[Paper Review] Long-duration superluminous supernovae at late times

Anders Jerkstrand, S. J. Smartt+14 more|arXiv (Cornell University)|Aug 9, 2016

Gamma-ray bursts and supernovae4 citations

TL;DR

This study analyzes late-time nebular spectra of long-duration Type Ic superluminous supernovae (SLSNe), including LSQ14an and SN 2015bn, using spectral modeling to constrain ejecta composition and structure. It finds that high oxygen- and magnesium-zone masses (>10 M⊙) and high electron densities (ne ≳ 10⁸ cm⁻³) point to explosions of massive CO cores with ZAMS masses >40 M⊙, supporting pair-instability or pulsational pair-instability scenarios over standard core-collapse models.

ABSTRACT

Nebular-phase observations and spectral models of Type Ic superluminous supernovae are presented. LSQ14an and SN 2015bn both display late-time spectra similar to galaxy-subtracted spectra of SN 2007bi, and the class shows strong similarity with broad-lined Type Ic SNe such as SN 1998bw. Near-infrared observations of SN 2015bn show a strong Ca II triplet, O I 9263, O I 1.13 um and Mg I 1.50 um, but no distinct He, Si, or S emission. The high Ca II NIR/[Ca II] 7291,7323 ratio of ~2 indicates a high electron density of n_e >~ 10^8 cm^{-3}. Spectral models of oxygen-zone emission are investigated to put constraints on the emitting region. Models require M(O-zone) >~ 10 Msun to produce enough [O I] 6300,6364 luminosity, irrespective of the powering situation and the density. The high oxygen-zone mass, supported by high estimated magnesium masses, points to explosions of massive CO cores, requiring M_ZAMS >~ 40 Msun. Collisions of pair-instability pulsations do not provide enough mass to account for the emission. [O II] and [O III] lines emerge naturally in many models, which strengthens the identification of broad [O II] 7320,7330, [O III] 4363, and [O III] 4959,5007 in some spectra. A small filling factor f

Motivation & Objective

• To understand the nucleosynthetic yields and core structure of long-duration Type Ic superluminous supernovae (SLSNe) through late-time nebular spectroscopy.
• To constrain the mass and density structure of the ejecta in SLSNe using observed emission lines and spectral modeling.
• To test competing explosion mechanisms—such as pair-instability or magnetar-powered models—against observed nebular line strengths and ratios.
• To determine the filling factor and clumpiness of the oxygen- and magnesium-rich zones from line luminosities.
• To assess the viability of different powering mechanisms by comparing observed [O I], [Ca II], and recombination line luminosities with model predictions.

Proposed method

• Acquisition and analysis of nebular-phase optical and near-infrared spectra from LSQ14an and SN 2015bn, including host-galaxy subtraction.
• Spectral modeling using photoionization equilibrium to derive ionization fractions and electron densities from line ratios, particularly the Ca II NIR/[Ca II] 7291,7323 ratio.
• Application of the ionization balance equation (N = α(T)Vne n(1−xn)) to model recombination line luminosities and infer neutral fraction and volume.
• Use of the escape optical depth τ = nxnσ(3/4π)¹ᐟ³V¹ᐟ³ to model radiative transfer effects and determine when ionization runs away.
• Modeling of [O I] 6300,6364, [O II] 7320,7330, [O III] 4363,4959,5007, Mg I] 4571, and O I 1.13 μm line luminosities to constrain mass and filling factor.
• Comparison of observed line strengths with synthetic models to infer ejecta mass, clumpiness (filling factor f ≲ 0.01), and electron density (ne ≳ 10⁸ cm⁻³).

Experimental results

Research questions

• RQ1 What is the mass of the oxygen-rich zone in long-duration SLSNe, and what does it imply about the progenitor core mass?
• RQ2 How do observed Ca II NIR line ratios constrain the electron density in the ejecta?
• RQ3 What is the filling factor of the O/Mg zone, and what does it reveal about ejecta clumpiness?
• RQ4 Can standard core-collapse or magnetar-powered models reproduce the observed nebular line luminosities?
• RQ5 What are the implications of the presence of [O II] and [O III] lines for the ionization structure and progenitor mass?

Key findings

• The oxygen-zone mass required to produce the observed [O I] 6300,6364 luminosity is M(O-zone) ≳ 10 M⊙, independent of the powering mechanism.
• The high Ca II NIR/[Ca II] 7291,7323 line ratio of ∼2 implies an electron density of ne ≳ 10⁸ cm⁻³, indicating a dense, compact ejecta region.
• The observed Mg I] 4571 and O I 1.13 μm line luminosities require a filling factor f ≲ 0.01 for the O/Mg zone, indicating strong clumpiness in the ejecta.
• The presence of [O II] 7320,7330 and [O III] 4363,4959,5007 lines is naturally reproduced in models, supporting their identification in observed spectra.
• The high oxygen and magnesium masses, combined with the high electron density, point to a progenitor with a ZAMS mass MZAMS ≳ 40 M⊙, inconsistent with standard core-collapse models.
• Pair-instability pulsation collisions do not produce sufficient mass to account for the observed emission, ruling out this mechanism as the primary source of the ejecta.