Esseldeurs, M., Mathis, S., & Decin, L. (2023, August). Towards a complete picture of the evolution of planetary systems around evolved stars. Complex Planetary Systems II.
@inproceedings{Esseldeurs2023b,
author = {{Esseldeurs}, Mats and {Mathis}, Stéphane and {Decin}, Leen},
title = {{Towards a complete picture of the evolution of planetary systems around evolved stars}},
keywords = {AGB stars, stellar winds and mass-loss, tides, star-planet interactions},
booktitle = {Complex Planetary Systems II},
year = {2023},
month = aug
}
Solar-like stars evolve through the Asymptotic Giant Branch (AGB) phase. This phase is characterized by increased radii, high luminosities, and significant mass loss. In order to understand the survival of companions during this phase, and explain the presence of planets orbiting white dwarfs, it is essential to examine the orbital evolution of these systems. Several physical mechanisms come into play for AGB stars, such as stellar mass-loss and tidal interactions between the star and its companion. On the one hand, evaluating mass-loss rates and accretion to the companion requires complex radiation-hydro-chemical simulations. On the other hand, the full history of tidal dissipation in low-mass stars during their late stages of evolution, which strongly depends on internal structure and boundary conditions, still requires dedicated studies. Finally a simultaneous treatment of winds and tides is required to predict a planet’s orbital evolution.
Esseldeurs, M., Siess, L., De Ceuster, F., Homan, W., Malfait, J., Maes, S., Konings, T., Ceulemans, T., & Decin, L. (2023). 3D simulations of AGB stellar winds. II. Ray-tracer implementation and impact of radiation on the outflow morphology. A&A, 674, A122.
@article{Esseldeurs2023a,
author = {{Esseldeurs}, M. and {Siess}, L. and {De Ceuster}, F. and {Homan}, W. and {Malfait}, J. and {Maes}, S. and {Konings}, T. and {Ceulemans}, T. and {Decin}, L.},
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.
Maes, S., Siess, L., Homan, W., Malfait, J., De Ceuster, F., Ceulemans, T., Donné, D., Esseldeurs, M., & Decin, L. (2022). Route towards complete 3D hydro-chemical simulations of companion-perturbed AGB outflows. The Origin of Outflows in Evolved Stars, 366, 227–233.
@inproceedings{Maes2022,
author = {{Maes}, Silke and {Siess}, Lionel and {Homan}, Ward and {Malfait}, Jolien and {De Ceuster}, Frederik and {Ceulemans}, Thomas and {Donn{\'e}}, Dion and {Esseldeurs}, Mats and {Decin}, Leen},
title = {{Route towards complete 3D hydro-chemical simulations of companion-perturbed AGB outflows}},
keywords = {Stars: AGB, Stars: winds, outflows, Methods: numerical, Astrophysics - Solar and Stellar Astrophysics, Astrophysics - Astrophysics of Galaxies},
booktitle = {The Origin of Outflows in Evolved Stars},
year = {2022},
volume = {366},
month = jan,
pages = {227-233},
doi = {10.1017/S1743921322000217},
archiveprefix = {arXiv},
eprint = {2206.12278},
primaryclass = {astro-ph.SR},
adsurl = {https://ui.adsabs.harvard.edu/abs/2022IAUS..366..227M},
adsnote = {Provided by the SAO/NASA Astrophysics Data System}
}
Low- and intermediate mass stars experience a significant mass loss during the last phases of their evolution, which obscures them in a vast, dusty envelope. Although it has long been thought this envelope is generally spherically symmetric in shape, recent high-resolution observations find that most of these stars exhibit complex and asymmetrical morphologies, most likely resulting from binary interaction. In order to improve our understanding about these systems, theoretical studies are needed in the form of numerical simulations. Currently, a handful of simulations exist, albeit they mainly focus on the hydrodynamics of the outflow. Hence, we here present the pathway to more detailed and accurate modelling of companion-perturbed outflows with, by discussing the missing but crucial physical and chemical processes. With these state-of-the-art simulations we aim to make a direct comparison with observations to unveil the true identity on the embedded systems.
Thesis
Esseldeurs, M. (2022). SPH Approach to Modelling Dust Attenuation of the Wind Acceleration in AGB Binaries. KU Leuven. Faculteit Wetenschappen.
@thesis{Esseldeurs2022,
language = {eng},
publisher = {KU Leuven. Faculteit Wetenschappen},
title = {SPH Approach to Modelling Dust Attenuation of the Wind Acceleration in AGB Binaries},
year = {2022},
author = {Esseldeurs, Mats},
address = {Leuven}
}
Circumstellar envelopes of asymptotic giant branch (AGB) stars have for a long time been modelled assuming a spherical symmetry. However, recent observations of the inner winds of these stars have shown that they exhibit a variety of complex structures, such as spirals, equatorial density enhancements, disks, bipolar outflows, etc. These structures are believed to originate from the presence of a companion, either stellar and/or planetary. Due to the inherently three-dimensional (3D) nature of this phenomenon, its investigation requires advanced 3D-simulations. It has already been shown that using hydrodynamic simulations, some of these wind morphologies can be obtained. However, the computational cost of truly selfconsistent calculations, including the crucial chemical and radiation processes, is currently still computationally prohibitive. Therefore, incremental modelling improvements using ever more refined approximations can significantly advance the quality and physical consistency of the simulations. One such problem revolves around the transfer of momentum from the stellar photons to the dust particles in the AGB wind. Until recently, this coupling has always either been ignored, or treated in the optically thin limit, which only requires knowledge on the local quantities. However, properly accounting for the dust opacity and its attenuation of the stellar radiation field can drastically affect the effective radiation pressure on the dust. In turn, this can significantly alter the dynamics and morphology of these stellar winds.
In the context of a large collaboration that revolves around upgrading the treatment of dusty winds within the smoothed-particle-hydrodynamics code PHANTOM, I have worked on levelling up the way in which dust acceleration is calculated. To calculate the attenuation of the stellar radiation field, knowledge on the distribution and optical properties of the matter in-between each particle and the star is required. To this end the ray-tracing algorithm of the radiative transfer code MAGRITTE was extracted, made compatible with the SPH philosophy, and coupled to PHANTOM. We investigated different options to speed up the ray-tracer with only a minimal loss in accuracy. We show that the best results are found when rays are traced outwards from the star in a uniform distribution, set by HEALPix. Because not all particles are struck by a ray, we also investigated different interpolation approaches. We find that interpolation scaling with the inverse square of the perpendicular distance to the four closest rays gives the most desirable result. Finally, we demonstrate the validity of our new approach in a simulation.