Nebular emission-line profiles of Type Ib/c Supernovae - probing the
  ejecta asphericity
  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many of them previously unpublished.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.
