A spectroscopic survey for lambda Bootis stars III. Final results

In the third paper of a series dedicated to the spectroscopic survey for ne w lambda Bootis stars, we present all new and confirmed members of the grou p as well as a detailed analysis of the observed sample. The nature of this small group of chemically peculiar stars of the upper main sequence still challenges our understanding of processes like diffusion, mass-loss and acc retion. The typical abundances pattern (nearly solar values for C, N, O and S whereas the Fe-peak elements are moderate to strong underabundant) can s till not be explained by any proposed theory. Hence, the significant increa se of new members gives the opportunity to investigate the group properties in more detail. We report the discovery of 26 new members of the group and the confirmation of 18 candidates from the literature. This almost triples the number of known lambda Bootis stars. The existence of one member in th e young open cluster NGC 2264 and four members in the Orion OB1 association proves that the lambda Bootis phenomenon already works at very early stage s of stellar evolution. Recent results from the Hipparcos mission have show n that the well established lambda Bootis stars of the Galactic field compr ise the whole area from the Zero Age Main Sequence to the Terminal Age Main Sequence (approximate to 10(9) yr for an A-type star). There is a continuo us transition between very young and rather evolved evolutionary stages. We find that the overall percentage of lambda Bootis type among all normal ty pe stars in the spectral range from B8 to F4 is 2% in the Galactic field as well as in open clusters. Furthermore, 44 metal-weak objects are listed wh ich might be connected with the lambda Bootis phenomenon. Our biased sample (chosen by photometric boxes) is not distinguished from all A-type stars i n the corresponding spectral region by the rotational velocity distribution . Only for the luminosity classes IV and III (especially for the cooler pro gram stars) the determined mean v sin i values are very high compared to th ose of the literature.