Red eminence: The intermediate-luminosity red transient AT 2022fnm

¹ Department of Physics and Astronomy, University of Turku, Vesilinnantie 5 20500, Finland
² School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
³ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, PR China
⁵ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, PR China
⁶ International Centre of Supernovae, Yunnan Key Laboratory, Kunming 650216, PR China
⁷ Dipartimento di Fisica e Astronomia “G. Galilei”, Università degli studi di Padova Vicolo dell’Osservatorio 3, 35122 Padova, Italy
⁸ School of Sciences, European University Cyprus, Diogenes Street, Engomi, 1516 Nicosia, Cyprus
⁹ European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago, Chile
¹⁰ Institut d’Estudis Espacials de Catalunya (IEEC), Gran Capità, 2-4, Edifici Nexus, Desp. 201, 08034 Barcelona, Spain
¹¹ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, 08193 Barcelona, Spain
¹² INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
¹³ Oskar Klein Centre, Department of Astronomy, Stockholm University, Albanova University Centre, 106 91 Stockholm, Sweden
¹⁴ Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
¹⁵ Physics Department, Tsinghua University, Beijing 100084, China
¹⁶ Beijing Planetarium, Beijing Academy of Science and Technology, Beijing 100044, China

Received: 10 October 2023
Accepted: 8 May 2024

Abstract

We present results from a five-month-long observing campaign of the unusual transient AT 2022fnm, which displays properties common to both luminous red novae (LRNe) and intermediate-luminosity red transients (ILRTs). Although its photometric evolution is broadly consistent with that of LRNe, no second peak is apparent in its light curve, and its spectral properties are more reminiscent of ILRTs. It has a fairly rapid rise time of 5.3 ± 1.5 d, reaching a peak absolute magnitude of −12.7 ± 0.1 (in the ATLAS o band). We find some evidence for circumstellar interaction, and a near-infrared excess becomes apparent at approximately +100 d after discovery. We attribute this to a dust echo. Finally, from an analytical diffusion toy model, we attempted to reproduce the pseudo-bolometric light curve and find that a mass of ∼4 M_⊙ is needed. Overall, the characteristics of AT 2022fnm are consistent with a weak stellar eruption or an explosion reminiscent of low-energy type IIP supernovae, which is compatible with expectations for ILRTs.