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1. Introduction

Ever since the pioneering work of Wilson (1963) and Kraft (1967), it has generally been accepted that stars of solar mass or less lose their angular momentum with time. Further research by van den Heuvel & Conti (1971) and Skumanich (1972) suggested that low-mass stars arrive on the main-sequence rotating rapidly, as a consequence of the conservation of angular momentum during the pre-main-sequence contraction phase. Drawing together both Ca II emission data (indicative of chromospheric activity in late-type stars) and projected rotational velocities (used to determine angular momenta) for a small number of stars in three open clusters, viz. Pleiades, Ursa Major and Hyades, and using the solar values, Skumanich derived empirical relationships between the afore-mentioned properties and age, which simply stated that both Ca II H & K emission reversals and stellar rotation declined with time according to an inverse power law. Such relationships were consistent with the theoretical predictions of Durney (1972) based on models in which rotational braking was caused by a stellar wind. This view remained unchallenged for more than a decade.

More recently, new observations have given rise to a new paradigm. Stauffer et al. (1984, 1985, 1987), following up the discovery of rapidly rotating K stars in the Pleiades (van Leeuwen et al. 1987), measured rotational tex2html_wrap_inline1225 for GKM-type dwarfs in the tex2html_wrap_inline1227Persei (age tex2html_wrap_inline1229 Myr), Pleiades (age tex2html_wrap_inline1231 Myr) and Hyades (age tex2html_wrap_inline1233 Myr) open clusters. These results showed that all late-type members of tex2html_wrap_inline1227Per exhibit a very large spread of rotational tex2html_wrap_inline1225 values, ranging from approximately tex2html_wrap_inline1239. By contrast, G-type Pleiads had tex2html_wrap_inline1225 close to or less than the observational limit of tex2html_wrap_inline1243 with rapid rotation only observed amongst the K- and M-types.

Inter-comparison of these two clusters suggests that a more rapid braking mechanism than that provided by the classical stellar wind scenario must be at work. This mechanism must be capable of braking the rotation of G-dwarfs on a time-scale of the order of the age difference between tex2html_wrap_inline1227 Per and the Pleiades, i.e. tex2html_wrap_inline1247 Myr. Furthermore, only moderately rapid rotation was detected in the oldest of the clusters, the Hyades, and then only in the M-type dwarfs. Comparison of these results supports the idea that, once this rapid phase of braking is complete, a power law relation may then apply.

These important conclusions have been based on the comparison of results found for three open clusters. In order to place constraints on possible braking mechanisms, further observations are required of young clusters with ages distributed over the critical range tex2html_wrap_inline1249. Clusters younger than, and of similar age to, tex2html_wrap_inline1227 Per are needed to confirm that the rapid braking of G-dwarfs is universal, while those intermediate in age between the Pleiades and Hyades will yield information on the time-scales for braking of progressively lower mass stars.

The authors have undertaken such a programme to investigate the distribution of stellar rotation in a number of open clusters. However, as a consequence of the intrinsic faintness of late-type dwarfs (tex2html_wrap_inline1253 at K0, tex2html_wrap_inline1255 at M0), it is necessary to restrict the study to clusters that are within approximately 400 pc of the Sun; otherwise the measurement of tex2html_wrap_inline1225 from high-resolution spectroscopy will not be observationally feasible for a sample of their late-type members. Due to their relative proximity, these clusters have a large extent on the sky and unambiguous identification of bona fide members is difficult. With this in mind, we have selected several target clusters for which we have obtained BVRI CCD photometry. Further details of the observational programme and background material can be found in Rolleston (1995).


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