AT 2025abao: The fourth luminous red nova in M 31

¹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy
² INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate (LC), Italy
³ American Association of Variable Star Observers (AAVSO), 185 Alewife Brook Parkway, Suite 410 Cambridge, MA 02138, USA
⁴ Università degli Studi di Padova, Dipartimento di Fisica e Astronomia, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁵ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, P.R. China
⁶ International Centre of Supernovae, Yunnan Key Laboratory, Kunming 650216, P.R. China
⁷ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Barcelona, Spain
⁸ Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
⁹ INAF – Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34143 Trieste, Italy
¹⁰ Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 København N, Denmark
¹¹ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 15 February 2026
Accepted: 24 March 2026

Abstract

We present photometric and spectroscopic observations of the luminous red nova (LRN) AT 2025abao, the fourth discovered in M 31. The LRN, associated with the asymptotic giant branch (AGB) star WNTR23bzdiq, was discovered during the fast rise following the minimum phase. It reached its peak at g = 15.1 mag (M_g = −9.5 ± 0.1 mag), and then it settled onto a long-duration plateau in the red bands, lasting 70 days, while it was slowly linearly declining in the blue bands. At the peak the object showed similarities with the canonical LRNe V838 Monocerotis, V1309 Scorpii, and the faint and fast-evolving AT 2019zhd, which is the third LRN in M31, though the later evolution is different. Spectroscopically, AT 2025abao evolved as a canonical LRN: the early spectra present a blue continuum with narrow Balmer lines in emission; at the peak, the spectral continuum had cooled to a yellow colour, with a photospheric temperature of 6000 K. Balmer lines had weakened, while absorption lines from metals (Fe I, Fe II, Sc II, Ba II, Ti II) had developed, and they were particularly broad from the UV Ca II H&K lines. Medium- and high-resolution spectra reveal narrow (∼50 km s⁻¹) absorption and broad (∼450 km s⁻¹) emission profiles in the Balmer lines, from a slower wind and a faster outflow, respectively. Finally, late-time spectra show an orange continuum (T ∼ 4000 − 5000 K), a return in strength of the Balmer lines and the formation of molecular absorption bands. AT 2025abao is the rare case of an LRN with detailed archival information regarding the progenitor system. For the first time, we obtained the spectral energy distribution in the infrared of the precursor of an LRN, which is consistent with that of an M giant/AGB. We propose that the dichotomy of light-curve behaviour in LRNe (two peaks vs. plateau) can be explained by the extent and H-richness of the common envelope.