2025A&A...703A.191K

Context. The canonical formation of second-generation (G2) stars in globular clusters (GCs) from gas enriched and ejected by G1 (primordial) polluters faces substantial challenges, namely (i) a mass-budget problem and (ii) uncertainty in the source(s) of the abundance anomaly of light elements (AALE) in G2 stars. The merger of G1 low-mass main-sequence (MS) binaries can overcome (i), but its ability to result in AALE remains unverified.
Aims. We present evidence of the merger process to explain AALE, based on highly probable merger remnants in the Galactic disc.
Methods. We focus on carbon-deficient red clump giants with low masses of 1.0 M_⊙<M ≲ 2.0 M_⊙, and hot He-intermediate subdwarfs of super-solar metallicity, both exhibiting G2-like AALE that are incompatible with a GC origin.
Results. The origin of such rare core He-burning stars as mergers of [MS star (MSS)]+[He white dwarf (HeWD)] binaries, which evolved from low-mass, high-mass ratio (MSS+MSS) binaries, is supported by models that evolve them to either horizontal branch (HB) stars or He subdwarfs via the red giant branch (RGB). Such binaries in the GC NGC 362 contain very young (∼4 Myr), extremely low-mass HeWDs, in contrast to much older (∼100 times) counterparts in open clusters. This agrees with the impact of the GC environment on the lifetime of hard binaries: (MSS+HeWD) systems merge there soon after forming from (MSS+MSS) binaries that have undergone the common envelope stage of evolution. Based on the number and lifetime of the (MSS+HeWD) binaries uncovered in NGC 362, the expected fraction of their progeny G2 RGB stars is estimated to be ≲ 10%.
Conclusions. The field merger remnants with G2-like AALE support the merger nature of at least a fraction of G2 stars in GCs. The specific channel [(MSS+MSS) - (MSS+HeWD) - merger product], supported by observations and models, is tentatively identified as the formation channel of the extreme G2 RGB component in GCs.