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

Abstract

Luminous red novae (LRNe) are potential signatures of common envelope evolution (CEE), a critical yet poorly understood phase in compact binary formation. AT 2025abao, an LRN in the Andromeda galaxy, originated from an asymptotic giant branch (AGB) progenitor and exhibited a prolonged pre-outburst transient. Lacking direct constraints on the companion, however, the system's evolutionary fate remains unclear. Here, we unravel the binary nature of AT 2025abao by modeling its complete evolution, from precursor to final outcome. We show that the eight-year infrared precursor arises from a $\approx 1.4_{-0.3}^{+0.2}{M}_{\odot }$ main-sequence companion inspiralling through the wind of a $\approx 6.9M_{\odot }$ AGB star. As CEE commences, the companion plunges into the envelope and launches a shock. Radiation hydrodynamic modeling of this outburst constrains the total ejecta mass to $\approx 0.92M_{\odot }$ and energy to $\approx 1.25\times 10^{47}$erg, revealing that only $\approx 0.57M_{\odot }$ becomes unbound. Subsequent radiative transfer simulations successfully reproduce the evolving spectral energy distribution. Crucially, our models indicate the main-sequence companion and AGB core survive in an eccentric orbit without merging at least 400 days after CEE onset. AT 2025abao may therefore be the first observed CEE event caught forming a close compact binary, bridging a long-standing observational gap in compact binary formation.

arXiv commentAccepted by ApJL, comments are very welcome. softwares and data are listed by the urls below https://github.com/czlovemath123/guangqi1d https://raw.githubusercontent.com/czlovemath123/guangqi1d/master/modules/lrne/wfst.dat 10.5281/zenodo.23149482