Aims. We present the first multi-epoch study that includes concurrent mid-infrared and radio interferometry of an oxygen-rich Mira star. Results. The modeling of our MIDI data results in phase-dependent continuum photospheric angular diameters of 9.0 ± 0.3 mas (phase 0.42), 7.9 ± 0.1 mas (0.55), 9.7 ± 0.1 mas (1.16), and 9.5 ± 0.4 mas (1.27). The dust shell can best be modeled with Al 2 O 3 grains using phase-dependent inner boundary radii between 1.8 and 2.4 photospheric radii. The dust shell appears to be more compact with greater optical depth near visual minimum (τ V ∼ 2.5), and more extended with lower optical depth after visual maximum (τ V ∼ 1.5). The ratios of the 43.1 GHz/42.8 GHz SiO maser ring radii to the photospheric radii are 2.2 ± 0.3/2.1 ± 0.2 (phase 0.44), 2.4 ± 0.3/2.3 ± 0.4 (0.55), and 2.1 ± 0.3/1.9 ± 0.2 (1.15). The maser spots mark the region of the molecular atmospheric layers just beyond the steepest decrease in the mid-infrared model intensity profile. Their velocity structure indicates a radial gas expansion. Conclusions. S Ori shows significant phase-dependences of photospheric radii and dust shell parameters. Al 2 O 3 dust grains and SiO maser spots form at relatively small radii of ∼1.8−2.4 photospheric radii. Our results suggest increased mass loss and dust formation close to the surface near the minimum visual phase, when Al 2 O 3 dust grains are co-located with the molecular gas and the SiO maser shells, and a more expanded dust shell after visual maximum. Silicon does not appear to be bound in dust, as our data show no sign of silicate grains.

Context. Asymptotic giant branch (AGB) stars are one of the major sources of dust in the universe. The formation of molecules and dust grains and their subsequent expulsion into the interstellar medium via strong stellar winds is under intense investigation. This is in particular true for oxygen-rich stars, for which the path of dust formation has remained unclear. Aims. We conducted spatially and spectrally resolved mid-infrared multi-epoch interferometric observations to investigate the dust formation process in the extended atmospheres of oxygen-rich AGB stars. Methods. We observed the Mira variable AGB stars S Ori, GX Mon, and R Cnc between February 2006 and March 2009 with the MIDI instrument at the VLT interferometer. We compared the data to radiative transfer models of the dust shells, where the central stellar intensity profiles were described by dust-free dynamic model atmospheres. We used Al 2 O 3 and warm silicate grains, following earlier studies in the literature. Results. Our S Ori and R Cnc data could be well described by an Al 2 O 3 dust shell alone, and our GX Mon data by a mix of an Al 2 O 3 and a silicate shell. The best-fit parameters for S Ori and R Cnc included photospheric angular diameters Θ Phot of 9.7 ± 1.0 mas and 12.3 ± 1.0 mas, optical depths τ V (Al 2 O 3 ) of 1.5 ± 0.5 and 1.35 ± 0.2, and inner radii R in of 1.9 ± 0.3 R Phot and 2.2 ± 0.3 R Phot , respectively. Best-fit parameters for GX Mon were Θ Phot = 8.7 ± 1.3 mas, τ V (Al 2 O 3 ) = 1.9 ± 0.6, R in (Al 2 O 3 ) = 2.1 ± 0.3 R Phot , τ V (silicate) = 3.2 ± 0.5, and R in (silicate) = 4.6 ± 0.2 R Phot . Our data did not show evidence of intra-cycle and cycle-to-cycle variability or of asymmetries within the error-bars and within the limits of our baseline and phase coverage. Conclusions. Our model fits constrain the chemical composition and the inner boundary radii of the dust shells, as well as the photospheric angular diameters. Our interferometric results are consistent with Al 2 O 3 grains condensing close to the stellar surface at about 2 stellar radii, co-located with the extended atmosphere and SiO maser emission, and warm silicate grains at larger distances of about 4-5 stellar radii. We verified that the number densities of aluminum can match that of the best-fit Al 2 O 3 dust shell near the inner dust radius in sufficiently extended atmospheres, confirming that Al 2 O 3 grains can be seed particles for the further dust condensation. Together with literature data of the mass-loss rates, our sample is consistent with a hypothesis that stars with low mass-loss rates form primarily dust that preserves the spectral properties of Al 2 O 3 , and stars with higher mass-loss rate form dust with properties of warm silicates.

We describe a combined dynamic atmosphere and maser propagation model of SiO maser emission in Mira variables. This model rectifies many of the defects of an earlier model of this type, particularly in relation to the infrared (IR) radiation field generated by dust and various wavelength-dependent, optically thick layers. Modelled masers form in rings with radii consistent with those found in very long baseline interferometry (VLBI) observations and with earlier models. This agreement requires the adoption of a radio photosphere of radius approximately twice that of the stellar photosphere, in agreement with observations. A radio photosphere of this size renders invisible certain maser sites with high amplification at low radii, and conceals high-velocity shocks, which are absent in radio continuum observations. The SiO masers are brightest at an optical phase of 0.1-0.25, which is consistent with observed phase lags. Dust can have both mild and profound effects on the maser emission. Maser rings, a shock and the optically thick layer in the SiO pumping band at 8.13 μm appear to be closely associated in three out of four phase samples.

Aims. We present J, H, K spectrally dispersed interferometry with a spectral resolution of 35 for the Mira variable S Orionis. We aim at measuring the diameter variation as a function of wavelength that is expected due to molecular layers lying above the continuumforming photosphere. Our final goal is a better understanding of the pulsating atmosphere and its role in the mass-loss process. Methods. Visibility data of S Ori were obtained at phase 0.78 with the VLTI/AMBER instrument using the fringe tracker FINITO at 29 spectral channels between 1.29 µm and 2.32 µm. Apparent uniform disk (UD) diameters were computed for each spectral channel. In addition, the visibility data were directly compared to predictions by recent self-excited dynamic model atmospheres. Results. S Ori shows significant variations in the visibility values as a function of spectral channel that can only be described by a clear variation in the apparent angular size with wavelength. The closure phase values are close to zero at all spectral channels, indicating the absence of asymmetric intensity features. The apparent UD angular diameter is smallest at about 1.3 µm and 1.7 µm and increases by a factor of ∼1.4 around 2.0 µm. The minimum UD angular diameter at near-continuum wavelengths is Θ UD = 8.1 ± 0.5 mas, corresponding to R ∼ 420 R . The S Ori visibility data and the apparent UD variations can be explained reasonably well by a dynamic atmosphere model that includes molecular layers, particularly water vapor and CO. The best-fitting photospheric angular diameter of the model atmosphere is Θ Phot = 8.3 ± 0.2 mas, consistent with the UD diameter measured at near-continuum wavelengths. Conclusions. The measured visibility and UD diameter variations with wavelength resemble and generally confirm the predictions by recent dynamic model atmospheres. These size variations with wavelength can be understood as the effects from water vapor and CO layers lying above the continuum-forming photosphere. The major remaining differences between observations and model prediction are very likely due to an imperfect match of the phase and cycle combination between observation and available models.