3mm & 7mm VLBA Observations of SiO masers: probing the close stellar environment of the bipolar Proto Planetary Nebulae OH231.8+4.2

We present milliarsecond-resolution maps of the SiO maser emission $v$=1 J=2–1 (3 mm) in the bipolar post-AGB nebula OH 231.8+4.2, and compare them with our previous observations of the $v$=2 J=1–0 line (7 mm). Our observations show that the SiO masers arise in several bright spots forming a structure elongated in a direction perpendicular to the symmetry axis of the nebula. This, and the complex velocity gradient observed, is consistent with the presence of an equatorial torus in rotation and with an infall of material towards the star.