We have used the Spitzer satellite to monitor the mid-IR evolution of SN
1987A over a 5 year period spanning the epochs between days 6000 and 8000 since
the explosion. The supernova (SN) has evolved into a supernova remnant (SNR)
and its radiative output is dominated by the interaction of the SN blast wave
with the pre-existing equatorial ring (ER). The mid-IR spectrum is dominated by
emission from ~180 K silicate dust, collisionally-heated by the hot X-ray
emitting gas with a temperature and density of ~5x10^6 K and 3x10^4 cm-3,
respectively. The mass of the radiating dust is ~1.2x10^(-6) Msun on day 7554,
and scales linearly with IR flux. The infrared to soft-X-ray flux ratio is
roughly constant with a value of 2.5. Gas-grain collisions therefore dominate
the cooling of the shocked gas. The constancy of of this ratio suggests that
very little grain processing or gas cooling have occurred throughout this
epoch. The shape of the dust spectrum remained unchanged during the
observations while the total flux increased with a time dependence of t^(0.87),
t being the time since the first encounter between the blast wave and the ER.
These observations are consistent with the transitioning of the blast wave from
free expansion to a Sedov phase as it propagates into the main body of the ER.Comment: Accepted for publication in the ApJ, 11 pages, 11 figure
