The Circumstellar Environment of Evolved Stars as seen by VLTI/MIDI

We present the results of the first mid-infrared interferometric observations of the Mira variable RR Sco with the VLTI/MIDI, together with K-band observations using VLTI/VINCI. The uniform-disk diameter was found to be 18 mas between 8 and 10 mu m, while it gradually increases at wavelengths longer than 10 Am to reach 24 mas at 13 mu m. These uniform-disk diameters in the mid-infrared are significantly larger than the K-band uniform-disk diameter of 10.2 +/- 0.5 mas measured using VLTI/VINCI, three weeks after the MIDI observations. Our model calculations show that optically thick emission from a warm molecular envelope consisting of H2O and SiO can cause the apparent mid-infrared diameter to be much larger than the continuum diameter, and this can explain the mid-infrared angular sizes roughly twice as large as that measured in the K band. The observed increase of the uniform-disk diameter longward of 10 mu m can be explained by an optically thin dust shell consisting of corundum and silicate grains.