Mid-infrared interferometry of the Mira variable RR Sco with the VLTI MIDI instrument

We present the results of the first mid-infrared interferometric observations of the Mira variable RR Sco with the MID-infrared Interferometer ( MIDI) coupled to the European Southern Observatory's (ESO) Very Large Telescope Interferometer (VLTI), together with K-band observations using VLTI VINCI. The observations were carried out in June 2003, when the variability phase of the object was 0.6, using two unit telescopes (UT1 and UT3), as part of the Science Demonstration Time (SDT) program of the instrument. Projected baseline lengths ranged from 73 to 102 m, and a spectral resolution of 30 was employed in the observations, which enabled us to obtain the wavelength dependence of the visibility in the region between 8 and 13 mum. The uniform-disk diameter was found to be 18 mas between 8 and 10 mum, while it gradually increases at wavelengths longer than 10 mum to reach 24 mas at 13 mum. The uniform-disk diameter between 8 and 13 mum is significantly larger than the K-band uniform-disk diameter of 10.2 +/- 0.5 mas measured using VLTI VINCI with projected baseline lengths of 15-16 m, 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. We find that the warm molecular envelope model extending to similar to2.3 R-star with a temperature of similar to1400 K and column densities of H2O and SiO of 3 x 10(21) cm(-2) and 1 x 10(20) cm(-2), respectively, can reproduce the observed uniform-disk diameters between 8 and 10 mum. The observed increase of the uniform-disk diameter longward of 10 mum can be explained by an optically thin dust shell consisting of silicate and corundum grains. The inner radius of the optically thin dust shell is derived to be 7-8 R-star with a temperature of similar to700 K, and the optical depth at 10 mum is found to be similar to0.025.