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 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 Type Ibc SNe with double-peak light curves and peak luminosities in the range ∼1042.2 to ∼1043 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 ∼1015 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 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 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 a pulsational-pair instability in very massive stars, requiring low metallicity, probably have not been observed yet.

Late-time H/He-poor circumstellar interaction in the normal-looking type-Ic supernova SN 2021ocs: an exposed oxygen-magnesium layer and extreme stripping of the progenitor

“…The strong narrow H emission lines defining the type-IIn SNe (Schlegel 1990) are clearly absent in SN 2021ocs. SNe Ibn show prominent narrow He emission lines and a rising blue part of the spectrum (Pastorello et al 2007), which originates from Fe emission (Dessart et al 2021(Dessart et al , 2022. Similarly this blue rise is also seen in SN 2021ocs, while strong He lines are absent except for the weak He I λ7065, and λ5876 which is blended with the Na I doublet λλ5890,5896.…”

“…late-time ejecta-CSM interaction (e.g. Maeda et al 2015;Dessart et al 2022) or magnetar spin-down (e.g. Afsariardchi et al 2021).…”

“…Furthermore, for the interaction to be dominating at late times, the amount of CSM must be low (see e.g. Dessart et al 2022). Assuming an ejecta velocity of 10 000 km s −1 , if CSM interaction started ∼100 d after the explosion then the distance traversed by the unimpeded SN ejecta would be ∼ 9 × 10 15 cm from the progenitor star.…”

Extreme Mass Loss in Low-mass Type Ib/c Supernova Progenitors

“…With the exception of the values derived for SN 2019hgp by Gal-Yam et al (2022); the type Icn SNe tend to produce more massive CSM than our models predict. Most importantly, these events likely require lower He mass fractions than ejected by our models (Dessart et al 2022), which typically have X He ∼ 0.8 and X CO ∼ 0.2.…”

An environmental analysis of the fast transient AT2018cow and implications for its progenitor and late-time brightness

“…Many observational studies (Ho et al 2021;Xiang et al 2021) reveal the similarities in the light curves and spectra between FBOTs and the Type Ibn SNe (i.e. the explosion of hydrogen-poor stars, possibly stripped in binaries, with strong ejecta-CSM interaction; Pastorello et al 2007Pastorello et al , 2008Hosseinzadeh et al 2017;, and theoretically, Dessart, Hillier, & Kuncarayakti (2022) showed that the standard-energy explosions of helium stars within dense CSM are reminiscent of FBOTs while the low-energy explosions are more consistent with Type Ibn SNe. Some other astronomers suggest that FBOTs have mildly or sub-relativistic outflows (e.g.…”