Volume 632, December 2019
|Number of page(s)||9|
|Section||Letters to the Editor|
|Published online||03 December 2019|
Letter to the Editor
The transitional gap transient AT 2018hso: new insights into the luminous red nova phenomenon⋆
Università degli Studi di Padova, Dipartimento di Fisica e Astronomia, Vicolo dell’Osservatorio 2, 35122 Padova, Italy
2 INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
3 School of Physics, O’Brien Centre for Science North, University College Dublin, Belfield, Dublin 4, Ireland
4 School of Physics, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland
5 Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
6 Department of Applied Physics, University of Cádiz, Campus of Puerto Real, 11510 Cádiz, Spain
7 Millennium Institute of Astrophysics (MAS), Nuncio Monsenor Sòtero Sanz 100, Providencia, Santiago, Chile
8 Departamento de Ciencias Fisicas, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago, Chile
9 SRON, Netherlands Institute for Space Research, Sorbonnelaan, 2, 3584 CA Utrecht, Netherlands
10 Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
11 Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
12 Nordic Optical Telescope, Apartado 474, 38700 Santa Cruz de La Palma, Spain
13 Depto. de Astrofísica, Centro de Astrobiología (INTA-CSIC), Camino Bajo del Castillo s/n, 28692 Madrid, Spain
14 DARK, Niels Bohr Institute, Lyngbyvej 2, 2100 Copenhagen, Denmark
15 INAF–Osservatorio Astronomico di Trieste, Via Giambattista Tiepolo 11, 34143 Trieste, Italy
16 Istituto di Astrofisica e Planetologia Spaziali (INAF), via del Fosso del Cavaliere 100, Roma 00133, Italy
17 The Oskar Klein Centre, Physics Department, Stockholm University, 106 91 Stockholm, Sweden
Accepted: 6 November 2019
Context. The absolute magnitudes of luminous red novae (LRNe) are intermediate between those of novae and supernovae (SNe), and show a relatively homogeneous spectro-photometric evolution. Although they were thought to derive from core instabilities in single stars, there is growing support for the idea that they are triggered by binary interaction that possibly ends with the merging of the two stars.
Aims. AT 2018hso is a new transient showing transitional properties between those of LRNe and the class of intermediate-luminosity red transients (ILRTs) similar to SN 2008S. Through the detailed analysis of the observed parameters, our study supports that it actually belongs to the LRN class and was likely produced by the coalescence of two massive stars.
Methods. We obtained ten months of optical and near-infrared photometric monitoring, and 11 epochs of low-resolution optical spectroscopy of AT 2018hso. We compared its observed properties with those of other ILRTs and LRNe. We also inspected the archival Hubble Space Telescope (HST) images obtained about 15 years ago to constrain the progenitor properties.
Results. The light curves of AT 2018hso show a first sharp peak (reddening-corrected Mr = −13.93 mag), followed by a broader and shallower second peak that resembles a plateau in the optical bands. The spectra dramatically change with time. Early-time spectra show prominent Balmer emission lines and a weak [Ca II] doublet, which is usually observed in ILRTs. However, the strong decrease in the continuum temperature, the appearance of narrow metal absorption lines, the great change in the Hα strength and profile, and the emergence of molecular bands support an LRN classification. The possible detection of a MI ∼ −8 mag source at the position of AT 2018hso in HST archive images is consistent with expectations for a pre-merger massive binary, similar to the precursor of the 2015 LRN in M101.
Conclusions. We provide reasonable arguments to support an LRN classification for AT 2018hso. This study reveals growing heterogeneity in the observables of LRNe than has been thought previously, which is a challenge for distinguishing between LRNe and ILRTs. This suggests that the entire evolution of gap transients needs to be monitored to avoid misclassifications.
Key words: binaries: close / stars: winds / outflows / stars: massive / supernovae: individual: AT 2018hso / supernovae: individual: AT 2017jfs / supernovae: individual: NGC4490-2011OT1
The light curves and the reduced spectra are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (18.104.22.168) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/632/L6
© ESO 2019
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