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

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 $-$16 to over 190 days relative to maximum light. Most spectra were obtained during the photospheric phases after the supernova explosion. For each spectrum, the pseudo-equivalent widths (pEWs) and blueshift velocities of principal lines were measured. We further investigated the common spectral features by analysing their velocity and strength correlations across all subtypes. We identify the feature near 6200~Å in SNe Ib as H$\mathrmα$ 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.

Figures (20)

• Figure 1: Pie charts of numbers of SNe (left) and numbers of spectra (right) by types of our sample.
• Figure 2: Distribution of the SN redshift (upper-left), number of spectra for a single SN (upper-right), phases of spectra (lower-left), and the phase of the first spectrum of each SN (lower-right).
• Figure 3: Pre-processing method for our spectral data. (a) The input spectrum after corrections of redshift; (b) Spectrum after binning into logarithmic scale with d$\lambda_{\mathrm{ln}} = 0.0015$; (c) Smoothed spectrum with window length of 21, the pseudo continuum is plotted as a dotted line; (d) The normalized spectrum by dividing the continuum in (c).
• Figure 4: The distribution of $t_{\mathrm{max},V}-t_{\mathrm{max},B}$ versus $\Delta m_{15,B}$ of our SESN sample. The vertical lines mark the average delay (solid line) and 1-$\sigma$ range (dashed lines). Different types are plotted in different colours which are denoted at the upper right corner.
• Figure 5: The schematic diagram illustrating the method of defining the spectral line range. For the smoothed spectrum (upper panel), the first derivative of flux with respect to wavelength is computed (lower panel). Local maxima and minima are identified where the sign of the derivative changes, marked with red upward-pointing and blue downward-pointing triangles, respectively. At certain wavelengths, distinct inflection points are observed where the derivative approaches zero without a sign change; these are indicated by green upward-pointing triangles. The wavelength range of the H$\upalpha$ line is thus delineated by purple colour, while the local pseudo-continuum is represented by the black dashed line.