Observations of the luminous red nova AT 2021biy in the nearby galaxy NGC 4631
Creators: name: Cai, Y. -Z.
name: Pastorello, A.
name: Fraser, M.
name: Wang, X. -F.
name: Filippenko, A. V.
name: Reguitti, A.
name: Patra, K. C.
name: Goranskij, V. P.
name: Barsukova, E. A.
name: Brink, T. G.
name: Elias-Rosa, N.
name: Stevance, H. F.
name: Zheng, W.
name: Yang, Y.
name: Atapin, K. E.
name: Benetti, S.
name: de Boer, T. J. L.
name: Bose, S.
name: Burke, J.
name: Byrne, R.
name: Cappellaro, E.
name: Chambers, K. C.
name: Chen, W. -L.
name: Emami, N.
name: Gao, H.
name: Hiramatsu, D.
name: Howell, D. A.
name: Huber, M. E.
name: Kankare, E.
name: Kelly, P. L.
name: Kotak, R.
name: Kravtsov, T.
name: Lander, V. Yu.
name: Li, Z. -T.
name: Lin, C. -C.
name: Lundqvist, P.
name: Magnier, E. A.
name: Malygin, E. A.
name: Maslennikova, N. A.
name: Matilainen, K.
name: Mazzali, P. A.
name: McCully, C.
name: Mo, J.
name: Moran, S.
name: Newsome, M.
name: Oparin, D. V.
name: Gonzalez, E. Padilla
name: Reynolds, T. M.
name: Shatsky, N. I.
name: Smartt, S. J.
name: Smith, K. W.
name: Stritzinger, M. D.
name: Tatarnikov, A. M.
name: Terreran, G.
name: Uklein, R. I.
name: Valerin, G.
name: Vallely, P. J.
name: Vozyakova, O. V.
name: Wainscoat, R.
name: Yan, S. -Y.
name: Zhang, J. -J.
name: Zhang, T. -M.
name: Zheltoukhov, S. G.
name: Dastidar, R.
name: Fulton, M.
name: Galbany, L.
name: Gangopadhyay, A.
name: Ge, H. -W.
name: Gutiérrez, C. P.
name: Lin, H.
name: Misra, K.
name: Ou, Z. -W.
name: Salmaso, I.
name: Tartaglia, L.
name: Xiao, L.
name: Zhang, X. -H.
Publisher: name: arXiv
Year: 2022
Subjects: subject: Solar and Stellar Astrophysics (astro-ph.SR)
subject: Astrophysics of Galaxies (astro-ph.GA)
subject: High Energy Astrophysical Phenomena (astro-ph.HE)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: We present an observational study of the luminous red nova (LRN) AT\,2021biy in the nearby galaxy NGC\,4631. The field of the object was routinely imaged during the pre-eruptive stage by synoptic surveys, but the transient was detected only at a few epochs from $\sim 231$\,days before maximum brightness. The LRN outburst was monitored with unprecedented cadence both photometrically and spectroscopically. AT\,2021biy shows a short-duration blue peak, with a bolometric luminosity of $\sim 1.6 \times 10^{41}$\,erg\,s$^{-1}$, followed by the longest plateau among LRNe to date, with a duration of 210\,days. A late-time hump in the light curve was also observed, possibly produced by a shell-shell collision. AT\,2021biy exhibits the typical spectral evolution of LRNe. Early-time spectra are characterised by a blue continuum and prominent H emission lines. Then, the continuum becomes redder, resembling that of a K-type star with a forest of metal absorption lines during the plateau phase. Finally, late-time spectra show a very red continuum ($T_{\mathrm{BB}} \approx 2050$ K) with molecular features (e.g., TiO) resembling those of M-type stars. Spectropolarimetric analysis indicates that AT\,2021biy has local dust properties similar to those of V838\,Mon in the Milky Way Galaxy. Inspection of archival {\it Hubble Space Telescope} data taken on 2003 August 3 reveals a $\sim 20$\,\msun\ progenitor candidate with log\,$(L/{\rm L}_{\odot}) = 5.0$\,dex and $T_{\rm{eff}} = 5900$\,K at solar metallicity. The above luminosity and colour match those of a luminous yellow supergiant. Most likely, this source is a close binary, with a 17--24\,\msun\ primary component.
description: 21 pages, 14 figures. Accepted by Astronomy and Astrophysics