2025A&A...698A.137S

Context. The origin of λ Boo stars is currently unknown. After several efforts by many authors, no bona fide λ Boo stars have been confirmed as members of open clusters. Their detection could provide an important test bed for a detailed study of λ Boo stars.
Aims. Our aim is to detect, for the first time, λ Boo stars as members of open clusters. The λ Boo class will be confirmed through a detailed abundance analysis, while the cluster membership will be evaluated using a multi-criteria analysis of probable members.
Methods. We cross-matched a homogeneous list of λ Boo stars with a Gaia DR3 catalog of open clusters and, notably, we found two candidate λ Boo stars in open clusters. We carried out a detailed abundance determination of the candidate λ Boo stars and additional cluster members via spectral synthesis. Stellar parameters were estimated by fitting observed spectral energy distributions (SEDs) with a grid of model atmospheres using the online tool VOSA, Gaia DR3 parallaxes, and the PARAM 1.3 interface. Then, the abundances were determined iteratively for 22 different species by fitting synthetic spectra using the SYNTHE program together with local thermodynamic equilibrium (LTE) ATLAS12 model atmospheres. 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 λ Boo stars. We also performed an independent cluster membership study using Gaia photometry and radial velocities with a multi-criteria analysis.
Results. For the first time, we present the surprising finding of two λ Boo stars as members of open clusters: HD 28548 belongs to the cluster HSC 1640 and HD 36726 belongs to the cluster Theia 139. This was confirmed using a detailed abundance analysis, while the cluster membership was independently analyzed using Gaia DR3 data and radial velocities. We compared the λ Boo star HD 36726 with other cluster members and showed that the λ Boo star was originally born with a near-solar composition. This also implies one of the highest chemical differences detected between two cluster members (∼0.5 dex). In addition, we suggest that the λ Boo peculiarity strongly depletes heavier metals, but could also slightly modify lighter abundances such as C and O. We also found that both λ Boo stars belong to the periphery of their respective clusters. This would suggest that λ Boo stars avoid the strong photoevaporation by UV radiation from massive stars in the central regions of the cluster. We preliminarily suggest that peripheral location appears to be a necessary, though not sufficient, condition for the development of λ Boo peculiarity. We also obtained a precise age determination for the λ Boo stars HD 28548 (26.3±1.4 Myr) and HD 36726 (33.1±1.1 Myr), which are among the most precise age determinations of λ Boo stars. We strongly encourage analyzing additional cluster members, which could provide important insights for the study of the origin of λ Boo stars.
Conclusions. We have confirmed, for the first time, that two λ Boo stars belong to open clusters. This remarkable finding could make open clusters excellent laboratories for studying the origin of λ Boo stars.