Type Ibn Supernovae: Characteristics and Origins

Updated 14 August 2025

Type Ibn supernovae are rare, fast-evolving explosions characterized by narrow He I emission lines and minimal hydrogen, signaling extreme pre-supernova mass loss.
Their rapidly declining light curves, often with double peaks, reflect complex interactions between the SN ejecta and a dense, confined He-rich circumstellar medium.
Diverse progenitor scenarios, from massive Wolf–Rayet stars to binary and ultra-stripped systems, underline the varied explosion mechanisms observed in these events.

Type Ibn supernovae (SNe Ibn) are a rare subclass of stripped-envelope stellar explosions characterized by prominent, relatively narrow helium (He I) emission lines with little or no hydrogen in their spectra and light curves that are fast-evolving and often overluminous compared to canonical SN Ib/c events. Observationally, SNe Ibn are defined by the clear spectroscopic signature of He I lines (with full width at half maximum, FWHM, typically ∼1000–5000 km s⁻¹), and by rapidly declining optical light curves. The dominant physical interpretation is that these events arise from core-collapse of massive stars that have undergone substantial pre-supernova (pre-SN) mass loss, creating a dense He-rich circumstellar medium (CSM) with which the SN ejecta interact. However, multiple channels—including both single and binary stellar evolution, as well as so-called “ultra-stripped” progenitors—are now recognized as plausible, and recent discoveries have broadened the diversity of their progenitor environments and explosion mechanisms.

1. Defining Observational Features

Type Ibn supernovae are distinguished by their spectroscopic and photometric properties:

Spectroscopy: SNe Ibn typically display persistent, relatively narrow (FWHM ∼1000–5000 km s⁻¹) He I emission lines (e.g., λ5876, λ6678, λ7065 Å), often with P Cygni profiles superposed on a blue continuum at early times [(Gorbikov et al., 2013); (Shivvers et al., 2016); (Hosseinzadeh et al., 2016)]. Hydrogen features are weak or absent, though transitional Ibn/IIn subtypes (with weak or ambiguous H α features) are recognized (Pastorello et al., 2015, Kool et al., 2020). Some spectra evolve from early-time P Cygni-dominated to emission-dominated profiles, accompanying a transition from optically thick to optically thin CSM (Gangopadhyay et al., 2019).
Light curves: Most SNe Ibn exhibit homogeneous, rapidly declining optical light curves, with decline rates clustered around 0.1 mag day⁻¹ during the first month after maximum (Hosseinzadeh et al., 2016). Outliers such as OGLE-2012-SN-006 and OGLE-2014-SN-131 show unusually broad or long-rising light curves, and some cases display double-peaked morphologies, indicative of complex CSM configuration and temporal variation in the power source [(Gorbikov et al., 2013); (Karamehmetoglu et al., 2017); (Dong et al., 7 May 2024)].
Bolometric evolution: Peak luminosities can range from ∼10⁴² erg s⁻¹ up to values exceeding 10⁴⁴ erg s⁻¹ for the most extreme (e.g., ASASSN-14ms), with total radiated energies ∼10⁵⁰ erg for the most luminous events (Vallely et al., 2017, Wang et al., 2021).

2. Physical Mechanisms and Energy Sources

The photometric and spectroscopic evolution of SNe Ibn is driven by the interaction of SN ejecta with a dense, He-rich circumstellar medium.

Shock breakout and CSM interaction: The early (and sometimes dominant) light curve peak often arises from the conversion of kinetic energy from the rapidly expanding SN ejecta into radiation through shock interaction with the CSM [(Gorbikov et al., 2013); (Pastorello et al., 2015); (Shivvers et al., 2016); (Wang et al., 2021)]. The first peak or bump in double-peaked SNe Ibn light curves is attributed to shock breakout in an optically thick CSM shell (as in iPTF13beo and SN2023fyq) [(Gorbikov et al., 2013); (Dong et al., 7 May 2024)].
- Theoretical scalings for the peak luminosity and rise time include:
$L_{\rm SN} \propto E^{5/6} M^{-1/2} R^{2/3} \kappa^{-1/3} T^{4/3}, \quad t_{\rm SN} \propto E^{-1/6} M^{1/2} R^{1/6} \kappa^{1/6} T^{-2/3}$

where $E$ is explosion energy, $M$ ejecta mass, $R$ CSM/ejecta radius, $\kappa$ opacity (∼0.2 cm² g⁻¹ for He-rich material), and $T$ temperature (Shivvers et al., 2016).
Radioactive decay of $^{56}$ Ni: The contribution from $^{56}$ Ni → $^{56}$ Co → $^{56}$ Fe radioactive decay can shape the slower, secondary peak or power later-time emission. However, many SNe Ibn decline too quickly post-maximum and show a large contrast between peak and late-time luminosity to be explained by radioactive decay alone—a hybrid CSM interaction plus radioactive decay model is often required (Moriya et al., 2016, Vallely et al., 2017, Ben-Ami et al., 2022).
Alternative power sources: In rare cases, magnetar spin-down or alternative central engines are considered, especially for exceptionally broad or luminous light curves (e.g., OGLE-2014-SN-131), but usually CSM interaction remains necessary to explain He I–rich emission (Karamehmetoglu et al., 2017).

3. Circumstellar Medium Properties and Pre-SN Mass Loss

The CSM structure and mass-loss history are critical for determining the SNe Ibn observables:

Mass loss rates and composition: Progenitors experience extreme mass loss (𝑀̇∼10⁻³–10⁻¹ M $_\odot$  yr⁻¹), leading to the rapid build-up of dense, He-rich CSM [(Gorbikov et al., 2013); (Pastorello et al., 2015); (Shivvers et al., 2016); (Wang et al., 2021)]. The wind velocities inferred from He I line FWHM range from ∼1000–3000 km s⁻¹, well-matched to Wolf–Rayet winds or binary-driven ejections (Moriya et al., 2016, Dessart et al., 2021).
CSM geometry: Observed narrow He I line widths and rapid SN light curve decline typically require the CSM to be spatially confined—a thin shell or dense disk distributed within ∼10¹⁴–10¹⁵ cm [(Gorbikov et al., 2013); (Hosseinzadeh et al., 2016); (Ben-Ami et al., 2022); (Dong et al., 7 May 2024)]. Multi-component CSM structures (an equatorial disk plus polar ejecta) as inferred from SN2023fyq and other SNe suggest complex binary-driven geometries (Dong et al., 7 May 2024).
Temporal mass-loss behavior: The CSM can be produced by short-lived (weeks to months) eruptive episodes immediately preceding explosion or by steady winds during late evolutionary phases. Long-duration pre-explosion outbursts (years) and rapid final brightening are seen as haLLMarks of binary interaction channels (Dong et al., 7 May 2024).

4. Progenitor Systems: Single, Binary, and Ultra-Stripped Scenarios

The origin of SNe Ibn progenitors is heterogeneous, with mounting evidence for contributions from both massive single stars and interacting binaries:

Single massive Wolf–Rayet stars: Classical scenario involves massive stars (ZAMS masses ≳30–60 M $_\odot$ ) that lose their H (and possibly part of their He) envelope via radiatively driven winds or pulsational mass loss, culminating in a WR star embedded in a He-rich CSM [(Gorbikov et al., 2013); (Pastorello et al., 2015); (Karamehmetoglu et al., 2017); (Wang et al., 2021)]. Observational evidence for extremely high pre-SN mass-loss rates and high-velocity winds support this picture for a subset of luminous SNe Ibn.
Interacting binaries (low-mass He-star channel): HST observations reveal binary companions in some nearby Type Ibn SNe, with the progenitor inferred to be a low-mass (∼2.5–3 M $_\odot$ ) He star, stripped via Roche lobe overflow or common-envelope (CE) evolution and transfer onto a companion, typically a 10–20 M $_\odot$ main-sequence star or, more rarely, a white dwarf (Sun et al., 2019, Ko et al., 1 Jun 2025). Population synthesis predicts that the event rate for this channel matches the observed SN Ibn rate (∼1–3% of CCSNe) (Ko et al., 1 Jun 2025). These systems naturally explain low-mass, He-rich CSM with short delay times and are consistent with SNe Ibn in both star-forming and older stellar hosts.
- The fate of the companion after explosion is generally survival as a main-sequence star; white dwarf counterparts are less common, but can explain events in old populations (Ko et al., 1 Jun 2025).
Merger and ultra-stripped scenarios: Mergers between He stars and compact objects can lead to common-envelope ejection and the production of dense CSM. Ultra-stripped SN progenitors—He stars with extremely low envelope mass (<0.1 M $_\odot$ $_{⊙}$ ) evolving in tight binaries—may undergo violent silicon shell burning weeks before collapse, ejecting >0.1 M $_\odot$ $_{⊙}$ He-rich material that produces the CSM for subsequent SN–CSM interaction, resulting in Ibn-like observables (Moriya et al., 7 Jul 2025).
- Ultra-stripped SNe Ibn characteristically display very low ejecta mass (~0.06 M $_\odot$ ), low explosion energy (∼9×10⁴⁹ erg), and prominent CSM interaction signatures (Moriya et al., 7 Jul 2025).
Host galaxies and non-massive progenitors: Some SNe Ibn occur in environments with low metallicity and negligible ongoing star formation (e.g., PS1-12sk, SN 2023tsz), which challenge the massive single-star scenario and motivate consideration of binaries or thermonuclear channels (Hosseinzadeh et al., 2019, Warwick et al., 21 Sep 2024).

5. Diversity and Classification within the Type Ibn Population

SNe Ibn display both photometric homogeneity and pronounced spectral diversity at peak.

Light curve diversity: While many SNe Ibn exhibit fast, smooth post-peak declines (~0.1 mag day⁻¹), a minority display long-lasting plateaus (e.g., SN 2020bqj) or very broad, slow-evolving light curves (OGLE-2014-SN-131), corresponding to variations in CSM mass, extent, and density (Karamehmetoglu et al., 2017, Kool et al., 2020). Double-peaked or multi-peaked light curves directly reflect the structure/timing of CSM ejection episodes [(Gorbikov et al., 2013); (Dong et al., 7 May 2024)].
Spectral diversity: At maximum light, two distinct spectroscopic subtypes are observed: (1) events with narrow P Cygni He I features embedded in a blue continuum, and (2) events dominated by broader He I emission lines, possibly affected by viewing angle or CSM optical depth (Hosseinzadeh et al., 2016). Late-time spectra often show strong Fe II emission, sometimes accompanied by O I, Si II, Mg II, Ca II, or N II, indicating solar metallicity and CSM mixing (Dessart et al., 2021).
Transitional events: Some SNe Ibn, including SN 2011hw and SN 2020bqj, present both Ibn and IIn features—indicative of progenitors not fully stripped of hydrogen and/or more complex mass-loss histories involving mixed CSM compositions (Kool et al., 2020).
Host galaxy outliers: SNe Ibn like SN 2023tsz and PS1-12sk are found in low-mass, low-SFR, low-metallicity hosts. This challenges the expectation of massive Wolf–Rayet progenitors and suggests the importance of alternative binary scenarios (Hosseinzadeh et al., 2019, Warwick et al., 21 Sep 2024).

6. Implications for Stellar Evolution and Explosion Mechanisms

Type Ibn SNe provide insight into the late stages of stellar and binary evolution, mass-loss physics, and explosion mechanisms:

Episodic and eruptive mass loss: The prevalence of dense, confined He-rich CSM in SNe Ibn implies that pre-SN outbursts or violent episodes (weeks to years before core collapse) are common across both high- and low-mass progenitor channels [(Gorbikov et al., 2013); (Dong et al., 7 May 2024)]. Enhanced mass loss via binary interaction (runaway Roche lobe overflow, merger, or CE ejection) is crucial for forming the CSM around low-mass He stars (Ko et al., 1 Jun 2025).
Low explosion energy and fallback: Modeling indicates that many SNe Ibn have lower explosion energy and synthesize small amounts of $^{56}$ Ni (<0.1 M $_\odot$ ), consistent with the possibility of significant fallback or, in some cases, nonterminal eruptions (Moriya et al., 2016).
Impacts of metallicity and environment: The dependency of line-driven wind mass-loss rates on metallicity (𝑀̇ ∝ Z⁰·⁷) means that forming single WR stars capable of losing their H and He envelopes is inhibited at low Z; binary evolution may thus dominate the progenitor channels in metal-poor environments (Warwick et al., 21 Sep 2024).
Predictions for companions: Binary population synthesis predicts that most Ibn SNe with low-mass He star progenitors will leave hydrogen-rich main-sequence stars as companions; a subset will have white dwarf companions and display longer delay times, potentially explaining events in old systems (Ko et al., 1 Jun 2025).

7. Future Directions and Open Questions

The ongoing discovery of peculiar SNe Ibn and increased diversity in host properties motivate extensive, multi-faceted observational and theoretical efforts:

Systematic early-time coverage (e.g., precursor monitoring, flash spectroscopy) constrains the timing, amount, and geometry of pre-SN mass loss (Dong et al., 7 May 2024).
Detailed hydrodynamic and radiative transfer modeling is required to disentangle the relative roles of CSM mass, velocity structure, and opacity in shaping light curves and spectra, and to understand the implications of metallicity and mixing for late-time Fe II emission (Dessart et al., 2021).
Direct progenitor and companion identification via high-resolution afterglow imaging (as in SN 2006jc and SN 2015G) remains a critical test of population synthesis predictions (Sun et al., 2019, Ko et al., 1 Jun 2025).
Host environment surveys across metallicity, stellar mass, and star-formation rate are necessary to quantify the relative contributions of massive single stars versus binary progenitors (Warwick et al., 21 Sep 2024).
Investigations of ultra-stripped and merger channels may connect SNe Ibn to the formation of double neutron star or other compact-object binaries, with broader implications for gravitational wave astrophysics and transient demographics (Moriya et al., 7 Jul 2025).
Modeling of rare subclasses and transitional events (e.g., Ibn/IIn, long-rising, double-peaked, precursor-rich) broadens constraints on late-stage stellar evolutionary phenomena, especially in interacting binaries [(Gorbikov et al., 2013); (Kool et al., 2020); (Dong et al., 7 May 2024)].

Type Ibn supernovae thus serve as laboratories for extreme mass-loss phenomena, binary evolution physics, and the final fates of both massive and moderately massive stars, bridging the gap between pure H-rich supernova–CSM interactions and classical stripped-envelope explosions.