Rotation and shell spectra among A-type dwarfs

Rotational velocities for 66 metallic-line and 123 normal A5-A9 IV or V stars are given and used to determine the frequency distributions of equatorial rotational velocities. Those distributions have only a 1.3 percent overlap, which is probably due to a small contamination of the two samples. If so, then the rotational velocity is a necessary and sufficient parameter to determine whether, within certain spectral type and age ranges, a star will have a well-developed metallic-line or normal spectrum. Among the 35 most rapidly rotating normal stars, eight were found to have shell spectra. Most of the shell stars occur during or just after the overall stellar contraction when the stars have depleted most of their core hydrogen; one shell star has apparently recently contracted to the main sequence. These shell stars seem to be the A-type counterparts of the Be stars. Subject headings: circumstellar shells - metallic-line stars - rotation, stellar