AT 2025abao: The fourth luminous red nova in M 31;
: Reguitti, A.; Pastorello, A.; Valerin, G.; Romanov, F. D.; Siviero, A.; Cai, Y.-Z.; Ciroi, S.; Elias-Rosa, N.; Iijima, T.; Kankare, E.; Koivisto, N.; Kravtsov, T.; Mason, E.; Matilainen, K.; Mura, A. C.; Ochner, P.; Reynolds, T. M.; Stritzinger, M. D.
Publisher: EDP Sciences
: 2026
Astronomy and Astrophysics
: A25
: 709
: 0004-6361
: 1432-0746
DOI: https://doi.org/10.1051/0004-6361/202659456
: https://doi.org/10.1051/0004-6361/202659456
: https://research.utu.fi/converis/portal/detail/Publication/526064336
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 (Mg = −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.
galaxies: individual: M 31, stars: AGB and post-AGB, supernovae: individual: AT 2025abao, techniques: photometric, techniques: spectroscopic
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AR, GV, YZC acknowledge financial support from the SOXS project (PI S. Campana). AR, AP, GV, NER acknowledge financial support from the PRIN-INAF 2022 “Shedding light on the nature of gap transients: from the observations to the models”. NER acknowledges support from the Spanish Ministerio de Ciencia e Innovación and the Agencia Estatal de Investigación 10.13039/501100011033 under the program Unidad de Excelencia María de Maeztu CEX2020-001058-M. TMR acknowledges support from the Research Council of Finland project 350458 and the Cosmic Dawn Center, funded by the Danish National Research Foundation under grant DNRF140. MDS is funded by the Independent Research Fund Denmark (grant nr. 10.46540/2032-00022B). YZC is supported by the National Natural Science Foundation of China (Grant N. 12303054), the National Key Research and Development Program of China (N. 2024YFA1611603), the Yunnan Fundamental Research Projects (N. 202501AS070078), and the International Centre of Supernovae, Yunnan Key Laboratory (N. 202302AN360001). Based on observations collected at Copernico and Schmidt telescopes (Asiago Ekar, Italy) of the INAF-Osservatorio Astronomico di Padova, and at the Galileo telescope of the Padova Univesity (Asiago Pennar, Italy). Support to ATLAS was provided by NASA grant NN12AR55G. Based on observations made with the NOT, owned and operated jointly by Aarhus University, Turku University, Oslo University, the University of Iceland and Stockholm University, under program 72-305 (PI Reguitti), and via the NUTS2 collaboration which is supported in part by the Instrument Centre for Danish Astrophysics, and the Finnish Centre for Astronomy with ESO via Academy of Finland grant nr. 306531. Based on observations made with the GTC. Based in part on observations made at the TNG, operated by INAF, under program A50TAC_41 (PI Valerin). All three telescopes are installed at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, on the island of La Palma. We acknowledge the use of data from the Swift data archive. FR is grateful to iTelescope.Net for providing observing time to use their remote telescopes and to the AAVSO who made it possible to use the AAVSOnet telescopes. SPHEREx is a joint project of the Jet Propulsion Laboratory and the California Institute of Technology, and is funded by the National Aeronautics and Space Administration.
