Evolution of rotation in main-sequence stars

Abstract

A review of the classical and modern measurements of axial rotation and their implications for the evolution of rotation of stars on the lower main sequence is given. From stars of main-sequence spectral type A through early K, the dependence of rotation on mass and age is investigated, from results for stars in open clusters and in the field. The high-mass, single, normal dwarf stars of spectral type A display a common mean projected rotational velocity dependent on their masses and re gardless of their ages. The angular momentum per unit mass in this range decreases slowly with decreasing mass.

At approximately mid-F spectral type, the rotation rate declines sharply then decreases again slowly with decreasing mass. The sudden decline in average rotation is probably precipitated by the effect of braking by magnetic torques in stellar winds. Along the lower main sequence, not only mass but also age determines rotation. Rotation slows with time at a given mass. Observational evidence is consistent with rotation slowing with the inverse square root of the main-sequence age of a star, <vsini> ∞ t^−1/2

Among the youngest main-sequence stars, those in the Pleiades near spectral type K2, rapid rotation (up to 150 km s⁻) is common. These stars have gained angular momentum during the radiative-track phase of their pre-main-sequence evolution, and should shed their apparently excessive angular momentum on a rapid timescale, a few x 107 years, shorter than that predicted by the t^−1/2 relationship.

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