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

TL;DR

This study spatially resolves the circumstellar environment of the post-AGB binary 89 Herculis using interferometry across optical and near-IR wavelengths, revealing a large optical flux contribution from a compact scattering region near the inner dust disk rim.

Contribution

First direct detection of significant optical circumstellar flux in a post-AGB binary, spatially resolving its emission region and providing new insights into disk structure and scattering processes.

Findings

Optical flux contribution of 35-40% detected and spatially resolved.

Emission region is compact, near the inner rim of the circumbinary dust disk.

Optical flux likely due to scattering in the puffed-up inner disk rim or bipolar outflow.

Abstract

Binary post-AGB stars are interesting laboratories to study both the evolution of binaries as well as the structure of circumstellar disks. 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 SED. A large interferometric data set, collected over the past decade and covering optical and near-IR wavelengths, is analyzed with simple geometric models. Combining the interferometric constraints with the photometry and the optical spectra, we reassess the energy budget of the post-AGB star and its circumstellar environment. We report the first (direct) detection of a large (35-40%) optical 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 emission region is rather compact and fits with(in) the inner rim of the circumbinary dust disk. This rim dominates our K band data through thermal emission and is rather compact, emitting significantly already at a radius of twice the orbital separation. We interpret the circumstellar optical flux as due to a scattering process, with the scatterers located in the extremely puffed-up inner rim of the disk and possibly also in a bipolar outflow seen pole-on. A non-LTE gaseous origin in an inner disk cannot be excluded but is considered highly unlikely. This direct detection of a significant amount of circumbinary 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.