Infant-phase reddening by surface Fe-peak elements in a normal Type Ia Supernova

TL;DR

This study reports the earliest detection of a Type Ia Supernova, revealing surface Fe-peak elements causing early reddening, which provides new insights into the explosion mechanisms and outer ejecta composition.

Contribution

It presents the first infant-phase observations of a normal Type Ia Supernova, showing surface Fe-peak elements influencing early light and color evolution, suggesting enhanced surface nuclear burning or mixing.

Findings

Detected the earliest signals of a Type Ia Supernova at -10.5 magnitude.

Observed a rapid redward color evolution between 1 and 12 hours after first light.

Identified surface Fe-peak elements as the cause of early reddening.

Abstract

Type Ia Supernovae are thermonuclear explosions of white dwarf stars. They play a central role in the chemical evolution of the Universe and are an important measure of cosmological distances. However, outstanding questions remain about their origins. Despite extensive efforts to obtain natal information from their earliest signals, observations have thus far failed to identify how the majority of them explode. Here, we present infant-phase detections of SN 2018aoz from a brightness of -10.5 absolute AB magnitudes -- the lowest luminosity early Type Ia signals ever detected -- revealing a hitherto unseen plateau in the $B$-band that results in a rapid redward color evolution between 1.0 and 12.4 hours after the estimated epoch of first light. The missing $B$-band flux is best-explained by line-blanket absorption from Fe-peak elements in the outer 1% of the ejected mass. The observed $B-V$ color evolution of the SN also matches the prediction from an over-density of Fe-peak elements in the same outer 1% of the ejected mass, whereas bluer colors are expected from a purely monotonic distribution of Fe-peak elements. The presence of excess nucleosynthetic material in the extreme outer layers of the ejecta points to enhanced surface nuclear burning or extended sub-sonic mixing processes in some normal Type Ia Supernova explosions.