Circumbinary disc interactions and stochastic dust obscuration in the post-asymptotic-giant-branch binary HD 213985

TL;DR

This study investigates the complex interactions between a circumbinary disc and stellar outflows in the eccentric binary HD 213985, revealing dust obscuration effects and variable photometric behavior driven by disc dynamics.

Contribution

It provides new insights into dust obscuration and variability mechanisms in post-AGB binary systems with circumbinary discs, supported by observational evidence.

Findings

Increased orbital variability amplitude observed in optical bands.

Irregular shape changes in light curves suggest dynamic disc interactions.

Short-term fluctuations indicate transient dust structures causing weather-like variability.

Abstract

HD 213985 is an eccentric binary system with a post-AGB primary and a jet-launching secondary star. We confirm that the system photometric variability is likely due to obscuration by the inner edge of a circumbinary disc, similar to RVb-type RV Tau stars. The system has shown an increase in the orbital variability amplitude in optical photometric bands, along with irregular changes in its shape that often started to appear skewed. Variability in the Na D lines suggests that this behaviour may be driven by interactions between the circumbinary disc and outflows through the L2 Lagrange point. Moreover, HD 213985 has exhibited episodes of short-term fluctuations whose appearance is not strictly related to the orbital phase. This variability is consistent with obscuration by transient dust structure leading to weather-like variability patterns.