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

Yize Dong, V. Ashley Villar, Anya Nugent, Griffin Hosseinzadeh, Ryan J. Foley, Christa Gall, Monica Gallegos-Garcia, Conor Ransome, Aidan Sedgewick, Daichi Tsuna, Stefano Valenti, Henna Abunemeh, Moira Andrews, Katie Auchettl, K. Azalee Bostroem, David A. Coulter, Thomas de Boer, Kaylee de Soto, Diego A. Farias, Joseph Farah, Danielle Frostig, Hua Gao, Alex Gagliano, Emily Hoang, D. Andrew Howell, Willem B. Hoogendam, Mark E. Huber, David O. Jones, Chien-Cheng Lin, Michael Lundquist, Curtis McCully, Darshana Mehta, Anthony L. Piro, Aravind P. Ravi, Nicolás Meza Retamal, César Rojas-Bravo, S. Karthik Yadavalli, Qinan Wang

TL;DR

This work analyzes SN 2024acyl, a typical Type Ibn with an unusually large host offset, and expands to a broader examination of spectral diversity in Type Ibn SNe around 20–35 days after peak. Through multi-wavelength photometry and extensive spectroscopy, the study models the light curve with a CSM+\mathrm{Ni} decay framework and characterizes post-peak spectral evolution, identifying two distinct groups based on $g-i$ color and He I line widths. The results imply diverse circumstellar environments and explosion energetics, possibly reflecting multiple progenitor channels, with SN 2024acyl classified as Group II. The findings challenge a single, massive-star origin scenario for Ibn SNe and underscore the need for systematic host studies and early-to-late-time spectroscopy to disentangle progenitor pathways and CSM configurations.

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.

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

TL;DR

This work analyzes SN 2024acyl, a typical Type Ibn with an unusually large host offset, and expands to a broader examination of spectral diversity in Type Ibn SNe around 20–35 days after peak. Through multi-wavelength photometry and extensive spectroscopy, the study models the light curve with a CSM+\mathrm{Ni} decay framework and characterizes post-peak spectral evolution, identifying two distinct groups based on color and He I line widths. The results imply diverse circumstellar environments and explosion energetics, possibly reflecting multiple progenitor channels, with SN 2024acyl classified as Group II. The findings challenge a single, massive-star origin scenario for Ibn SNe and underscore the need for systematic host studies and early-to-late-time spectroscopy to disentangle progenitor pathways and CSM configurations.

Abstract

In this paper, we first present observations of SN~2024acyl, a normal Type Ibn supernova 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 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 ( 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.

Paper Structure

This paper contains 20 sections, 13 figures.

Table of Contents

  1. Introduction
  2. Observations
  3. Photometric Observations
  4. Spectrocopic Observations
  5. Photometric Evolution
  6. Spectroscopic Evolution
  7. Host Environments of SN 2024acyl
  8. Global Host Environment
  9. Local Host Environment
  10. Implications from Host Environments
  11. Spectral Diversity in Type Ibn SNe
  12. Spectral Diversity in SNe Ibn A Month After Maximum
  13. Connections To The Spectral Diversity Presented In Hosseinzadeh2017ApJ...836..158H
  14. Implications For The Progenitors of SNe Ibn
  15. Summary and Conclusions
  16. ...and 5 more sections

Figures (13)

  • Figure 1: Left: Stacked $w$-band image from PS1 and PS2 taken before the SN explosion. There is no source detected at the SN position down to 24.5 mag. The blue circle, centered on the SN location, has a radius of 10 kpc and is shown for visual reference. Middle: Composite $gri$ image of SN 2024acyl obtained with the Gemini-North Observatory on 2025 February 22. The position of SN 2024acyl is indicated by white tick markers. Right: A zoom-in on the region around SN 2024acyl. Seven extended sources near the SN are labeled, with their estimated redshifts indicated.
  • Figure 2: Photometric evolution of SN 2024acyl with respect to the maximum light. Detections with S/N $>4$ are shown as large filled symbols, those with $3 < \mathrm{S/N} \leq 4$ as hollow symbols, and non-detections with $\mathrm{S/N} \leq 3$ as smaller downward limits. Epochs of spectra are marked by black lines along the bottom axis.
  • Figure 3: $r/R$-band light curve comparison between SN 2024acyl and 27 Type Ibn SNe with well-sampled light curves. The $^{56}$Co decay rate is shown as a dashed line for comparison. The Type Ibn SNe used in this plot includes: PS1-12sk Sanders2013ApJ...769...39S, SN 2002ao Pastorello2008MNRAS.389..113P, SN 2018bcc Karamehmetoglu2021AA...649A.163K, SN 2020bqj Kool2021AA...652A.136K, SN 2020nxt Wang2024MNRAS.530.3906W, SN 2023emq Pursiainen2023ApJ...959L..10P, SN 2014av Pastorello2016MNRAS.456..853P, SN 2019uo Gangopadhyay2020ApJ...889..170G, PTF12ldy Hosseinzadeh2017ApJ...836..158H, SN 2018jmt Wang2024AA...691A.156W, iPTF15ul Hosseinzadeh2017ApJ...836..158H, iPTF14aki Hosseinzadeh2017ApJ...836..158H, iPTF15akq Hosseinzadeh2017ApJ...836..158H, SN 2021jpk Pellegrino2022ApJ...926..125P, ASASSN-15ed Pastorello2015MNRAS.453.3649P, SN 2015U Tsvetkov2015IBVS.6140....1TPastorello2015MNRAS.454.4293PHosseinzadeh2017ApJ...836..158H, SN 2019deh Pellegrino2022ApJ...926..125P, PTF11rfh Hosseinzadeh2017ApJ...836..158H, OGLE-2012-SN-006 Pastorello2015MNRAS.449.1941P, SN 2023fyq Dong2024ApJ...977..254DBrennan2024AA...684L..18B, SN 2019kbj Ben-Ami2023ApJ...946...30B, SN 2006jc Pastorello2007Natur.447..829P, SN 2023tsz Warwick2025MNRAS.536.3588W, SN 2018gjx Prentice2020MNRAS.499.1450P, SN 2015G Shivvers2017MNRAS.471.4381SHosseinzadeh2017ApJ...836..158H, and SN 2010al Pastorello2015MNRAS.449.1921P,
  • Figure 4: Spectrocopic evolution of SN 2024acyl. The phase is measured from the $o$-band maximum. The spectra shown here have been smoothed with a second order Savitzky-Golay filter, and the gray background lines are the original spectra.
  • Figure 5: Spectral evolution of SN 2024acyl at -4.1 days, -2.8 days and -1.6 days compared to SN 2010al and SN 2019uo. The inset shows the evolution of the flash ionization lines He II $\lambda4686$ and N III $\lambda4640$/C III $\lambda4650$, which become weaker over time, while He II $\lambda4686$ develops a P-Cygni profile at -2.0 days.
  • ...and 8 more figures