A physically motivated classification of stripped-envelope supernovae

Abstract

The classification of stripped-envelope supernovae (SE-SNe) is revisited using modern data-sets. Spectra are analysed using an empirical method to "blindly" categorise SNe according to spectral feature strength and appearance. This method makes a clear distinction between SNe that are He-rich (IIb/Ib) and He-poor (Ic) and further analysis is performed on each subgroup. For He-rich SNe the presence of H becomes the focus. The strength, velocity, and ratio between absorption and emission of H$α$ is measured, along with additional analysis of He I lines, in order to categorise the SNe. The He-poor SNe are ordered according to the number of absorption features $N$ present in the spectra, which is a measure of the degree of line blending. The kinetic energy per unit mass $E_\mathrm{k}/M_\mathrm{ej}$ is strongly affected by mass at high velocity and such situations principally occur when the outer density profile of the ejecta is shallow, leading to the blending of lines. Using the results, the existing SE-SN taxonomic scheme is adapted. He-rich SNe are split into four groups, IIb, IIb(I), Ib(II), and Ib, which represent H-rich to H-poor SNe. The SNe Ic category of broad-lined Ic (Ic-BL) is abandoned in favour of quantifying the line blending via $\left $ before peak. To better reflect the physical parameters of the explosions, the velocity of Si II at peak and the half-luminosity decay time $t_{+1/2}$ are included to give SNe Ic a designation of Ic-$\left \left(v_\mathrm{p,SiII}/t_{+1/2}\right)$.