Water vapor and silicon monoxide maser observations in the protoplanetary nebula OH 231.8+4.2

Context.OH 231.8+4.2 is a well studied preplanetary nebula (pPN) around a binary stellar system that shows a remarkable bipolar outflow. Aims.To study the structure and kinematics of the inner 10–80 AU nebular regions probed by SiO and H2O maser emission, where the agents of wind collimation are expected to operate, in order to gain insights into the, yet poorly known, processes responsible for the shaping of bipolar pPNe. Methods.We performed high-resolution observations of the H2O 6$_{1,6}$–5$_{2,3}$ and 28SiO $v=2$, $J=1$–0 maser emissions with the Very Long Baseline Array. The absolute position of both emission distributions were recovered using the phase referencing technique, and accurately registered in HST optical images. Results.Maps of both masers were produced and compared. H2O maser clumps are found to be distributed in two areas of 20 mas in size spatially displaced by ~60 milli-arcs along an axis oriented nearly north-south. SiO masers are tentatively found to be placed between the two H2O maser emitting regions, probably indicating the position of the Mira component of the system. Conclusions.The SiO maser emission traces an inner equatorial component with a diameter of 12 AU, probably a disk rotating around the M-type star. Outwards, we detect in the H2O data a pair of polar caps, separated by 80 AU. We believe that the inner regions of the nebula probably have been altered by the presence of the companion, leading to an equator-to-pole density contrast that may explain the lack of H2O masers and strong SiO maser emission in the denser, equatorial regions.