Ejecta, Rings, and Dust in SN 1987A with JWST MIRI/MRS

Abstract

Supernova (SN) 1987A is the nearest supernova in ∼400 yr. Using the JWST MIRI Medium Resolution Spectrograph, we spatially resolved the ejecta, equatorial ring (ER), and outer rings in the mid-infrared 12,927 days (35.4 yr) after the explosion. The spectra are rich in line and dust continuum emission, both in the ejecta and the ring. The broad emission lines (280–380 km s−1 FWHM) that are 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 dust 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 ii], [Ar ii], [Fe ii], and [Ni ii]. With the exception of [Fe ii] 25.99 μm, these all originate from the ejecta close to the ring and are likely to be excited by X-rays from the interaction. The [Fe ii] 5.34 to 25.99 μm line ratio indicates a temperature of only a few hundred K in the inner core, which is consistent with being powered by 44 Ti decay.

JWST MIRI Imager Observations of Supernova SN 1987A

“…For the F1000W and F1800W filters, emission lines make a negligible contribution (Table 1). The contributions of synchrotron and free-free emission are also small except perhaps at wavelengths shorter than 8 μm (Jones et al 2023). It would seem that the most likely explanation for the small discrepancy at the two longest wavelengths between the Imager filter fluxes and the equivalent MRS filter fluxes is a difference in the background estimation, given that background flux levels rise steeply to longer wavelengths.…”

“…In Figure 5 we have superimposed on our image obtained through the F2550W filter the contours of the [Fe II]λ25.99 μm line from the MRS data. The MRS data clearly showed that this prominent line is coming mostly from the inner ejecta and the ejecta close to the reverse shock (Jones et al 2023), while the dust which dominates the F2550W filter is from the ring. The Imager data, which have a spatial resolution ∼2 times better than the MRS data, confirm this conclusion, with the dust-dominated F2550W image showing a drop in emission inside the ER, whereas the [Fe II]λ25.99-dominated contours peak inside the ER.…”

“…Figure 3 shows the central 65 × 65 pixels in each of the F560W, F1000W, F1800W, and F2550W filters. For each filter, we have superimposed the MIRI-MRS image contours of the strong [Ar II] 6.985 μm line (Fransson et al 2024), which were derived from the data presented in Jones et al (2023). Table 1 lists the in-band photometric fluxes measured for each of the four filters for SN 1987A's ER and ejecta.…”

“…Table 1 lists the in-band photometric fluxes measured for each of the four filters for SN 1987A's ER and ejecta. These were measured using the same on-source and background apertures as defined by Jones et al (2023) for their "total" MRS spectrum of SN 1987A. An updated version of the "total" MRS spectrum is shown in Figure 4.…”

“…ejecta only. Moreover, fitting a blackbody model to the mid-IR observations is not justified as the mid-IR fluxes are dominated by optically thin emission from silicate dust grains in the ER (Arendt et al 2016;Jones et al 2023).…”