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 (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.