Multi-Epoch Mid-Infrared Interferometric Observations of the Oxygen-rich Mira Variable Star RR Aql with the VLTI/MIDI Instrument

This work is part of an on-going program of multi-epoch simultaneous observations of a sample of four evolved stars - RR Aql, GX Mon, S On, and AH Sco - using the VLTI and VLBA facilities. Different pulsating layers and their relative positions are monitored across phases and cycles. Here, we present mid-infrared interferometric observations of the oxygen-rich Mira variable RR Aql at 13 epochs covering 4 pulsation cycles with the MIDI instrument at the VLTI. We modeled the observed data using dust-free self-excited dynamic model atmospheres combined with an ad-hoc radiative transfer model of the dust shell. This study represents the first comparison between interferometric observations and dynamic models over an extended range of pulsation phases covering several cycles. We show that the combination of the models describe the observed data well. We determine the best dust-free dynamical model atmosphere and the best dust shell parameters including the optical depth of the two examined dust species (Al2O3 and silicates), the inner boundary radii of the dust shells, the density distribution, and the continuum photospheric angular diameter for all epochs. The results indicate that silicate is the major chemical component in the dust shell. We also compare the data with equivalent uniform disk (UD) and Gaussian FWHM diameters.