Infrared Continuum and Line Evolution of the Equatorial Ring around SN 1987A

Abstract: Spitzer observations of SN 1987A have now spanned more than a decade. Since day ~4,000, mid-infrared (mid-IR) emission has been dominated by that from shock-heated dust in the equatorial ring (ER). From 6,000 to 8,000 days after the explosion, Spitzer observations included broadband photometry at 3.6 - 24 micron, and low and moderate resolution spectroscopy at 5 - 35 micron. Here we present later Spitzer observations, through day 10,377, which include only the broadband measurements at 3.6 and 4.5 micron. These data show that the 3.6 and 4.5 micron brightness has clearly begun to fade after day ~8,500, and no longer tracks the X-ray emission as well as it did at earlier epochs. This can be explained by the destruction of the dust in the ER on time scales shorter than the cooling time for the shocked gas. We find that the evolution of the late time IR emission is also similar to the now fading optical emission. We provide the complete record of the IR emission lines, as seen by Spitzer prior to day 8,000. The past evolution of the gas as seen by the IR emission lines seems largely consistent with the optical emission, although the IR [Fe II] and [Si II] lines show different, peculiar velocity structures.