The mid-IR spatially resolved environment of OH 26.5+0.6 at maximum luminosity

We present observations of the famous OH/IR star OH 26.5+0.6 obtained using the Mid-Infrared Interferometric Instrument MIDI at the European Southern Observatory (ESO) Very Large Telescope Interferometer VLTI. Emission of the dusty envelope, spectrally dispersed at a resolution of 30 from 8 mum to 13.5 mum, appears resolved by a single dish UT telescope. In particular the angular diameter increases strongly within the silicate absorption band. Moreover an acquisition image taken at 8.7 mum exhibits, after deconvolution, a strong asymmetry. The axis ratio is 0.75±0.07 with the FWHM of the major and minor axis which are 286 mas and 214 mas respectively. The measured PA angle, 95° ±6°, is reminiscent of the asymmetry in the OH maser emission detected at 1612 MHz. In interferometric mode the UT1-UT3 102 m baseline was employed to detect the presence of the star. No fringes were found with a detection threshold estimated to be about 1% of the total flux of the source, i.e. 5-8 Jy. These observations were carried out during the maximum luminosity phase of the star, when the dust shell is more diluted and therefore the chance to detect the central source maximized. We modeled the dusty environment based on the work of Justtanont et al. (1996). In particular, the failure to detect fringes provides strong constraints on the opacities in the inner regions of the dust shell or in the close vicinity of the star.