Jet formation in post-AGB binaries

Confronting cold magnetohydrodynamic disc wind models with observations^⋆

¹ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
² Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
³ Department of Physics & Astronomy, School of Mathematical and Physical Sciences, Macquarie University, Sydney, NSW 2109, Australia
⁴ Astronomy, Astrophysics and Astrophotonics Research Centre, Macquarie University, Sydney, NSW 2109, Australia
⁵ Center for Interdisciplinary Exploration & Research in Astrophysics (CIERA), Physics and Astronomy, Northwestern University, Evanston, IL 60202, USA

Received: 16 February 2024
Accepted: 12 June 2024

Abstract

Context. Jets are launched from many classes of astrophysical objects, including post-asymptotic giant branch (post-AGB) binaries with a circumbinary disc. Despite dozens of detections, the formation of these post-AGB binary jets and their connection to the inter-component interactions in their host systems remains poorly understood.

Aims. Building upon the previous paper in this series, we consider cold self-similar magnetohydrodynamic (MHD) disc wind solutions to describe jets that are launched from the circumcompanion accretion discs in post-AGB binaries. Resulting predictions are matched to observations. This both tests the physical validity of the MHD disc wind paradigm and reveals the accretion disc properties.

Methods. Five MHD solutions are used as input to synthesise spectral time-series of the Hα line for five different post-AGB binaries. A fitting routine over the remaining model parameters is developed to find the disc wind models that best fit the observed time-series.

Results. Many of the time-series’ properties are reproduced well by the models, though systematic mismatches, such as overestimated rotation, remain. Four targets imply accretion discs that reach close to the secondary’s stellar surface, while one is fitted with an unrealistically large inner radius at ≳20 stellar radii. Some fits imply inner disc temperatures over 10 000 K, seemingly discrepant with a previous observational estimate from H band interferometry. This estimate is, however, shown to be biased. Fitted mass-accretion rates range from ∼10⁻⁶ − 10⁻³ M_⊙/yr. Relative to the jets launched from young stellar objects (YSOs), all targets prefer winds with higher ejection efficiencies, lower magnetizations and thicker discs.

Conclusions. Our models show that current cold MHD disc wind solutions can explain many of the jet-related Hα features seen in post-AGB binaries, though systematic discrepancies remain. This includes, but is not limited to, overestimated rotation and underestimated post-AGB circumbinary disc lifetimes. The consideration of thicker discs and the inclusion of irradiation from the post-AGB primary, leading to warm magnetothermal wind launching, might alleviate these.