A study in scarlet

II. Spectroscopic 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 – OATS, Via G.B. Tiepolo 11, 34143 Trieste, Italy
³ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
⁴ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, P.R. China
⁵ International Centre of Supernovae, Yunnan Key Laboratory, Kunming 650216, P.R. China
⁶ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, P.R. China
⁷ Graduate Institute of Astronomy, National Central University, 300 Jhongda Road, 32001 Jhongli, Taiwan
⁸ SRON, Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁹ Department of Astrophysics/IMAPP, Radboud University Nijmegen, P.O. Box 9010 6500 GL Nijmegen, The Netherlands
¹⁰ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, E-08193 Barcelona, Spain
¹¹ School of Physics, O’Brien Centre for Science North, University College Dublin, Belfield Dublin 4, Ireland
¹² The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, SE-10691 Stockholm, Sweden
¹³ Hiroshima Astrophysical Science Center, Hiroshima University, Hiroshima, Japan
¹⁴ Department of Physics, Florida State University, 77 Chieftan Way, Tallahassee, FL 32306, USA
¹⁵ Las Cumbres Observatory, 6740 Cortona Dr. Suite 102, Goleta, CA 93117, USA
¹⁶ Department of Physics, University of California, Santa Barbara, CA 93106, USA
¹⁷ Cardiff Hub for Astrophysics Research and Technology, School of Physics & Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
¹⁸ INAF, Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, I-80131 Napoli, Italy
¹⁹ DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen, Denmark
²⁰ Caltech/IPAC, Mailcode 100-22, Pasadena, CA 91125, USA
²¹ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
²² Department of Astronomy, AlbaNova University Center, Stockholm University, SE-10691 Stockholm, Sweden
²³ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
²⁴ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild Str. 1, D-85748 Garching, Germany
²⁵ Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak, Nainital 263001, India
²⁶ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
²⁷ School of Physics, Trinity College Dublin, College Green, Dublin 2, Ireland
²⁸ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA
²⁹ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
³⁰ Astrophysics Research Centre, School of Mathematics and Physics, Queens University Belfast Belfast BT7 1NN, UK
³¹ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark
³² INAF – Osservatorio Astronomico d’Abruzzo, Via M. Maggini snc, Teramo I-64100, Italy
³³ Department of Physics, University of California, Davis, CA 95616, USA
³⁴ European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago, Chile
³⁵ Millennium Institute of Astrophysics, Nuncio Monsenor Sótero Sanz 100, Providencia, 8320000 Santiago, Chile
³⁶ INAF-Osservatorio Astrofisico di Catania, Via Santa Sofia 78, I-95123 Catania, Italy
³⁷ Instituto de Astrofísica, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago RM, Chile
³⁸ ICRANet, Piazza della Repubblica 10, I-65122 Pescara, Italy
³⁹ Institut für Theoretische Physik, Goethe Universität, Max-von-Laue-Str. 1, 60438 Frankfurt am Main, Germany
⁴⁰ INFN-TIFPA, Trento Institute for Fundamental Physics and Applications, Via Sommarive 14, I-38123 Trento, Italy
⁴¹ Institut d’Estudis Espacials de Catalunya (IEEC), E-08034 Barcelona, Spain
⁴² Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
⁴³ Center for Astrophysics, Harvard & Smithsonian, Cambridge, Massachusetts MA 02138, US
⁴⁴ The NSF AI Institute for Artificial Intelligence and Fundamental Interactions, 77 Massachusetts Avenue, Cambridge, USA
⁴⁵ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
⁴⁶ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, 2800 Kongens Lyngby, Denmark
⁴⁷ Istituto Nazionale di Astrofisica – Osservatorio Astronomico di Roma, Via Frascati 33, I-00040 Monteporzio Catone, Italy
⁴⁸ Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo Institutes for Advanced Study, The University of Tokyo, Kashiwa, Chiba 277-8583, Japan
⁴⁹ INFN, Sezione di Perugia, Via A. Pascoli s/n, I-06125 Perugia, Italy
⁵⁰ Dipartimento di Fisica e Astronomia “G. Galilei”, Università degli studi di Padova Vicolo dell’Osservatorio 3, I-35122 Padova, Italy
⁵¹ IAASARS, National Observatory of Athens, Metaxa & Vas. Pavlou St., 15236 Penteli, Athens, Greece
⁵² Department of Astronomy, University of Virginia, Charlottesville, VA 22904, USA
⁵³ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
⁵⁴ Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
⁵⁵ Cosmic Dawn Center (DAWN), Rådmandsgade 64, 2200 Copenhagen N., Denmark
⁵⁶ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 København N, Denmark
⁵⁷ Manipal Centre for Natural Sciences, Manipal Academy of Higher Education, Manipal, 576104 Karnataka, India
⁵⁸ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
⁵⁹ Space Telescope Science Institute, Baltimore, MD, USA
⁶⁰ 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: 26 September 2024

Abstract

Aims. We investigate the spectroscopic characteristics 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 unveiling the physical origin of these events based on the analysis of the collected datasets.

Methods. We present the extensive optical and near-infrared (NIR) spectroscopic monitoring of four ILRTs, namely NGC 300 2008OT-1, AT 2019abn, AT 2019ahd and AT 2019udc. First we focus on the evolution of the most prominent spectral features observed in the low-resolution spectra. We then present a more detailed description of the high-resolution spectrum collected for NGC 300 2008OT-1 with the Very Large Telescope equipped with UVES. Finally, we describe our analysis of late-time spectra of NGC 300 2008OT-1 and AT 2019ahd through comparisons with both synthetic and observed spectra.

Results. Balmer and Ca lines dominate the optical spectra, revealing the presence of slowly moving circumstellar medium (CSM) around the objects. The line luminosity of Hα, Hβ, and Ca II NIR triplet presents a double peaked evolution with time, possibly indicative of interaction between fast ejecta and the slow CSM. The high-resolution spectrum of NGC 300 2008OT-1 reveals a complex circumstellar environment, with the transient being surrounded by a slow (∼30 km s⁻¹) progenitor wind. At late epochs, optical spectra of NGC 300 2008OT-1 and AT 2019ahd show broad (∼2500 km s⁻¹) emission features at ∼6170 Å and ∼7000 Å which are unprecedented for ILRTs. We find that these lines originate most likely from the blending of several narrow lines, possibly of iron-peak elements.