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

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 ( ρ CSM ∝ r −3 ), inferring a rapid increase in the mass-loss rate toward the SN as a generic property of the SN Ibn progenitors. From the derived ejecta and CSM properties, we argue that massive Wolf–Rayet stars with an initial mass of ≳18 M ⊙ can be a potential class of the progenitors. The present work also indicates the existence of a 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.