The Pleiades and α Persei Clusters

The upper–main-sequence members of the Pleiades and α Persei clusters, considered as members of the Local Association, yield mean parallaxes that are only 4% larger than the mean values from Hipparcos observations. The (log T eff , M V ) diagram reveals that in the temperature range from 6000 to 8000 K, the Hyades and α Persei main-sequence members are nearly identical and several tenths of a magnitude brighter than similar stars on the Pleiades main sequence. The departure of the Pleiades main sequence cannot be traced to either age or heavy-element abundance differences in the range thought to apply to these clusters. A 50% increase in the helium abundance of Pleiades over Hyades stars could account for the luminosity difference. Alternative explanations are that the Pleiades cluster is rejected from supercluster membership and/or that the Hipparcos parallax results for the Pleiades are in error by some 10%.