Stripped-Envelope Supernovae (SESNe)

• SESNe are core-collapse events marked by the loss of outer hydrogen—and sometimes helium—layers, leading to diverse spectral types like IIb, Ib, Ic, and Ic-BL.
• Detailed spectral diagnostics and light curve analyses help constrain ejecta mass, explosion energetics, and core composition through photometric and NIR observations.
• Studies of SESNe illuminate the roles of binary interactions, stellar winds, and envelope stripping, advancing our understanding of massive star evolution and explosion physics.

Stripped-envelope supernovae (SESNe) are a class of core-collapse supernovae characterized by the substantial depletion or complete removal of the progenitor star's outer hydrogen and, in some cases, helium envelopes prior to explosion. This heterogeneous class encompasses SNe IIb, Ib, Ic, Ic-BL, Ibn, and related subtypes, unified by the absence or marked weakness of hydrogen features in their spectra. SESNe serve as key probes of stellar evolution, binary interactions, and the endpoints of massive star life cycles. Their observed diversity in photometric, spectroscopic, and environmental characteristics reflects the complex physics of progenitor mass loss, explosion energy deposition, nucleosynthesis, and circumstellar interaction.

1. Taxonomy, Spectral Classification, and Diversity

The SESNe family comprises several spectral subtypes that are distinguished by detailed spectral diagnostics, with modern surveys providing comprehensive taxonomic schemes grounded in empirical and physically motivated classification criteria (Prentice et al., 2017, Holmbo et al., 2023, Shahbandeh et al., 2021). The canonical types are defined as follows:

• Type IIb: Show hydrogen features that are strong pre-maximum but decline post-maximum, indicative of partial envelope stripping.
• Type Ib: Lack hydrogen lines in early-time spectra but exhibit He I features, corresponding to progenitors that have lost their hydrogen layer but retain helium.
• Type Ic: Lack both hydrogen and helium lines, consistent with complete removal of the outer envelopes.
• Type Ic-BL: Distinguished by extremely broad absorption lines and high ejecta velocities; commonly associated with long gamma-ray bursts and often interpreted as originating from highly energetic, massive progenitor explosions.

Physically motivated classification now utilizes spectral metrics such as the equivalent width (EW) and absorption-to-emission ratio (f_em/f_abs) of the Hα region for IIb/Ib events, and the discrete number of absorption features N within key wavelength bands to quantify blending in He-poor (Ic) SNe (Prentice et al., 2017). These parameters, combined with light-curve half-luminosity decay time (t_{+1/2}) and photospheric velocities (notably v_p,SiII), yield type labels that encode physical conditions, e.g., Ic-6(11/9) for a SN Ic with ⟨N⟩ = 6, peak Si II velocity 11,000 km/s, and t_{+1/2} = 9 days.

Near-infrared (NIR) spectroscopy reveals a sharp dichotomy between "He-rich" (IIb/Ib) and "He-poor" (Ic/Ic-BL) events, with the He I λ2.0581 μm line acting as an effective tracer of helium mass retention (Shahbandeh et al., 2021). Approximately half of He-poor events show faint He I NIR features, suggesting incomplete stripping or excitation effects.

These classification advances facilitate robust template construction and better discrimination among SESNe, especially utilizing libraries of continuum-removed, phase-resolved spectral templates for SNID and similar cross-correlation classifiers (Liu et al., 2014).

2. Physical Origin: Envelope Stripping Mechanisms and Progenitor Pathways

SESNe emerge as a consequence of advanced mass loss in massive stars (M_{ZAMS} ≳ 8 M_⊙) prior to core collapse. Two principal mass-loss channels determine the final envelope configuration and thus the SN subtype (Fang et al., 2018, Sun et al., 2022):

1. Binary interaction (mass-insensitive hydrogen stripping):
   • Through Roche-lobe overflow or stable mass transfer, a binary companion removes the hydrogen envelope, a mechanism effective for a broad range of initial masses.
   • Progenitors stripped by this pathway yield Type IIb (hydrogen-poor, He-rich), Ib (hydrogen-free, He-rich), and, occasionally, Ibn (hydrogen-free, He-enriched with CSM interaction) SNe.
2. Stellar winds or eruptive mass loss (mass-sensitive helium stripping):
   • Powerful winds act more efficiently in higher-mass stars (M_{ZAMS} ≳ 17 M_⊙), leading to additional loss of the outer helium layer—crucial for forming Type Ic/Ic-BL objects.
   • SN Ic progenitors are thus both hydrogen- and helium-deficient, and environmental age-dating shows that they arise in regions with systematically younger stellar populations, confirming a higher-mass bias (Sun et al., 2022).

This "hybrid envelope-stripping" scenario is strongly supported by nebular-phase spectroscopy: the [O I]/[Ca II] ratio, a tracer of the CO core mass, is systematically higher in SNe Ic, indicating an origin in more massive progenitors (Fang et al., 2018). Meanwhile, the progenitor masses for IIb/Ib events generally lie below 17–25 M_⊙, often overlapping the mass range of SN II progenitors.

3. Light Curve Morphology, Explosion Energetics, and Ejecta Structure

The photometric evolution of SESNe encodes crucial information about the explosion physics, diffusion timescales, ejecta mass, and energy. Bolometric light curves—constructed from multi-band photometry—are modeled using a combination of analytic prescriptions (Arnett-type models) and late-time gamma-ray deposition models, each sensitive to different physical parameters (Prentice et al., 2018, Haynie et al., 2023, Lu et al., 17 Nov 2024).

• Ejecta Mass and Energy: Analytical formulas connect the diffusion time (proportional to the width of the light curve) and photospheric velocity to the ejecta mass, though the accuracy is strongly impacted by assumptions about opacity, ^{56}Ni mixing, and helium recombination. For example, the commonly used relation:

$$M_{ej} = \frac{1}{2} \frac{\beta c}{\kappa} v_{ph} t_r^2$$

(with $\beta \simeq 13.8$, $\kappa$ = constant opacity)

may overestimate the ejecta mass, especially for high-mass progenitors, by up to a factor of 2.6. Instead, ejecta mass constraints with ∼20% accuracy are achieved using late-time tail slopes governed by gamma-ray leakage (Haynie et al., 2023, Lu et al., 17 Nov 2024).

• ^{56}Ni Synthesis and Peak Luminosity:
  • SESNe typically synthesize more ^{56}Ni than Type II SNe (by a factor of ∼3), inferred from both direct tail-fitting methods and improved analytic calibrations (Afsariardchi et al., 2020).
  • Observed peak luminosities sometimes exceed the maximum achievable in standard neutrino-driven explosion models, especially in normal Ibc SNe, indicating either underestimated energy sources (e.g., early shock-cooling or magnetar input) or a deficiency in theoretical modeling (Sollerman et al., 2021). In many cases, 7–50% of the peak luminosity must be attributed to sources beyond radioactive decay.
• Photometric Diversity and Multiband Evolution:
  • Recent work has produced robust UV–NIR light curve templates for multiple SESNe subtypes using Gaussian Process regression on data from hundreds of SNe (Khakpash et al., 2 May 2024).
  • Subtypes such as SNe Ibn and Ic-BL exhibit significantly faster rises and declines (as measured by Δm_{15} and Δm_{-10}) than normal SNe Ib, IIb, or Ic.
  • The incidence of double-peaked light curves, particularly among some IIb and Ib events, is increasingly recognized: the first peak may be associated with shock-cooling, surface ^{56}Ni, or alternative energy injection mechanisms, while the second corresponds to the main ^{56}Ni-powered phase (Sharma et al., 4 Jul 2025).
  • Statistical analysis confirms correlations between the two peak luminosities in such double-peaked SESNe, with Δt^{21} (peak separation) and ΔM^{21} (peak magnitude difference) mapping subgroups to distinct energy sources (magnetar, double‐nickel, fallback, or CSM interaction scenarios).

4. Nebular Spectra, Progenitor Core Mass, and Ejecta Geometry

The late nebular phase, when the ejecta is optically thin, provides access to the unambiguous measurement of inner core composition, geometry, and explosion energetics:

• [O I] and [Ca II] Diagnostics:
  • The [O I] λλ6300,6363 emission profile (width and morphology) encodes the expansion velocity and asymmetry of the oxygen-rich core, while the [O I]/[Ca II] flux ratio is a strong function of the CO core mass (Fang et al., 2022, Fang et al., 2023).
  • Empirically, the velocity-width of [O I] scales with the CO core mass, leading to a derived energetic scaling [$E_K \propto M_{CO}^{1.39 \pm 0.09}$], with both He-rich and He-poor (Ic/Ic-BL) SESNe following the same energy–core-mass relation.
• Explosion Geometry and Morphological Classes:
  • Gaussian, narrow-core, double-peaked, and asymmetric [O I] morphologies are all observed, with deviations from spherical symmetry (bipolar or clumpy ejecta) being common in all SESNe types.
  • The fraction of double-peaked profiles (indicative of toroidal or highly axisymmetric geometry) decreases with increasing [O I]/[Ca II], suggesting a trend toward more "spherical" inner cores as the progenitor mass increases.
• Implications for Explosion Mechanisms:
  • The consistent M_CO–E_K scaling between SESN subtypes implies a universal explosion mechanism, with differences in spectral appearance resulting primarily from outer layer stripping rather than intrinsic inner dynamics (Fang et al., 2023). This supports models in which neutrino-driven and turbulence-aided core-collapse processes are modified by the final core mass and density profile (Lu et al., 17 Nov 2024).

5. Binary Demographics, Companion Constraints, and Population Synthesis

Direct HST imaging and population-synthesis modeling have begun to clarify the prevalence and nature of binary companions at the time of explosion (Zapartas et al., 18 Aug 2025). The current statistical picture is as follows:

• Binary Origin Prevalence:
  • Population synthesis with the POSYDON framework predicts that 80–90% of Type Ib/c and 60–85% of IIb SESNe retain a rapidly rotating, main-sequence stellar companion at explosion, with most of the stripping occurring via stable, rotationally limited mass transfer (typical accretion efficiencies $\beta_{rot} \sim 4-7\%$).
  • Observations confirm that the fraction of SESNe with detected companions generally matches model predictions given image depth, with most non-detections explained by faint, undetectable companions.
• Inefficient Accretion and Wolf-Rayet Non-Explodability:
  • The observed faintness of the companions, combined with model likelihood analysis, disfavors strongly conservative mass transfer (efficiency $\beta = 100\%$), arguing instead for rotationally limited, inefficient transfer.
  • The results further support the conclusion that the most massive hydrogen- and helium-stripped Wolf-Rayet stars, subject to strong winds at high metallicity, are more likely to undergo failed core collapse (direct black hole formation) rather than explosive SESN events.
• Metallicity Effects:
  • The fraction of SESNe with detectable companions is inversely correlated with metallicity, given enhanced wind-driven mass loss at high Z. At low metallicity, binary stripping is the dominant pathway, explaining the high companion fractions seen in SESN populations even in metal-poor environments.
• Implications for Progenitor Rotation:
  • The main-sequence companions in SESN-producing binaries are predicted to rotate at a high fraction ($\omega/\omega_{crit} \gtrsim 0.8$) of breakup, implying significant spin-up during mass transfer and potential future evolutionary consequences (e.g., as gravitational-wave sources).

6. Central Engines, Extreme Events, and Alternative Power Sources

While normal SESNe can often be explained within the radioactive decay paradigm, several sub-populations require additional energy input or unusual ejecta/envelope configurations:

• Magnetar-driven Events:
  • Millisecond magnetars appear to power the most energetic SESNe subclasses—Type Ic superluminous SNe, broad-lined Ic, and fast blue optical transients—through injection of rotational energy (E_{rot,i}) on spin-down timescales. Detailed binary evolution models show that tidal spin-up and moderate angular-momentum transport in close binaries naturally lead to fast-rotating cores compatible with this scenario (Hu et al., 2023, Kumar, 12 Dec 2024).
  • Magnetar-powered light-curve models, constrained via χ²-minimization against broad multi-band observations, yield spin periods and field strengths (P_i, B) consistent with these central engine requirements, with explosion energies frequently exceeding those achievable in standard delayed-neutrino mechanisms, sometimes necessitating recourse to jet-driven (jittering jet) explosion models.
• Double-Peaked and Transiently Powered SESNe:
  • The landscape of double-peaked light curves reveals events where neither shock-cooling nor a canonical ^{56}Ni distribution suffices to explain the observed evolution (Sharma et al., 4 Jul 2025). Subgroups within this population are attributable to double-nickel structures (near-surface and deeply buried ^{56}Ni), magnetar input, fallback accretion, or strong circumstellar interaction.
  • Mapping the separation and luminosity difference (Δt^{21}, ΔM^{21}) between peaks provides an effective phase-space to infer the dominant powering mechanism.
• Systematic Tensions and Ongoing Debates:
  • The discovery of normal SNe Ibc exceeding the predicted theoretical maximum luminosity (even after accounting for plausible bolometric corrections and host extinction) introduces a persistent tension with explosion models limited by neutrino-driven ^{56}Ni production (Sollerman et al., 2021, Afsariardchi et al., 2020).

7. Future Directions, Template Construction, and Survey Implications

Current and next-generation surveys (e.g., LSST, ZTF) will vastly expand SESNe samples, posing both challenges and opportunities:

• Classification and Template Infrastructure:
  • The expansion of template libraries (e.g., SNID, multi-filter GP-templated light curves) is essential for efficient and accurate photometric and spectroscopic classification in the low-cadence, follow-up-limited era (Liu et al., 2014, Khakpash et al., 2 May 2024).
  • Machine-learning approaches, supported by data-driven empirical templates, are expected to improve subtype discrimination, but caution is warranted due to differences between real and simulated SN light curves, as identified in PLAsTiCC/ELAsTiCC simulation comparisons.
• Companion Demographics as Progenitor Benchmarks:
  • Increasing the sample of direct binary companion detections and non-detections, especially with time-resolved HST and future facility imaging, will sharpen constraints on mass-transfer physics, accretion efficiency, and the rate of successful versus failed explosions in massive Wolf-Rayet stars.
• Nebular-phase Spectroscopy and Explosion Modeling:
  • Systematic nebular spectroscopy and coupled radiative transfer/hydrodynamics (e.g., with TARDIS, SNEC, FLASH) will continue to calibrate proxies for progenitor core mass and explosion energy, while continued theoretical refinement is needed in opacity treatment and mixing prescriptions (Lu et al., 17 Nov 2024).
• Statistical Analyses and Population Synthesis:
  • The integration of observed and predicted companion luminosity distributions, rotation distributions, and metallicity-dependent frequency of SESNe will refine the inferred evolutionary channels, the role of inefficient accretion, and the fate of high-mass stripped progenitors (Zapartas et al., 18 Aug 2025).

The SESNe family, though defined by common spectroscopic signatures of severe envelope stripping, incorporates a rich set of physical pathways, explosion morphologies, and central engine properties. Ongoing development of classification schemes, systematic photometric and spectroscopic surveys, and population-synthesis benchmarking are converging to clarify the interplay between binary interaction, mass loss, progenitor structure, and the diverse manifestations of massive star death.