Forbidden hugs in pandemic times

III. Observations of the luminous red nova AT 2021biy in the nearby galaxy NGC 4631^⋆

¹ Physics Department and Tsinghua Center for Astrophysics (THCA), Tsinghua University, Beijing 100084, PR China
e-mail: yzcai789@163.com, wang_xf@mail.tsinghua.edu.cn
² INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ School of Physics, O’Brien Centre for Science North, University College Dublin, Belfield, Dublin 4, Ireland
⁴ Beijing Planetarium, Beijing Academy of Science and Technology, Beijing 100044, PR China
⁵ Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
⁶ Departamento de Ciencias Fisicas, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago, Chile
⁷ Millennium Institute of Astrophysics (MAS), Nuncio Monsenor Sòtero Sanz 100, Providencia, Santiago 8320000, Chile
⁸ Nagaraj-Noll-Otelline Graduate Fellow, University of California, Berkeley, CA 94720, USA
⁹ Sternberg Astronomical Institute of the Moscow State University, Moscow 119234, Russia
¹⁰ Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnij Arkhyz 369167, Russia
¹¹ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, 08193 Barcelona, Spain
¹² The Department of Physics, The University of Auckland, Private Bag 92019, Auckland, New Zealand
¹³ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast, BT7 1NN Northern Ireland, UK
¹⁴ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
¹⁵ Department of Astronomy, The Ohio State University, 140 W. 18th Avenue, Columbus, OH 43210, USA
¹⁶ Center for Cosmology and AstroParticle Physics (CCAPP), The Ohio State University, 191 W.Woodruff Avenue, Columbus, OH 43210, USA
¹⁷ Las Cumbres Observatory, 6740 Cortona Dr. Suite 102, Goleta, CA 93117, USA
¹⁸ Department of Physics, University of California, Santa Barbara, CA 93106, USA
¹⁹ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138-1516, USA
²⁰ The NSF AI Institute for Artificial Intelligence and Fundamental Interactions, Cambridge, USA
²¹ School of Physics and Astronomy, University of Minnesota, 116 Church Street SE, Minneapolis, MN 55455, USA
²² Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
²³ Turku Collegium for Science, Medicine and Technology, University of Turku, 20014 Turku, Finland
²⁴ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
²⁵ School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 101408, PR China
²⁶ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
²⁷ Faculty of Physics, Moscow State University, 1 bldg. 2, Leninskie Gory, Moscow 119991, Russia
²⁸ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
²⁹ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild Str. 1, 85741 Garching, Germany
³⁰ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 København N, Denmark
³¹ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
³² Università degli Studi di Padova, Dipartimento di Fisica e Astronomia, Vicolo dell’Osservatorio 2, 35122 Padova, Italy
³³ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, PR China
³⁴ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, PR China
³⁵ Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
³⁶ Hiroshima Astrophysical Science Centre, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
³⁷ University of Chinese Academy of Sciences, Beijing 100049, PR China
³⁸ Finnish Centre for Astronomy with ESO (FINCA), 20014 University of Turku, Finland
³⁹ Aryabhatta Research Institute of Observational Sciences, Manora Peak, Nainital 263 001, India
⁴⁰ School of Physics and Astronomy, Sun Yat-sen University, Zhuhai 519082, PR China
⁴¹ Department of Physics, College of Physical Sciences and Technology, Hebei University, Baoding 071002, PR China
⁴² Key Laboratory of High-precision Computation and Application of Quantum Field Theory of Hebei Province, Hebei University, Baoding 071002, PR China

Received: 1 July 2022
Accepted: 12 August 2022

Abstract

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 ∼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 ∼1.6 × 10⁴¹ 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_BB ≈ 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 Hubble Space Telescope data taken on 2003 August 3 reveals a ∼20 M_⊙ progenitor candidate with log (L/L_⊙) = 5.0 dex and T_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 M_⊙ primary component.