Characteristic velocities of stripped-envelope core-collapse supernova cores

TL;DR

This study measures the inner ejecta velocities of 56 stripped-envelope core-collapse supernovae using nebular spectra, revealing a typical velocity around 4500 km/s and exploring the asphericity in broad-lined supernovae through 3D spectral modeling.

Contribution

It provides the first systematic measurement of core velocities in a large sample of stripped-envelope CC-SNe and investigates their asphericity using synthetic 3D nebular spectra.

Findings

Inner ejecta velocities cluster around 4500 km/s with small scatter.

Broad-lined SNe show higher and more variable core velocities.

Evidence suggests asphericity in the inner ejecta of broad-lined SNe.

Abstract

The velocity of the inner ejecta of stripped-envelope core-collapse supernovae (CC-SNe) is studied by means of an analysis of their nebular spectra. Stripped-envelope CC-SNe are the result of the explosion of bare cores of massive stars ($\geq 8$ M$_{\odot}$), and their late-time spectra are typically dominated by a strong [O {\sc i}] $\lambda\lambda$6300, 6363 emission line produced by the innermost, slow-moving ejecta which are not visible at earlier times as they are located below the photosphere. A characteristic velocity of the inner ejecta is obtained for a sample of 56 stripped-envelope CC-SNe of different spectral types (IIb, Ib, Ic) using direct measurements of the line width as well as spectral fitting. For most SNe, this value shows a small scatter around 4500 km s$^{-1}$. Observations ($< 100$ days) of stripped-envelope CC-SNe have revealed a subclass of very energetic SNe, termed broad-lined SNe (BL-SNe) or hypernovae, which are characterised by broad absorption lines in the early-time spectra, indicative of outer ejecta moving at very high velocity ($v \geq 0.1 c$). SNe identified as BL in the early phase show large variations of core velocities at late phases, with some having much higher and some having similar velocities with respect to regular CC-SNe. This might indicate asphericity of the inner ejecta of BL-SNe, a possibility we investigate using synthetic three-dimensional nebular spectra.