Revealing the Diversity of Type IIn Supernova Progenitors through Their Environments

Type IIn supernovae (SNe IIn) are hydrogen-rich explosions embedded in dense circumstellar medium, which gives rise to their characteristic narrow hydrogen emission lines. The nature of their progenitors and pre-explosion mass loss remains, however, poorly understood. This study analyzes the local stellar environments of a volume-limited sample (z < 0.02) of 31 SNe IIn using high-resolution Hubble Space Telescope imaging. The environments are found to be highly diverse and are classified into three categories: inside star-forming regions (Class 1), outside but near star-forming regions (Class 2), and regions with no obvious star formation (Class 3). Bright SNe IIn (M_peak < −19.5 mag) predominantly occur in Class 1 environments, supporting the origin of very massive progenitors, while the faint SNe IIn (M_peak < −15.5 mag) are associated with the Class 2 and 3 environments, consistent with the less massive progenitors. SNe IIn with intermediate peak magnitudes (−19.5 mag < M_peak < −15.5 mag) appear in all three types of environments. Furthermore, directly detected SN IIn progenitors are systematically brighter and/or bluer than the youngest stellar populations in their environments, implying a nonquiescent pre-explosion state or past binary interactions. These results point to multiple progenitor channels of SNe IIn, spanning a wide range of masses, evolutionary stages, and potential binary interaction histories.