  The rotation period of classical T Tauri stars (CTTS) represents a
longstanding puzzle. While young low-mass stars show a wide range of rotation
periods, many CTTS are slow rotators, spinning at a small fraction of break-up,
and their rotation period does not seem to shorten, despite the fact that they
are actively accreting and contracting. Matt & Pudritz (2005) proposed that the
spin-down torque of a stellar wind powered by a fraction of the accretion
energy would be strong enough to balance the spin-up torque due to accretion.
Since this model establishes a direct relation between accretion and ejection,
the observable stellar parameters (mass, radius, rotation period, magnetic
field) and the accretion diagnostics (accretion shock luminosity), can be used
to constraint the wind characteristics. In particular, since the accretion
energy powers both the stellar wind and the shock emission, we show in this
letter how the accretion shock luminosity L_UV can provide upper limits to the
spin-down efficiency of the stellar wind. It is found that luminous sources
with L_UV > 0.1 L_Sun and typical dipolar field components < 1 kG do not allow
spin equilibrium solutions. Lower luminosity stars (L_UV < 0.1 L_Sun) are
compatible with a zero-torque condition, but the corresponding stellar winds
are still very demanding in terms of mass and energy flux. We therefore
conclude that accretion powered stellar winds are unlikely to be the sole
mechanism to provide an efficient spin-down torque for accreting classical T
Tauri stars.
