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 Pleia des in the context of other young clusters (alpha Persei and the Hyade s) and pre-main-sequence (PMS) stars (in Taurus-Auriga and Orion) in o rder to examine how the angular momentum of a star like the Sun evolve s during its early life on the main sequence. The rotation of PMS star s can be evolved into distributions like those seen in the young clust ers if there is only modest, rotation-independent angular momentum los s 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 fo r 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 ex perience. Simple scaling relations (like the Skumanich relation) appli ed to the observed surface rotation rates of young solar-type stars ca nnot reproduce the way in which the Pleiades evolves into the Hyades, especially the dramatic convergence in rotation rates seen among the l owest masses. Also, the Hyades has a strongly mass-dependent distribut ion 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 i nvoking 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 distribu tions of rotational velocities. However, much better observations of P MS stars-especially those found under conditions akin to open clusters -are needed to better determine the initial conditions that these star s actually experience.