Spectral dataset of stripped-envelope supernovae from the Tsinghua supernova group

Context. The extent of envelope stripping in progenitor stars is directly reflected in the diversity of spectral features that are observed in stripped-envelope supernovae (SESNe). Aims. Through extensive spectral observation and analysis, we aim to clarify the statistical differences between the subclasses of SESNe. Methods. The Tsinghua supernova group obtained 249 optical spectra of 62 SESNe from 2010 to 2020, covering phases from –16 to over 190 days relative to maximum light. Most spectra were obtained during the photospheric phases after the supernova explosion. For each spectrum, the pseudo-equivalent widths and blueshift velocities of the principal lines were measured. We further investigated the common spectral features by analyzing their velocity and strength correlations across all subtypes. Results. We identified the feature near 6200 Å in SNe Ib as H α through a comparison with SNe IIb and Ic. This resolves inconsistent interpretations in the literature. Our finding reveals prevalent residual hydrogen in SNe Ib, further supporting a continuous stripping sequence from SNe IIb to Ib. The velocity among different subtypes of stripped-envelope SNe increases, with SNe IIb exhibiting the lowest line velocities, followed by Ib, Ic, and Ic-BL. Typically, the O I lines in SNe Ic/Ic-BL are stronger than those seen in SNe IIb/Ib. In nebular phases, the Ca II emission dominates O I in SNe IIb/Ib, while O I is stronger in SNe Ic, including in the He-rich SN 2016coi. This spectral dichotomy implies that progenitors of SNe Ic (BL) have more massive CO cores and hence higher initial masses.