Mid-infrared interferometric monitoring of evolved stars

Abstract

Aims. We present a unique multi-epoch infrared interferometric study of the oxygen-rich Mira variable RR Aql in comparison to radiative transfer models of the dust shell. We investigate flux and visibility spectra at 8-13 μm with the aim of better understanding the pulsation mechanism and its connection to the dust condensation sequence and mass-loss process. Methods. We obtained 13 epochs of mid-infrared interferometry with the MIDI instrument at the VLTI between April 2004 and July 2007, covering minimum to pre-maximum pulsation phases (0.45-0.85) within four cycles. The data are modeled with a radiative transfer model of the dust shell where the central stellar intensity profile is described by a series of dust-free dynamic model atmospheres based on self-excited pulsation models. We examined two dust species, silicate and Al 2 O 3 grains. We performed model simulations using variations in model phase and dust shell parameters to investigate the expected variability of our mid-infrared pho-tometric and interferometric data. Results. The observed visibility spectra do not show any indication of variations as a function of pulsation phase and cycle. The observed photometry spectra may indicate intracycle and cycle-to-cycle variations at the level of 1-2 standard deviations. The pho-tometric and visibility spectra of RR Aql can be described well by the radiative transfer model of the dust shell that uses a dynamic model atmosphere describing the central source. The best-fitting model for our average pulsation phase of Φ V = 0.64 ± 0.15 includes the dynamic model atmosphere M21n (T model = 2550 K) with a photospheric angular diameter of θ Phot = 7.6 ± 0.6 mas, and a silicate dust shell with an optical depth of τ V = 2.8 ± 0.8, an inner radius of R in = 4.1 ± 0.7 R Phot , and a power-law index of the density distribution of p = 2.6 ± 0.3. The addition of an Al 2 O 3 dust shell did not improve the model fit. However, our model simulations indicate that the presence of an inner Al 2 O 3 dust shell with lower optical depth than for the silicate dust shell can not be excluded. The photospheric angular diameter corresponds to a radius of R phot = 520 +230 −140 R and an effective temperature of T eff ∼ 2420 ± 200 K. Our modeling simulations confirm that significant intracycle and cycle-to-cycle visibility variations are not expected for RR Aql at mid-infrared wavelengths within our uncertainties. Conclusions. We conclude that our RR Aql data can be described by a pulsating atmosphere surrounded by a silicate dust shell. The effects of the pulsation on the mid-infrared flux and visibility values are expected to be less than about 25% and 20%, respectively, and are too low to be detected within our measurement uncertainties.