Rotation and evolution of A stars: looking for progenitors of cool Ap stars

Abstract

We explore to what extent the study of the projected rotational velocities of normal A stars might reveal the existence of progenitors of cool Ap stars, i.e. of slowly rotating normal young A stars. Since according to a recent work, magnetic Ap stars with masses ranging from 1.7 to 2.5 M_sun are never found on the ZAMS in the HR diagram, but have completed at least 30% of their MS lifetime, one may suspect that some normal young A stars will become Ap later on. Such progenitors are expected to rotate slowly, as is the case of magnetic Ap stars. We show that an unambiguous proof of their existence would require a much larger sample than is available today. A sample of A stars with homogeneous determinations of vsin i and with precise Hipparcos parallaxes is examined. It is shown that no statistically significant difference can be found between the vsin i distributions of young and old stars, and that the data are compatible with conservation of angular momentum in the case of rigid-body rotation. The vsin i distribution of young A stars is compatible with a Maxwellian distribution of equatorial velocities, while this is less so of the older A stars.