Testing the accretion scenario of lambda Boo stars

Abstract

Context. The group of λ Boo stars is known for years, however the origin of its chemical peculiarity is still strongly debated. Aims. Our aim is to test the accretion scenario of λ Boo stars. This model predicts that a binary system with two early-type stars passing through a diffuse cloud should both display the same superficial peculiarity. Methods. Via spectral synthesis, we carried out a detailed abundance determination of three multiple systems hosting a candidate λ Boo star: the remarkable triple system HD 15164/65/65C and the two binary systems HD 193256/281 and HD 198160/161. Stellar parameters were initially estimated using Strömgren photometry or literature values and then refined by requiring excitation and ionization balances of Fe lines. The abundances were determined iteratively for 24 different species by fitting synthetic spectra using the SYNTHE program together with local thermodynamic equilibrium (LTE) ATLAS12 model atmospheres. Specific opacities were calculated for each star, depending on its arbitrary composition and microturbulence velocity v micro through the opacity sampling (OS) method. The abundances of the light elements C and O were corrected by Non-LTE effects. The complete chemical pattern of the stars were then compared to those of λ Boo stars.Results. The abundance analysis of the triple system HD 15164/65/65C shows a clear λ Boo object (HD 15165) and two objects with near solar composition (HD 15164 and 15165C). Notably, the presence of a λ Boo star (HD 15165) together with a near solar early-type object (HD 15164) is difficult to explain under the accretion scenario. Also, the solar-like composition derived for the late-type star of the system (HD 15165C) could be used, for the first time, as a proxy for the initial composition of the λ Boo stars. This could help to constrain any model of λ Boo stars formation and not only the accretion scenario. The abundance analysis of the binary system HD 193256/281 shows no clear λ Boo components, while the analysis of HD 198160/161 shows two mild-λ Boo stars. Then, by carefully reviewing abundance analysis of all known binary systems with candidate λ Boo stars from literature and including the systems analyzed here, we find no binary/multiple system having two clear "bonafide" λ Boo stars, as expected from the accretion scenario. The closer candidates to show two λ Boo-like stars are the binary systems HD 84948, HD 171948 and HD 198160; however, in our opinion they show mild rather than clear λ Boo patterns. Conclusions. We performed for the first time a complete analysis of a triple system which includes a λ Boo candidate. Our results brings little support to the accretion scenario of λ Boo stars. Then, there is an urgent need of additional binary and multiple systems to be analyzed through a detailed abundance analysis, in order to test the accretion model of λ Boo stars.