The Chromospheric Age Dependence of the Birthrate, Composition, Motions, and Rotation of Late F and G Dwarfs within 25 Parsecs of the Sun

Chromospheric ages are calculated for 115 stars. Reported uncertainties of about 10% in the Vaughan-Preston S values lead to accidental errors in the calculated ages ranging from about 50% at 3×10⁸yr to about 15% at 10¹⁰yr. Rotational velocities are plotted against chromospheric age to reveal the rotational spin-down trajectories of solar-type stars of various masses. Also, the stellar birthrate, the [Fe/H] ratios, and the peculiar velocities of the sample stars are presented as a function of chromospheric age. The results confirm that a star's rotation depends only on its mass and age. The smooth, continuous decline of both a star's rotation and its activity across the Vaughan-Preston gap is demonstrated. The simples and only remaining tenable interpretation of the gap is that it is caused by time variations in the stellar birthrate.