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
for GKM-type dwarfs in the
Persei (age
Myr),
Pleiades (age
Myr) and Hyades (age
Myr) open clusters.
These results showed that all late-type members of
Per exhibit a
very large spread of rotational
values, ranging from
approximately
. By contrast, G-type Pleiads had
close to or less than the observational limit of
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 Per
and the Pleiades, i.e.
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 . Clusters
younger than, and of
similar age to,
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 ( at K0,
at M0),
it is necessary to restrict the study to clusters that are within
approximately 400 pc of the Sun; otherwise the measurement of
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).