An interferometric study of the post-AGB binary 89 Herculis

Context. Binary post asymptotic giant branch (post-AGB) stars are interesting laboratories to study both the evolution of binar ies as well as the structure of circumstellar disks. Aims. A multiwavelength high angular resolution study of the prototypical object 89 Herculis is performed with the aim of identifying and locating the different emission components seen in the spectral energy distribution. Methods. A large interferometric data set, collected over the past de cade and covering optical and near-infrared (near-IR) wavelengths, is analyzed in combination with the spectral energy distribution (SED) and flux-calibrated optical spectra. I n this first paper only simple geometric models are applied to fit the interfero metric data. Combining the interferometric constraints with the photometry and the optical spectra, we re-assess the energy budget of the post-AGB star and its circumstellar environment. Results. We report the first (direct) detection of a large (35-40%) opt ical circumstellar flux contribution and spatially resolve its emission region. Given this large amount of reprocessed and/or redistributed optical light, the fitted size of the emissi on region is rather compact and fits with(in) the inner rim of the circumbinary du st disk. This rim dominates our K band data through thermal emission and is rather compact, emitting significantly already at a ra dius of twice the orbital separation. We interpret the circu mstellar optical flux as due to a scattering process, with the scatterers locat ed in the extremely puffed-up inner rim of the disk and possibly also in a bipolar outflow seen pole-on. A non local thermodynamic equi librium (non-LTE) gaseous origin in an inner disk cannot be excluded but is considered highly unlikely. Conclusions. This direct detection of a significant amount of circumbinar y light at optical wavelengths poses several significant questions regarding our understanding of both post-AGB binaries and the physics in their circumbinary disks. Although the identification of the source of emission/scattering remains inconclusive without further study on this and similar objects, the implications are manifold.