The Turn-On Point of the Pleiades Cluster

Abstract

A still increasing amount of newly discovered characteristics of the Pleiades G, K and M dwarfs indicate that the “turn-on” point of this cluster is situated near to spectral type K2, corresponding to a mass of 0.75 to 0.8 M_⊙. This is very close to the expected position based on the cluster age, as derived from the B and early A-star members of the Pleiades, and stellar contraction times. This also means, however, that the pre-main-sequence stars with masses up to 0.3 M_⊙ are found very close to the main-sequence, contrary to the theoretical predictions. The “turn-on” point in the Pleiades is characterized by a halt to contraction, a rapid disappearance of very rapid rotation, the disappearance of circumstellar shells, polarization and emission lines, the disappearance of flare activity, and the occurrence of X-ray emission. The very rapid rotation is, in addition, shortly before the “turn-on” point accompanied by a strong (angular-velocity dependent) longitudinal surface flux and stellar wind modulation, causing rotational modulation of lightcurves. Closely related phenomena have been observed when more massive stars with radiative envelopes evolve away from the main-sequence and become rapidly rotating stars with deep convective envelopes.