Probing stellar mergers through asteroseismology and models of carbon-deficient red giants

Binary and multiple-star systems account for about half of all solar‑type stars, and thus contribute significantly to Galactic chemical enrichment. Studying binary products is key to understanding their complex outcomes. We show here that the enigmatic weak G-band stars, also known as carbon-deficient giants (CDGs), are most likely binary merger products and offer a unique opportunity to study stellar mergers.

CDGs are rare G- and K-type giants with an extreme deficiency of carbon that cannot be explained by single-star evolution alone. While the first dredge-up reduces ¹²C by only 30%, the depletion in CDGs is far more extreme. Previous attempts to determine their masses and evolutionary states were inconclusive due to degeneracies between the stellar parameters of the SGB, RGB, RC, and EAGB phases  in the HR diagram.

We use asteroseismology combined with spectroscopy, and astrometry to decipher the nature of CDGs. From 129 CDGs observed by Kepler, K2, and TESS, we detect solar-like oscillations in 43 stars. By measuring νmax and applying seismic scaling relations, we derive precise masses and find that 79% are low-mass (M ≲ 2 M⊙). Furthermore, the seismic HR diagram reveals that 98% of these stars are in the core helium-burning or EAGB phase.

Combined with orbital constraints, we identify mergers of helium white dwarfs with RGB stars, possibly in hierarchical triples, as the dominant formation channel. We show initial comparisons between predictions from merger and envelope‑stripping models and asteroseismic observations. Future measurements of Δν, 𝜀, 𝛿𝜈01, 𝛿𝜈02 and ΔΠ1 will help resolve remaining degeneracies and constrain internal structure, shedding new light on a century‑old puzzle in stellar evolution.