The pioneering work of Wilson (1963) and Kraft (1967) showed that stars
of solar mass and less lose their angular momentum with time. Further
research by van den Heuvel & Conti (1971) and Skumanich (1972) quantified
the rate of loss. They considered both the strength of emission in the
Ca II resonance lines as a proxy of stellar rotation (via magnetic
field generation) as well as a small number of direct measurements
of stars in three open clusters of increasing age, viz. the Pleiades,
Ursa Major and the Hyades. These data along with similar measurements
obtained for the Sun were used to show that stellar rotation decayed with
age according to a power law with an index close to -0.5. This result was
consistent with models of Durney (1972) in which rotational braking was
caused by a stellar wind.
This wind-braking model became universally accepted for almost a decade and a half until it was challenged by the work of Stauffer et al. (1984, 1985, 1991a). These investigations involved the measurement of rotational for large numbers of GKM-type dwarfs in Persei ( Myr), the Pleiades ( Myr) and the Hyades ( Myr). Their results suggested that (a) rotational braking was mass dependent, ie. the more massive stars braking fastest, (b) that in the youngest clusters not all stars rotated rapidly and that many were rotating at a rate below the detection threshold of the data (), and (c) that the time scale for braking the rotation of solar mass stars is comparable to, or even shorter than, the age difference between the Per and Pleiades open clusters.
The fact that, even at the young age of the Per system, many solar mass stars are slow rotators suggests that the initial distribution of angular momentum on the zero-age main-sequence is a function of the earlier evolutionary history of individual stars. For example, the interaction of the star with its environment during the late stages of contraction and the presence of circumstellar disks may play an important role (Li & Cameron 1993). Thus, the understanding of angular momentum evolution probably involves a detailed consideration of the interaction of stars with their environments and whether these include the formation of massive disks or not. It will also depend upon the validity of the assumption that each cluster can be taken as representative of clusters of that age in general.
Furthermore, it is apparent that the braking of the more massive fast rotators is extremely rapid. Any discussion of the timescales involved must inevitably take into account uncertainties in the cluster ages. For instance, in the case of Per and the Pleiades, uncertainities in their ages are probably comparable to, or even larger than, their difference in age.
Thus, it seems clear that further observations of a number of open clusters of various ages is vital in order to advance our understanding of the subject matter outlined above. The present authors have undertaken an observational programme of open clusters, both somewhat younger than Per and between the ages of the Pleiades and Hyades, with a view to increasing the data available for this discussion. In our selection of clusters for investigation it is important to bear in mind the following considerations. In order that the measurement of will be feasible for individual stars (with presently available instrumentation), the study was restricted to clusters closer than 400 pc (ie. an M0 dwarf should be brighter than V=17.0). However at such distances, open clusters have large extents on the sky resulting in a serious confusion between cluster stars and background objects. Hence, our programme should ideally consist of at least two steps, 1) the identification and elimination of background non-cluster members and then, 2) measurement of the rotational properties of resultant candidate members.
The first step may involve one or more of the following techniques, viz. multicolour photometry, proper motion studies, low-resolution spectroscopic classification and radial velocity measurements. This present paper is the first in a series identifying candidate members of appropriate young open clusters from the comparison of multicolour photometry and theoretical isochrones. A more detailed discussion of the points raised above has been given in Rolleston (1995).