Discovery of lambda Boo stars in open clusters

Abstract

The origin of lambda Boo stars is currently unknown. After several efforts by many authors, no bona fide lambda Boo stars have been confirmed as members of open clusters. Their detection could provide an important test bed for a detailed study of lambda Boo stars. Results. For the first time, we present the surprising finding of two lambda 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 lambda Boo star HD 36726 with other cluster members and showed that the lambda 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 lambda Boo peculiarity strongly depletes heavier metals, but could also slightly modify lighter abundances such as C and O. We also found that both lambda Boo stars belong to the periphery of their respective clusters. This would suggest that lambda 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 lambda Boo peculiarity. We also obtained a precise age determination for the lambda Boo stars HD 28548 (26.3 Myr) and HD 36726 (33.1 Myr). Conclusions. We have confirmed, for the first time, that two lambda Boo stars belong to open clusters. This remarkable finding could make open clusters excellent laboratories for studying the origin of lambda Boo stars.

18 pages, 14 figures, 15 tables, A&A accepted

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