A search for circumstellar gas around normal A stars and Lambda Bootis stars

We have searched for interstellar or circumstellar absorption lines in the center of Ca II K towards bright A-type stars that are mostly within 80 pc of the Sun. Narrow absorption features are found in about 30 % of the 28 normal main-sequence A stars and 18 metal-deficient λ Bootis stars studied. We have determined surface gravities and projected rotational velocities. Most of the stars with detectable Ca K features have comparatively low gravities and high projected rotational velocities. This correlation with stellar properties implies that most of the narrow absorption features are of circumstellar rather than interstellar origin. The preference of low gravity and rapid rotation furthermore suggests that most of the gas shells around A stars develop in the pre-main-sequence phase of evolution, and disappear largely before the star arrives at the ZAMS. Among the normal A stars studied, about 50 % are known to have dust disks. Unlike A stars with circumstellar gas, these dusty stars do not prefer low log g and high v sin i. This results in an apparent lack of correlation between gas and dust, and indicates that normal A stars with gas shells and those with dust disks are not in the same evolutionary stage. We conjecture that dust disks tend to develop after most of the gas has disappeared.