Boron Abundances in Early B Dwarfs of the Galactic Open Cluster NGC 3293

New boron abundances or upper limits have been determined for 8 early-B stars in the young Galactic open cluster NGC 3293, using ultraviolet spectra obtained by the Hubble Space Telescope Cosmic Origins Spectrograph. With previous observations, there are now 18 early-B stars in this cluster with boron measurements. Six of the newly observed stars have projected rotational velocities greater than 200 km/s, allowing new constraints on rotationally driven mixing in main-sequence stars. When comparing to synthetic model populations, we find that the majority of our sample stars agree well with the predicted trends of stronger boron depletion for larger rotation and for larger mass or luminosity. Based on those, a smaller than the canonical rotational mixing efficiency,(fc = 0.0165 vs the more standard value of 0.033), appears to be required. However, our five most slowly rotating stars are not well explained by rotational mixing, and we speculate that they originate from binary mergers.