On the main sequence, it has long been known that large mean rotational velocities are common among the early-type stars and that these velocities decline steeply in the F-star region, from 150 km s-1 to less than 10 km s-1 in the cooler stars (see Figure 1.6). As was shown in Section 6.3.2, the observed projected velocities indicate that the mean value of the total angular momentum (J) closely follows the simple power law (J) a M2 for stars earlier than spectral type F0, which corresponds to about 1.5M0 (see Figure 6.7). The difficulty is not to account for such a relation, which probably reflects the initial distribution of angular momentum, but to explain why it does not apply throughout the main sequence. It has been suggested that the break in the mean rotational velocities beginning at about spectral type F0 might be due to the systematic occurrence of planets around the low-mass stars (M < 1.5M0), with most of the initial angular momentum being then transferred to the planets. Although this explanation has retained its attractiveness well into the 1960s, there is now ample evidence that it is not the most likely cause of the remarkable decline of rotation in the F-star region along the main sequence. Indeed, following Schatzman's (1962) original suggestion, there is now widespread agreement that this break in the rotation curve can be attributed to angular momentum loss through magnetized winds and/or sporadic mass ejections from stars with deep surface convection zones. This interaction between rotation and surface activity, which is the basis for understanding much of the evolution of low-mass stars, will be considered in Section 7.2.

Now, as was shown by Wilson (1963), the average intensity of Ca II emission in a late-type dwarf and, hence, the general degree of its chromospheric activity bear an inverse relationship to its age. A similar trend was found by Kraft (1967) in the rotational velocities of late-F and early-G dwarfs. From a detailed examination of these data, Skumanich (1972) has shown that both rotational velocities and Ca II emission decline with advancing age according to a t-1/2 law (see Eq. [1.7]). This coincidence strongly suggests that there exists a deep physical connection between rotation and surface activity among the low-mass stars. Further complexity was added to the problem when van Leeuwen and Alphenaar (1982) announced the discovery of a number of rapidly rotating G- and K-dwarfs in the Pleiades, with equatorial velocities up to 170 km s-1. This important result led to a flurry of interest in the rotational evolution of these low-mass stars, which spin down faster than predicted by Skumanich's empirical law shortly upon arriving on the main sequence. In Section 7.3 we shall briefly review the new rotational velocity data for T Tauri stars and late-type dwarfs in young open clusters (see also Section 1.3). The major theoretical models developed to clarify these new findings will be considered in Section 7.4.

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