obtain spectra of the subcoronal O VI emission lines, which lie at a wavelength where the photospheric continuum of the middle 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 companion, and the high L_X/L(O VI) 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 ∼ 8300 K, in agreement with conventional stellar structure models.