Abundance Stratification in Type Iax SN 2020rea with TARDIS

TL;DR

This paper uses the 1D Monte Carlo radiative-transfer code TARDIS to model the spectral evolution of Type Iax SN 2020rea from -7 to +21 days around maximum light. It finds that early-time spectra require stratified, velocity-dependent abundances, while later epochs are consistent with a more homogeneous inner ejecta, indicating a transition away from complete mixing. The uniform-abundance models can reproduce many features at late times but fail pre-maximum, whereas a velocity-stratified abundance profile improves the fit for early epochs, particularly in the 3400–4200 Å region and the 6000 Å feature. The results challenge pure deflagration scenarios that predict fully mixed ejecta and point to a more nuanced explosion history for SNe Iax, emphasizing the need for multi-epoch abundance tomography across more events.

Abstract

Using the 1D Monte Carlo-based radiative transfer code TARDIS, we investigate the spectral evolution of the Type Iax supernova (SN) 2020rea from -7 days before to +21 days after maximum light. Our best-fit models indicate stratified, velocity-dependent abundances at early times, successfully reproducing most observed spectral features. As the SN evolves, the ejecta transition from a layered to a more homogeneous composition, posing an alternative to pure deflagration models that predict fully mixed ejecta. These results highlight the need for further investigation, as current pure deflagration models cannot fully explain the origin or spectral properties of Type Iax SNe like SN 2020rea.

Figures (4)

• Figure 1: Observed (grey) and modelled (red) spectra for six different epochs of SN 2020rea, assuming a uniform abundance profile. The marked lines were identified by the TARDIS line identification tool. These were compared with li_sn_2003 and barna_type_2018. Fe III and Co III lines in the earlier epochs transform into their corresponding lower ionisation states as the SN evolves.
• Figure 2: The observed spectra of SN 2020rea obtained between $-$7 and +20.9 days (w.r.t. $g_\text{max}$) compared to the best-fitting tardis models assuming stratified and uniform abundance profile.
• Figure 3: The best-fit chemical abundance structure for the synthetic spectrum. The dotted lines show the best-fit abundance values used in the uniform abundance model.
• Figure 4: Best-fit modelled spectra with uniform and stratified abundances compared with the observed spectrum of SN 2020rea (at $-$0.9 days w.r.t $g_\text{max}$). The shaded bars highlight the (a) 3400--4200 Å region where the stratified abundance model provides a better fit, (b) the peaks at $\sim$ 4700 Å and the (c) "W" feature $\sim$ 6000 Å being reproduced, which were missing in the uniform abundance model.