Spectral Classification of Faint Members of the Hyades and Pleiades and the Dating Problem in Galactic Clusters.

In young clusters, the assumption that all members of a cluster are coeval leads to a conflict between the upper turnoff age and the age inferred from the location of faint members with respect to the normal main sequence. A similar result is found in the Pleindes: stars fainter than about G2 should lie above the main sequence if the H-burning age of 6 x 10 years is applicable, yet no systematic elevation of the main sequence is observed at least as far as type KS. Furthermore, there is no sign of T Tauri stars or dMe stars among the faint members of the Pleiades to about m,, = 160. In explanation, it is proposed that the assumption of near-simultaneous origin at all masses be replaced by the hypothesis that there may exist a major spread in the starting times of the members of a cluster or association. It is proposed that star formation at intermediate and low masses can go on in a large dark cloud for a very long time. The Taurus-Auriga dark nebulae are to be identified with a cloud in this condition But when a high- luminosity 0-type star forms in this volume, the hydrogen in its vicinity is ionized, the dust is evaporated, and a high degree of kinetic activity and turbulence is produced that brings an end to most star formation in this region. The Orion Nebula is in this state today. If the stars inside the dissolving cloud form a dynamically stable group when the gas is largely swept away, then the result is a stable cluster. If not, the result may be an expanding association. This particular feature was anticipated by a suggestion of Zwicky (1953). In this picture, the H-burning age of a cluster gives the time since the last generation of bright stars was formed and not the total time since stars have been forming within the cluster. In the case of the Pleiades, these two intervals are about 6 x 10 and >2 2 x 10 years, respectively. It is estimated that the mass density of stars within the Taurus-Auriga clouds is comparable with that of representative star clusters such as the Pleiades and Hyades, a correspondence that supports the hypothesis An alternative explanation of some of these observations is that the true time scale of gravitational contraction may be much shorter than the classical value But it is not clear that all the observational data can be satisfied simply by shortening the time scale.