Spectral Dataset of Stripped-Envelope Supernovae from the Tsinghua Supernova Group

TL;DR

This paper presents a comprehensive spectral dataset of 62 stripped-envelope supernovae, analyzing their spectral features to understand progenitor properties and subclass differences.

Contribution

It provides extensive spectral observations and analysis, clarifying spectral feature interpretations and revealing trends across supernova subclasses.

Findings

Residual hydrogen is prevalent in SNe Ib, supporting a continuous stripping sequence.

Velocity increases from SNe IIb to Ic, indicating more massive progenitors for SNe Ic.

Nebular phase spectra show different element dominance, implying varying progenitor core masses.

Abstract

The extent of envelope stripping in the progenitor stars is directly reflected in the diversity of spectral features observed in stripped-envelope supernovae (SESNe). Through extensive spectral observation and analysis, we aim to clarify the statistical differences between the subclasses of SESNe. The Tsinghua Supernova group obtained 249 optical spectra of 62 SESNe during the years from 2010 to 2020, covering phases from $-$16 to over 190 days relative to maximum light. Most spectra were obtained during the photospheric phases after the supernova explosion. For each spectrum, the pseudo-equivalent widths (pEWs) and blueshift velocities of principal lines were measured. We further investigated the common spectral features by analysing their velocity and strength correlations across all subtypes. We identify the feature near 6200~\AA\ in SNe Ib as H$\mathrm{\alpha}$ through comparison with SNe IIb and Ic, which resolves inconsistent literature interpretations. Our finding reveals prevalent residual hydrogen in SNe Ib, further supporting a continuous stripping sequence from SNe IIb to Ib. We observe a trend in increasing velocity among different subtypes of stripped-envelope SNe, 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 over [O I] in SNe IIb/Ib while [O I] is stronger in SNe Ic, including 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.