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

The Tsinghua Supernova Group analyzed 249 optical spectra of 62 stripped-envelope supernovae to establish statistical differences among subtypes, confirming a continuous envelope-stripping sequence from SNe IIb to Ic, identifying residual hydrogen in SNe Ib, and revealing that SNe Ic progenitors possess more massive CO cores based on distinct velocity trends and nebular emission line ratios.

Original authors: Danfeng Xiang, Xiaofeng Wang, Jujia Zhang, Shengyu Yan, Han Lin, Liming Rui, Jun Mo, Xinghan Zhang, Hanna Sai, Cheng Miao, Gaobo Xi, Zhihao Chen, Fangzhou Guo, Xiaoran Ma, Gaici Li, Tianmeng Zhang, Li

Published 2026-03-16

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CC BY 4.0

Original authors: Danfeng Xiang, Xiaofeng Wang, Jujia Zhang, Shengyu Yan, Han Lin, Liming Rui, Jun Mo, Xinghan Zhang, Hanna Sai, Cheng Miao, Gaobo Xi, Zhihao Chen, Fangzhou Guo, Xiaoran Ma, Gaici Li, Tianmeng Zhang, Liyang Chen, Jialian Liu, Wenxiong Li, Xulin Zhao, Fang Huang, Yongzhi Cai, Weili Lin, Jie Lin, Chengyuan Wu, Maokai Hu, Cuiying Song, Jicheng Zhang, Qiqi Xia, Zhitong Li, Linyi Li, Kaicheng Zhang, Qian Zhai, Juncheng Chen, Zhou Fan, Jianning Fu, Shengbang Qian, Hong Wu, Xue-Bing Wu, Huawei Zhang, Junbo Zhang, Liyun Zhang, Jie Zheng

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a grand theater, and massive stars are the lead actors. When these actors reach the end of their lives, they don't just fade away; they explode in a spectacular finale called a supernova.

For a long time, astronomers have been trying to figure out the "backstory" of these explosions. Specifically, they wanted to know: How much of the star's outer "costume" did it lose before the explosion?

This paper is like a massive, detailed detective report from the Tsinghua Supernova Group (a team of astronomers in China). They spent a decade (2010–2020) watching 62 of these exploding stars and taking 249 "snapshots" (spectra) of their light. Here is what they found, explained simply:

1. The Great Costume Strip-Off

Think of a massive star as wearing a heavy winter coat (Hydrogen) and a thick sweater (Helium) over its core.

Type IIb: The star lost its heavy coat but kept the sweater.
Type Ib: The star lost the coat and most of the sweater, but maybe a little bit of the sweater is still there.
Type Ic: The star stripped off everything, down to the bare skin (Carbon and Oxygen core).
Type Ic-BL: These are the "extreme strippers," exploding with incredible speed and energy.

The team wanted to see if this was a smooth, continuous process (like peeling an onion layer by layer) or if the stars just randomly decided to lose different amounts of clothing.

2. The Big Mystery: The "Ghost" at 6200 Å

In the early days of a Type Ib explosion, there is a weird dark spot in the light spectrum around the color red (6200 Å). For years, astronomers argued about what this spot was:

Theory A: It's a leftover piece of the Hydrogen coat (H-alpha).
Theory B: It's just a different element (Silicon) acting weird.

The Verdict: The Tsinghua team acted like forensic scientists. They compared the "ghost" in Type Ib stars to Type IIb stars (which definitely have Hydrogen) and Type Ic stars (which definitely don't).

Result: They proved the ghost is Hydrogen.
Why it matters: This means Type Ib stars didn't lose all their Hydrogen. They kept a thin layer. This confirms that the stripping process is a continuous spectrum. It's not a jump from "fully dressed" to "naked"; it's a gradual undressing.

3. The Speed Limit

The team noticed a clear pattern in how fast the debris was flying out:

Type IIb: The slowest runners.
Type Ib: A bit faster.
Type Ic: Even faster.
Type Ic-BL: The Usain Bolts of the supernova world.

The Analogy: Imagine throwing a ball. If you throw a heavy, bulky object (a star with lots of outer layers), it moves slower. If you throw a lightweight, stripped-down object, it flies much faster. The fact that the "naked" stars (Ic) are the fastest suggests they were likely born from heavier, more massive stars that had more energy to begin with.

4. The "After-Party" (Nebular Phase)

After the initial flash, the explosion settles down into a "nebular phase" (like the quiet after a party). By looking at the light emitted by the cooling gas, the team could weigh the star's core.

The Clue: They looked at the ratio of Oxygen light to Calcium light.
The Finding: Type Ic stars (the "naked" ones) had way more Oxygen than Type IIb/Ib stars.
The Conclusion: This proves that Type Ic stars came from much heavier parents. They had massive cores that were completely stripped of their outer layers, leaving behind a heavy, dense core that exploded with more force.

5. Special Cases

The paper also highlighted some "maverick" stars:

SN 2016coi: A star that looked like a "naked" Type Ic but was actually wearing a tiny bit of a Helium sweater. It's the perfect bridge between the two types.
SN 2019ehk: A star that seemed to be a Type Ib but was actually a rare "Calcium-rich" explosion, possibly from a different kind of star system entirely (like two white dwarfs crashing).

The Bottom Line

This paper is a massive step forward in understanding how stars die. By gathering a huge library of data, the Tsinghua team showed us that:

Stripping is a gradient: Stars don't just snap from "dressed" to "naked"; they peel off layers gradually.
More mass = More speed: The stars that lose the most layers are the most massive and explode the fastest.
Hydrogen is everywhere: Even in stars thought to be "Hydrogen-free," a little bit often remains.

It's like finally having the complete instruction manual for how massive stars shed their clothes before their final, explosive dance.

1. Problem Statement

Stripped-Envelope Supernovae (SESNe) are core-collapse events where the progenitor star has lost its hydrogen (H) and/or helium (He) envelopes prior to explosion. While the classification into subtypes (IIb, Ib, Ic, Ic-BL) is based on spectral features, the physical continuity of the envelope stripping process and the specific progenitor properties (mass, binary interaction) remain debated.
Key open questions include:

Is the transition from H-rich to H-poor/He-poor a continuous sequence or distinct categories?
What is the true nature of the absorption feature near 6200 Å in SNe Ib (is it H $\alpha$ or Si II)?
How do spectral line velocities and intensities correlate across subtypes to reveal differences in ejecta structure and progenitor mass?
Do SNe Ic progenitors possess significantly more massive Carbon-Oxygen (CO) cores than IIb/Ib progenitors?

2. Methodology

The authors present a homogeneous dataset and analysis pipeline developed by the Tsinghua Supernova Group.

Dataset Construction:
- Observations: 249 optical spectra of 62 SESNe collected between 2010 and 2020.
- Instruments: 2.16-m telescope at Beijing Xinglong Observatory (XLT) and 2.4-m telescope at Lijiang Observatory (LJT).
- Coverage: Phases range from -16 to >190 days relative to maximum light.
- Sample Composition: 12 SNe IIb, 20 SNe Ib, 19 SNe Ic, and 11 SNe Ic-BL.
- Data Reduction: Standard IRAF pipelines were used for bias, flat-field, wavelength (He/Ar, Fe/Ar), and flux calibration. Telluric absorption was removed.
Pre-processing & Analysis:
- Homogenization: Spectra were converted to the rest frame, binned logarithmically ( $d\lambda_{ln} = 0.0015$ ), smoothed (Savitzky-Golay filter), and normalized against a pseudo-continuum.
- Phase Determination: Maximum light dates were derived from photometric light curves (B/V bands) or estimated via spectral cross-correlation using SNID.
- Line Measurements: Pseudo-equivalent widths (pEWs) and expansion velocities (blueshifts of absorption minima) were measured for principal lines (H $\alpha$ , He I, Si II, O I, Ca II, Fe II, etc.).
- Statistical Tools: Spearman's rank correlation coefficients ( $\rho$ ) were calculated to analyze correlations between line velocities and intensities across subtypes.

3. Key Contributions

Resolution of the 6200 Å Feature: The paper definitively identifies the absorption feature near 6200 Å in early-phase SNe Ib as H $\alpha$ , resolving conflicting literature interpretations that suggested it might be Si II.
Comprehensive Statistical Analysis: By analyzing a large, homogeneous sample, the study quantifies the evolutionary trends of line strengths and velocities, moving beyond case studies to population-level statistics.
Nebular Phase Diagnostics: The inclusion of 11 nebular-phase spectra allows for a robust comparison of [O I] and [Ca II] luminosities, serving as a proxy for progenitor CO core mass.

4. Key Results

A. Spectral Identification and Evolution

H $\alpha$ in SNe Ib: The feature near 6200 Å is identified as H $\alpha$ . While SNe Ib show residual hydrogen, its strength declines rapidly before maximum light, unlike SNe IIb where H $\alpha$ strengthens until after maximum. This supports a continuous stripping sequence from IIb to Ib, where Ib progenitors retain a thin H envelope.
Velocity Trends: A clear velocity gradient exists across subtypes: SNe IIb < Ib < Ic < Ic-BL.
- SNe Ic-BL exhibit the highest velocities, implying intrinsically higher explosion energies.
- SNe Ib show systematically higher H $\alpha$ velocities than SNe IIb, suggesting a thinner hydrogen shell in Ib progenitors.
Line Correlations:
- Velocities of common lines (Fe II, O I, Ca II) are positively correlated across all subtypes, consistent with homologous expansion.
- Correlations are strongest in SNe Ic and Ic-BL, suggesting more compact progenitor cores compared to the more dispersed ejecta of IIb/Ib.

B. Line Intensity and Composition

Oxygen vs. Helium: O I lines are strongest in SNe Ic/Ic-BL, intermediate in Ib, and weakest in IIb.
He I Evolution: He I lines generally decline after maximum in SNe Ib. The He I $\lambda$ 7281 line shows no clear weakening, likely due to contamination from the emerging [Ca II] emission.
Differentiation: In the early phase ( $t < +10$ days), SNe IIb and Ib are distinguishable in the H $\alpha$ -Fe II diagnostic space: SNe IIb have strong H $\alpha$ and weak Fe II, while SNe Ib have weaker H $\alpha$ and stronger Fe II.

C. Nebular Phase and Progenitor Mass

Dominant Emission: In the nebular phase ( $>90$ days), [Ca II] emission dominates over [O I] in SNe IIb and Ib. Conversely, [O I] dominates over [Ca II] in SNe Ic (including the He-rich SN 2016coi).
Progenitor Mass Implication: The ratio $L_{[O I]} / L_{[Ca II]}$ is a tracer of the CO core mass. The significantly higher ratios in SNe Ic indicate that Ic progenitors have more massive CO cores and higher initial masses compared to IIb/Ib progenitors.
SN 2016coi: Despite showing He I features in the photospheric phase (making it a "Ib-BL"), its nebular spectrum resembles a typical SN Ic with strong [O I], confirming it shares the massive progenitor characteristics of the Ic-BL class.

5. Significance

This work provides a critical, homogeneous dataset that bridges the gap between theoretical models and observational reality for SESNe.

Progenitor Channel: The results strongly favor binary interaction as the dominant channel for SESNe, as the continuous stripping sequence (IIb $\to$ Ib $\to$ Ic) and the mass differences inferred from nebular lines align better with binary evolution models than single-star evolution.
Classification Clarity: The identification of H $\alpha$ in SNe Ib clarifies the spectral transition between IIb and Ib, reducing misclassification in future surveys.
Mass Constraints: The nebular analysis provides robust evidence that SNe Ic (and Ic-BL) originate from significantly more massive stars than SNe IIb and Ib, refining our understanding of the upper mass limits for core-collapse supernovae.
Resource: The dataset is made publicly available via the Weizmann Interactive Supernova Data Repository (WiseREP), serving as a foundational resource for future statistical studies and machine learning applications in supernova astrophysics.