Luminous red novae: Stellar mergers or giant eruptions?^⋆

¹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
e-mail: andrea.pastorello@inaf.it
² INAF – Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34143 Trieste, Italy
³ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str. 1 85748 Garching, Germany
⁴ European Organisation for Astronomical Research in the Southern Hemisphere (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
⁵ School of Physics, O’Brien Centre for Science North, University College Dublin, Belfield, Dublin 4, Ireland
⁶ Osservatorio Astronomico di Monte Maggiore, Via Montemaggiore 3, 47016, Predappio Forlì-Cesena, Italy
⁷ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁸ Institute of Space Sciences (ICE, CSIC), Campus UAB, Camí de Can Magrans s/n, 08193 Cerdanyola del Vallès (Barcelona), Spain
⁹ Institut d’Estudis Espacials de Catalunya (IEEC), c/Gran Capità 2-4, Edif. Nexus 201, 08034 Barcelona, Spain
¹⁰ Department of Physics and Astronomy, University of Turku, Vesilinnantie 5, 20014 Turku, Finland
¹¹ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK
¹² The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova 10691 Stockholm, Sweden
¹³ California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
¹⁴ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Quantum, Vesilinnantie 5 20014, Finland
¹⁵ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
¹⁶ Lajatico Astronomical Centre, Via dei Mulini a Vento, 56030, Lajatico Pisa, Italy
¹⁷ Coral Towers Observatory, Unit 38 Coral Towers, 255 Esplanade, Cairns 4870, Australia
¹⁸ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
¹⁹ Dipartimento di Fisica e Astronomia, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
²⁰ INFN – Sezione di Padova, Via Marzolo 8, 35131 Padova, Italy
²¹ Center for Astrophysics, Guangzhou University, Guangzhou 510006, PR China
²² Lunar and Planetary Lab, Department of Planetary Sciences, University of Arizona, Tucson, AZ 85721, USA
²³ Osservatorio Astronomico di Monte Agliale, Via Cune Motrone, 55023 Borgo a Mozzano, Lucca, Italy
²⁴ Las Campanas Observatory – Carnagie Institution of Washington, Colina el Pino, Casilla 601, La Serena, Chile
²⁵ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, LC, Italy
²⁶ Osservatorio Astronomico del Col Druscié, 32043 Cortina d’Ampezzo, Italy
²⁷ School of Physics & Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
²⁸ Gemini Observatory, Southern Operations Center, c/o AURA, Casilla 603, La Serena, Chile
²⁹ School of Physics, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland
³⁰ Department of Applied Physics, University of Cádiz, Campus of Puerto Real, 11510 Cádiz, Spain
³¹ Departamento de Ciencias Fisicas, Universidad Andres Bello, Avda. Republica 252, Santiago, Chile
³² Millennium Institute of Astrophysics (MAS), Nuncio Monseor Sótero Snz 100, Providencia, Santiago, Chile
³³ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) and Department of Physics and Astronomy, Northwestern University, Evanston, IL 60208, USA
³⁴ Minnesota Institute for Astrophysics, University of Minnesota, Minneapolis, MN 55455, USA

Received: 31 May 2019
Accepted: 5 August 2019

Abstract

We present extensive datasets for a class of intermediate-luminosity optical transients known as luminous red novae. They show double-peaked light curves, with an initial rapid luminosity rise to a blue peak (at −13 to −15 mag), which is followed by a longer-duration red peak that sometimes is attenuated, resembling a plateau. The progenitors of three of them (NGC 4490−2011OT1, M 101−2015OT1, and SNhunt248), likely relatively massive blue to yellow stars, were also observed in a pre-eruptive stage when their luminosity was slowly increasing. Early spectra obtained during the first peak show a blue continuum with superposed prominent narrow Balmer lines, with P Cygni profiles. Lines of Fe II are also clearly observed, mostly in emission. During the second peak, the spectral continuum becomes much redder, Hα is barely detected, and a forest of narrow metal lines is observed in absorption. Very late-time spectra (∼6 months after blue peak) show an extremely red spectral continuum, peaking in the infrared (IR) domain. Hα is detected in pure emission at such late phases, along with broad absorption bands due to molecular overtones (such as TiO, VO). We discuss a few alternative scenarios for luminous red novae. Although major instabilities of single massive stars cannot be definitely ruled out, we favour a common envelope ejection in a close binary system, with possibly a final coalescence of the two stars. The similarity between luminous red novae and the outburst observed a few months before the explosion of the Type IIn SN 2011ht is also discussed.