EDP Sciences
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Volume 370, Number 3, May II 2001
Page(s) 1056 - 1063
Section Diffuse matter in space
DOI http://dx.doi.org/10.1051/0004-6361:20010292

A&A 370, 1056-1063 (2001)
DOI: 10.1051/0004-6361:20010292

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

R. A. Jansen and P. Jakobsen

Astrophysics Division, Space Science Department of ESA, ESTEC, NL-2200 AG Noordwijk, The Netherlands
    e-mail: rjansen@astro.estec.esa.nl; pjakobse@astro.estec.esa.nl

(Received 11 January 2001 / Accepted 19 February 2001)

The evolution of the shape and size of the ejecta of SN 1987A is analyzed over a period of ~8 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 ( $\lambda\simeq5000$ Å) the ejecta became progressively more elongated, reaching an ellipticity of $\epsilon\simeq
0.25$ by day 4000. In the near-UV ( $\lambda\simeq2500$ Å), the ejecta remained closely circular ( $\epsilon\leq 0.1$) and ~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 MgI $\lambda$2852, MgII $\lambda\lambda$2795,2802Åand several FeII 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).

Key words: supernovae: individual (SN 1987A) -- supernova remnants

Offprint request: R. A. Jansen, rjansen@astro.estec.esa.nl

© ESO 2001