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Session 45 - Supernovae.
Display session, Tuesday, January 14
We have measured the expansion and shape of the SN 1987A debris with HST images. Broadband and narrow emission images covering a span of 6 years obtained between 1990 August 23 (day 1278) and 1996 September 1 (day 3479) are used. We fitted the observed debris profile to a surface brightness function which decreases with angular radius \rho as F(\rho) \propto 1 / [1+(\rho/\rho_0)^\alpha]. The best-fit observed diameter, 2\rho_0, at day 3271 (1996 Feburary 6) for the F555W (broadband V) image is found to be 199 \pm 6 mas, and the power index, \alpha, to be 3.58 \pm 0.11. The debris size varies with wavelength and is larger in the UV than in the optical with \rho_UV/\rho_V = 1.6 \pm 0.2. The expansion of the debris is consistent with a linear expansion with time with the power index, \alpha, decreasing with time. The angular rate of expansion at the optical wavelengths at the distance of the LMC (50 kpc), 2700 --- 3000 kms^-1, corresponds reasonably well with the observed widths of the emission lines (\sim2500 kms^-1). With the post-service mission data, we are also able to study the asphericity of the supernova debris by fitting an elliptical surface brightness function. The major-to-minor axis ratio is found to be 1.19 \pm 0.02 on day 3271 for the V band, with the major axis corresponding to a position angle 20.2\deg \pm 3.1\deg. The direction of the debris elongation is consistent with the picture in which the long axis of the supernova debris is perpendicular to the plane of the circumstellar (inner) ring which, for any viewing angle, appears along the minor axis of the ring in the sky. Detailed examination of the deconvolved WFPC2 images show that the debris is now becoming resolved into two opposed blobs above and below the plane of the inner ring and is dim in the center, causing the observed asphericity. The two blobs of ejected materials along the polar direction suggest that the plane of the inner ring may be the dim equatorial belt of the supernova. This provides important clues to the study of the dynamics of the supernova explosion.
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