2022A&A...658A.130D

TypeIbn supernovae (SNe) are a mysterious class of transients whose spectra exhibit persistently narrow HeI lines, and whose bolometric light curves are typically fast evolving and overluminous at peak relative to standard TypeIbc SNe. We explore the interaction scenario of such TypeIbn SNe by performing radiation-hydrodynamics and radiative-transfer calculations. We find that standard-energy helium-star explosions within dense wind-like circumstellar material (CSM) can reach a peak luminosity of a few 10⁴⁴ erg s^–1 on day timescales, which is reminiscent of exceptional events such as AT 2018cow. Similar interactions but with weaker winds can lead to TypeIbc SNe with double-peak light curves and peak luminosities in the range ∼10^42.2 to ∼10⁴³ erg/s. In contrast, the narrow spectral lines and modest peak luminosities of most TypeIbn SNe are suggestive of a low-energy explosion in an initially ≤5 M_☉ helium star, most likely arising from interacting binaries and colliding with a massive helium-rich, probably ejecta-like, CSM at ∼10¹⁵ cm. Nonlocal thermodynamic equilibrium radiative-transfer simulations of a slow-moving dense shell born out and powered by the interaction compare favorably to TypeIbn SNe such as 2006jc, 2011hw, or 2018bcc at late times and suggest a composition made of about 50% helium, a solar metallicity, and a total ejecta and CSM mass of 1-2 M_☉. A lower fractional helium abundance leads to weak or absent HeI lines and thus excludes more massive configurations for observed TypeIbn SNe. Further, the dominance of FeII emission below 5500 Å seen in TypeIbn SNe at late times is not predicted at low metallicity. Hence, despite their promising properties, TypeIbn SNe from a pulsational-pair instability in very massive stars, requiring low metallicity, probably have not been observed yet.