A spectroscopic survey for λ Bootis stars

Abstract

In recent years, the chemically peculiar (CP) stars of the upper main sequence have become a fruitful field for the testing of astrophysical theories. Processes such as diffusion, convection and mass loss have been developed theoretically and introduced into models. The group of λ Bootis stars, however, is remarkable among the chemically peculiar stars as they are nonmagnetic, Population I, A to F-type dwarfs which show significant underabundances of metals (except for C, N, O and S). Unfortunately, the small number of confirmed members of the λ Bootis class makes a sound statistical analysis of their properties impossible. Thus, it is still difficult to decide between the two theories - mass loss with diffusion and the accretion theory - which have been proposed to explain the origin of these stars. We therefore have started a spectroscopic survey to find new λ Bootis stars in the field as well as in open clusters and associations. The presence of λ Bootis stars in open clusters and associations would permit a determination of the ages of these stars, and thus would yield an important test for distinguishing between the two theories. In this paper we describe the selection of candidates using photometric criteria, the basic requirements, spectroscopic follow-up observations and results from the first three observing runs. Special care was taken to avoid misclassification of our programme stars (e.g. λ Bootis stars are often confused with intermediate Population II, He-weak or high stars), using a refined MK system. The discovery of at least six new λ Bootis stars (including three in the Orion OB1 association) shows the efficacy of our selection criteria.