Asphericity in supernova explosions from late-time spectroscopy

Core- collapse supernovae (CC-SNe) are the explosions that announce the death of massive stars. Some CC- SNe are linked to long- duration gamma- ray bursts ( GRBs) and are highly aspherical. One important question is to what extent asphericity is common to all CC- SNe. Here we present late- time spectra for a number of CC- SNe from stripped- envelope stars and use them to explore any asphericity generated in the inner part of the exploding star, near the site of collapse. A range of oxygen emission- line profiles is observed, including a high incidence of double- peaked profiles, a distinct signature of an aspherical explosion. Our results suggest that all CC- SNe from stripped- envelope stars are aspherical explosions and that SNe accompanied by GRBs exhibit the highest degree of asphericity.