Intermediate-luminosity red transients: Spectrophotometric properties and connection to electron-capture supernova explosions^⋆

¹ Physics Department and Tsinghua Center for Astrophysics (THCA), Tsinghua University, Beijing 100084, PR China
e-mail: yzcai@mail.tsinghua.edu.cn
² INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
e-mail: andrea.pastorello@inaf.it
³ Università degli Studi di Padova, Dipartimento di Fisica e Astronomia, Vicolo dell’Osservatorio 2, 35122 Padova, Italy
⁴ School of Physics, O’Brien Centre for Science North, University College Dublin, Belfield, Dublin 4, Ireland
e-mail: morgan.fraser@ucd.ie
⁵ INAF-Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁶ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Barcelona, Spain
⁷ Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100101, PR China
⁸ CAS Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
⁹ Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
¹⁰ Departamento de Ciencias Fisicas, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago, Chile
¹¹ Millennium Institute of Astrophysics (MAS), Nuncio Monsenor Sòtero Sanz 100, Providencia, Santiago, Chile
¹² Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast, BT7 1NN Northern Ireland, UK
¹³ Institute for Astronomy, University of Hawai’i at Manoa, 2680 Woodlawn Dr. Hawai’i, Honolulu, HI 96822, USA
¹⁴ Institute of Astrophysics of the Canary Islands, C/ Vía Láctea, s/n, Tenerife, Spain
¹⁵ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
¹⁶ Telescopio Nazionale Galileo, Fundación Galileo Galilei – INAF, Rambla José Ana Fernández Pérez, 7, 38712 Breña Baja, TF, Spain
¹⁷ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹⁸ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild Str. 1, 85741 Garching, Germany
¹⁹ Department of Applied Physics, University of Cádiz, Campus of Puerto Real, 11510 Cádiz, Spain
²⁰ School of Physics, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland
²¹ Department of Physics, Anhui Normal University, Wuhu, Anhui 241002, PR China
²² Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
²³ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA
²⁴ Department of Physics and Astronomy, University of California, 1 Shields Avenue, Davis, CA 95616-5270, USA
²⁵ School of Chemistry and Physics, University of KwaZulu-Natal, Westville Campus, Private Bag X54001, Durban 4000, South Africa
²⁶ NAOC-UKZN Computational Astrophysics Centre (NUCAC), University of KwaZulu-Natal, Durban 4000, South Africa
²⁷ Beijing Planetarium, Beijing Academy of Science and Technology, Beijing 100044, PR China
²⁸ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010 6500 GL Nijmegen, The Netherlands
²⁹ SRON, Netherlands Institute for Space Research, Sorbonnelaan, 2, 3584CA Utrecht, The Netherlands
³⁰ Physics Department, Technion, Haifa 32000, Israel
³¹ Departamento de Astronomía, Universidad de Chile, Camino del Observatorio 1515, Las Condes, Santiago, Chile
³² Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, 08193 Barcelona, Spain
³³ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
³⁴ Xinjiang Astronomical Observatory, 150 Science-1 Street, Urumqi 830011, PR China
³⁵ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
³⁶ Department of Astronomy, Shanghai Jiao Tong University, Shanghai 200240, PR China
³⁷ School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
³⁸ Itagaki Astronomical Observatory, Yamagata, Yamagata 990-2492, Japan
³⁹ Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
⁴⁰ Gemini Observatory/NSF’s NOIRLab, Casilla 603, La Serena, Chile
⁴¹ INAF-Osservatorio Astronomico d’Abruzzo, via M. Maggini snc, 64100 Teramo, Italy
⁴² The Oskar Klein Centre, Department of Physics, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
⁴³ Xingming Observatory, Mount Nanshan, Xinjiang, PR China

Received: 14 April 2021
Accepted: 29 July 2021

Abstract

We present the spectroscopic and photometric study of five intermediate-luminosity red transients (ILRTs), namely AT 2010dn, AT 2012jc, AT 2013la, AT 2013lb, and AT 2018aes. They share common observational properties and belong to a family of objects similar to the prototypical ILRT SN 2008S. These events have a rise time that is less than 15 days and absolute peak magnitudes of between −11.5 and −14.5 mag. Their pseudo-bolometric light curves peak in the range 0.5–9.0 × 10⁴⁰ erg s⁻¹ and their total radiated energies are on the order of (0.3–3) × 10⁴⁷ erg. After maximum brightness, the light curves show a monotonic decline or a plateau, resembling those of faint supernovae IIL or IIP, respectively. At late phases, the light curves flatten, roughly following the slope of the ⁵⁶Co decay. If the late-time power source is indeed radioactive decay, these transients produce ⁵⁶Ni masses on the order of 10⁻⁴ to 10⁻³ M_⊙. The spectral energy distribution of our ILRT sample, extending from the optical to the mid-infrared (MIR) domain, reveals a clear IR excess soon after explosion and non-negligible MIR emission at very late phases. The spectra show prominent H lines in emission with a typical velocity of a few hundred km s⁻¹, along with Ca II features. In particular, the [Ca II] λ7291,7324 doublet is visible at all times, which is a characteristic feature for this family of transients. The identified progenitor of SN 2008S, which is luminous in archival Spitzer MIR images, suggests an intermediate-mass precursor star embedded in a dusty cocoon. We propose the explosion of a super-asymptotic giant branch star forming an electron-capture supernova as a plausible explanation for these events.