The Compact Circumstellar Material around OH 231.8+4.2^*

We have observed the bipolar post-asymptotic giant branch candidate OH 231.8+4.2, using the mid-infrared interferometer MIDI and the infrared camera with the adaptive optics system NACO on the Very Large Telescope. An unresolved core (<200 mas in FWHM) is found at the center of OH 231.8+4.2 in the 3.8 μm image. This compact source is resolved with the interferometer. We used two 8 m telescopes with four different baselines, which cover projected baseline lengths from 62 to 47 m, and projected position angles from 112° to 131° that are almost perpendicular to the bipolar outflow. Fringes from 8 to 9 μm and from 12 to 13.5 μm were clearly detected, while strong silicate self-absorption allows only marginal detection of visibilities between 9 and 12 μm. The fringes from the four baselines consistently show the presence of a compact circumstellar object with an inner radius of 30-40 mas, which is equivalent to 40-50 AU at 1.3 kpc. This clearly shows that the mid-infrared compact source is not the central star (3 AU) but circumstellar material. The measured size of the circumstellar material is consistent with the size of such disks calculated by hydrodynamic models, implying that the circumstellar material may be in a disk configuration.