Nebular phase observations of the type-Ib supernova iPTF13bvn favour a binary progenitor

TL;DR

Late-time nebular observations of supernova iPTF13bvn suggest its progenitor was a lower-mass star in a binary system, challenging the idea of a single Wolf-Rayet star origin.

Contribution

This study provides new nebular phase spectral data and estimates the progenitor's core oxygen mass, supporting a binary progenitor scenario over a massive Wolf-Rayet star.

Findings

Progenitor's core oxygen mass estimated at ≤0.7 M☉

Progenitor's initial mass likely 15-17 M☉

Supports binary progenitor hypothesis

Abstract

Aims. We present and analyse late-time observations of the type-Ib supernova with possible pre-supernova progenitor detection, iPTF13bvn, taken at $\sim$300 days after the explosion, and discuss these in the context of constraints on the supernova's progenitor. Previous studies have proposed two possible natures for the progenitor of the supernova, i.e. a massive Wolf-Rayet star or a lower-mass star in close binary system. Methods. Our observations show that the supernova has entered the nebular phase, with the spectrum dominated by Mg~I]$\lambda\lambda$4571, [O~I]$\lambda\lambda$6300, 6364, and [Ca~II]$\lambda\lambda$7291, 7324 emission lines. We measured the emission line fluxes to estimate the core oxygen mass and compare the [O~I]/[Ca~II] line ratio with other supernovae. Results. The core oxygen mass of the supernova progenitor was estimated to be $\lesssim$0.7 M$_\odot$, which implies initial progenitor mass not exceeding $\sim$15 -- 17 M$_\odot$. Since the derived mass is too small for a single star to become a Wolf-Rayet star, this result lends more support to the binary nature of the progenitor star of iPTF13bvn. The comparison of [O~I]/[Ca~II] line ratio with other supernovae also shows that iPTF13bvn appears to be in close association with the lower-mass progenitors of stripped-envelope and type-II supernovae.