A Low-Mass Helium Star Progenitor Model for the Type Ibn SN 2020nxt

TL;DR

This study models the progenitor of the Type Ibn SN 2020nxt, suggesting it originated from a low-mass helium star that lost its envelope shortly before explosion, challenging the idea that all SNe Ibn come from massive Wolf-Rayet stars.

Contribution

It introduces a new progenitor model for SNe Ibn based on multi-wavelength data and advanced spectral modeling, proposing a low-mass helium star origin.

Findings

A $ extless 4 M_ ext{sun}$ helium star model fits the data.

The progenitor lost about 1 $M_ ext{sun}$ of helium-rich envelope before collapse.

Some SNe Ibn may not originate from massive Wolf-Rayet stars.

Abstract

A growing number of supernovae (SNe) are now known to exhibit evidence for significant interaction with a dense, pre-existing, circumstellar medium (CSM). SNe Ibn comprise one such class that can be characterised by both rapidly evolving light curves and persistent narrow He I lines. The origin of such a dense CSM in these systems remains a pressing question, specifically concerning the progenitor system and mass-loss mechanism. In this paper, we present multi-wavelength data of the Type Ibn SN 2020nxt, including $HST$/STIS ultraviolet spectra. We fit the data with recently updated CMFGEN models designed to handle configurations for SNe Ibn. The UV coverage yields strong constraints on the energetics and, when combined with the CMFGEN models, offer new insight on potential progenitor systems. We find the most successful model is a $\lesssim4 {\rm M}_\odot$ helium star that lost its $\sim 1\,{\rm M}_\odot$ He-rich envelope in the years preceding core collapse. We also consider viable alternatives, such as a He white dwarf merger. Ultimately, we conclude at least some SNe Ibn do not arise from single, massive ($>30 {\rm M}_\odot$) Wolf-Rayet-like stars.