DAY-640 INFRARED LINE AND CONTINUUM MEASUREMENTS - DUST FORMATION IN SN-1987A

We have measured day 640-645 line and continuum spectra of [Ni II] 6.6 mum, [Ne II] 12.8 mum (line emission was not detected), and [Fe II] 1 7.9 and 26.0 mum from SN 1987A. The high velocity feature at upsilon(H VF) approximately 3900 km s-1 found in both of our day 410 [Fe II] spe ctra is again detected in the day 640 [Ni II] spectrum, although the s ignal-to-noise of the day 640 [Fe II] spectra is insufficient to show this feature. The continuum fluxes provide clear evidence for the form ation of dust between day 410 and day 640 and are best fitted by a gra ybody spectrum with a temperature of 342 +/- 17 K at day 640 and a sur face area corresponding to a minimum dust velocity upsilon(dust) = 191 0 +/- 170 km s-1. Optically thin dust emissivity laws proportional to lambda-1 or lambda-2 are inconsistent with the data. Either the dust g rains are large (radius a much greater than 4 mum) and radiate like in dividual blackbodies, or else they are located in clumps optically thi ck in the 6-26 mum range. The [Ni ii] 6.6 mum line flux yields a minim um Ni+ mass of 5.8 +/- 1.6 x 10(-4) M., and a Ni/Fe abundance ratio of 0.06 +/- 0.02, equal to the solar value. The ratio of the two [Fe II] line profiles implies a gas temperature 2600 +/- 700 K, a drop of 180 0 +/- 800 K from our day 410 measurement. The [Fe II] 26.0 mum line fl ux has decreased by a factor of 2 and the day 640 [Ni II] profile is b lueshifted by -440 +/- 270 km s-1, relative to observations before day 500. We show that the decrease in the [Fe II] flux and the blueshift are not produced by a decrease in electron scattering optical depth, e lectron density, or temperature, but rather are probably due to obscur ation by the same dust which produces the infrared continuum. This sup ports the interpretation that the dust spectrum is produced by optical ly thick clumps. We discuss possible explanations for the discrepancy between the mass of Fe+ detected and the total iron mass required to p ower the light curve. The decrease in the [Fe II] fluxes relative to t he decrease required to account for the blueshifts of optical lines fr om non-iron-group elements and the similarity between upsilon(dust) an d the Ni+ expansion velocity imply a spatial association between the d ust clumps and the iron-group elements. In addition, the larger bluesh ift observed for the near and far-infrared, heavy metal transitions re lative to non-iron-group lines suggests that the iron-group elements a re somewhat segregated from lighter elements such as the Mg0 and O0 re sponsible for shorter wavelength lines. We speculate that FeS may be a n important constituent of the dust. A comparison of our line profiles with radiative transfer models shows that while power law and exponen tial density distributions yield reasonable fits to the data, polytrop e distributions provided significantly worse agreement. The best fits require a substantial fraction of the iron to be undetectable, and are consistent with maximum expansion velocities of upsilon(max) approxim ately 3000 km s-1.