Detection of [C I] Emission in Nebular Spectra of a Peculiar Type Ia Supernova 2022pul

TL;DR

This study reports the first detection of narrow [C I] emission in the nebular spectra of a peculiar super-Chandrasekhar-mass Type Ia supernova, providing insights into its explosion mechanism and ejecta composition.

Contribution

It presents new late-time optical observations of SN 2022pul, including the first identification of [C I] emission, and discusses implications for explosion models involving carbon and white dwarf mergers.

Findings

Detection of asymmetric [C I] emission at +515 days.

Carbon mass comparable to oxygen in the ejecta.

Supports explosion models with significant unburnt carbon.

Abstract

SN 2022pul gains special attention due to its possible origin of a super-Chandarsekhar-mass white dwarf explosion (or called a 03fg-like type Ia supernova), which shows prominent [O I], [Ne II], and [Ca II] lines in its late-time spectra taken at $\sim+$300 days after the peak brightness. In this paper, we present new optical observations for this peculiar object, extending up to over 500 days after the peak brightness. In particular, in the $t\approx+515$ days spectrum, we identified for the first time the presence of narrow emission from [C I] $\lambda\lambda9824, 9850$, which appears asymmetric and quite similar to the accompanied [O I] $\lambda6300$ line in strength and profile. Based on the violent merger model that accounts well for previous observations but leaves little carbon in the center of the ejecta, this carbon line can be reproduced by increasing the degree of clumping in the ejecta and setting the carbon mass the same as that of oxygen ($\sim$0.06 $M_{\odot}$) in the innermost region ($\lesssim 2000$ km s$^{-1}$). In principle, the central carbon could come from the secondary white dwarf (WD) if it is ignited when hit by the shockwave of the explosion of the primary WD and explodes as a Ca-rich supernova, whereas pure deflagration of a super-Chandarsekhar-mass WD can account for such unburnt carbon more naturally.