Spectro-Imaging of the Asymmetric Inner Molecular and Dust Shell Region of the Mira Variable W Hya with MIDI/VLTI11Based on observations made with the Very Large Telescope Interferometer (VLTI) at the Paranal Observatory under program IDs 083.D-0294, 084.D-0334, and 085.D-0201.

Abstract

We have observed W Hya, one of the closest and best-studied oxygen-rich evolved stars, in the dust sensitive mid-IR spectral domain with the interferometric instrument MIDI. Images could be obtained for the first time with MIDI in 25 wavelengths bins with the image reconstruction software MiRA using only the modulus of the visibilities. This still remains one of the few cases in which images could be successfully recovered due to the difficulties inherent to optical/infrared interferometry concerning the sparseness of the UV-plane and the missing Fourier phase information. Different regularization terms were compared and the influence of the UV-coverage was investigated. The lack of Fourier phase information, however, still limits the interpretation of the images. W Hya appears clearly nonsymmetric and the size is wavelength dependent. The photosphere, molecular layers, and dust formation zone could be resolved with an photospheric Gaussian FWHM diameter of 42 ± 2 mas (corresponding to 4.1 AU) and a dust layer of presumably amorphous aluminum oxide (Al2O3) at around two photospheric radii. The position angle of the major axis of the elongated structure could be determined to be (15 ± 10)° with a less well defined axis ratio between 0.4 and 0.6 showing that the dust forms primarily along a N–S axis. By comparing the elongated structure seen with MIDI with the Herschel/PACS 70 μm image at much larger scales, one can conclude that the asymmetry in the mass-loss most likely originates in the very close vicinity of the star and is thus not due to an interaction with the ambient media.