The origin and evolution of the normal Type Ia SN 2018aoz with infant-phase reddening and excess emission

TL;DR

SN 2018aoz, a Type Ia supernova with early excess emission and surface iron-peak elements, provides insights into its progenitor and explosion mechanism, favoring a white dwarf companion and challenging some explosion models.

Contribution

This study advances understanding of SN 2018aoz's origin by analyzing its evolution to nebular phase, constraining progenitor and explosion models with new observational limits.

Findings

Excess emission may originate from surface iron-peak decay and ejecta interaction.

Nebular limits favor a white dwarf companion over a binary merger.

Asymmetric Chandrasekhar-mass explosion is less consistent with constraints.

Abstract

SN~2018aoz is a Type Ia SN with a $B$-band plateau and excess emission in the infant-phase light curves $\lesssim$ 1 day after first light, evidencing an over-density of surface iron-peak elements as shown in our previous study. Here, we advance the constraints on the nature and origin of SN~2018aoz based on its evolution until the nebular phase. Near-peak spectroscopic features show the SN is intermediate between two subtypes of normal Type Ia: Core-Normal and Broad-Line. The excess emission could have contributions from the radioactive decay of surface iron-peak elements as well as ejecta interaction with either the binary companion or a small torus of circumstellar material. Nebular-phase limits on H$\alpha$ and He~I favour a white dwarf companion, consistent with the small companion size constrained by the low early SN luminosity, while the absence of [O~I] and He~I disfavours a violent merger of the progenitor. Of the two main explosion mechanisms proposed to explain the distribution of surface iron-peak elements in SN~2018aoz, the asymmetric Chandrasekhar-mass explosion is less consistent with the progenitor constraints and the observed blueshifts of nebular-phase [Fe~II] and [Ni~II]. The helium-shell double-detonation explosion is compatible with the observed lack of C spectral features, but current 1-D models are incompatible with the infant-phase excess emission, $B_{\rm max}-V_{\rm max}$ color, and absence of nebular-phase [Ca~II]. Although the explosion processes of SN~2018aoz still need to be more precisely understood, the same processes could produce a significant fraction of Type Ia SNe that appear normal after $\sim$ 1 day.