A&A 639, A104 (2020) Astronomy https://doi.org/10.1051/0004-6361/202038019 & c ESO 2020 Astrophysics The Carnegie Supernova Project II Observations of the luminous red nova AT 2014ej?,?? M. D. Stritzinger1, F. Taddia1, M. Fraser2, T. M. Tauris3,1, C. Contreras1,4, S. Drybye1,5, L. Galbany6, S. Holmbo1, N. Morrell4, A. Pastorello7, M. M. Phillips4, G. Pignata8,9, L. Tartaglia10, N. B. Suntzeff11,12, J. Anais4, C. Ashall13, E. Baron14, C. R. Burns15, P. Hoeflich13, E. Y. Hsiao13, E. Karamehmetoglu1, T. J. Moriya16,17, G. Bock18, A. Campillay4, S. Castellón4, C. Inserra19, C. González4, P. Marples20, S. Parker21, D. Reichart22, S. Torres-Robledo23,4, and D. R. Young24 1 Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark e-mail: [email protected] 2 School of Physics, O’Brien Centre for Science North, University College Dublin, Belfield, Dublin 4, Ireland 3 Aarhus Institute of Advanced Studies (AIAS), Aarhus University, Høegh-Guldbergs Gade 6B, 8000 Aarhus C, Denmark 4 Las Campanas Observatory, Carnegie Observatories, Casilla 601, La Serena, Chile 5 Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Spain 6 Departamento de Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain 7 INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy 8 Departamento de Ciencias Fisicas, Universidad Andres Bello, Avda. Republica 252, Santiago, Chile 9 Millennium Institute of Astrophysics, Santiago, Chile 10 The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova 10691, Stockholm, Sweden 11 The George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University, College Station, TX 877843, USA 12 Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843, USA 13 Department of Physics, Florida State University, Tallahassee, FL 32306, USA 14 Homer L. Dodge Department of Physics and Astronomy, University of Oklahoma, 440 W. Brooks, Rm 100, Norman, OK 73019-2061, USA 15 Observatories of the Carnegie Institution for Science, 813 Santa Barbara St, Pasadena, CA 91101, USA 16 National Astronomical Observatory of Japan, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan 17 School of Physics and Astronomy, Faculty of Science, Monash University, Clayton, VIC 3800, Australia 18 Runaway Bay, Gold Coast, Queensland, Australia 19 School of Physics & Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK 20 Leyburn Observatory, Queensland 4129, Australia 21 Parkdales Observatory, Oxford, New Zealand 22 University of North Carolina at Chapel Hill, Campus Box 3255, Chapel Hill, NC 27599-3255, USA 23 SOAR Telescope, La Serena 1700000, Chile 24 Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK Received 25 March 2020 / Accepted 15 May 2020 ABSTRACT We present optical and near-infrared broadband photometry and optical spectra of AT2014ej from the Carnegie Supernova Project- II. These observations are complemented with data from the CHilean Automatic Supernova sEarch, the Public ESO Spectroscopic Survey of Transient Objects, and from the Backyard Observatory Supernova Search. Observational signatures of AT2014ej reveal that it is similar to other members of the gap-transient subclass known as luminous red novae (LRNe), including the ubiquitous double- hump light curve and spectral properties similar to that of LRN SN 2017jfs. A medium-dispersion visual-wavelength spectrum of AT2014ej taken with the Magellan Clay telescope exhibits a P Cygni Hα feature characterized by a blue velocity at zero intensity of ≈110 km s−1 and a P Cygni minimum velocity of ≈70 km s−1. We attribute this to emission from a circumstellar wind. Inspection of pre-outbust Hubble Space Telescope images yields no conclusive progenitor detection. In comparison with a sample of LRNe from the literature, AT2014ej lies at the brighter end of the luminosity distribution. Comparison of the ultra-violet, optical, infrared light curves of well-observed LRNe to common-envelope evolution models from the literature indicates that the models underpredict the luminosity of the comparison sample at all phases and also produce inconsistent timescales of the secondary peak. Future efforts to model LRNe should expand upon the current parameter space we explore here and therefore may consider more massive systems and a wider range of dynamical timescales. Key words. supernovae: individual: AT2014ej – supernovae: general ? This paper includes data gathered with the 6.5 meter Magellan telescopes at Las Campanas Observatory, Chile. ?? Photometry and spectra are presented in this paper are available on WISeREP. Article published by EDP Sciences A104, page 1 of 19 A&A 639, A104 (2020) 1. Introduction This is the second of two papers by the Carnegie Supernova Project-II (CSP-II) that each present a case study of a gap tran- 1' sient. Gap transients are located within the so-called “luminos- -42°30' ity gap” parameter space that is devoid of bright novae (MV . −10 mag) and the least luminous core-collapse supernovae (SN; ∼ − peak MV 16 mag). Because the visual wavelength spectra of AT2014ej gap transients (in the past also often referred to as intermedi- ate luminosity optical transients) typically resemble the spectra 35' of interacting type IIn SNe, they were historically commonly called SN imposters (e.g., Schlegel 1990; Filippenko 1997; Van Dyk et al. 2000). As the population of gap transients with decent observational data has increased, it has become apparent that Declination (J2000) Declination N the diversity displayed among the literature sample suggests dis- 40' tinctly different populations (see Kulkarni & Kasliwal 2009; Kochanek et al. 2012a; Kashi & Soker 2016; Pastorello & Fraser E 2019, for more detailed discussion). 23h16m40s 20s 00s 15m40s Today, at least three known populations of gap transients are Right Ascension (J2000) recognized in the luminoisty gap. These include classical lumi- nous blue variable (LBV) outbursts, intermediate-luminosity red Fig. 1. Finding chart of NGC 7552 hosting AT2014ej constructed from transients (ILRTs), and luminous red novae (LRNe). LBVs are a single Swope r-band image. The position of the transient is indicated thought to be related to eruptions of massive luminous stars (see with a dotted circle with a zoom-in of the area enclosed shown in the Smith et al. 2011). As shown in greater detail in Stritzinger et al. upper right corner inset. Optical and NIR local sequence stars are indi- (2020, hereafter Paper I), the ILRT subtype is well represented cated with yellow and red circles, respectively. by NGC 300-2008-OT and SN 2008S, and has been linked to asymptotic giant branch (S-AGB) stars (Prieto et al. 2008, 2009; time parameter space populated by both ILRTs and LRNe is cur- Thompson et al. 2009; Botticella et al. 2009; Kochanek 2011; rently unknown. Adams et al. 2016; Doherty et al. 2017) that die as electron- Similar to the ILRT SNhunt120 presented in Paper I, here in capture supernovae (Miyaji et al. 1980; Nomoto 1984; Miyaji Paper II, we add to the small but growing sample of LRNe with & Nomoto 1987; Hashimoto et al. 1993; Kitaura et al. 2006; our observational data set of AT2014ej. The data set is based Poelarends et al. 2008). Other models appearing in the litera- largely on observations performed by the CSP-II (Phillips et al. ture for ILRTs consist of moderately massive stars experienc- 2019). This includes optical light curves and some near-IR (NIR) ing super-Eddington winds and/or giant outbursts (e.g., Smith photometry, as well as one low- and one medium-dispersion et al. 2009; Humphreys et al. 2011), or massive stars donat- visual wavelength spectrum. The CSP-II data are clearly com- ing material to a main-sequence star, leading to the release of plemented with unfiltered images from the Backyard Observa- gravitational energy (e.g., Kashi et al. 2010). Finally, a lead- tory Supernova Search1 (BOSS) and the CHilean Automatic ing model for the origins of LRNe, which all display a ubiq- Supernova sEarch (CHASE; Reichart et al. 2005), as well as a uitous double-humped light curve (Pastorello et al. 2019a), handful of visual wavelength spectra and optical/NIR broadband consists of the ejection of a common envelope by a massive photometric measurements obtained by the Public ESO Spec- binary system (e.g., Blagorodnova et al. 2017) upon coales- troscopic Survey of Transient Objects (PESSTO; Smartt et al. cence (Smith et al. 2016; Metzger & Pejcha 2017; Lipunov et al. 2015). In addition, archival images of the host galaxy NGC 7552 2017; Mauerhan et al. 2018). However, other models have also obtained with the Hubble Space Telescope (HST) are examined been proposed in the past to account for LRNe, particularly in order to compute progenitor limits. within articles that have studied the Galactic LRN archetype V838 Mon. These include, among others, outbursts from mas- sive stars (Tylenda 2005), accretion of low-mass stars onto 2. AT 2014ej solar-mass main-sequence companions (Soker & Tylenda 2003; Tylenda & Soker 2006; Kashi et al. 2010; Kashi & Soker 2016; 2.1. Discovery, distance, and reddening Soker 2020), or even giant stars that accrete relatively massive PSN J23160979−4234575 (hereafter we refer to this transient planets (Retter & Marom 2003). by its Transient Name Server designation AT2014ej) was dis- The locations of the three gap-transient subtypes in the lumi- covered in NGC 7552 on 2014 September 24.46 UT by Peter nosity versus decay time (defined as the time interval in which Marples during the course of the BOSS (Bock et al. 2014) and the r-band light curve drops one magnitude from peak) parame- was confirmed to be ∼17.8 ± 0.4 magnitude in unfiltered image ter space are shown in Fig. 1 of Paper I. In general, the various taken by G.