The early discovery of SN 2017ahn: signatures of persistent interaction in a fast declining Type II supernova

TL;DR

This paper presents detailed early and late observations of SN 2017ahn, revealing persistent interaction signatures with a complex circumstellar medium, and supports models linking linearly declining Type II supernovae to massive, hydrogen-depleted progenitors.

Contribution

It provides comprehensive high-cadence data on a nearby Type II supernova, demonstrating persistent circumstellar interaction and supporting progenitor models involving hydrogen-depleted massive stars.

Findings

Early flash spectroscopy shows high-ionization emission features.

Evidence of a dense circumstellar shell interacting with ejecta.

Supports models of linearly declining Type II supernovae from hydrogen-poor progenitors.

Abstract

We present high-cadence, comprehensive data on the nearby ($D\simeq33\,\rm{Mpc}$) Type II SN 2017ahn, discovered within $\sim$1 day of explosion, from the very early phases after explosion to the nebular phase. The observables of SN 2017ahn show a significant evolution over the $\simeq470\,\rm{d}$ of our follow-up campaign, first showing prominent, narrow Balmer lines and other high-ionization features purely in emission (i.e. flash spectroscopy features), which progressively fade and lead to a spectroscopic evolution similar to that of more canonical Type II supernovae. Over the same period, the decline of the light curves in all bands is fast, resembling the photometric evolution of linearly declining H-rich core-collapse supernovae. The modeling of the light curves and early flash spectra suggest a complex circumstellar medium surrounding the progenitor star at the time of explosion, with a first dense shell produced during the very late stages of its evolution being swept up by the rapidly expanding ejecta within the first $\sim6\,\rm{d}$ of the supernova evolution, while signatures of interaction are observed also at later phases. Hydrodynamical models support the scenario in which linearly declining Type II supernovae are predicted to arise from massive yellow super/hyper giants depleted of most of their hydrogen layers.