S. Thomas

Context . The outer rings (ORs) of Supernova SN 1987A were ejected some 20 000 years before the explosion. Their characterisation is crucial for constraining the properties of the progenitor of this famous SN. Aims . While numerous studies have investigated the SN ejecta, equatorial ring (ER), and reverse shocks in detail, very few were dedicated to the ORs. We aim to fill this gap and investigate the ORs’ physical properties. We analysed data obtained over a long temporal period, from multiple instruments, and over a wide wavelength range from the optical to mid-infrared of the northern and southern ORs (NOR and SOR) of SN 1987A. We combined observations taken with the Hubble Space Telescope (HST) between 1994 and 2022, with VLT/MUSE in 2023, and with the James Webb Space Telescope (JWST) in 2022 and 2024. Methods . We measured emission flux in different regions of the ORs in narrow-band HST and broad- and narrow-band JWST/NIRCam filter images. We extracted optical and mid-infrared spectra for the ORs in the MUSE and JWST/MIRI/MRS data and measured emission-line fluxes. We analysed the evolution of the ORs clumps’ morphology over time with HST. Results . The optical light curves of the ORs have shown a steady decline with time over the last 30 years, both in the H α and [O III ] λ 5007 filters. This behaviour is expected as the ORs were ionised by the initial SN UV flash and have been fading since then. The observations do not show any sign of interaction of the SN ejecta with the ORs, which would appear as a brightening in the ORs’ optical light curve. We estimated the decay times for [O III ] to be 900 and 630–730 days for the NOR and SOR, respectively, and for H α +[N II ] to be 15 870 and 7160 days for the NOR and SOR, respectively. We provide the first mid-resolution spectra of the ORs in the optical and mid-infrared. We constrained the temperature from the optical [N II ] lines to 13 400–16 900 K and 11 800–14 500 K for the NOR and SOR, respectively. We constrained the electron density from the optical [S II ] lines to 610–670 cm ⁻³ and 720–790 cm ⁻³ for the NOR and SOR, respectively. Conclusions . The spectra of the ORs differ significantly from the spectrum of the ER, not only in the detected lines, but also in the line ratios of given lines. The ORs will likely keep on fading in the coming years until the SN ejecta sweep up the ORs. Continued monitoring of SN 1987A and its ring system in the X-ray to mid-infrared is essential to capture this instant.