We have observed the bipolar post-AGB 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 the OH 231.8+4.2 in the 3.8 \mu m image .
This compact source is resolved with the interferometer .
We used two 8-meter telescopes with four different baselines , which cover projected baseline lengths from 62 to 47 meters , and projected position angles from 112 to 131 degrees that are almost perpendicular to the bipolar outflow .
Fringes from 8 to 9 \mu m and from 12 to 13.5 \mu m were clearly detected , whilst the strong silicate self-absorption allows only marginal detection of visibilities between 9 and 12 \mu m. 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 the circumstellar material may have a disk configuration .