LSQ13fn: A type II-Plateau supernova with a possibly low metallicity progenitor that breaks the standardised candle relation^⋆

¹ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK
e-mail: jpolshaw01@qub.ac.uk
² Laboratoire Lagrange, Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Boulevard de l’Observatoire, CS 34229, 06304 Nice Cedex 4, France
³ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK
⁴ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, 76100 Rehovot, Israel
⁵ Department of Astronomy and the Oskar Klein Centre, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
⁶ Department of Physics, Yale University, New Haven, CT 06250-8121, USA
⁷ INAF-Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁸ INAF-Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁹ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
¹⁰ Millennium Institute of Astrophysics, Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
¹¹ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
¹² School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK
¹³ Departamento de Ciencias Físicas, Universidad Andrés Bello, Avda. República 252, 32349 Santiago, Chile
¹⁴ University of California Davis, 1 Shields Avenue, Davis, CA 95616 06520, USA

Received: 2 November 2015
Accepted: 29 January 2016

Abstract

We present optical imaging and spectroscopy of supernova (SN) LSQ13fn, a type II supernova with several hitherto-unseen properties. Although it initially showed strong symmetric spectral emission features attributable to He ii, N iii, and C iii, reminiscent of some interacting SNe, it transitioned into an object that would fall more naturally under a type II-Plateau (IIP) classification. However, its spectral evolution revealed several unusual properties: metal lines appeared later than expected, were weak, and some species were conspicuous by their absence. Furthermore, the line velocities were found to be lower than expected given the plateau brightness, breaking the SN IIP standardised candle method for distance estimates. We found that, in combination with a short phase of early-time ejecta-circumstellar material interaction, metal-poor ejecta, and a large progenitor radius could reasonably account for the observed behaviour. Comparisons with synthetic model spectra of SNe IIP of a given progenitor mass would imply a progenitor star metallicity as low as 0.1 Z_⊙. LSQ13fn highlights the diversity of SNe II and the many competing physical effects that come into play towards the final stages of massive star evolution immediately preceding core-collapse.