A spectroscopic survey for λ Bootis stars

Abstract

Bootis stars comprise only a small number of all A-type stars and are characterized as nonmagnetic, Population i, late B to early F-type dwarfs which show signicant underabundances of metals whereas the light elements (C, N, O and S) are almost normal abundant compared to the Sun. In the second paper on a spectroscopic survey for Bootis stars, we present the spectral classications of all program stars observed. These stars were selected on the basis of their Stromgren uvby colors as Bootis candidates. In total, 708 objects in six open clusters, the Orion OB1 association and the Galactic eld were classied. In addition, 9 serendipity non-candidates in the vicinity of our program stars as well as 15 Guide Star Catalogue stars were observed resulting in a total of 732 classied stars. The 15 objects from the Guide Star Catalogue are part of a program for the classication of apparent variable stars from the Fine Guidance Sensors of the Hubble Space Telescope. A grid of 105 MK standard as well as \pathological stars guarantees a precise classication. A comparison of our spectral classication with the extensive work of Abt & Morrell (1995) shows no signicant dierences. The derived types are 0.23 0.09 (rms error per measurement) subclasses later and 0.30 0.08 luminosity classes more luminous than those of Abt & Morrell (1995) based on a sample of 160 objects in common. The estimated errors of the means are 0.1 subclasses. The characteristics of our sample are discussed in respect to the distribution on the sky, apparent visual magnitudes and Stromgren uvby colors.