Shaping of the outflows of evolved low-mass stars by wind-companion interactions

Jolien Malfait ¹ , Lionel Siess ² , Mats Esseldeurs ¹ , Frederik De Ceuster ¹ , Silke Maes ¹ , Thomas Ceulemans ¹ , Leen Decin ¹

Abstract

At the end of their lives, low and intermediate mass stars scatter their envelope throughout the interstellar medium via a dust-driven stellar wind. For decades, modelling endeavours of these outflows have assumed that these winds are spherically symmetric. However, recent high-spatial resolution observations reveal that the winds of these Asymptotic Giant Branch (AGB) stars typically possess a high degree of complexity, including spirals, disks, clumps, bipolar outflows, etc., and showing strong resemblance with the intricate morphologies of their successors being Post-AGB stars and planetary nebulae.

Many of these observed wind structures are believed to be formed primarily by the impact of one or more companion stars or planets, that remain hidden within the stellar wind. If neglecting the 3D structures and the impact of these companions, systematic errors may occur in the estimate of critical stellar evolution parameters such as the mass-loss rate.
With sophisticated 3D hydrodynamical PHANTOM modelling tools, we show how these wind structures can be formed by the gravitational interaction of one or multiple stellar or planetary companions with an AGB outflow, in various configurations.

The ultimate goal of our PHANTOM simulations is to compare them to high-resolution observations, to unravel how and by which binary and triple configuration the observed wind structures can be created. Therefore, we post-process our hydrodynamical models with the radiative transfer code MAGRITTE, to create synthetic observations, and reveal the similarities with actual ALMA observations.
Although there are many difficulties embedded within this forward-modelling approach due to the extreme complexity of the observed stellar outflows, our simulations offer us a novel gateway for understanding the complex wind structures of evolved stars. This will help us constraining the fundamental stellar and wind parameters, which are key ingredients for predicting their further evolution.

Hydrosimulations - triple systems

Malfait et al. in prep.

Hydrosimulation of wind-companion interactions in hierarchical triple: AGB + close companion + far companion

Density distribution in slice through orbital plane

Note: if movies don't work, they should work in a different browser

Movie of density distribution in orbital plane slice

M_\rm{AGB} = 1.6 \, M_\odot; \, M_\rm{c1} = 0.4 \, M_\odot; \, M_\rm{c2} = 15 \, M_\odot; \, a_1 = 5\, \rm{au};\, a_2 = 35 \, \rm{au}; \, v_{\rm{w}} = 15 \, \rm{km/s}; \, \dot{M} = 1.1 \cdot 10^{-6} \, M_\odot/ \rm{yr}

The inner companion creates a dense inner spiral, that is shaped as a snail-shell due to the orbit of the inner binary around the common center-of-mass of the system. The outer companion creates another spiral structure on top of this inner snail-shell spiral.

Hydrosimulations - accretion disks

Malfait et al. in prep.

Hydrosimulations revealing the formation of an accretion disk around a solar-mass companion.

M_\text{AGB} = 1.5 \, M_\odot; \, M_\text{comp} = 1.0 \, M_\odot; \, a = 6 \, \rm{au}; \, v_\text{w} = 10 \, \rm {km/s}; \, \dot{M} = 10^{-7} \, M_\odot/\rm{yr}

Density distribution in edge-on slice, zoomed in on the flaring accretion disk around the accreting companion sink particle

Density distribution in a slice through the orbital plane, revealing the accretion disk and bow shock around the companion.

Density distribution in the orbital plane slice, focussed on the accretion disk. Velocity vectors are annotated in white.

Movie of orbital plane density distribution evolution, showing how the accretion disk forms around the companion sink particle.

Big black cirle is AGB, small black dot is companion sink particle. Black arrows indicate velocity field.

Note: if movies don't work, they should work on a different bowser

All simulations are performed with SPH code Phantom (Price et al. 2018), and visualised with Splash (Price et al. 2007)

Hydrosimulations - binary stars

Hydrosimulations of wind-companion interactions in the outflow of an AGB star. Malfait et al. 2021, Malfait et al. in prep.

e = 0.0, density distribution in slice trhough orbital plane

e = 0.50, density distribution in slice through orbital plane

e = 0.0, movie of density distribution in slice through orbital plane

e = 0.5, movie of density distribution in slice through orbital plane

M_\text{AGB} = 1.5 \, M_\odot; \, M_\text{comp} = 1.0 \, M_\odot; \, a = 6 \, \rm{au}; \, v_\text{w} = 10 \, \rm {km/s}; \, \dot{M} = 10^{-7} \, M_\odot/\rm{yr}

Our Smoothed-Particle-Hydrodynamic simulations show how spirals, and more complex structures form naturally through gravitational wind-companion interaction in the outflow of low-mass evolved stars. Simulations are performed with Phantom (Price et al. 2018), and visualised with Splash (Price et al. 2007)

For more details, see Malfait et al. 2021, Maes et al. 2021., Esseldeurs et al. 2023 (see QR codes)

Follow contributed talk by Mats Esseldeurs S13e on Thursday at 11h45!

Note: if movies don't work, they should work on a different browser

Observations: asymmetric outflows

Hydrosimulations - triple systems

Hydrosimulations - accretion disks

Hydrosimulations - binary stars

Coupling Observations & Hydrosimulations

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