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 flash ofthe SN illuminated a ring-like structure in the circumstellar material at about0.65 ly from the SN. The interaction between the stellar winds from the SNprogenitor is considered to be the candidate for the formation of thecircumstellar structure. In the case that the stellar winds are sphericallysymmetric, the interaction should result in a shell-like structure. However, inthis paper we show that the magnetic field in the winds causes an anisotropywhich leads to the formation of a ring-like structure. When the fast wind ofthe blue supergiant phase of the progenitor sweeps up the surrounding slow windof the red-supergiant phase, the magnetic field as well as the wind materialare piled up in the interaction region. Since the magnetic energy increases inproportion to the square of the amplitude, the magnetic field exhibits itseffect prominently at the interaction region; due to the magnetic pressureforce the material at lower latitudes is compressed into a ring-like structure.It is suggested that this magnetic process can also explain the newly observedpair of rings of the SN 1987A nebula.Comment: 18 pages LaTeX, 3 PostScript figure