The stratification in the envelope of SN 1987 A⋆

We present measurements of the absorption trough velocities in P Cygni-type and absorption-line profiles in SN 1987 A during the first 111 days, are compared these with the corresponding velocities at the photosphere. The expanding envelope shows a considerable stratification: strongest absorption in prominent lines occurs at radii 2.5–4 times larger than the photospheric radius, while weaker lines show radii of maximum absorption coinciding with the photosphere. Absorption lines, as well as the photosphere, are found to show certain distinct phases of velocity evolution, reflecting both the slope of the initial density law and changes in excitation or ionization conditions. A comparison with the model calculations of Höflich and Lucy for the atmosphere of SN 1987A shows that both the assumption of homologous expansion and the estimates of the initial density distribution made in these models are essentially justified; however, the models of Höflich predict systematically larger photospheric velocities than are observed.