The late-time expansion of the ejecta of SN 1987A

The evolution of the shape and size of the ejecta of SN 1987A is analyzed over a period of similar to8 years based on HST images and spectra taken between 1278 and 4336 days after the supernova outburst. We combine both proprietary and archival HST data obtained with the FOC. WFPC2 and STIS. The low resolution near-UV prism FOC spectrum obtained at day 3043 has: not been described previously. Although the FWHM of the ejecta grew linearly over the time span studied, the appearance of the SN envelope also changed markedly with wavelength. At visible wavelengths (lambda similar or equal to 5000 Angstrom) the ejecta became progressively more elongated, reaching an ellipticity of epsilon similar or equal to 0.25 by day 4000. In the near-UV (lambda similar or equal to 2500 Angstrom). the ejecta remained closely circular (epsilon less than or equal to 0.1) and similar to 50% larger in angular extent than in the visible. The FOC prism observations show that the large extent of the SN envelope is confined to a grouping of resonance lines spanning Mg I lambda 2852, Mg II lambda lambda 2795,2802 Angstrom and several Fe II multiplets - thereby confirming that the larger size of the debris in the near-UV is due to scattering in these optically thick transitions compared to the optically thin forbidden and semi-forbidden transitions that dominate the visible spectrum. The available data are not of sufficient quality to detect the slight deviation from linear expansion expected for the outermost regions of the near-UV images as predicted by Chugai et al. (1997).