THE POLARIZED WATER MASER IN ORION - A PROTOPLANETARY RING

We present observations of the polarized water maser-flare source in O rion during the last years of its life, when we found low intensity bu t very high polarization. We also propose a geometrical model in which the maser source is formed in the central part of an edge-on protopla netary ring. This model explains the linear variation of the maser vel ocity with position, detected in VLBI observations, as the projection of the Keplerian orbital velocity in the direction of the line of sigh t. It also explains the high brightness temperatures as the amplificat ion of the radiation emitted by the back part of the ring by the front of it. Variability will be due to inhomogeneities in the ring, strong emission will occur when dense clumps of material are aligned in the observer's direction, the life-time of this strong emission will depen d on the size of the clumps and on their relative velocities. Polariza tion properties are possibly due to both geometry and magnetic fields. From the observed velocity gradient and from saturation consideration s we derive a lower limit of 15 M. for the central star, a diameter fo r the ring of 80 AU and a width of 1.6 AU.