Spectral analysis of the 91bg-like Type Ia SN 2005bl: Low luminosity, low velocities, incomplete burning

TL;DR

This study analyzes the spectral features of the underluminous 91bg-like SN 2005bl, revealing low velocities, incomplete burning, and a lower kinetic energy compared to typical Type Ia supernovae, using spectral modeling and modified explosion profiles.

Contribution

It provides a detailed spectral analysis of SN 2005bl, demonstrating the need for modified density profiles and lower kinetic energy models to explain its properties.

Findings

SN 2005bl shows lack of burning products in outer layers.

Spectra favor a reduced kinetic energy by ~30%.

Inconsistent with complete burning, indicating incomplete Si or O burning.

Abstract

The properties of underluminous type Ia supernovae (SNe Ia) of the 91bg subclass have yet to be theoretically understood. Here, we take a closer look at the structure of the dim SN Ia 2005bl. We infer the abundance and density profiles needed to reproduce the observed spectral evolution between -6 d and +12.9 d with respect to B maximum. Initially, we assume the density structure of the standard explosion model W7; then we test whether better fits to the observed spectra can be obtained using modified density profiles with different total masses and kinetic energies. Compared to normal SNe Ia, we find a lack of burning products especially in the rapidly-expanding outer layers (v>~15000 km/s). The zone between ~8500 and 15000 km/s is dominated by oxygen and includes some amount of intermediate mass elements. At lower velocities, intermediate mass elements dominate. This holds down to the lowest zones investigated in this work. This fact, together with negligible-to-moderate abundances of Fe-group elements, indicates large-scale incomplete Si burning or explosive O burning, possibly in a detonation at low densities. Consistently with the reduced nucleosynthesis, we find hints of a kinetic energy lower than that of a canonical SN Ia: The spectra strongly favour reduced densities at >~13000 km/s compared to W7, and are very well fitted using a rescaled W7 model with original mass (1.38 M_sun), but a kinetic energy reduced by ~30 % (i.e. from 1.33e51 erg to 0.93e51 erg).