2002A&A...396..641A

Detailed elemental abundances were derived for twenty bona fide λ Bootis as well as two MK standard stars. Other than LTE abundances for ten elements (including C and O), NLTE values for Na were determined. The group of λ Bootis stars consists of non-magnetic, Population I, late B to early F-type dwarfs with a typical abundance pattern (Fe-peak elements being underabundant whereas C, N, O and S being almost solar abundant). Since classification resolution spectroscopy in the optical domain is not capable of determining the abundance of the light elements, a detailed abundance analysis is the ultimate test for the membership of an object to this group. Another important point is the detection of apparent spectroscopic binary systems in which two solar abundance objects mimic one metal-weak star, as proposed as a working hypothesis by Faraggiana & Bonifacio (1999A&A...349..521F). From twenty program stars we are able to confirm or establish the membership for nine objects (HD 23258, HD 36726, HD 40588, HD 74911, HD 84123, HD 91130, HD 106223, HD 111604 and HD 290799). Five stars (HD 90821, HD 98772, HD 103483, HD 108765 and HD 261904) can be definitely ruled out as being members of the λ Bootis group whereas no unambiguous decision can be drawn for another six stars (HD 66684, HD 105058, HD 120500, HD 141851, HD 201184 and HD 294253). One very important result is the apparent overabundances found for Na which cannot be explained by accretion or mass-loss alone.