THE ASYMMETRIC RADIO REMNANT OF SN 1987A

We present seven years of radio observations of SN 1987A made with the Australia Telescope Compact Array. At 1.4, 2.4, 4.8, and 8.6 GHz, the flux density of the radio remnant has increased monotonically since e mission was redetected 1200 days after the explosion. On day 3200, the remnant was expanding at 2800 +/- 400 km s(-1) which we interpret as indicating significant deceleration of the fastest moving ejecta. Sinc e day 1787, the spectral index has remained constant at alpha = -0.95 +/- 0.04 (S proportional to v(alpha)). These observations are all cons istent with the shock having encountered a denser shocked component of the progenitor's stellar wind. At the current rate of expansion, the shock is expected to encounter the inner optical ring in the year 2006 +/- 3, in line with predictions made by hydrodynamic simulations. Usi ng superresolution, we have also obtained 9 GHz images of the remnant (resolution approximate to 0.'' 5) at four epochs. The emission is dis tributed around the rim of a nearly circular shell, but has become inc reasingly asymmetric with time. There are two ''hot spots'' to the eas t and west, aligned along the major axis of the optical ring. This mor phology is most likely indicative of an axisymmetric circumstellar med ium into which the shock is expanding, consistent with present underst anding of the progenitor star and its environment. The two hot spots a re increasing in flux density at different rates, which may indicate d irectional anisotropies in the ejecta. We believe that the northern an d southern regions of the remnant are encountering a shocked wind that is less dense and also further from the progenitor star than that in the bright regions of emission. As a result, these regions should even tually brighten and/or extend.