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

TL;DR

This study models the light curves of Type Ibn supernovae to understand their progenitors, revealing steep circumstellar density profiles and suggesting Wolf-Rayet stars as likely progenitors, while predicting a hidden population of UV-bright transients.

Contribution

A new light curve model for SNe Ibn that explains their post-peak behavior and infers progenitor properties, highlighting the role of CSM density gradients and mass-loss rates.

Findings

SN Ibn light curves are explained by shock transition from cooling to adiabatic regimes.

Steep CSM density gradient (~ r^{-3}) indicates increasing mass-loss rates before explosion.

Massive Wolf-Rayet stars (>~18 Msun) are likely progenitors.

Abstract

Type Ibn Supernovae (SNe Ibn) show signatures of strong interaction between the SN ejecta and hydrogen-poor circumstellar matter (CSM). Deriving the ejecta and CSM properties of SNe Ibn provides a great opportunity to study the final evolution of massive stars. In the present work, we present a light curve (LC) model for the ejecta-CSM interaction, taking into account the processes in which the high-energy photons originally created at the forward and reverse shocks are converted to the observed emission in the optical. The model is applied to a sample of SNe Ibn and `SN Ibn' rapidly evolving transients. We show that the characteristic post-peak behavior commonly seen in the SN Ibn LCs, where a slow decay is followed by a rapid decay, is naturally explained by the transition of the forward-shock property from cooling to adiabatic regime without introducing a change in the CSM density distribution. The (commonly-found) slope in the rapid decay phase indicates a steep CSM density gradient (rho_CSM ~ r^{-3}), inferring a rapid increase in the mass-loss rate toward the SN as a generic properties of the SN Ibn progenitors. From the derived ejecta and CSM properties, we argue that massive Wolf-Rayet stars with the initial mass of >~ 18 Msun can be a potential class of the progenitors. The present work also indicates existence of currently missing population of UV-bright rapid transients for which the final mass-loss rate is lower than the optical SNe Ibn, which can be efficiently probed by future UV missions.