A study in scarlet

I. Photometric properties of a sample of intermediate-luminosity red transients

¹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy
² INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
³ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, PR China
⁴ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, P.R. China
⁵ International Centre of Supernovae, Yunnan Key Laboratory Kunming 650216, PR China
⁶ Graduate Institute of Astronomy, National Central University, 300 Jhongda Road, 32001 Jhongli, Taiwan
⁷ SRON, Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA, Leiden, The Netherlands
⁸ Department of Astrophysics/IMAPP, Radboud University Nijmegen, P.O. Box 9010 6500 GL, Nijmegen, The Netherlands
⁹ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, E-08193 Barcelona, Spain
¹⁰ School of Physics, O’Brien Centre for Science North, University College Dublin, Belfield, Dublin 4, Ireland
¹¹ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
¹² Hiroshima Astrophysical Science Center, Hiroshima University, Higashi-Hiroshima, Japan
¹³ Department of Physics, Florida State University, 77 Chieftan Way, Tallahassee, FL 32306, USA
¹⁴ Las Cumbres Observatory, 6740 Cortona Dr. Suite 102, Goleta, CA 93117, USA
¹⁵ Department of Physics, University of California, Santa Barbara, CA 93106, USA
¹⁶ School of Physics & Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
¹⁷ INAF, Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, I-80131 Napoli, Italy
¹⁸ DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen, Denmark
¹⁹ Caltech, Mail Code 220-6, Pasadena, CA 91125, USA
²⁰ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
²¹ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
²² Max-Planck-Institut für Astrophysik, Karl-Schwarzschild Str. 1, D-85748 Garching, Germany
²³ Aryabhatta Research Institute of observational sciences, Manora Peak, Nainital 263001, India
²⁴ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
²⁵ School of Physics, Trinity College Dublin, College Green, Dublin 2, Ireland
²⁶ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA
²⁷ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
²⁸ Astrophysics Research Centre, School of Mathematics and Physics, Queens University Belfast Belfast BT7 1NN, UK,
²⁹ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark
³⁰ INAF – Osservatorio Astronomico d’Abruzzo, via M. Maggini snc, Teramo I-64100, Italy
³¹ Department of Physics, University of California, Davis, CA 95616, USA
³² European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago, Chile
³³ Millennium Institute of Astrophysics, Nuncio Monsenor Sótero Sanz 100, Providencia, 8320000 Santiago, Chile
³⁴ INAF-Osservatorio Astrofisico di Catania, Via Santa Sofia 78, I-95123 Catania, Italy
³⁵ Instituto de Astrofísica, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago RM, Chile
³⁶ ICRANet, Piazza della Repubblica 10, I-65122 Pescara, Italy
³⁷ Institut für Theoretische Physik, Goethe Universität, Max-von-Laue-Str. 1, 60438 Frankfurt am Main, Germany
³⁸ INFN-TIFPA, Trento Institute for Fundamental Physics and Applications, Via Sommarive 14, I-38123 Trento, Italy
³⁹ Institut d’Estudis Espacials de Catalunya (IEEC), E-08034 Barcelona, Spain
⁴⁰ Center for Astrophysics, Harvard & Smithsonian, Cambridge, Massachusetts, MA 02138, USA
⁴¹ The NSF AI Institute for Artificial Intelligence and Fundamental Interactions, 77 Massachusetts Avenue, Cambridge, USA
⁴² Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
⁴³ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, 2800Kgs. Lyngby, Denmark
⁴⁴ Dipartimento di Fisica e Astronomia “G. Galilei”, Università degli studi di Padova Vicolo dell’Osservatorio 3, I-35122 Padova, Italy
⁴⁵ IAASARS, National Observatory of Athens, Metaxa & Vas. Pavlou St., 15236 Penteli, Athens, Greece
⁴⁶ Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA
⁴⁷ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
⁴⁸ Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
⁴⁹ Cosmic Dawn Center (DAWN), Rådmandsgade 64, 2200 Copenhagen N., Denmark
⁵⁰ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 København N, Denmark
⁵¹ Manipal Centre for Natural Sciences, Manipal Academy of Higher Education, Manipal, – 576104 Karnataka, India
⁵² Indian Institute Of Astrophysics, 100 Feet Rd, Santhosapuram, 2nd Block, Koramangala, Bengaluru, Karnataka 560034, India

^⋆ Corresponding author; giorgio.valerin@inaf.it

Received: 31 July 2024
Accepted: 21 January 2025

Abstract

Aims. We investigate the photometric characteristics of a sample of intermediate-luminosity red transients (ILRTs), a class of elusive objects with peak luminosity between that of classical novae and standard supernovae. Our goal is to provide a stepping stone in the path to reveal the physical origin of such events, thanks to the analysis of the datasets collected.

Methods. We present the multi-wavelength photometric follow-up of four ILRTs, namely NGC 300 2008OT-1, AT 2019abn, AT 2019ahd, and AT 2019udc. Through the analysis and modelling of their spectral energy distribution and bolometric light curves, we inferred the physical parameters associated with these transients.

Results. All four objects display a single-peaked light curve which ends in a linear decline in magnitudes at late phases. A flux excess with respect to a single blackbody emission is detected in the infrared domain for three objects in our sample, a few months after maximum. This feature, commonly found in ILRTs, is interpreted as a sign of dust formation. Mid-infrared monitoring of NGC 300 2008OT-1 761 days after maximum allowed us to infer the presence of ∼10⁻³–10⁻⁵ M_⊙ of dust, depending on the chemical composition and the grain size adopted. The late-time decline of the bolometric light curves of the considered ILRTs is shallower than expected for ⁵⁶Ni decay, hence requiring an additional powering mechanism. James Webb Space Telescope observations of AT 2019abn prove that the object has faded below its progenitor luminosity in the mid-infrared domain, five years after its peak. Together with the disappearance of NGC 300 2008OT-1 in Spitzer images seven years after its discovery, this supports the terminal explosion scenario for ILRTs. With a simple semi-analytical model we tried to reproduce the observed bolometric light curves in the context of a few solar masses ejected at few 10³ km s⁻¹ and enshrouded in an optically thick circumstellar medium.