2025A&A...696A.123A

Context. The origin of λ Boo stars is currently unknown. Very few of them are known as members of multiple systems, which could provide benchmark laboratories to study their origin.
Aims. Our goal is to find new candidate λ Boo stars that belong to binary systems. Then, a detailed abundance determination of some candidates could confirm their true λ Boo nature, while the composition of any late-type companions could be used as a proxy for the initial composition of the λ Boo stars.
Methods. We cross-matched a homogeneous list of candidate λ Boo stars with a recent Gaia eDR3 catalog of resolved binaries. Then we carried out a detailed abundance determination via spectral synthesis of three of these systems that host a candidate λ Boo star and a late-type companion: HD 98069 + UCAC4 431-054639; HD 87304 + CD-33 6615B; HD 153747 + TYC 7869-2003-1. The stellar parameters were estimated by fitting the observed spectral energy distributions (SEDs) with a grid of model atmospheres using the online tool VOSA, together with Gaia eDR3 parallaxes and the PARAM 1.3 interface. Then the abundances were determined iteratively for 31 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 patterns of the stars were then compared to those of the λ Boo stars.
Results. We obtained a group of 19 newly identified binary systems that contain a candidate λ Boo star, allowing us to approximately double the number of λ Boo stars currently known in multiple systems. This important group could be used in further studies of λ Boo stars. For the first time we performed a detailed abundance analysis of three of these binary systems that include a candidate λ Boo star and a late-type companion. We confirmed the true λ Boo nature of the three early-type stars (HD 87304, HD 98069, and HD 153747), and obtained mostly a solar-like composition for their late-type components. Adopting as a proxy the late-type stars, we showed that the three λ Boo stars were initially born with a solar-like composition. This is an important constraint for any scenario trying to explain the origin of λ Boo stars. The present work provides three numerical examples of possible starting and ending compositions to test formation models of λ Boo stars. In addition, the solar-like composition of the late-type stars supports the idea that λ Boo stars are Population I objects; however, we caution that other explanations are also possible.
Conclusions. We performed, for the first time, a detailed abundance analysis of binary systems including a λ Boo star and a late-type companion. We obtained a solid indication that λ Boo stars were born from a solar-like composition and established an important constraint to test the formation models of λ Boo stars.