[Paper Review] Binary post-AGB stars and their Keplerian discs

H. Van Winckel, T. Lloyd Evans+3 more|arXiv (Cornell University)|Apr 7, 2006

Astronomy and Astrophysical Research1 references3 citations

TL;DR

This study presents strong evidence that post-AGB stars in a sample of 51 objects are binary systems hosting stable, Keplerian, dusty circumbinary discs. Multi-wavelength observations, including interferometry and Spitzer spectroscopy, confirm that hot dust near sublimation temperature and compact, structured discs are ubiquitous, indicating disc formation via non-conservative mass transfer or wind capture in binary interactions.

ABSTRACT

In this contribution we give a progress report on our systematic study of a large sample of post-AGB stars. The sample stars were selected on the basis of their infrared colours and the selection criteria were tuned to discover objects with hot dust in the system. We started a very extensive, multi-wavelength programme which includes the analysis of our radial velocity monitoring; our optical high-resolution spectra; our groundbased N-band spectral data as well as the Spitzer full spectral scans; the broad-band SED and the high spatial-resolution interferometric experiments with the VLTI. In this contribution we highlight the main results obtained so far and argue that all systems in our sample are indeed binaries, which are surrounded by dusty Keplerian circumbinary discs. The discs play a lead role in the evolution of the systems.

Motivation & Objective

• To systematically identify and characterize post-AGB stars with hot dust and binary companions using infrared color selection.
• To determine the nature and geometry of circumstellar dust in these systems, particularly whether it forms bound, Keplerian discs.
• To investigate the role of binarity in shaping the late evolution of post-AGB stars and the formation of circumbinary discs.
• To probe the thermal and chemical history of dust grains in these discs through spectroscopic and interferometric analysis.
• To assess whether disc formation mechanisms such as wind capture or non-conservative mass transfer are viable in shaping these systems.

Proposed method

• Selected 51 post-AGB stars using infrared color criteria tuned to detect hot dust, including serendipitous binaries, IRAS-excess RV Tauri stars, and newly identified candidates.
• Conducted multi-wavelength observations: radial velocity monitoring, high-resolution optical spectroscopy, N-band ground-based spectroscopy, Spitzer full spectral scans, and broad-band SED modeling.
• Employed VLTI/MIDI interferometry to resolve the spatial structure of the circumstellar emission, measuring compact sizes and probing mineralogical composition via spectrally dispersed fringes.
• Modeled the SEDs to infer dust temperature distribution and constrain the presence of gravitationally bound material, ruling out ongoing mass loss.
• Analyzed dust mineralogy using Spitzer and interferometric data to detect crystallinity gradients and assess grain processing history.
• Used radial velocity monitoring and spectral analysis to confirm binary nature and constrain orbital parameters.

Experimental results

Research questions

• RQ1 Are the observed hot dust excesses in post-AGB stars due to gravitationally bound, Keplerian discs rather than ongoing mass loss?
• RQ2 What is the origin of the compact, hot dust emission seen in the SEDs of these systems, and what does it imply about the disc geometry?
• RQ3 How do the physical and chemical properties of dust grains—such as crystallinity and thermal history—vary across the disc, and what do they reveal about formation mechanisms?
• RQ4 Is there a connection between the orbital parameters of the binary systems and the observed dust properties or SED morphology?
• RQ5 What role do binary interactions play in truncating AGB evolution and forming stable circumbinary discs?

Key findings

• All 51 sample post-AGB stars exhibit broad infrared excesses starting in the H or K band, indicating the presence of hot dust near sublimation temperature, consistent with a compact, Keplerian disc.
• Interferometric observations with VLTI/MIDI show that the dust emission is extremely compact, with an upper limit of 18 AU for the diameter of the emitting region in SX Cen.
• The dust in the discs is predominantly oxygen-rich, and the innermost regions show evidence of high-temperature processing, with crystalline silicates detected in some systems.
• In some stars, a radial gradient in crystallinity is observed, indicating that the inner disc is more processed than the outer regions, while in others, both amorphous and crystalline grains are uniformly distributed.
• Spectral energy distribution modeling confirms that the dust excess cannot be explained by ongoing mass loss, but rather by a stable, bound reservoir of dust in a Keplerian disc.
• The lack of correlation between dust spectral properties and stellar parameters such as effective temperature or orbital period suggests that disc evolution and grain processing are complex and not yet fully predictable.