The status of Galactic field λ Bootis stars in the post‐ Hipparcos era

The λ Bootis stars are Population I, late B‐ to early F‐type stars, with moderate to extreme (up to a factor 100) surface underabundances of most Fe‐peak elements and solar abundances of lighter elements (C, N, O and S). To put constraints on the various existing theories that try to explain these peculiar stars, we investigate the observational properties of λ Bootis stars compared with a reference sample of normal stars. Using various photometric systems and Hipparcos data, we analyse the validity of standard photometric calibrations, elemental abundances, and Galactic space motions. There crystallizes a clear picture of a homogeneous group of Population I objects found at all stages of their main‐sequence evolution, with a peak at about 1 Gyr. No correlation of astrophysical parameters such as the projected rotational velocities or elemental abundances with age is found, suggesting that the a priori unknown mechanism, which creates λ Bootis stars, works continuously for late B‐ to early F‐type stars in all stages of main‐sequence evolution. Surprisingly, the sodium abundances seem to indicate an interaction between the stars and their local environment.