Revealing the diversity of Type IIn supernova progenitors through their environments

TL;DR

This study constrains Type IIn SN progenitors by examining their local environments with high-resolution HST imaging for 31 events at z<0.02. A three-class framework (Class 1: inside star-forming regions; Class 2: nearby; Class 3: old/quiet) reveals that Bright SNe IIn preferentially inhabit Class 1 regions, while Faint SNe IIn reside in Classes 2–3, implying multiple progenitor channels across a broad mass range. Normal SNe IIn appear in all environments, suggesting diverse mass-loss mechanisms and evolutionary histories. Direct progenitor detections corroborate a non-quiescent or binary-interaction origin, reinforcing the conclusion that SNe IIn arise from a heterogeneous set of progenitors rather than a single, very massive-star channel. The environmental approach provides a robust framework for interpreting SNe IIn diversity and sets the stage for future, larger multi-wavelength surveys to refine progenitor pathways.

Abstract

Type IIn supernovae (SNe IIn) are hydrogen-rich explosions embedded in dense circumstellar medium (CSM), which gives rise to their characteristic narrow hydrogen emission lines. The nature of their progenitors and pre-explosion mass loss remains, however, poorly understood. Using high-resolution Hubble Space Telescope (HST) imaging, we analyze the local stellar environments of a volume-limited sample (z < 0.02) of 31 SNe IIn. The environments of SNe IIn are found to be very diverse; the SN could reside within a star-forming region (Class 1), outside a star-forming region (Class 2), or in much older environments without any obvious signs of star formation (Class 3). The bright SNe IIn (Mpeak < -19.5 mag) predominantly occur in Class 1 environments, indicative of very massive progenitors, while the faint SNe IIn (Mpeak < -15.5 mag) are associated with Classes 2 and 3 environments, suggesting the least massive progenitors. Meanwhile, normal SNe IIn with -19.5 < Mpeak < -15.5 mag occur in all three types of environments, suggesting a diversity in their progenitor mass, lifetime, and evolutionary pathways. Moreover, the directly detected SN IIn progenitors are systematically brighter and/or bluer than the youngest stellar populations in their environments, suggesting that they were either in a non-quiescent state when observed or had experienced binary interactions. These results point to a significantly diverse origin for progenitors of SNe IIn, spanning a wide range of masses, evolutionary stages, and potential binary interaction histories.

Figures (19)

• Figure 1: The environment of SN 2005gl serves as an example of a Class 1 environment, locating within a star-forming region. (a) SDSS $u/g/r$ composite image of the host galaxy, centered on the SN. The white square outlines an 800 pc $\times$ 800 pc zoomed-in region. (b, c) HST images of the SN site. (d, e) Spatial distribution of stars detected in the HST images. Symbol sizes represent absolute magnitudes corrected for Galactic extinction. The orange contour traces a 3$\sigma$ density enhancement above the local average. Blue circles in panels (b--e) indicate a 50 pc radius centered on the derived SN site, considering the positional uncertainties of the SN in Section \ref{['sec:pin']}. The green elongated circle in panel (d) outlines the nearest stellar overdensity region to the SN, which is spatially distinct from the dense region southeast of the SN. Stars within this circle are considered to be coeval with the SN progenitor and are used for comparison in analysis in Section \ref{['sec:env_prog']}.
• Figure 2: Similar to Figure \ref{['fig:class1_05gl']}, but for SN 2000cl. No stellar overdensity is detected within 50 pc of the SN; however, a significant overdensity of stars is detected within 300 pc. This distance approximately corresponds to the spatial displacement within which 90$\%$ of progenitors that have undergone binary interactions. 2025Wagg2019Renzo. Therefore, environments such as that of SN 2000cl are defined as Class 2, indicating locations outside star-forming regions.
• Figure 3: Similar to Figures \ref{['fig:class1_05gl']} and \ref{['fig:class2_00cl']}, but for SN 2015bh. This environment lacks strong evidence for star-forming regions within 300 pc and is therefore defined as Class 3.
• Figure 4: Peak absolute magnitude versus local environment class for 31 SNe IIn at $z < 0.02$ with HST high-resolution imaging. The local environments of SNe are classified into three types based on resolved stellar populations: inside star-forming regions, outside star-forming regions, or regions lacking obvious star-forming regions. Those SNe for which the classification of their host environment is still preliminary are indicted with $^{*}$. Following the SNe IIn light curve census of 2025Ransome, we classify their peak absolute magnitudes as: bright ($M_{\rm peak}<-20$), normal ($-20<M_{\rm peak}<-16$), or faint ($M_{\rm peak}>-16$). The 2009ip-like SNe (showing two luminous peaks) are marked with ${\dagger}$. SNe whose progenitors have been directly detected are indicated with circles; their progenitors and environmental stars are analyzed in detail in Section \ref{['sec:env_prog']}.
• Figure 5: Color-absolute magnitude diagrams of the resolved stellar populations surrounding the site of seven SNe IIn with progenitor detections. Each panel is labeled with the SN name and its environmental Class. The directly detected progenitors are marked in red, with asterisks for well-constrained detections and horizontal stripes for less certain ones. For Class 1, the progenitors are compared to stars in the nearest star-forming region. For Class 2, we plot stars within 300 pc of the SN site. The star-forming population is shown in orange, and the background population is shown in gray (see text). The Class 3 environments lack obvious star-forming regions, therefore surrounding stars within 300 pc are shown. The classification of SN 2009ip is uncertain (see discussion in the text). The only point source detected within 300 pc of its position has been studied in detail by 2016Smith. Corrections have been applied for both Galactic extinction (SFD map) and host extinction (in literature) for the progenitor and surrounding stellar. The widths of stripes and errorbars represent 1$\sigma$ uncertainties. parsec single-star isochrones, with labelled initial masses, are overlaid.