FORMATION OF THE RING-LIKE STRUCTURE IN THE SN 1987A NEBULA DUE TO THE MAGNETIC PRESSURE OF THE TOROIDAL FIELD

Several weeks after the explosion of supernova (SN) 1987A, the UV flas h of the SN illuminated a ring-like structure in the circumstellar mat erial at about 0.65 ly from the SN. The interaction between the stella r winds from the SN progenitor is considered to be the candidate for t he formation of the circumstellar structure. In the case that the stel lar winds are spherically symmetric, the interaction should result in a shell-like structure. However, in this paper we show that the magnet ic field in the winds causes an anisotropy which leads to the formatio n of a ring-like structure. When the East wind of the blue supergiant phase of the progenitor sweeps up the surrounding slow wind of the red -supergiant phase, the magnetic field as well as the wind material are piled up in the interaction region. Since the magnetic energy increas es in proportion to the square of the amplitude, the magnetic field ex hibits its effect prominently at the interaction region; due to the ma gnetic pressure force the material at lower latitudes is compressed in to a ring-like structure. It is suggested that this magnetic process c an also explain the newly observed pair of rings of the SN 1987A nebul a.