Planet engulfment in the chemically anomalous HD 129171/HD 129209 pair

¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
² Universidade de São Paulo, Instituto de Astronomia, Geofísica e Ciências Atmosféricas (IAG/USP), Departamento de Astronomia, Rua do Matão 1226, Cidade Universitária, 05508-090, SP, Brazil
³ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
⁵ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China
⁶ School of Physics and Astronomy, Monash University, Clayton 3800, Australia
⁷ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 22 February 2026
Accepted: 21 May 2026

Abstract

Context. Binary systems composed of stars with similar parameters are expected to have identical chemical composition. However, many chemically anomalous pairs have been found in the literature, such as the binary HD 129171/HD 129209. It is still unclear whether these anomalies originate from inhomogeneities of protostellar clouds, with important implications for chemical tagging and theories of star formation, or if they are caused by a planet engulfment event suffered by one binary component.

Aims. In this work, we measured precise differential abundances for the system HD 129171/HD 129209 to explore the planet engulfment hypothesis proposed in the literature. We focused on the Be abundance in particular, showing that this element can serve as a diagnostic of engulfment events for solar-type stars.

Methods. Atmospheric parameters were determined imposing spectroscopic equilibrium of iron lines. Masses and ages were estimated with the isochronal method. The Li, Be, N, and O abundances were determined via spectral synthesis. Other elemental abundances (up to Zn) were determined by equivalent width measurements. The spectra adopted in the analysis was gathered using UVES/ESO.

Results. We confirm the large difference in [Fe/H] (0.120 ± 0.004 dex) and A(Li) (−1.00 ± 0.02 dex) among the members of the pair, as well as the trend between differential abundances and condensation temperature of the elements. The binary system also shows detectable differences in Be abundances (−0.20 ± 0.04 dex). The abundance pattern of the pair is reasonably reproduced by an engulfment model of 11.2 M_⊕ of rocky material.

Conclusions. The difference in chemical abundances of the HD 129171/HD 129209 pair provides strong evidence in favor of the planet engulfment scenario. In this context, the detection of a Be difference among chemically inhomogeneous binary systems can be used as a diagnostic of rocky material ingestion experienced by a member of the pair.