Spectral Diversity in Type Ibn Supernovae and the Large Host Offset of SN2024acyl

TL;DR

This study analyzes the spectral diversity of Type Ibn supernovae, highlighting two distinct groups with different spectral and photometric properties, and investigates the peculiar case of SN2024acyl with a large host offset and possible unique progenitor characteristics.

Contribution

It identifies two spectral groups among Type Ibn supernovae and characterizes SN2024acyl as belonging to the less common, redder, broader-line group, suggesting diverse progenitor properties and explosion mechanisms.

Findings

SN2024acyl has a large host offset (~35 kpc) and low local star formation.

Type Ibn SNe show two spectral groups with distinct colors and line widths.

Spectral diversity may reflect differences in circumstellar material and progenitor systems.

Abstract

In this paper, we first present observations of SN~2024acyl, a normal Type Ibn supernova with a large projected offset ($\sim$35~kpc) from its host galaxy. The low star-formation rate measured at the explosion site raises the possibility that the progenitor of SN~2024acyl may not have been a massive star. We then examine, more broadly, the spectral diversity of Type Ibn supernovae around 20--35 days after peak brightness and identify two distinct groups: Group I, which shows bluer rest-frame optical color and narrower He~I emission lines; and Group II, which shows redder rest-frame optical color and broader He~I lines. Group~I also tends to show higher peak luminosities. The diversity we identify appears to be closely connected to the diversity observed around peak and to persist into late phases ($>80$ days after peak). Given its redder color and broader He~I lines, we classify SN~2024acyl as belonging to Group II. Based on the current dataset, we find no clear connection between this spectral diversity and either the host environments of Type Ibn SNe or their pre-explosion activity. The observed diversity in Type Ibn SNe likely reflects differences in circumstellar material properties and/or explosion energetics. These differences could result from a range of progenitor properties, such as different helium star mass, orbital period and companion type if they are in binary systems, and may indicate fundamentally diverse progenitors. Whether a continuous distribution exists between the two groups remains to be determined and will require further data to explore.