The Chemical Homogeneity of Single-Lined Spectroscopic Binaries in Open Clusters

TL;DR

This study investigates the chemical homogeneity of single-lined spectroscopic binaries in open clusters using APOGEE data, revealing that most are chemically similar to single stars, but some show signs of pollution affecting age estimates.

Contribution

It provides the first detailed comparison of chemical abundances between SB1 binaries and single stars in open clusters, demonstrating chemical homogeneity despite binarity.

Findings

SB1s are generally chemically homogeneous with single stars.

Some binaries with UV excess show elevated [C/N], affecting age estimates.

No correlation between orbital separation and chemical enrichment.

Abstract

Using SDSS-V DR19 Milky Way Mapper APOGEE data, we measure the impact that close binarity has on surface chemistry across the Hertzsprung-Russell diagram in a broad set of abundances by studying single-lined spectroscopic binaries (SB1s) in open clusters. We derive binary membership and orbital parameters for 103 SB1s by analysing APOGEE radial velocities with The Joker and UltraNest. We perform a detailed abundance analysis with BACCHUS to derive abundances in fourteen chemical species: Si, Fe, C, N, O, Na, Mg, Al, Ca, Ti, Cr, Ni, Ce, and Nd. Leveraging the assumptions of chemical homogeneity in open clusters, we compare the surface abundances of SB1s to non-binary stars at similar evolutionary states. We find that a subset of binaries with significant UV excess have a $\Delta$[C/N] that is 0.2--0.5 dex higher than expected, resulting in overestimated [C/N]-based ages for those stars. This points to pollution from an evolved companion and has implications for [C/N]-based age studies of the broader Milky Way. At the population level, we find that SB1s in our sample can be treated as statistically chemically homogeneous with their single-star counterparts, and we find no connection between orbital separation and chemical enrichment or depletion. We show that at separations up to ~5 pc, co-eval stars can be considered chemically homogeneous with one another within current abundance precisions, regardless of multiplicity.