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VLTI/AMBER spectro-interferometric imaging of VX Sagittarii's inhomogenous outer atmosphere
Aims: We aim to explore the photosphere of the very coollate-type star VX Sgr and in particular the characterization ofmolecular layers above the continuum forming photosphere. Methods: We obtained interferometric observations with the VLTI/AMBERinterferometer using the fringe tracker FINITO in the spectral domain1.45-2.50 ?m with a spectral resolution of ?35 and baselinesranging from 15 to 88 m. We performed independent image reconstructionfor different wavelength bins and fit the interferometric data with ageometrical toy model. We also compared the data to 1D dynamical modelsof Miras atmosphere and to 3D hydrodynamical simulations of redsupergiant (RSG) and asymptotic giant branch (AGB) stars. Results: Reconstructed images and visibilities show a strong wavelengthdependence. The H-band images display two bright spots whose positionsare confirmed by the geometrical toy model. The inhomogeneities arequalitatively predicted by 3D simulations. At ?2.00 ?m and in theregion 2.35-2.50 ?m, the photosphere appears extended and the radiusis larger than in the H band. In this spectral region, the geometricaltoy model locates a third bright spot outside the photosphere that canbe a feature of the molecular layers. The wavelength dependence of thevisibility can be qualitatively explained by 1D dynamical models of Miraatmospheres. The best-fitting photospheric models show a good match withthe observed visibilities and give a photospheric diameter of Theta=8.82± 0.50 mas. The H2O molecule seems to be the dominantabsorber in the molecular layers. Conclusions: We show that theatmosphere of VX Sgr seems to resemble Mira/AGB star model atmospheresmore closely than do RSG model atmospheres. In particular, we seemolecular (water) layers that are typical of Mira stars.Based on the observations made with VLTI-ESO Paranal, Chile under theprograms IDs 081.D-0005(A, B, C, D, E, F, G, H).
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