Rotational velocities of A-type stars. III. Velocity distributions

TL;DR

This study analyzes the rotational velocity distributions of single A-type stars, revealing bimodal patterns likely caused by angular momentum evolution, with notable absence of slow rotators among certain spectral types.

Contribution

It provides the first detailed, bias-corrected velocity distributions for a large, homogeneous sample of single A-type stars, highlighting evolutionary effects on stellar rotation.

Findings

Bimodal velocity distributions in late B and early A-type stars.

Lack of slow rotators among intermediate and late A-type stars.

Evolutionary effects influence the shape of rotational velocity distributions.

Abstract

Aim - In this work, a sample of vsini of B9 to F2-type main sequence single stars has been built from highly homogeneous vsini parameters determined for a large sample cleansed from objects presenting the Am and Ap phenomenon as well as from all known binaries. The aim is to study the distributions of rotational velocities in the mass range of A-type stars for normal single objects. Methods - Robust statistical methods are used to rectify the vsini distributions from the projection effect and the error distribution. The equatorial velocity distributions are obtained for an amount of about 1100 stars divided in six groups defined by the spectral type, under the assumption of randomly orientated rotational axes. Results - We show that late B and early A-type main-sequence stars have genuine bimodal distributions of true equatorial rotational velocities due probably to phenomena of angular momentum loss and redistribution the star underwent before reaching the main sequence. A striking lack of slow rotators is noticed among intermediate and late A-type stars. The bimodal-like shape of their true equatorial rotational velocity distributions could be due to evolutionary effects.