SIMBAD references

We present precision astrometry of the supernova 1987A which, for the first time, accurately registers high-resolution optical and radio images to better than 100milliarcsecond (mas). We have used preliminary results from the Hipparcos satellite, along with astrometric photographic plates, to redetermine the optical position of SN1987A, and have made VLBI observations of radio stars to link the Hipparcos and VLBI reference frames in the vicinity of SN1987A. This has allowed us to register the optical and radio images with an accuracy of ∼70 mas. We find that the centre of the radio emission coincides with the position of the optical star at the centre of SN1987A much more closely than previously found and conclude that the radio emission is due to interaction between an expanding shock front and the surrounding medium. However, a marginally significant displacement of the radio emission towards the east (140mas) may indicate that the shock wave has expanded asymmetrically.