10 m spectral observations of SN1987A: the first year

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Abstract
We present spectra of SN1987A in the 10 μm region taken on days 257 and 366–371 after core collapse, which supplement those published earlier for days 54, 87 and 164. From about 9 months the hydrogenic lines and the ionic fine structure lines attributed to [Ne II], [Ni II] and [Co II] (earlier tentatively attributed to [S IV]) have been effectively optically thin, and the [Co II] line strength and development yields an estimate of the parental 56Ni mass of 0.075 M⊙. The velocities appropriate to these two regions, hydrogenic and ionic, are ≃ 2000 and ≃ 1500 km s−1, respectively. The hydrogen/helium envelope is partially ionized at the few per cent level and this provides the free–free continuum and hydrogenic lines. There is relative transparency in the mid-infrared, while the bound electrons provide the X-ray opacity which is still considerable at day 370. The continuum appears to have become optically thin from about 6 months, and there may be a contribution from low ionization middle mass elements and the iron group. Apart from an excess between 8–9 μm which has been tentatively attributed to molecular SiO, there is no evidence of departure from the emission expected from an optically thin plasma, and no sign of any emission from dust.

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