Helium stars exploding in circumstellar material and the origin of Type Ibn supernovae

Abstract

Type Ibn supernovae (SNe) are a mysterious class of transients whose spectra exhibit persistently narrow He i lines, and whose bolometric light curves are typically fast evolving and overluminous at peak relative to standard Type Ibc SNe. We explore the interaction scenario of such Type Ibn 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 on day timescales a peak luminosity of a few 10 44 erg s −1 , reminiscent of exceptional events like AT 2018cow. Similar interactions but with weaker winds can lead to Type Ibc SNe with double-peak light curves and peak luminosities in the range ∼ 10 42.2 to ∼ 10 43 erg s −1 . In contrast, the narrow spectral lines and modest peak luminosities of most Type Ibn 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 ∼ 10 15 cm. Nonlocal thermodynamic equilibrium radiative-transfer simulations of a slow-moving dense shell born out and powered by the interaction compare favorably to Type Ibn SNe like 2006jc, 2011hw, or 2018bcc at late times and suggest a composition made of about 50% helium, a solar metallicity, and a total ejecta/CSM mass of 1-2 M . A lower fractional helium abundance leads to weak or absent He i lines and thus excludes more massive configurations for observed Type Ibn SNe. Further, the dominance of Fe ii emission below 5500 Å seen in Type Ibn SNe at late times is not predicted at low metallicity. Hence, despite their promising properties, Type Ibn SNe from pulsational-pair instability in very massive stars, which requires low metallicity, have probably not yet been observed.

Properties of Type Ibn Supernovae: Implications for the Progenitor Evolution and the Origin of a Population of Rapid Transients

“…A simplified assumption of constant conversion efficiency (which basically assumes full thermalization of the dissipated energy) has been adopted in analytical treatment (Chatzopoulos et al 2012;Moriya et al 2013Moriya et al , 2014 which are frequently used to model the LCs of the interaction-powered SNe including SNe Ibn (e.g., Pellegrino et al 2021). Widely used numerical LC codes also assume full thermalization at the FS and RS (Morozova et al 2015;Moriya et al 2017;Dessart et al 2021). As we show in the present work, the conversion efficiency should be generally timedependent (see also Chugai 2009Chugai , 2018, and this effect is important in calculating the LCs for SNe Ibn.…”

“…However, further testing the ideas has been limited by a lack of our detailed knowledge on how their characteristic observational properties are explained. Especially, despite the general idea that their light curves (LCs) are powered by the interaction between the SN ejecta and H-poor CSM, the optical LC model for SNe Ibn based on this scenario has been largely unexplored, without which the properties of the SN ejecta and the CSM cannot be quantified; while there are some LC models for SNe Ibn with different levels of sophistication (Tominaga et al 2008;Pellegrino et al 2021;Dessart et al 2021), some key processes are still likely missing (see Section 3). This is the topic of the present work; we construct the (general) LC model for the SN-CSM interaction scenario, especially addressing how the kinetic energy dissipated by the interaction is converted into the thermal radiation mostly in the optical (and UV/NIR) as we observe.…”