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 o ver a period of similar to8 years based on HST images and spectra taken bet ween 1278 and 4336 days after the supernova outburst. We combine both propr ietary 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 d escribed 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 A ngstrom) the ejecta became progressively more elongated, reaching an ellipt icity of epsilon similar or equal to 0.25 by day 4000. In the near-UV (lamb da 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 e xtent than in the visible. The FOC prism observations show that the large e xtent of the SN envelope is confined to a grouping of resonance lines spann ing 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 t he near-UV is due to scattering in these optically thick transitions compar ed to the optically thin forbidden and semi-forbidden transitions that domi nate the visible spectrum. The available data are not of sufficient quality to detect the slight deviation from linear expansion expected for the oute rmost regions of the near-UV images as predicted by Chugai et al. (1997).