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