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C. Sanchez Contreras (JPL), J.F. Desmurs, V. Bujarrabal, F. Colomer, J. Alcolea (National Astronomical Observatory of Spain)
Contrarily to the circumstellar envelopes around Asymptotic Giant Branch (AGB) stars, protoplanetary and planetary nebulae (PPNe & PNe; the subsequent evolutionary stages to the AGB) show conspicuous asymmetries and collimated jets whose origins are not yet well understood. To explain the evolution of post-AGB objects from spherical AGB envelopes, several models have postulated the presence of dense rings or disks very close to the central stars: such disks would be the main agents of the collimation and acceleration of the stellar wind. However, these inner equatorial disks have not been observed up to now.
We present high-resolution (\sim\,0.3\,mas) maps of the SiO maser emission at 43\,GHz obtained with the Very Long Baseline Array (VLBA) of the PPN \mbox{OH\,231.8+4.2}. These observations have provided by the first time the structure and kinematics of the close stellar surroundings in a PPN.
Our observations of OH231.8 reveal the SiO maser emission arising from several 1013\,cm-size spots forming a structure elongated in the direction perpendicular to the symmetry axis of the nebula. Such a distribution is consistent with a clumpy, equatorial torus of gas with a radius of ~\,5\,AU around the central star. A complex velocity gradient is found along the torus. Such gradient is not well reproduced with pure rotation and suggest infall of material towards the star. The rotation and infalling velocities deduced are ~\,6 and ~\,10\,km\,s-1, respectively. In order to correctly interpret the complex SiO emission in OH231.8 we intend to analyze the polarization of the maser spots, particularly relevant to place the central star and to investigate eventual magentic fields in the stellar surroundings.
This work is partially funded through grant PB96-0104 to the spanish NAO and a Caltech postdoctoral fellowship to C.S.C.