Submilliarcsecond-resolution mapping of the 43 GHz SiO maser emission in the bipolar post-AGB nebula OH231.8+4.2

We present similar to0.3 milliarcsec-resolution maps of the SiO (v = 2,J = 1-0) maser emission in the bipolar post-AGB nebula OH231.8+4.2 obtained with the Very Long Baseline Array. These observations have provided for the first time the structure and kinematics of the close stellar environment in a proto-Planetary Nebula. Our observations reveal the SiO maser emission arising in several bright spots of less than similar to10(13) cm in size forming a structure elongated in the direction perpendicular to the symmetry axis of the nebula. Such a distribution is consistent with an equatorial torus with a radius of similar to6 AU around the central star. A complex velocity gradient is found along the torus, which suggests rotation and infall of material towards the star. The rotation and infalling velocities deduced are of the same order and range between similar to7 and similar to10 km s(-1). From our data, we estimate the mass of the SiO torus and the central star, as well as a stringent upper limit to the present stellar mass-loss rate.