Inhomogeneities in Molecular Layers of Mira Atmospheres
NAVAL OBSERVATORY WASHINGTON DC
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Aims. We investigate the structure and shape of the photospheric and molecular layers of the atmospheres of four Mira variables. Methods. We obtained near-infrared K-band spectro-interferometric observations of the Mira variables R Cnc, X Hya, W Vel, and RW Vel with a spectral resolution of about 1500 using the AMBER instrument at the VLTI. We obtained concurrent JHKL photometry using the Mk II instrument at the SAAO. Results. The Mira stars in our sample are found to have wavelength-dependent visibility values that are consistent with earlier low-resolution AMBER observations of S Ori and with the predictions of dynamic model atmosphere series based on self-excited pulsation models. The corresponding wavelength-dependent uniform disk UD diameters show a minimum near the near-continuum bandpass at 2.25 mum. They then increase by up to 30 toward the H2O band at 2.0 mum and by up to 70 at the CO bandheads between 2.29 mum and 2.48 mum. The dynamic model atmosphere series show a consistent wavelength-dependence, and their parameters such as the visual phase, effective temperature, and distances are consistent with independent estimates. The closure phases have significantly wavelength-dependent and non-zero values at all wavelengths indicating deviations from point symmetry.