Abstract
We present ∼0.3 milliarcsec-resolution maps of the SiO (v = 2, J = 1-0) maser emission in the bipolar post-AGB nebula OH 231.8+4.2 obtained with the Very Long Baseline Array. These observations have provided for the firsttime 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 ∼10 1 3 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 ∼6 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 ∼7 and ∼10 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.
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