VLTI/PIONIER survey of disks around post-AGB binaries. Dust sublimation physics rules!

Abstract: Post-AGB binaries are surrounded by circumbinary disks of gas and dust that are similar to protoplanetary disks found around young stars. We aim to understand the structure of these disks and identify the physical phenomena at play in their very inner regions. We want to understand the disk-binary interaction and to further investigate the comparison with protoplanetary disks. We have conducted an interferometric snapshot survey of 23 post-AGB binaries in the near-infrared (H-band) using VLTI/PIONIER. We have fitted the multiwavelength visibilities and closure phases with purely geometrical models with an increasing complexity in order to retrieve the sizes, temperatures and flux ratios of the different components All sources are resolved and the different components contributing to the H-band flux are dissected. The environment of these targets is very complex: 13/23 targets need models with thirteen or more parameters to fit the data. We find that the inner disk rims follow and extend the size-luminosity relation established for disks around young stars with an offset toward larger sizes. The measured temperature of the near-infrared circumstellar emission of post-AGB binaries is lower (Tsub~1200K) than for young stars, probably due to a different dust mineralogy and/or gas density in the dust sublimation region. The dusty inner rims of the circumbinary disks around post-AGB binaries are ruled by dust sublimation physics. Additionally, a significant amount of the circumstellar H-band flux is over-resolved (14 targets have more than 10% of their non-stellar flux over-resolved) hinting for more structure from a yet unknown origin (disk structure or outflow). The amount of over-resolved flux is larger than around young stars. Due to the complexity of these targets, interferometric imaging is a necessary tool to reveal the interacting inner regions in a model-independent way.

• The paper uses VLTI/PIONIER data to resolve complex circumbinary disks around post-AGB binaries, finding that the inner disk edge is primarily governed by dust sublimation physics rather than binary interaction.
• Post-AGB circumbinary disks show inner rim sizes larger and temperatures lower than expected based on comparisons with young star disks, suggesting different physical conditions affecting dust sublimation.
• Analysis reveals a significant amount of over-resolved non-stellar flux whose origin is uncertain, and disk inclinations correlate with photometric variability seen in some post-AGB binaries.

VLTI/PIONIER Survey of Disks Around Post-AGB Binaries

The VLTI/PIONIER survey investigates the circumbinary disks commonly observed around post-asymptotic giant branch (post-AGB) binary stars. The study aims to provide insights into the structural complexities and physical phenomena taking place in the innermost regions of these disks, with an emphasis on their interactions with the central binary system and the comparison with protoplanetary disks.

Methodology

Using the Very Large Telescope Interferometer (VLTI) equipped with the PIONIER instrument, the survey executed interferometric observations in the near-infrared H-band of 23 post-AGB binaries. The research utilizes multiwavelength visibility and closure phase data and applies progressively complex geometric models to decipher these observations. This analysis allows for the determination of the sizes, temperatures, and flux contributions of different disk components.

Key Findings

1. Resolution of Circumbinary Disks: All observed systems were resolved, and multi-component models were required to fit the data for 13 out of 23 targets, indicating complex disk environments. The analysis highlights that many systems need at least 13 parameters for adequate modeling, reflecting the intricate nature of these circumbinary structures.
2. Size-Luminosity Relation: The inner disk rims of post-AGB circumbinary disks exhibit a size-luminosity relationship that aligns with that of disks around young stars but with an offset towards larger sizes. This suggests that dust sublimation rather than dynamical interaction with the binary governs the inner disk edge.
3. Temperature Anomalies: The measured temperatures of the circumstellar emission are lower (around 1200 K) than typical young star environments. This discrepancy could be attributed to differing dust grain properties or lower gas densities affecting the dust sublimation temperature.
4. Significant Over-Resolved Flux: A notable fraction of non-stellar flux is over-resolved in these systems, with 14 targets displaying more than 10% of non-stellar flux in this category. The origin of this extended emission remains uncertain, potentially pointing to additional complex disk structures or outflows.
5. Disk Inclination and Morphology: The analysis indicates a distribution of disk inclinations; highly inclined systems align with observed photometric phenomena such as RVb variability, supporting the hypothesis of disk-induced variability due to variable extinction along the line of sight.

Implications and Future Research

The study's results reinforce the notion that circumbinary disks around post-AGB binaries share several defining characteristics with protoplanetary disks despite their differing origins. However, the persistent lower temperature offsets and enlarged inner disk radii in post-AGB systems necessitate a reevaluation of the physical conditions presumed in these environments, particularly regarding dust composition and local density effects.

Future research could focus on direct imaging techniques, like millimeter observations with ALMA, to elucidate the spatial structure and physical mechanics governing these emissions. The noted substantial over-resolved flux also warrants further investigation, potentially through enhanced spatial resolution and improved modeling approaches, to ascertain its origin and implications for the disk's dynamical and chemical processes. Overall, these findings contribute to the broader understanding of disk evolution and characteristics across different stellar evolutionary phases.