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Session 17 - Supernovae.
Display session, Monday, January 15
North Banquet Hall, Convention Center
We have studied the expansion and the shape of the debris of SN 1987A with observations from HST\/ cameras. Broadband and narrow emission images covering a span of 4.5 years (from August 1990 to March 1995) are obtained before (with FOC and PC) and after (with WFPC2) COSTAR installment. The high resolution of the images allows the structure of the supernova debris to be revealed. We fitted the heavily limb-darkened image profiles of the debris to a surface brightness profile which decreases with angular radius \rho as \phi (\rho) \propto 1 / [1 + (\rho/\rho_0)^\alpha], where the power index, \alpha \simeq 4. The diameter of the debris, 2\rho_0, at day 2932 (March 3, 1995) is around 200 mas for the broadband optical images. However, the apparent debris size varies with wavelength and is found to be \sim 20% larger in the ultraviolet wavelengths. The expansion of the debris is consistent with a homologous linear expansion with time. The rate of expansion corresponds reasonably well with the observed widths of the emission lines (\sim 2500 km s^-1) at the distance of the LMC.
With the post-COSTAR images, we are able to study the shape of the supernova debris by measuring its ellipticity and orientation at several wavelengths. We found the debris to be asymmetric at all wavelengths. The major-to-minor axes ratio is found to be \sim 1.2, with the major axis corresponding to a position angle of 165\hbox^\circ The elongation axis of the supernova debris lies very close to axis perpendicular to the plane of the circumstellar ring (the minor axis of the ring in the sky).
