Letter to the Editor

Disentangling the origin of chemical differences using GHOST

¹ Instituto de Ciencias Astronómicas, de la Tierra y del Espacio (ICATE-CONICET), C.C 467, 5400 San Juan, Argentina
e-mail: saffe.carlos@gmail.com
² Universidad Nacional de San Juan (UNSJ), Facultad de Ciencias Exactas, Físicas y Naturales (FCEFN), San Juan, Argentina
³ Instituto de Investigación Multidisciplinar en Ciencia y Tecnología, Universidad de La Serena, Raúl Bitrán 1305, La Serena, Chile
⁴ Departamento de Física y Astronomía, Universidad de La Serena, Av. Cisternas 1200 N, La Serena, Chile
⁵ Gemini Observatory/NSF’s NOIRLab, Casilla 603, La Serena, Chile
⁶ Observatorio Astronómico de Córdoba (OAC), Laprida 854, X5000BGR Córdoba, Argentina
⁷ Laboratório Nacional de Astrofísica (LNA/MCTI), Rua Estados Unidos 154, Itajubá, MG, Brazil
⁸ The Observatories of the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101, USA
⁹ Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina

Received: 18 January 2024
Accepted: 13 February 2024

Abstract

Aims. We explore different scenarios to explain the chemical difference found in the remarkable giant-giant binary system HD 138202 + CD−30 12303. For the first time, we suggest how to distinguish these scenarios by taking advantage of the extensive convective envelopes of giant stars.

Methods. We carried out a high-precision determination of stellar parameters and abundances by applying a full line-by-line differential analysis on GHOST high-resolution spectra. We used the FUNDPAR program with ATLAS12 model atmospheres and specific opacities calculated for an arbitrary composition through a doubly iterated method. Physical parameters were estimated with the isochrones package and evolutionary tracks were calculated via MIST models.

Results. We found a significant chemical difference between the two stars (Δ[Fe/H] ∼ 0.08 dex), which is largely unexpected considering the insensitivity of giant stars to planetary ingestion and diffusion effects. We tested the possibility of engulfment events by using several different combinations of stellar mass, ingested mass, metallicity of the engulfed object and different convective envelopes. However, the planetary ingestion scenario does not seem to explain the observed differences. For the first time, we distinguished the source of chemical differences using a giant-giant binary system. By ruling out other possible scenarios such as planet formation and evolutionary effects between the two stars, we suggest that primordial inhomogeneities might explain the observed differences. This remarkable result implies that the metallicity differences that were observed in at least some main-sequence binary systems might be related to primordial inhomogeneities rather than engulfment events. We also discuss the important implications of finding primordial inhomogeneities, which affect chemical tagging and other fields such as planet formation. We strongly encourage the use of giant-giant pairs. They are a relevant complement to main-sequence pairs for determining the origin of the observed chemical differences in multiple systems.