Radiative-transfer models for explosions from rotating and non-rotating single WC stars

Implications for SN 1998bw and LGRB/SNe

¹ Unidad Mixta Internacional Franco-Chilena de Astronomía (CNRS UMI 3386)Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago Chile
e-mail: Luc.Dessart@oca.eu
² Department of Physics and Astronomy & Pittsburgh Particle Physics, Astrophysics, and Cosmology Center (PITT PACC), University of Pittsburgh, 3941 O’Hara Street, Pittsburgh, PA 15260, USA
³ Department of Physics and Astronomy, Seoul National University, Gwanak-ro 1, Gwanak-gu, 151-742 Seoul, Republic of Korea
⁴ Racah Institute of Physics, The Hebrew University, 91904 Jerusalem, Israel

Received: 27 March 2017
Accepted: 30 April 2017

Abstract

Using 1D, non-local thermodynamic equilibrium and time-dependent radiative transfer simulations, we study the ejecta properties required to match the early- and late-time photometric and spectroscopic properties of supernovae (SNe) associated with long-duration γ-ray bursts (LGRBs). Matching the short rise time, narrow light curve peak and extremely broad spectral lines of SN 1998bw requires a model with ≲3 M_⊙ ejecta but a high explosion energy of a few 10⁵² erg and 0.5 M_⊙ of ⁵⁶Ni. The relatively high luminosity, presence of narrow spectral lines of intermediate mass elements, and low ionisation at the nebular stage, however, are matched with a more standard C-rich Wolf-Rayet (WR) star explosion, an ejecta of ≳10 M_⊙, an explosion energy ≳10⁵¹ erg, and only 0.1 M_⊙ of ⁵⁶Ni. As the two models are mutually exclusive, the breaking of spherical symmetry is essential to match the early- and late-time photometric and spectroscopic properties of SN 1998bw. This conclusion confirms the notion that the ejecta of SN 1998bw is highly aspherical on large scales. More generally, with asphericity, the energetics and ⁵⁶Ni masses of LGRB/SNe are reduced and their ejecta masses are increased, favouring a massive fast-rotating Wolf-Rayet star progenitor. Contrary to persisting claims in favour of the proto-magnetar model for LGRB/SNe, such progenitor/ejecta properties are compatible with collapsar formation. Ejecta properties of LGRB/SNe inferred from 1D radiative-transfer modelling are fundamentally flawed.