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 3D simulations 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 prescriptions of radiative transfer, with different degrees of complexity, including the free-wind, the geometrical, the Lucy, and the attenuation prescription. By comparing the results to predictions from the 3D radiative transfer code Magritte, I have found that the Lucy prescription 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.

Tidal Dissipation in Evolved Stars

Building on our understanding of AGB stellar winds, we turn to the crucial process of tidal dissipation in evolved stars. As these stars expand and develop deep convective envelopes, their interactions with orbiting companions become dominated by tidal forces. These tides can transfer angular momentum, leading to dramatic changes in the orbits of planets and binary companions.

To accurately capture these effects, we employ ab-initio modelling of tidal dissipation, directly linking the internal structure and evolution of the star to the efficiency of tidal energy loss. By moving beyond simplified prescriptions and instead using detailed stellar models, we can quantify how convection, oscillations, and the changing stellar envelope shape the tidal response.

This physically motivated approach allows us to accurately predict how tidal interactions influence the fate of companions: whether they spiral in and are engulfed, or survive as the star evolves. Our ab-initio tidal dissipation models thus provide the essential bridge between the physical processes in evolved stars and the orbital evolution of their planetary systems, setting the stage for understanding the diverse outcomes observed in post-AGB systems.

• Esseldeurs, et al. (2024). Tidal dissipation in evolved low- and intermediate-mass stars. A&A, 690, A266.
  PDF DOI BIB Abstract

  @article{Esseldeurs2024_short,
    author = {Esseldeurs, {et al.}},
    title = {{Tidal dissipation in evolved low- and intermediate-mass stars}},
    journal = {A&A},
    keywords = {methods: numerical, planet-star interactions, binaries: close, stars: evolution, planetary systems, Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Earth and Planetary Astrophysics},
    year = {2024},
    month = oct,
    volume = {690},
    eid = {A266},
    pages = {A266},
    doi = {10.1051/0004-6361/202449648},
    archiveprefix = {arXiv},
    eprint = {2407.10573},
    primaryclass = {astro-ph.SR},
    adsurl = {https://ui.adsabs.harvard.edu/abs/2024A&A...690A.266E},
    adsnote = {Provided by the SAO/NASA Astrophysics Data System}
  }
  

  Context. As the observed occurrence for planets or stellar companions orbiting low- and intermediate-mass evolved stars is increasing, so is the importance of understanding and evaluating the strength of their interactions. This is important for the further evolution of both our own Earth-Sun system and most of the observed exoplanetary systems. One of the most fundamental mechanisms behind this interaction is the tidal dissipation in these stars, as it is one of the engines of the orbital and rotational evolution of star-planet and star-star systems.
  Aims. This article builds upon previous works that studied the evolution of the tidal dissipation along the pre-main sequence and the main sequence of low- and intermediate-mass stars and found a strong link between the structural and rotational evolution of stars and tidal dissipation. This article provides, for the first time, a complete picture of tidal dissipation along the entire evolution of low- and intermediate-mass stars, including the advanced phases of evolution.
  Methods. Using stellar evolutionary models, the internal structure of the star was computed from the pre-main sequence all the way up to the white dwarf phase for stars with initial masses between 1 and 4 Msun. Using this internal structure, the tidal dissipation was computed along the entire stellar evolution. Tidal dissipation was separated into two components: the dissipation of the equilibrium (non-wave-like) tide and the dissipation of the dynamical (wave-like) tide. For evolved stars, the dynamical tide is constituted by progressive internal gravity waves. The evolution of the tidal dissipation was investigated for both the equilibrium and dynamical tides, and the results were compared.
  Results. The significance of both the equilibrium and dynamical tide dissipation becomes apparent within distinct domains of the parameter space. The dissipation of the equilibrium tide is dominant when the star is large or the companion is far from the star. Conversely, the dissipation of the dynamical tide is important when the star is small or the companion is close to the star. The size and location of these domains depend on the masses of both the star and the companion, as well as on the evolutionary phase.
  Conclusions. Both the equilibrium and the dynamical tides are important in evolved stars, and therefore both need to be taken into account when studying the tidal dissipation in evolved stars and the evolution of the planetary and/or stellar companions orbiting them.

Orbital evolution of companions to AGB stars

In addition to simulating the winds of AGB stars and modelling their internal tidal dissipation, I investigate how these processes together shape the orbits of planets and companions. The significant mass-loss and structural changes during the AGB phase, combined with strong tidal forces, can dramatically alter the fate of any objects in orbit. Whether a companion survives, spirals inward and is engulfed, or escapes to a wider orbit depends on the delicate balance between tidal dissipation, mass-loss, and accretion processes.

Our research advances the field by combining ab-initio tidal dissipation calculations with detailed 3D hydrodynamic simulations of the wind and the gravitational influence of companions. This comprehensive approach allows us to accurately model not only the mass-loss rates and accretion efficiency, but also the evolving tidal forces that govern orbital evolution.

By integrating these effects, we can build coherent models of planetary systems around evolved AGB stars, providing new insights into the survival of planets, the formation of close binaries, and the diversity of systems observed around white dwarfs and post-AGB stars.