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Studies using direct detection of periods via rotational modulation
have been carried out on stars in
several young clusters which are near the zero--age main sequence (ZAMS),
including the Hyades, the Pleiades
and 
Persei (see Radick 1988, Schaller et al. 1992 and Prosser 1992
respectively). The ages of these clusters are about 700, 70 and 50Myr
respectively. Comparison of these data leads to the following
conclusions (Soderblom 1996):
Persei and the Pleiades,
contain ultra--fast rotators (UFRs) with
km/s at all masses.
When combining these results with those on TTs, problems develop.
If one sets the initial rotation periods, masses and radii observed for TTs
and then allows them to evolve forward, the resulting distribution of rotation
periods doesn't resemble the distribution of rotation periods observed in young
clusters.
Barnes and Sofia (1996) argue the UFRs cannot be produced if Skumanich--like
spin down occurs on the pre--main sequence. They argue that the magnetic field
saturates beyond a threshold rotational velocity and has a very strong
dipole. This limits the rate at which angular momentum transfer
occurs through the magnetic field and causes UFRs to
exist in cases of relatively short disk survival.
Once stars reach the main sequence, all stars experience a
Skumanich--like slowdown due to angular momentum loss via their
winds (Endal & Sofia 1981). Recent work (cf. Barry 1988,
Walter & Barry 1991)
indicate that the spin--down rate is a
function of the mass and is proportional to 
(where A is
a constant). The result is that younger stars are expected to spin
down more quickly and older stars spin down more slowly than expected
with the Skumanich law.
In order to explain the slow down of the UFRs to the rate of the
other slower rotators,
core--envelope decoupling has been suggested (cf. Li & Collier--Cameron
1993, Strom 1994).
In the models, the core decouples from the envelope of a
young star. The core is
free to spin--up in accordance with 
, without a
noticeable change to the stellar surface rotation.
According to such
models, in time, angular momentum in transferred out to the envelope.
Meanwhile, all stars slow down due to wind losses. This model is
tunable, depending on the coupling between the core and the envelope.
Stars on their radiative tracks can either speed up or slow down their
observed rotational velocities. By moving the decoupling radius for
stars of various ages and
various masses, the observed distributions can be reproduced.
This seems to be a case of too many free parameters, and not enough
data. The models can be constrained somewhat by including younger
stars into the database. These stars will provide fiducial positions
for rotation period as a function of age. The data on the young stars will
teach us exactly what the usual rotation rate is as a function of mass
and age.
