SN 2018gjx reveals that some SNe Ibn are SNe IIb exploding in dense circumstellar material

Abstract

We present the data and analysis of SN 2018gjx, an unusual low-luminosity transient with three distinct spectroscopic phases. Phase I shows a hot blue spectrum with signatures of ionised circumstellar material (CSM), Phase II has the appearance of broad SN features, consistent with those seen in a Type IIb supernova at maximum light, and Phase III is that of a supernova interacting with helium-rich CSM, similar to a Type Ibn supernova. This event provides an apparently rare opportunity to view the inner workings of an interacting supernova. The observed properties can be explained by the explosion of a star in an aspherical CSM. The initial light is emitted from an extended CSM (∼ 4000 R ), which ionises the exterior unshocked material. Some days after, the SN photosphere envelops this region, leading to the appearance of a SN IIb. Over time, the photosphere recedes in velocity space, revealing interaction between the supernova ejecta and the CSM that partially obscures the supernova nebular phase. Modelling of the initial spectrum reveals a surface composition consistent with compact H-deficient Wolf-Rayet and LBV stars. Such configurations may not be unusual, with SNe IIb being known to have signs of interaction so at least some SNe IIb and SNe Ibn may be the same phenomena viewed from different angles or, possibly with differing CSM configurations.

A pre-explosion extended effervescent zone around core collapse supernova progenitors

“…This case requires values of β > 1 to achieve high densities in the outer effervescent zone. Prentice et al (2020) study the type IIb (envelopestripped) SN 2018gjx and argue that its CSM was nonspherical, probably a torus, and extended to about 20 − 30 AU at explosion. They further estimate the mass in the CSM to be ≈ 0.004 − 0.014M and the mass loss rate from the progenitor that formed this CSM to be ≈ 0.01 − 0.05M yr −1 .…”

“…In some cases the pre-explosion wind starts only several months before explosion (e.g., Bruch et al 2020). Prentice et al (2020) claim that the CSM around the envelopestripped SN 2018gjx was ejected within four months from explosion. In that case the CSM is non-spherical, most likely indicating a pre-explosion binary interaction (Prentice et al 2020).…”

A Pre-explosion Extended Effervescent Zone around Core-collapse Supernova Progenitors

“…By compact CSM, I refer to CSM with which the ejecta collides within days after the explosion, unlike SN 1987A, where a collision occurs years after the explosion. In principle, compact dense CSM of CCSN progenitors might result from an enhanced mass loss rate that starts years to weeks before explosion, possibly accompanied by a pre-explosion outburst (e.g., Foley et al 2007;Pastorello et al 2007;Smith et al 2010;Margutti et al 2014;Ofek et al 2014;Svirski & Nakar 2014;Tartaglia et al 2016;Yaron et al 2017;Wang, Wang, & Dai 2019;Bruch et al 2020;Prentice et al 2020;Strotjohann et al 2021;Jacobson-Galán et al 2022), an extended long-lived dense zone above the stellar photosphere (e.g., Dessart et al 2017), an extended accelerated zone of the wind (e.g., Moriya et al 2017Moriya et al , 2018 or by a long-lived extended dense zone of uprising and falling gas parcels or streams above the stellar photosphere (e.g., Soker , 2023Fuller & Tsuna 2024; see further discussion by Fuller & Tsuna 2024). SN 2024ggi did not experience an outburst within years before the explosion (e.g., Shrestha et al 2024).…”