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Binary post-AGB stars are interesting laboratories to study both theevolution of binaries as well as the structure of circumstellar disks. Amultiwavelength high angular resolution study of the prototypical object 89Herculis is performed with the aim of identifying and locating the differentemission components seen in the SED. A large interferometric data set,collected over the past decade and covering optical and near-IR wavelengths, isanalyzed with simple geometric models. Combining the interferometricconstraints with the photometry and the optical spectra, we reassess the energybudget of the post-AGB star and its circumstellar environment. We report thefirst (direct) detection of a large (35-40%) optical circumstellar fluxcontribution and spatially resolve its emission region. Given this large amountof reprocessed and/or redistributed optical light, the fitted size of theemission region is rather compact and fits with(in) the inner rim of thecircumbinary dust disk. This rim dominates our K band data through thermalemission and is rather compact, emitting significantly already at a radius oftwice the orbital separation. We interpret the circumstellar optical flux asdue to a scattering process, with the scatterers located in the extremelypuffed-up inner rim of the disk and possibly also in a bipolar outflow seenpole-on. A non-LTE gaseous origin in an inner disk cannot be excluded but isconsidered highly unlikely. This direct detection of a significant amount ofcircumbinary light at optical wavelengths poses several significant questionsregarding our understanding of both post-AGB binaries and the physics in theircircumbinary disks. Although the identification of the source ofemission/scattering remains inconclusive without further study on this andsimilar objects, the implications are manifold.

astronomy and astrophysics

An interferometric study of the post-AGB binary 89 Herculis I Spatially  resolving the continuum circumstellar environment at optical and near-IR  wavelengths with the VLTI, NPOI, IOTA, PTI, and the CHARA Array