A paradoxical gap in the relative ages of T Tauri stars

The frequency distribution of T Tauri stars of different Youth (relative age) shows a pronounced gap at 5 percent of their time to the zero-age main sequence. This gap, which occurs in all of the four major T Tauri associations, is too large to be filled by unclassifiable veiled stars. It is nearly vertical on the Hertzsprung-Russell diagram, is centered near spectral class K5, and lies close to the transition between the convective and radiative tracks of the pre-main-sequence stars. It is noted that the range of variability seems unrelated to the other observational features of these stars.