Type Ibn Supernovae

• Type Ibn supernovae are a rare subclass of core-collapse events defined by prominent narrow He I emission lines and weak hydrogen features from a helium-rich CSM.
• Their light curves exhibit rapid rise times and declines with peak luminosities around –18 to –19 mag, indicating confined, dense circumstellar environments.
• Diverse progenitor channels—from massive Wolf–Rayet stars to binary-stripped helium cores—highlight complex pre-supernova mass-loss processes and explosion dynamics.

Type Ibn supernovae (SNe Ibn) are a rare subclass of stripped-envelope core-collapse supernovae whose defining characteristic is the predominance of helium-rich, hydrogen-poor circumstellar medium (CSM) interaction signatures. Their spectra exhibit relatively narrow He I emission lines (FWHM ≈ 1,000–5,000 km s⁻¹) and very weak or absent hydrogen features. The distinctive photometric and spectroscopic properties of SNe Ibn elucidate the mass-loss and core-collapse processes at the endpoints of massive star evolution, while also revealing substantial diversity in progenitor scenarios and explosion dynamics.

1. Classification and Observational Definition

Type Ibn SNe are classified by the presence of prominent, relatively narrow He I emission lines in early-time spectra, typically with FWHM between 1,000 and 2,000 km s⁻¹, and minimal or undetectable hydrogen lines (Pastorello et al., 2015, Karamehmetoglu et al., 2017, Inoue et al., 12 Dec 2024, Hosseinzadeh et al., 2016). These lines arise from unshocked or mildly shocked, He-rich CSM illuminated by the shock front driven by core-collapse ejecta. The key diagnostic features include:

• Strong He I emission lines (especially λ5876, λ6678, λ7065 Å), often with P Cygni profiles in the earliest spectra.
• Weak or absent hydrogen Balmer emission, distinguishing SNe Ibn from Type IIn (hydrogen-rich CSM) and Type Ib (no narrow lines) (Pastorello et al., 2015, Pastorello et al., 2015, Inoue et al., 12 Dec 2024).
• Intermediate-width ejecta features (O I, Ca II, [O I] at late times) often emerging after the CSM is overtaken.
• Double-peaked or plateau light curves in rare, transitional objects (Ibn/IIn, e.g., SN 2011hw, SN 2020bqj) indicating mixed CSM composition or geometry (Kool et al., 2020, Pastorello et al., 2015).

Spectroscopically, events are further categorized by the width and morphology of He I features—narrow, symmetric features with blue continua (“Group I”) versus broader, redder lines and emission from inner ejecta (“Group II”) (Dong et al., 6 Nov 2025).

2. Photometric Characteristics and Light-curve Morphologies

The photometric evolution of SNe Ibn reveals a surprising degree of homogeneity within the broader diversity of CSM-interacting transients (Hosseinzadeh et al., 2016, Maeda et al., 2022, Pastorello et al., 2015):

• Rise times: Typically 5–20 days to peak optical brightness (e.g., I-band rise of ~14 d for OGLE-2012-SN-006 (Pastorello et al., 2015); ≲10–15 d is common, but rare slow risers reach ~42 d (Karamehmetoglu et al., 2017)).
• Peak luminosity: Clustered around absolute magnitudes M_R ≈ –18 to –19; rare events (ASASSN-14ms) exceed M_V ≈ –20.5 (Vallely et al., 2017).
• Decline rates: Post-peak declines are rapid, Δm ≈ 0.05–0.2 mag d⁻¹ over the first month (Hosseinzadeh et al., 2016, Pastorello et al., 2015), with some exhibiting an initial fast drop followed by a flattening, then steepening (“t^–1 to t^–3” behavioral transitions) (Maeda et al., 2022, Pastorello et al., 2015).
• Light curve morphology: Most SNe Ibn display single, fast-rising, fast-declining peaks, but a subset (“transitional” Ibn/IIn) exhibit double peaks or extended plateaus lasting up to ~40 days (SN 2020bqj, (Kool et al., 2020); SN 2011hw, (Pastorello et al., 2015)).
• Late-time behavior: In many cases, late-time luminosity does not follow pure ^56Co decay, remaining flatter due to continued shock interaction or eventually converging toward a radioactive tail (Pastorello et al., 2015, Pastorello et al., 2015).

Table: Representative Photometric Parameters

Property	Typical Value	Extreme Value / Outlier
Rise time	5–15 d	≥42 d (OGLE-2014-SN-131)
M_peak (R-band)	–18…–19 mag	–20.5 (ASASSN-14ms)
Decline rate	0.05–0.2 mag d⁻¹	>0.2 mag d⁻¹ (LSQ13ccw)
Plateau	None/short (~few d)	≈40 d (SN 2020bqj)

This photometric homogeneity implies confined CSM shells rather than extended wind media (Hosseinzadeh et al., 2016, Pastorello et al., 2015).

3. Spectral Evolution and CSM Diagnostics

The key spectral characteristics of SNe Ibn are governed by dynamical interaction with He-rich CSM (Pastorello et al., 2015, Pastorello et al., 2015, Dessart et al., 2021):

• Narrow He I emission at velocities v_FWHM ≈ 1,000–2,000 km s⁻¹ (CSM origin); sometimes as broad as ≈5,000 km s⁻¹ in more energetic or less massive CSM environments (Karamehmetoglu et al., 2017, Dong et al., 6 Nov 2025).
• Blue continuum at early times (T_BB ≳ 10,000–15,000 K) (Pastorello et al., 2015, Ben-Ami et al., 2022).
• Broad O I, Ca II, [O I] lines emerging at later phases from SN ejecta (Pastorello et al., 2015, Pastorello et al., 2015).
• Fe II–dominated pseudo-continuum below 5500 Å at late times; strong Fe II emission requires near-solar metallicity, excluding most pulsational-pair-instability scenarios at low-Z (Dessart et al., 2021).
• Electron-scattering wings on He I in high optical-depth CSM, producing symmetric or asymmetric broad bases (Dong et al., 6 Nov 2025).
• Coronal lines, faint H lines: In transitional Ibn/IIn events, weak H features (H α FWHM ≲ 2,000 km s⁻¹), [Fe VII] or [Ne IV], are indicative of partially H-rich or photoionized CSM (Pastorello et al., 2015, Kool et al., 2020).

The temporal evolution from ~narrow CSM-dominated emission to broader, ejecta-dominated features is modulated by CSM density, mass, and composition (Inoue et al., 12 Dec 2024, Dessart et al., 2021).

4. Progenitor Scenarios and Circumstellar Mass Loss

The requirement of a dense, H-poor, He-rich CSM pinpoints advanced binary and/or single-star Wolf–Rayet (WR) evolutionary stages, but multiple progenitor channels are now evidenced by direct and indirect studies:

• Single massive WR stars: Initially thought to dominate, with eruptive or nuclear flash-driven shell ejection in the last ≲1–10 years (Pastorello et al., 2015, Karamehmetoglu et al., 2017, Maeda et al., 2022).
• Binary-stripped, low-mass He stars: Rapid population synthesis and resolved companion detections (e.g., SN 2006jc, M_2 ≈ 12 M_⊙ (Sun et al., 2019, Ko et al., 1 Jun 2025)) now show that close binaries yielding low-mass He stars (M_He ≈ 2.5–3.0 M_⊙) in orbits ≲300 R_⊙ can quantitatively reproduce the observed Ibn rates; these stars lose their He envelopes through Case B mass transfer and spring a dense He CSM via Roche-lobe overflow in the last ≲100 Myr (Ko et al., 1 Jun 2025, Sun et al., 2019).
• Compact-object mergers: Post-common-envelope NS/BH–He star mergers yield dense, He-rich CSM via envelope stripping but likely account for only a minority of events (Ko et al., 1 Jun 2025, Moriya et al., 7 Jul 2025).
• Ultra-stripped progenitors: Binary He stars losing nearly all envelope mass via Roche-lobe overflow/Si-burning-driven pre-SN mass ejection; when a flash-driven shell ejection precedes a low-mass, low-energy explosion (E ≈ 10⁵⁰ erg, M_ej ≈ 0.06 M_⊙), the CSM-dominated light curve mimics Type Ibn phenomena (Moriya et al., 7 Jul 2025).
• Uncommon thermonuclear/degenerate origins: In rare cases (PS1-12sk), location in passive environments without recent star formation supports scenarios involving He-shell detonations on WDs or He-star+WD interactions, though these do not explain the bulk of the population (Hosseinzadeh et al., 2019, Dong et al., 6 Nov 2025).

In all dominant channels, mass loss must occur at Ṁ ≳ 10⁻³–10⁻² M_⊙ yr⁻¹ in the final 0.1–10 years before collapse, feeding a CSM with density ρ_CSM ≈ 10⁻¹⁴–10⁻¹³ g cm⁻³ at r ≈ 10¹⁵ cm (Moriya et al., 2016, Baer-Way et al., 8 Sep 2025, Dessart et al., 2021).

5. Circumstellar Interaction Physics and Light-curve Modeling

Radiation from SNe Ibn is powered by the deposition of ejecta kinetic energy into the optically thick, He-rich CSM shell through radiative shocks (Maeda et al., 2022, Pastorello et al., 2015, Dessart et al., 2021):

• Shock-powered luminosity: The energy conversion can be described by $L_{\text{shock}} \propto (\dot{M} / v_w) v_s^3$ where v_s is the shock velocity (Pastorello et al., 2015, Moriya et al., 2016).
• CSM density profile: ρ_CSM(r) ∝ r^–s with s ≈ 2 (wind) or ≈3 (accelerated pre-SN mass loss) (Maeda et al., 2022, Inoue et al., 12 Dec 2024). Observations favor steep s ≳ 2.5–3 profiles, requiring rapid mass-loss increase toward explosion.
• Light-curve models: Hybrid models combining Ni decay with CSM interaction (Chatzopoulos et al. prescription) yield shell masses M_CSM ≈ 0.1–1 M_⊙ (Ben-Ami et al., 2022, Vallely et al., 2017, Kool et al., 2020), with energy diffusion controlled by t_diff ≈ √2 κ M_ej / (β c v_ej).
• Transition “breaks”: Two-stage declines, with L ∝ t^–1 (radiative regime) transitioning to L ∝ t^–3 (adiabatic regime) at late times (Maeda et al., 2022).
• Role of radio/X-ray emission: Radio observations constrain the radial density and timescale of pre-SN mass loss (e.g., SN 2023fyq: Ṁ ≈ 4×10⁻³ M_⊙ yr⁻¹ at r ≈ 1×10¹⁶ cm, 0.7–3 yrs pre-explosion) (Baer-Way et al., 8 Sep 2025). X-ray modeling probes CSM composition and C/O stripping in the progenitor (Inoue et al., 12 Dec 2024).

CSM mass, structure, and composition directly impact the emergent spectra and light-curve morphology, and variations yield a continuum from canonical to outlier SNe Ibn (Dong et al., 6 Nov 2025, Kool et al., 2020, Ben-Ami et al., 2022).

6. Progenitor Diversity, Host Environments, and Population Implications

Type Ibn SNe display a range of host environments and explosion sites, reinforcing the diversity of progenitor channels (Ko et al., 1 Jun 2025, Dong et al., 6 Nov 2025, Hosseinzadeh et al., 2019):

• Classic population: Majority found in star-forming (spiral/irregular) galaxies with a wide range of metallicities (Z ≈ 0.3–2 Z_⊙), consistent with massive WR or post-LBV progenitors (Pastorello et al., 2015, Vallely et al., 2017).
• Objects with large host offsets and low local SFR: PS1-12sk and SN 2024acyl exploded in regions with ΣSFR < 3.6×10⁻⁴ M⊙ yr⁻¹ kpc⁻², incompatible with high-mass WR lifetimes, supporting lower-mass binary or degenerate origins (Hosseinzadeh et al., 2019, Dong et al., 6 Nov 2025).
• Population synthesis: Low-mass He-star (M_He ≈ 2.5–3 M_⊙) binary channel can match the observed ~1–2% fraction of SNe Ibn among CCSNe, with MS companions comprising ~90% and WD companions up to ~10% (delayed by up to ∼100 Myr) (Ko et al., 1 Jun 2025). Common envelope mergers/compact object channels are subdominant.

Table: Progenitor & Environment Summary

Channel	Progenitor mass	Environment
Massive WR	M_ZAMS ≳ 18–40 M_⊙	Star-forming
Binary Low-mass He star	M_He ≈ 2.5–3 M_⊙	Star-forming/Passive
Compact-object mergers	Lower	All, including passive
Degenerate WD scenario	<1 M_⊙	Elliptical/Old

Transitional subclasses (Ibn/IIn) and outliers are linked to partial hydrogen retention, peculiar mass loss, or rare progenitor configurations (Pastorello et al., 2015, Kool et al., 2020).

7. Current Frontiers and Diversity in SNe Ibn

Recent studies show that SNe Ibn comprise a spectrum of explosion energies, CSM densities, and progenitor histories, manifesting as observable diversity in spectral and photometric evolution:

• Spectral bifurcation: Blue, narrow-lined (“Group I”) and redder, broader-lined (“Group II”) SNe correspond to differences in CSM density, shell mass, and explosion energetics; Group I tends to be more luminous and retains interaction signatures longer (Dong et al., 6 Nov 2025).
• CSM geometry and asymmetry: Electron-scattering profiles and the coexistence of ejecta and interaction signatures at late times suggest a range of CSM geometries (shells, aspherical configurations, clumping) (Dong et al., 6 Nov 2025).
• Precursor eruptions: Direct precursor outbursts are not universal but, when detected (e.g., SN 2006jc), demonstrate massive shell ejection within 1–2 years of explosion (Sun et al., 2019, Baer-Way et al., 8 Sep 2025).
• Rapid transients and C/O-rich cases: Models predict a family of UV-bright, rapidly evolving transients arising from lower-density or more deeply stripped progenitors (Maeda et al., 2022, Inoue et al., 12 Dec 2024).
• Open questions: The dichotomy between classic WR single-star and diverse binary-channeled (including ultra-stripped or degenerate) progenitors is not fully resolved and may reflect a continuum rather than discrete classes (Dong et al., 6 Nov 2025, Ben-Ami et al., 2022).

In summary, Type Ibn supernovae are defined by interaction with a dense, helium-rich, hydrogen-poor CSM, with rapid photometric evolution, characteristic narrow/intermediate He I emission, and light curves and spectra that reflect the complex interplay among mass-loss physics, binary evolution, explosion energy, and CSM structure. The emerging picture is of a population marked by significant heterogeneity in both progenitor and explosion properties, spanning massive WR stars, binary-stripped low-mass He-cores, and, in rare environments, compact-object or thermonuclear origins. Ongoing population studies, high-cadence multi-wavelength surveys, and detailed radiative transfer and hydrodynamic modeling continue to refine the boundaries and diversity of these enigmatic explosions (Pastorello et al., 2015, Karamehmetoglu et al., 2017, Maeda et al., 2022, Dessart et al., 2021, Ko et al., 1 Jun 2025, Moriya et al., 7 Jul 2025, Dong et al., 6 Nov 2025).