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^{⋆,⋆⋆,⋆⋆⋆}

¹ Instituut voor Sterrenkunde (IvS), KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
e-mail: Michel.Hillen@ster.kuleuven.be
² Belgian Institute for Space Aeronomy, 1180 Brussels, Belgium
³ Laboratoire Lagrange, UMR7293, Univ. Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, 06300 Nice, France
⁴ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁵ University of Michigan, 941 Dennison Building, 500 Church Street, Ann Arbor, MI 48109-1090, USA
⁶ The CHARA Array of Georgia State University, Mt. Wilson Observatory, Mt. Wilson, CA 91023, USA
⁷ Department of Physics, Central Michigan University, Mt. Pleasant, MI 48859, USA
⁸ Physical Research Laboratory, Navrangpura, 380009 Ahmedabad, India
⁹ US Naval Observatory, Flagstaff Station, 10391 W. Naval Obs. Rd., Flagstaff, AZ 86001, USA

Received: 1 April 2013
Accepted: 25 July 2013

Abstract

Context. Binary post-asymptotic giant branch (post-AGB) stars are interesting laboratories to study both the evolution of binaries 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 decade and covering optical and near-infrared wavelengths, is analyzed in combination with the spectral energy distribution and flux-calibrated optical spectra. In this first paper only simple geometric models are applied to fit the interferometric 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%) 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-local thermodynamic equilibrium gaseous origin in an inner disk cannot be excluded but is considered highly unlikely.

Conclusions. 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.