Nebular emission-line profiles of Type Ib/c Supernovae - probing the ejecta asphericity

TL;DR

This study analyzes late-time spectra of Type Ib/c supernovae to investigate ejecta asphericity, revealing that at least half are aspherical and that line profiles suggest complex ejecta geometries and matter distribution.

Contribution

It provides a detailed spectral analysis of 98 spectra from 39 supernovae, classifies ejecta geometries, and challenges the applicability of bipolar jet models for all cases.

Findings

At least 50% of Type Ib/c supernovae are aspherical.

Line profiles indicate high-velocity blobs and possible neutron-star kicks.

Residual opacity explains observed blueshifts in line centroids.

Abstract

In order to assess qualitatively the ejecta geometry of stripped-envelope core-collapse supernovae, we investigate 98 late-time spectra of 39 objects, many of them previously unpublished. We perform a Gauss-fitting of the [O I] 6300, 6364 feature in all spectra, with the position, full width at half maximum (FWHM) and intensity of the 6300 Gaussian as free parameters, and the 6364 Gaussian added appropriately to account for the doublet nature of the [O I] feature. On the basis of the best-fit parameters, the objects are organised into morphological classes, and we conclude that at least half of all Type Ib/c supernovae must be aspherical. Bipolar jet-models do not seem to be universally applicable, as we find too few symmetric double-peaked [O I] profiles. In some objects the [O I] line exhibits a variety of shifted secondary peaks or shoulders, interpreted as blobs of matter ejected at high velocity and possibly accompanied by neutron-star kicks to assure momentum conservation. At phases earlier than ~200d, a systematic blueshift of the [O I] 6300, 6364 line centroids can be discerned. Residual opacity provides the most convincing explanation of this phenomenon, photons emitted on the rear side of the SN being scattered or absorbed on their way through the ejecta. Once modified to account for the doublet nature of the oxygen feature, the profile of Mg I] 4571 at sufficiently late phases generally resembles that of [O I] 6300, 6364, suggesting negligible contamination from other lines and confirming that O and Mg are similarly distributed within the ejecta.