Unravelling the Nature of AT\,2025abao: From Precursor to Final Fate

Luminous red novae are potential signatures of common envelope evolution, a critical but poorly understood phase in the formation of compact binaries. AT 2025abao, a luminous red nova in the Andromeda galaxy, originated from an asymptotic giant branch progenitor and displayed a prolonged pre-outburst transient, but the lack of direct constraints on its companion left the system's evolutionary fate unclear.

To address this, the researchers modeled the system's complete evolution from precursor to final outcome. They show that the eight-year infrared precursor arose from an approximately 1.4 (+0.2/-0.3) solar-mass main-sequence companion inspiralling through the wind of an approximately 6.9 solar-mass AGB star. As common envelope evolution commenced, the companion plunged into the envelope and launched a shock, which the team modeled with radiation hydrodynamics.

The outburst modeling constrains the total ejecta mass to about 0.92 solar masses and the energy to about 1.25 × 10^47 erg, with only about 0.57 solar masses becoming unbound. Subsequent radiative transfer simulations successfully reproduced the evolving spectral energy distribution. Crucially, the models indicate that the main-sequence companion and the AGB core survive in an eccentric orbit without merging at least 400 days after common envelope evolution onset.

AT 2025abao may therefore be the first observed common envelope evolution event caught in the act of forming a close compact binary, bridging a long-standing observational gap in compact binary formation.