Stellar masses and mass ratios for Gaia open cluster members

¹ Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona (UB), C Martí i Franquès, 1, 08028 Barcelona, Spain
² Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (UB), C Martí i Franquès, 1, 08028 Barcelona, Spain
³ Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC, 08860 Castelldefels (Barcelona), Spain
⁴ Astrophysics Division, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China
⁵ School of Astronomy and Space Science, University of Chinese Academy of Sciences, No. 19A, Yuquan Road, Beijing 100049, China
⁶ Universidade da Coruña (UDC), Department of Computer Science and Information Technologies, Campus de Elviña s/n, 15071, A Coruña, Galiza, Spain

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

Received: 2 October 2025
Accepted: 5 December 2025

Abstract

Context. Unresolved binaries in star clusters can bias stellar and cluster mass estimates. A proper treatment is therefore essential for studying the cluster dynamics and evolution.

Aims. We develop a fast and robust framework for jointly deriving stellar masses and multiplicity statistics of member stars, together with optimal cluster parameters.

Methods. We used Gaia DR3 parallaxes together with multi-band photometry of open cluster (OC) members to infer stellar masses and binary mass ratios through simulation-based inference (SBI), while iteratively fitting the cluster parameters. The validation of our SBI framework on simulated clusters demonstrates that the inclusion of infrared photometry significantly improves the detection of low mass-ratio binaries. The minimum mass-ratio threshold for reliably identifying unresolved binaries depends on the cluster properties and the available photometry, but typically lies below q = 0.5.

Results. Applying our method to 42 well-populated OCs, we derived a catalogue of stellar masses and mass ratios for 27 201 stars achieving typical uncertainties of 0.08 in q and 0.01 M_⊙ in the primary stellar mass. We analysed the archetype OCs M67 and NGC 2360 in detail, including mass segregation and mass-ratio distribution, and derived multiplicity fractions for the rest of the sample. We find evidence that the high mass ratio (q ≥ 0.6) binary fraction is strongly correlated with the age and weakly anti-correlated with the cluster metallicity. Furthermore, the variation in the binary fraction with stellar mass in OCs strongly agrees with the observed dependence for field stars with masses higher than ≳0.6 M_⊙.

Conclusions. Our work paves a path for future population-level investigations of multiplicity statistics and precision stellar masses in extended OC samples.