Spectral Diversity in Type Ibn Supernovae and the Large Host Offset of SN 2024acyl

Le résumé fourni par la source

Abstract In this paper, we first present observations of SN 2024acyl, a normal Type Ibn supernova (SN), with a large projected offset (∼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 (SNe Ibn) 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 the 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 SNe Ibn or their pre-explosion activity. The observed diversity in SNe Ibn 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.

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