THE EVOLUTION OF ANGULAR-MOMENTUM AMONG ZERO-AGE MAIN-SEQUENCE SOLAR-TYPE STARS

We consider a survey of rotation among F, G, and K dwarfs of the Pleiades in the context of other young clusters (alpha Persei and the Hyades) and pre-main-sequence (PMS) stars (in Taurus-Auriga and Orion) in order to examine how the angular momentum of a star like the Sun evolves during its early life on the main sequence. The rotation of PMS stars can be evolved into distributions like those seen in the young clusters if there is only modest, rotation-independent angular momentum loss prior to the ZAMS. Even then, the ultrafast rotators (UFRs, or ZAMS G and K dwarfs with v sin i greater-than-or-similar-to 30 km s-1) must owe their extra angular momentum to their conditions of formation and to different angular momentum loss rates above a threshold velocity, for it is unlikely that these stars had angular momentum added as they neared the ZAMS, nor can a spread in ages within a cluster account for the range of rotation seen. Only a fraction of solar-type stars are thus capable of becoming UFRs, and it is not a phase that all stars experience. Simple scaling relations (like the Skumanich relation) applied to the observed surface rotation rates of young solar-type stars cannot reproduce the way in which the Pleiades evolves into the Hyades, especially the dramatic convergence in rotation rates seen among the lowest masses. Also, the Hyades has a strongly mass-dependent distribution of rotation, a dependence that is subtle or absent in the Pleiades, at least for (B - V) greater-than-or-similar-to 0.6. We argue that invoking internal differential rotation in these ZAMS stars can explain several aspects of the observations and thus can provide a consistent picture of ZAMS angular momentum evolution. Models with gradual core-envelope recoupling during the early main sequence lifetime of a solar-type star reproduce the qualitative features of the observed distributions of rotational velocities. However, much better observations of PMS stars-especially those found under conditions akin to open clusters-are needed to better determine the initial conditions that these stars actually experience.