Forbidden hugs in pandemic times

IV. Panchromatic evolution of three luminous red novae^⋆

¹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
e-mail: andrea.pastorello@inaf.it
² Universitá degli Studi di Padova, Dipartimento di Fisica e Astronomia, Vicolo dell’Osservatorio 2, 35122 Padova, Italy
³ School of Physics, O’Brien Centre for Science North, University College Dublin, Belfield, Dublin 4, Ireland
⁴ Instituto de astrofísica, Facultad de Ciencias Exactas, Universidad Andres Bello, Fernández Concha 700, Las Condes, Santiago, Chile
⁵ Millennium Institute of Astrophysics (MAS), Nuncio Monsenor Sótero Sanz 100, Providencia, Santiago 8320000, Chile
⁶ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Barcelona, Spain
⁷ Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
⁸ Miller Institute for Basic Research in Science, University of California, Berkeley, CA 94720, USA
⁹ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
¹⁰ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
¹¹ Cosmic Dawn Center (DAWN), Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 København N, Denmark
¹² Department of Physics and Astronomy, University of California, Davis, 1 Shields Avenue, Davis, CA 95616-5270, USA
¹³ Gemini Observatory, 670 North A’ohoku Place, Hilo, HI 96720-2700, USA
¹⁴ Department of Physics, Virginia Tech, 850 West Campus Drive, Blacksburg, VA 24061, USA
¹⁵ DIRAC Institute, Department of Astronomy, University of Washington, 3910 15th Avenue NE, Seattle, WA 98195-0002, USA
¹⁶ Las Cumbres Observatory, 6740 Cortona Dr. Suite 102, Goleta, CA 93117, USA
¹⁷ Department of Physics, University of California, Santa Barbara, CA 93106, USA
¹⁸ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, PR China
¹⁹ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, PR China
²⁰ Physics Department and Tsinghua Center for Astrophysics (THCA), Tsinghua University, Beijing 100084, PR China
²¹ School of Physics, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland
²² European Centre for Theoretical Studies in Nuclear Physics and Related Areas (ECT*), Fondazione Bruno Kessler, 38123, Trento, Italy
²³ INFN-TIFPA, Trento Institute for Fundamental Physics and Applications, Via Sommarive 14, 38123 Trento, Italy
²⁴ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
²⁵ Institut d’Estudis Espacials de Catalunya (IEEC), 08034, Barcelona, Spain
²⁶ Aryabhatta Research Institute of observational sciencES, Manora Peak, Nainital 263 002, India
²⁷ Hiroshima Astrophysical Science Centre, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
²⁸ Gran Telescopio Canarias (GRANTECAN), Cuesta de San José s/n, 38712 Breña Baja, La Palma, Spain
²⁹ Instituto de Astrofísica de Canarias, Vía Láctea s/n, 38200 La Laguna, Tenerife, Spain
³⁰ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, 20014 Turku, Finland
³¹ Department of Physics and Astronomy, University of North Carolina, 120 East Cameron Avenue, Chapel Hill, NC 27599, USA
³² Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138-1516, USA
³³ The NSF AI Institute for Artificial Intelligence and Fundamental Interactions, Cambridge, MA 02139-4307, USA
³⁴ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
³⁵ Department of Physics, Florida State University, 77 Chieftan Way, Tallahassee, FL 32306, USA
³⁶ Department of Physics and Astronomy, Rutgers, The State University of New Jersey, 136 Frelinghuysen Road, Piscataway, NJ 08854, USA
³⁷ Turku Collegium for Science, Medicine and Technology, University of Turku, 20014 Turku, Finland
³⁸ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, Evanston, IL 60208, USA
³⁹ Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
⁴⁰ School of Astronomy and Space Science, University of Chinese Academy of Sciences, Beijing 101408, PR China
⁴¹ W. M. Keck Observatory, 65-1120 Ma-malahoa Highway, Kamuela, HI 96743-8431, USA
⁴² The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
⁴³ Astrophysics Research Institute, Liverpool John Moores University, ic2, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁴⁴ Max-Planck Institut für Astrophysik, Karl-Schwarzschild-Str. 1, 85741 Garching, Germany
⁴⁵ Department of Applied Physics, University of Cádiz, Campus of Puerto Real, 11510 Cádiz, Spain
⁴⁶ Las Campanas Observatory, Carnegie Observatories, Casilla 601, La Serena, Chile
⁴⁷ Graduate Institute of Astronomy, National Central University, 300 Zhongda Road, Zhongli, Taoyuan 32001, Taiwan
⁴⁸ The Observatories of the Carnegie Institution for Science, 813 Santa Barbara St., Pasadena, CA 91101, USA
⁴⁹ Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
⁵⁰ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁵¹ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA
⁵² Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK
⁵³ Chinese Academy of Sciences, South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100101, PR China
⁵⁴ Beijing Planetarium, Beijing Academy of Science and Technology, Beijing 100044, PR China
⁵⁵ Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA

Received: 4 August 2022
Accepted: 13 December 2022

Abstract

We present photometric and spectroscopic data on three extragalactic luminous red novae (LRNe): AT 2018bwo, AT 2021afy, and AT 2021blu. AT 2018bwo was discovered in NGC 45 (at about 6.8 Mpc) a few weeks after the outburst onset. During the monitoring period, the transient reached a peak luminosity of 10⁴⁰ erg s⁻¹. AT 2021afy, hosted by UGC 10043 (∼49.2 Mpc), showed a double-peaked light curve, with the two peaks reaching a similar luminosity of 2.1(±0.6)×10⁴¹ erg s⁻¹. Finally, for AT 2021blu in UGC 5829 (∼8.6 Mpc), the pre-outburst phase was well-monitored by several photometric surveys, and the object showed a slow luminosity rise before the outburst. The light curve of AT 2021blu was sampled with an unprecedented cadence until the object disappeared behind the Sun, and it was then recovered at late phases. The light curve of LRN AT 2021blu shows a double peak, with a prominent early maximum reaching a luminosity of 6.5 × 10⁴⁰ erg s⁻¹, which is half of that of AT 2021afy. The spectra of AT 2021afy and AT 2021blu display the expected evolution for LRNe: a blue continuum dominated by prominent Balmer lines in emission during the first peak, and a redder continuum consistent with that of a K-type star with narrow absorption metal lines during the second, broad maximum. The spectra of AT 2018bwo are markedly different, with a very red continuum dominated by broad molecular features in absorption. As these spectra closely resemble those of LRNe after the second peak, AT 2018bwo was probably discovered at the very late evolutionary stages. This would explain its fast evolution and the spectral properties compatible with that of an M-type star. From the analysis of deep frames of the LRN sites years before the outburst, and considerations of the light curves, the quiescent progenitor systems of the three LRNe were likely massive, with primaries ranging from about 13 M_⊙ for AT 2018bwo, to 14₋₁⁺⁴ M_⊙ for AT 2021blu, and over 40 M_⊙ for AT 2021afy.