The Relationship Between the BÖöhm-Vitense Gap and Stellar Activity in Open Clusters

Using published UBV photometry of nine open clusters and chromospheric activity data in five clusters, we have performed the most complete study to date of the supposed Böhm-Vitense gap in main-sequence late-A and early-F stars. Using the χ 2 test on the distribution of B - V colors of the main-sequence stars of these clusters, we located gaps at the 0.90 probability level or greater in eight out of the nine clusters. Six of the nine clusters show a gap near B - V = 0.35 at this level. In four nearby clusters, Alpha Persei, Pleiades, Coma, and Praesepe, we compare the gaps with measurements of chromospheric activity via the He I λ5876 (D3) line. If the Böhm-Vitense gap is related to the onset of surface convection zones, as has been suggested, we would expect to find a direct relationship between the onset of activity and the gaps. However, in all four cases we find stars on both sides of the gap showing strong activity. The observed gaps do not appear to correlate directly with any known physical mechanism and thus either point to an unknown mechanism or a statistical anomaly.