3D simulations of AGB stellar winds

In my research, I focus on understanding the behavior of stars during the Asymptotic Giant Branch (AGB) phase, which is a crucial stage in their evolution. During this phase, stars with an initial mass below approximately 8 solar masses develop a strong stellar wind due to radiation pressure on newly formed dust grains.

Recent observations have shown that AGB outflows display significant morphological complexities, which are most likely caused by the interaction with a companion. In order to better understand and describe these morphologies, I have developed a 3D simulation of AGB stellar winds.

My simulation takes into account both the radiation force in dust-driven winds and the impact of a companion on the AGB wind morphology. To achieve this, I have implemented a ray-tracer for radiative transfer in the smoothed particle hydrodynamics (SPH) code Phantom. This method allows for the creation of a 3D map of the optical depth around the AGB star.

I have compared the effects of four different descriptions of radiative transfer, with different degrees of complexity, including the free-wind approximation, the geometrical approximation, the Lucy approximation, and the attenuation approximation. By comparing the results to predictions from the 3D radiative transfer code Magritte, I have found that the Lucy approximation provides the most accurate results for most of the model, although it does not account for all effects.

Overall, my research highlights the critical role of the radiation force in dust-driven AGB winds, impacting the velocity profile and morphological structures. These findings provide important insights into the behavior of stars during the AGB phase and contribute to our understanding of the evolution of stars.

• Esseldeurs, et al. (2023). 3D simulations of AGB stellar winds. II. Ray-tracer implementation and impact of radiation on the outflow morphology. A&A, 674, A122.
  PDF DOI BIB Abstract

  @article{Esseldeurs2023a_short,
    author = {Esseldeurs, {et al.}},
    title = {{3D simulations of AGB stellar winds. II. Ray-tracer implementation and impact of radiation on the outflow morphology}},
    journal = {A&A},
    keywords = {stars: winds, outflows, methods: numerical, hydrodynamics, stars: AGB and post-AGB, radiative transfer, Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Astrophysics of Galaxies},
    year = {2023},
    month = jun,
    volume = {674},
    eid = {A122},
    pages = {A122},
    doi = {10.1051/0004-6361/202346282},
    archiveprefix = {arXiv},
    eprint = {2304.09786},
    primaryclass = {astro-ph.SR},
    adsurl = {https://ui.adsabs.harvard.edu/abs/2023A&A...674A.122E},
    adsnote = {Provided by the SAO/NASA Astrophysics Data System}
  }
  

  Context. Stars with an initial mass below ∼ 8 M⊙ will evolve through the Asymptotic Giant Branch (AGB) phase, during which they develop a strong stellar wind, due to radiation pressure on newly formed dust grains. Recent observations have revealed significant morphological complexities in AGB outflows, which are most probably caused by the interaction with a companion.
  Aims. We aim for a more accurate description of AGB wind morphologies by accounting for both the radiation force in dust-driven winds and the impact of a companion on the AGB wind morphology.
  Methods. We present the implementation of a ray-tracer for radiative transfer in the smoothed particle hydrodynamics (SPH) code Phantom. Our method allows for the creation of a 3D map of the optical depth around the AGB star. The effects of 4 different descriptions of radiative transfer, with different degrees of complexity, are compared: the free-wind approximation, the geometrical approximation, the Lucy approximation, and the attenuation approximation. Finally, we compare the Lucy and attenuation approximation to predictions with the 3D radiative transfer code Magritte.
  Results. The effects of the different radiative transfer treatments are analysed considering both a low and high mass-loss rate regime and this both in the case of a single AGB star, as well as for an AGB binary system. For both low and high mass-loss rates, the velocity profile of the outflow is modified when going from the free-wind to the geometrical approximation, also resulting in a different wind morphology for AGB binary systems. In the case of a low mass-loss rate, the effect of the Lucy and attenuation approximation is negligible due to the low densities but morphological differences appear in the high mass-loss rate regime. By comparing the radiative equilibrium temperature and radiation force to the predictions from Magritte, we show that for most of the model the Lucy approximation works best, although close to the companion artificial heating occurs; and that it fails to simulate the shadow cast by the companion. The attenuation approximation leads to a stronger absorption of the radiation field yielding lower equilibrium temperature, weaker radiation force but produces the shadow cast by the companion. From the predictions of the 3D radiative transfer code Magritte, we also conclude that a radially directed radiation force is a reasonable assumption.
  Conclusions. The radiation force plays a critical role in dust-driven AGB winds, impacting the velocity profile and morphological structures. For low mass-loss rates, the geometrical approximation suffices, but for high mass-loss rates a more rigorous method is required. Among the studied approaches, the Lucy approximation provides the most accurate results, although it does not account for all effects.

Planetary systems around evolved stars

Studying the same AGB stars, I also investigate any planets or companions orbiting around them. Their significant mass-loss and changes in their physical characteristics can have a significant impact on any companions orbiting around them. To fully understand the evolution of planetary systems, we need to study the AGB phase and the role that companions play in this process.

Most AGB stars have at least one companion, and the sudden changes in the star’s characteristics can completely transform the planetary or binary system. To understand whether the companion survives this phase and explain the presence of planets orbiting around White Dwarfs, we need to study their orbital evolution. This involves taking into account the stellar mass-loss rate, mass accretion efficiency onto the companion, and tidal interactions between the star and its companion.

Previous research has only addressed these processes by using simple models that are now considered outdated. Our research focuses on improving this treatment by studying the dynamical tide, which can exceed the dissipation of the equilibrium tide by three orders of magnitude, and by using complex 3D hydrodynamic simulations that account for all components of the wind launching mechanism and the gravitational perturbation of the companion. This enables us to accurately model the mass-loss rates, mass accretion efficiency, and morphological structure of the AGB surroundings created by the companion.

Our research will help build coherent models of planetary systems orbiting evolved AGB stars, shedding light on the fate of planetary systems and their companions as stars evolve.