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We have used the Spitzer satellite to monitor the mid-IR evolution of SN1987A over a 5 year period spanning the epochs between days 6000 and 8000 sincethe explosion. The supernova (SN) has evolved into a supernova remnant (SNR)and its radiative output is dominated by the interaction of the SN blast wavewith the pre-existing equatorial ring (ER). The mid-IR spectrum is dominated byemission from ~180 K silicate dust, collisionally-heated by the hot X-rayemitting 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 isroughly constant with a value of 2.5. Gas-grain collisions therefore dominatethe cooling of the shocked gas. The constancy of of this ratio suggests thatvery little grain processing or gas cooling have occurred throughout thisepoch. The shape of the dust spectrum remained unchanged during theobservations 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 fromfree expansion to a Sedov phase as it propagates into the main body of the ER.

Five Years of Mid-Infrared Evolution of the Remnant of SN 1987A: The Encounter Between the Blast Wave and the Dusty Equatorial Ring