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Description
(from Publ. Astron. Soc. Pac. 109, 759 (1997)) We analyze these rotation rates for a dependence on both mass and age. We compare the average rotation speeds of the field stars with mean velocities for young stars in Orion, the {Alpha} Persei cluster, the Pleiades, and the Hyades. The average rotation speeds of stars more massive than ~1.6 Msol experience little or no change during the evolutionary lifetimes of these stars on the zero age main sequence (ZAMS) or within the ZAMS band. Less massive stars in the range between 1.6 Msol and 1.3 Msol also show little decline in mean rotation rate while they are on the main sequence, and at most a factor of two decrease in velocity as they evolve off the main sequence. The e-folding time for the loss of angular momentum by the latter group of stars is at least 1-2 billion years. This inferred characteristic time scale for spindown is far longer than the established rotational braking time for solar-type stars with masses below ~1.3 Msol. We conclude from a comparison of the trends in rotation with trends in chromospheric and coronal activity that the overall decline in mean rotation speed along the main sequence, from ~2 Msol down to ~1.3 Msol, is imposed during the pre-main-sequence phase of evolution, and that this pattern changes little thereafter while the star resides on the main sequence. The magnetic activity must therefore play only a minor role in determining the rotation rates of the intermediate-mass stars, either because a solar-like dynamo is weak or absent, or else the geometry of the magnetic field is appreciably less effective in removing angular momentum from these stars.
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