Supernova spectra below strong circumstellar interaction

¹ The Oskar Klein Centre, Department of PhysicsStockholm University, AlbaNova, 10691 Stockholm, Sweden
e-mail: giorgos@dark-cosmology.dk
² Department of Particle Physics & Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
³ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁴ Carnegie Observatories, Las Campanas Observatory, Colina El Pino, Casilla 601, Chile
⁵ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
⁶ Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, 606-8502 Kyoto, Japan
⁷ Kavli Institute for the Physics and Mathematics of the Universe (WPI), Todai Institutes for Advanced Study, University of Tokyo, Kashiwanoha 5-1-5, Kashiwa, 277-8583 Chiba, Japan
⁸ Argelander Institute for Astronomy, University of Bonn, Auf dem Hgel 71, 53121 Bonn, Germany
⁹ Space Sciences Laboratory, University of California Berkeley, Berkeley, CA 94720, USA
¹⁰ E. O. Lawrence Berkeley National Lab, 1 Cyclotron Rd., Berkeley, CA 94720, USA
¹¹ Department of Astronomy, University of Texas, Austin, TX 78712-0259, USA
¹² The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden

Received: 7 June 2013
Accepted: 16 November 2014

Abstract

We construct spectra of supernovae (SNe) interacting strongly with a circumstellar medium (CSM) by adding SN templates, a black-body continuum, and an emission-line spectrum. In a Monte Carlo simulation we vary a large number of parameters, such as the SN type, brightness and phase, the strength of the CSM interaction, the extinction, and the signal to noise ratio (S/N) of the observed spectrum. We generate more than 800 spectra, distribute them to ten different human classifiers, and study how the different simulation parameters affect the appearance of the spectra and their classification. The SNe IIn showing some structure over the continuum were characterized as “SNe IInS” to allow for a better quantification. We demonstrate that the flux ratio of the underlying SN to the continuum f_V is the single most important parameter determining whether a spectrum can be classified correctly. Other parameters, such as extinction, S/N, and the width and strength of the emission lines, do not play a significant role. Thermonuclear SNe get progressively classified as Ia-CSM, IInS, and IIn as f_V decreases. The transition between Ia-CSM and IInS occurs at f_V ~ 0.2−0.3. It is therefore possible to determine that SNe Ia-CSM are found at the (un-extincted) magnitude range −19.5 >M> −21.6, in very good agreement with observations, and that the faintest SN IIn that can hide a SN Ia has M = −20.1. The literature sample of SNe Ia-CSM shows an association with 91T-like SNe Ia. Our experiment does not support that this association can be attributed to a luminosity bias (91T-like being brighter than normal events). We therefore conclude that this association has real physical origins and we propose that 91T-like explosions result from single degenerate progenitors that are responsible for the CSM. Despite the spectroscopic similarities between SNe Ibc and SNe Ia, the number of misclassifications between these types was very small in our simulation and mostly at low S/N. Combined with the SN luminosity function needed to reproduce the observed SN Ia-CSM luminosities, it is unlikely that SNe Ibc constitute an important contaminant within this sample. We show how Type II spectra transition to IIn and how the Hα profiles vary with f_V. SNe IIn fainter than M = −17.2 are unable to mask SNe IIP brighter than M = −15. A more advanced simulation, including radiative transfer, shows that our simplified model is a good first order approximation. The spectra obtained are in good agreement with real data.