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We investigate the molecular and dusty environment of OH/IR stars in order tocharacterize the mass-loss process during the tip-AGB superwind phase.Employing the AMBER instrument at the VLT Interferometer we obtainednear-infrared H- and K-band spectro-interferometric observations of the threeOH/IR stars IRAS 13479-5436, IRAS 14086-6907 and IRAS 17020-5254 with aspectral resolution of about 35. We use a two-component geometrical model,consisting of a uniform disk and a Gaussian disk, to obtain characteristicangular sizes of the central stellar sources and their dust envelopes, as wellas the flux ratios between these components. Angular uniform disk diameters ofthe three central components of the objects above have values between 3.2 masand 5.4 mas. For their dust envelopes, we find FWHM values between 17.1 mas and25.2 mas. According to distance estimates from the literature, the centralstellar components have radii between 900 R_sun and 1400 R_sun, while theirdust envelopes reach FWHM values between 9000 R_sun and 13000 R_sun. Thevisibility functions of all three sources exhibit wavelength variations thatresemble those of earlier VLTI/AMBER observations of semi-regular and Miravariable AGB stars. These are interpreted as characteristic of atmosphericmolecular layers lying above the photosphere. We also find that the dustenvelopes have a clearly larger optical depth than those known for Mira stars.We interpret this as an expected result of the "superwind" phase, the final 10000 to 30 000 years of AGB-evolution, when the mass-loss rate increases by afactor of 10-100. By their different optical depths, the three dust shellsstudied here may represent different stages of the "superwind" and differentinitial masses.

Near-infrared spectro-interferometry of three OH/IR Stars with the VLTI/AMBER instrument