Why Are the K Dwarfs in the Pleiades So Blue?

The K dwarfs in the Pleiades fall nearly mag below a main-sequence isochrone when plotted in a color-magnitude diagram utilizing V magnitude as the luminosity index and B - V as the color index. This peculiarity has been known for 40 years but has gone unexplained and mostly ignored. When compared to Praesepe members, the Pleiades K dwarfs again are subluminous (or blue) in a color-magnitude diagram using B - V as the color index. However, using V - I as the color index, stars in the two clusters are coincident to M V ∼ 10; using V - K as the color index, Pleiades late K and M stars fall above the main-sequence locus defined by Praesepe members. We believe that the anomalous spectral energy distributions for the Pleiades K dwarfs, as compared to older clusters, are a consequence of rapid stellar rotation and may be primarily due to spottedness. If so, the required areal filling factor for the cool component has to be very large (≥50%). Weak-lined T Tauri stars have similar color anomalies, and we suspect that this is a common feature of all very young K dwarfs (spectral type >K3). The peculiar spectral energy distribution needs to be considered in deriving accurate pre–main-sequence isochrone-fitting ages for clusters like the Pleiades, since the age derived will depend on the temperature index used.