A consolidated catalogue of lambda Bootis stars

lambda Bootis stars challenge our understanding of diffusion and accretion processes related to stars and their circumstellar environment, and they are interesting components of the classical instability strip. Attempts to derive group properties with statistical methods are severely limited by the small number of unambiguously identified lambda Bootis stars. In general, the subject appears to be obscured by incorrect memberships and it is therefore essential to provide a sufficiently large catalogue of definitive group members before modeling the lambda Bootis phenomenon. This paper describes the first steps towards this goal, based on our current knowledge of well investigated members, leading to a concise definition of lambda Bootis stars: {Pop I hydrogen burning metal poor (except of C,N,O and S) A-type stars.} The definition does not depend on phenomenological features, like flux depressions, colour excesses, v sin i values, etc. Based on a new homogeneous catalogue with 45 lambda Bootis stars, we discuss classification criteria which can be used for a spectroscopic and photometric all-sky survey for lambda Bootis stars in the field and in clusters of different ages. Based on observations obtained at ESO-La,Silla, CTIO, Observatoire de Haute-Provence, Osservatorio Astronomico di Padua-Asiago, Univ. Toronto Southern Observatory, Observatorio do Pico dos Dias-LNA/CNPq/MCT (Brazil). Table 1 is also available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html.