Supernova (SN) 1987A is the nearest supernova in $\sim$400 years. Using the { \em JWST} MIRI Medium Resolution Spectrograph, we spatially resolved the ejecta, equatorial ring (ER) and outer rings in the mid-infrared 12,927 days after the explosion. The spectra are rich in line and dust continuum emission, both in the ejecta and the ring. Broad emission lines (280-380~km~s$^{ -1}$ FWHM) seen from all singly-ionized species originate from the expanding ER, with properties consistent with dense post-shock cooling gas. Narrower emission lines (100-170~km~s$^{ -1}$ FWHM) are seen from species originating from a more extended lower-density component whose high ionization may have been produced by shocks progressing through the ER, or by the UV radiation pulse associated with the original supernova event. The asymmetric east-west dust emission in the ER has continued to fade, with constant temperature, signifying a reduction in dust mass. Small grains in the ER are preferentially destroyed, with larger grains from the progenitor surviving the transition from SN into SNR. The ER is fit with a single set of optical constants, eliminating the need for a secondary featureless hot dust component. We find several broad ejecta emission lines from [Ne~{ \sc ii}], [Ar~{ \sc ii}], [Fe~{ \sc ii}], and [Ni~{ \sc ii}]. With the exception of [Fe~{ \sc ii}]~25.99$\mu$m, these all originate from the ejecta close to the ring and are likely being excited by X-rays from the interaction. The [Fe~{ \sc ii}]~5.34$\mu$m to 25.99$\mu$m line ratio indicates a temperature of only a few hundred K in the inner core, consistent with being powered by ${ }^{ 44}$Ti decay.
Categories: astro-ph.HE astro-ph.GA astro-ph.SR