Statistical Properties of the Nebular Spectra of 103 Stripped-envelope Core-collapse Supernovae*

Abstract

We present an analysis of the nebular spectra of 103 stripped-envelope (SE) supernovae (SNe) collected from the literature and observed with the Subaru Telescope from 2002 to 2012, focusing on [O i ] λλ 6300, 6363. The line profile and width of [O i ] are employed to infer the ejecta geometry and the expansion velocity of the inner core; these two measurements are then compared with the SN subtypes, and further with the [O i ]/[Ca ii ] ratio, which is used as an indicator of the progenitor CO core mass. Based on the best-fit results of the [O i ] profile, the objects are classified into different morphological groups, and we conclude that the deviation from spherical symmetry is a common feature for all types of SESNe. There is a hint (at the ∼1 σ level) that the distributions of the line profile fractions are different between canonical SESNe and broad-line SNe Ic. A correlation between [O i ] width and [O i ]/[Ca ii ] is discerned, indicating that the oxygen-rich material tends to expand faster for objects with a more massive CO core. Such a correlation can be utilized to constrain the relation between the progenitor mass and the kinetic energy of the explosion. Further, when [O i ]/[Ca ii ] increases, the fraction of objects with Gaussian [O i ] profile increases, while those with double-peaked profile decreases. This phenomenon connects ejecta geometry and the progenitor CO core mass.