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We show here that the rotation period data in open clusters allow theempirical determination of an expression for the rate of loss of angularmomentum from cool stars on the main sequence. One significant component of theexpression, the dependence on rotation rate, persists from prior work; othersdo not. The expression has a bifurcation, as before, that corresponds to anobserved bifurcation in the rotation periods of coeval open cluster stars. Thedual dependencies of this loss rate on stellar mass are captured by twofunctions, $f(B-V)$ and $T(B-V)$, that can be determined from the rotationperiod observations. Equivalent masses and other [UBVRIJHK] colors are providedin Table 1. Dimensional considerations, and a comparison with appropriatecalculated quantities suggest interpretations for $f$ and $T$, both of whichappear to be related closely (but differently) to the calculated convectiveturnover timescale, $\tau_c$, in cool stars. This identification enables us towrite down symmetrical expressions for the angular momentum loss rate and thedeceleration of cool stars, and also to revive the convective turnovertimescale as a vital connection between stellar rotation and stellar activityphysics.

Angular Momentum Loss from Cool Stars: An Empirical Expression and Connection to Stellar Activity