Five years of mid-infrared evolution of the remnant of SN 1987A: The encounter between the blast wave and the dusty equatorial ring

We have used the Spitzer satellite to monitor the mid-IR evolution of SN 1987A over a five year period spanning the epochs between days ∼6000 and 8000 since the explosion. The supernova (SN) has evolved into a supernova remnant 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 ∼5 × 10 ⁶ K and ∼3 × 10⁴ cm^-3, respectively. The mass of the radiating dust is ∼1.2 × 10^-6M⊙ on day 7554 and scales linearly with IR flux. Comparison of the IR data with the soft X-ray flux derived from Chandra observations shows that the IR-to-X-ray flux ratio, IRX, is roughly constant with a value of 2.5. Gas-grain collisions therefore dominate the cooling of the shocked gas. The constancy of IRX is most consistent with the scenario that very little grain processing or gas cooling has occurred throughout this epoch. The shape of the dust spectrum remained unchanged during the observations while the total flux increased by a factor of ∼5 with a time dependence of t′^0.87±0.20, 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, as also suggested by X-ray observations. The constant spectral shape of the IR emission provides strong constraints on the density and temperature of the shocked gas in which the interaction takes place. Silicate grains, with radii of ∼0.2μm and temperature of T ∼ 180 K, best fit the spectral and temporal evolution of the ∼8-30μm data. The IR spectra also show the presence of a secondary population of very small, hot (T ≳ 350 K), featureless dust. If these grains spatially coexist with the silicates, then they must have shorter lifetimes. The data show slightly different rates of increase of their respective fluxes, lending some support to this hypothesis. However, the origin of this emission component and the exact nature of its relation to the silicate emission is still a major unsolved puzzle.