Evolution Models of CO White Dwarf–Asymptotic Giant Branch Star Merger Remnants

Abstract Common envelope evolution is a critical but still poorly understood phase in binary evolution. It plays a key role in forming close binaries such as hot subdwarfs, double white dwarfs (WDs), X-ray binaries, and double neutron stars. However, its outcomes remain highly uncertain. Depending on the efficiency of envelope ejection, a system may either survive as a close binary or undergo a complete merger. In this work, we investigate the postmerger evolution of systems where a CO WD merges with the core of an asymptotic giant branch (AGB) star. A grid of merger remnant models with various core and envelope masses is constructed. At the onset of evolution, the CO core contracts and undergoes off-center carbon ignition, producing an inwardly propagating carbon flame. For remnants with a relatively low mass of CO core, the flame phase is followed by core contraction and subsequent H shell burning. For more massive CO cores, the carbon flame reaches the center and is soon followed by off-center neon burning, which is expected to eventually lead to core-collapse supernovae. The merger remnants occupy nearly the same region on the H-R diagram as ordinary AGB or super-AGB stars, exhibiting similar surface properties. Although their surface abundance may differ slightly from those of normal AGB stars, depending on the initial core and envelope masses, these differences are strongly reduced once mass loss is taken into account. We suggest that some giant-like stars, including candidates for Thorne–Żytkow objects (e.g., HV 2112), might alternatively be explained as AGB–WD merger remnants.