The Evolution of the Mid-infrared Spectrum of SN 1987A Observed with JWST MIRI/MRS

Supernova (SN) 1987A provides a unique laboratory for investigating many aspects of SN physics and evolution. An observation at Day 12927 (35.4 yr) since the explosion with the Mid-Infrared Instrument (MIRI) Medium Resolution Spectrometer (MRS) on the James Webb Space Telescope provided the first spatially resolved spectroscopic study of SN 1987A in the mid-infrared (mid-IR), yielding insights into the evolution of dust, the ejecta, the equatorial ring (ER), and shocks in the system. Here we present a second epoch with MIRI/MRS at Day 13311 (36.4 yr), allowing the mid-IR spatially resolved spectroscopic temporal evolution of SN 1987A to be probed for the first time. Analysis of the ER-dominated dust continuum showed little evolution between Days 12927 and 13311. However, spatial analysis reveals that the inner ER is fading while the outermost regions are brightening. Broad ejecta emission lines detected at Day 12927 are evolving rapidly, driven by the recent onset of ejecta–ER interaction in the northeast and southwest portions of the ER. Most lines from the ER show no change during the 384 days between epochs, although some, such as [Ne ii] and [Ar ii], have faded. We identify mid-IR H₂ emission associated with the ejecta for the first time. Using near- and mid-IR [Fe ii] lines as density and temperature diagnostics of the ejecta in the interaction region, we find it likely that the dense, inner Fe-rich ejecta has now reached the reverse shock. Continued monitoring of SN 1987A is essential to observe the evolving ejecta–ER interaction and dust components.