Wild behaviour near the finish line: the Type Ibn SN 2020able and its stratified environment

Abstract

We present spectrophotometric observations of the Type Ibn supernova SN~2020able, along with a comprehensive analysis and modelling of its evolution. The transient occurred in SDSS J092602.93+243115.1, a faint host galaxy characterised by significantly sub-solar metallicity ($12+\log (O/H)\simeq 8.28\mathrm{dex}$). The early photometric evolution and high peak luminosity ($L_{peak}\simeq 4.4\times 10^{43}\mathrm{erg}{\mathrm{s}}^{-1}$) indicate an efficient conversion of ejecta kinetic energy into radiation via interaction with the circumstellar medium. Light-curve modelling suggests that this interaction involves ejecta with a mass of $M_{\mathrm{ej}}\simeq 1.3M_{\odot }$ impacting a shell of $M_{\mathrm{CSM}}\simeq 0.6M_{\odot }$. However, interaction is found to be geometrically and temporally confined within a few days after explosion, with the later evolution of the transient being dominated by the free expansion of the ejecta. Spectroscopic analysis reveals a complex, stratified circumstellar environment. In addition to the innermost shocked region, we identify two additional He-rich shells: a dense inner one ($n_{\mathrm{e}}\simeq 1.4\times 10^9\mathrm{c}{\mathrm{m}}^{-3}$) located at $R\gtrsim 9.3\times 10^{14}\mathrm{cm}$, and a more extended outer component reaching $R_{out}\simeq 3.3\times 10^{15}\mathrm{cm}$ and expanding at approximately $1.2\times 10^3\mathrm{km}{\mathrm{s}}^{-1}$. The physical parameters of the former suggest it was produced by a discrete pre-supernova eruptive event similar to that of SN~2006jc, which expelled approximately $1.8\times 10^{-2}{M}_{\odot }$ of material. Conversely, the expansion velocity of the outermost shell is consistent with the escape velocity of WR stars, pointing towards a late WN to WC-type progenitor as the most viable candidate for SN~2020able.

Accepted for publication on Astronomy & Astrophysics