Luminous Red Novae: Stellar Mergers or Giant Eruptions?

Creators: Pastorello, A.; Mason, E.; Taubenberger, S.; Fraser, M.; Cortini, G.; Tomasella, L.; Botticella, M. T.; Elias-Rosa, N.; Kotak, R.; Smartt, S. J.; Benetti, S.; Cappellaro, E.; Turatto, M.; Tartaglia, L.; Djorgovski, S. G.; Drake, A. J.; Berton, M.; Briganti, F.; Brimacombe, J.; Bufano, F.; Cai, Y.-Z.; Chen, S.; Christensen, E. J.; Ciabattari, F.; Congiu, E.; Dimai, A.; Inserra, C.; Kankare, E.; Magill, L.; Maguire, K.; Martinelli, F.; Morales-Garoffolo, A.; Ochner, P.; Pignata, G.; Reguitti, A.; Sollerman, J.; Spiro, S.; Terreran, G.; Wright, D. E.

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.

Additional Information

© 2019 ESO. Article published by EDP Sciences. Received 31 May 2019; Accepted 5 August 2019; Published online 24 September 2019. We dedicate this work to our friend Alex Dimai. His enthusiasm and competence will remain as a precious gift for those who had the priviledge of working with him. We thank the referee Elena Barsukova for the insightful comments that improved the manuscript. We are grateful to Marco Fiaschi for his observations at the Asiago Telescopes, Stefano Valenti and Mattias Ergon for their observations at ESO-La Silla, and Avet Harutyunyan for his support with the observations with TNG. We also acknowledge M. MacLeod and O. Pejcha for helpful discussions, and N. Blagorodnova for providing the data of M 101−2015OT1, and for useful suggestions. SB, LT, PO, MT, MTB are partially supported by the PRIN-INAF 2017 "Towards the SKA and CTA era: discovery, localisation and physics of transient sources" (PI M. Giroletti). NER acknowledges support from the Spanish MICINN grant ESP2017-82674-R and FEDER funds. YC is supported by the China Scholarship Council. KM is supported by STFC through an Ernest Rutherford Fellowship. ST is supported by TRR33 "The Dark Universe" of the German Research Foundation. SJS acknowledges funding from ERC Grant 291222 and STFC grant Grant Ref: ST/P000312/1. SGS, AJD, and the CRTS survey have been supported by the NSF grants AST-1313422, AST-1413600, and AST-1749235. KM acknowledges support from H2020 through an ERC Starting Grant (758638). Support for GP is provided by the Ministry of Economy, Development, and Tourism's Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics, MAS. This article is based on observations made with the following facilities: the Italian Telescopio Nazionale Galileo operated on the island of La Palma by the Fundación Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias; the Nordic Optical Telescope, operated by the Nordic Optical Telescope Scientific Association at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofísica de Canarias; the William Herschel Telescope and the Isaac Newton Telescope, which are operated on the island of La Palma by the Isaac Newton Group of Telescopes in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias; the Gran Telescopio Canarias, installed at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, in the island of La Palma; the Calar Alto 2.2 m Telescope of the Centro Astronómico Hispano-Alemán, Almería, Spain; the 1.93 m OHP telescope of the Observatoire de Haute-Provence; the 3.56 m New Technology Telescope and the Dutch 0.9 m telescopes at ESO-La Silla; the Copernico and the Schmidt telescopes (Asiago, Italy) of the INAF – Osservatorio Astronomico di Padova; the 8.4 m Large Binocular Telescope at Mt. Graham (Arizona, USA); the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia, Inovaçãos e Comunicações do Brasil (MCTIC/LNA), the U.S. National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU); the Liverpool Telescope which is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias with financial support from the UK Science and Technology Facilities Council; the 2 m Faulkes North Telescope of the Las Cumbres Observatory Global Telescope Network (LCOGTN). This paper used data obtained with the MODS spectrographs built with funding from NSF grant AST-9987045 and the NSF Telescope System Instrumentation Program (TSIP), with additional funds from the Ohio Board of Regents and the Ohio State University Office of Research. The LBT is an international collaboration among institutions in the United States, Italy and Germany. LBT Corporation partners are: The University of Arizona on behalf of the Arizona university system; Istituto Nazionale di Astrofisica, Italy; LBT Beteiligungsgesellschaft, Germany, representing the Max-Planck Society, the Astrophysical Institute Potsdam, and Heidelberg University; The Ohio State University, and The Research Corporation, on behalf of The University of Notre Dame, University of Minnesota and University of Virginia. This paper is also based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programmes 184.D-1140 and 184.D-1151. This research has made use of the NASA/ IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research has also made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. We acknowledge the usage of the HyperLeda database (http://leda.univ-lyon1.fr).

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