10 μm spectral observations of SN 1987A: interpretation of the infrared excess

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Abstract
We present spectra of SN 1987A in the 10μm region; at these wavelengths the photometric flux is much weaker than in the visible and near infrared, and emission from the lower density regions of the expanding envelope becomes correspondingly more important. On days 54, 86 and 164, 8–13μm spectra have revealed a free–free emission component from the expanding envelope, along with emission lines of hydrogen and lines tentatively identified with [S IV] and [Ne II]. While all the lines increased in strength during this period, those of the fine structure lines did so dramatically. We attribute the appearance of [S IV], its rapid increase in strength, and the flattening of the continuum slope, to early signs of a developing nebular or coronal phase. There was no evidence by day 164 for dust emission either as an echo from pre-existing material, or from condensation in the supernova ejecta.

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