Towards a complete picture of the evolution of planetary systems around evolved stars

About 95% of all stars in the galaxy have an initial mass lower than 8 solar masses. When these stars evolve off the main sequence, they will go through the Asymptotic Giant Branch (AGB) phase, just before turning into a white dwarf. This phase is characterized by increased radii, high luminosities, intense pulsations, and significant mass loss. In order to get a complete picture of the evolution of planetary systems from the birth to the end of their host stars, as well as to understand the survival of planetary or stellar 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 studying orbital evolution around AGB stars, such as the significant stellar mass-loss rate, the efficiency of mass accretion onto the companion, and the tidal interactions between the star and its companion.

3D radiation-hydro-chemical simulations of AGB stellar outflows

AGB stars lose a significant amount of mass through the so-called dust-driven wind. Pulsations at the surface of AGB stars push material into sufficiently high layers where it is cool enough for molecules to condensate into dust. These dust particles are opague, such that they can be pushed outward by radiation pressure. Finally the dust particles collide with the remaining gas particles and create a steady outflow of material. These outflows from AGB stars exhibit complex structures, such as arcs, spirals. The current theory suggests that these complexities are caused by an unseen companion. To understand this phenomenon, complex 3D radiation-hydro-chemical simulations are necessary to understand the influence a companion can have on the morphological structures of AGB outlows. These simulations will also allow us to investigate the impact of the companion on the star’s mass loss rate and the efficiency of accretion onto the companion. However, the existing simulations are computationally demanding, and ongoing efforts are concentrated on enhancing the computational speed.

Identifying tidal dissipation mechanisms around evolved stars

Tidal dissipation encompasses two components: equilibrium and dynamical tides. The former occurs due to hydrostatic displacement induced by the ellipsoidal deformation triggered by the companion. Its energy is dissipated because of turbulent friction in convective layers, generating transfer of angular momentum between the spin and the orbit. The latter involves other effects (sometimes more significant than the equilibrium tide), such as pulsations, which are influenced by the star’s internal structure, which can be seen on the figure on the left. AGB stars possess a convective envelope exciting inertial modes (only for stellar companions, as planetary companions don’t spin up the the star sufficiently) and a radiative core exciting low-frequency gravity waves in response to tides. The figure on the right shows the important frequencies in this regard, showing the Brunt–Väisälä frequency, the Lamb frequency, and the tidal frequency our earth would have if it were to orbit an AGB star. The dissipation of these waves varies depending on the boundary conditions chosen for the star. While for main-sequence stars static boundary conditions are adequate, the substantial mass loss of AGB stars necessitates the exploration of more suitable boundary conditions, like a dynamical (mass losing) outer boundary.

The interplay between winds, pulsations, and tides

Tidal dissipation and mass loss are not problems that can be treated separatly. The AGB’s dust-driven winds are initiated by pulsations, while the presence of tides may induce additional pulsations. Hence tides can result in an increased mass-loss rate. On the other hand, mass loss may dissipate energy stored in tidal waves moving passing through the surface of the star (which is moddeled by a dynamical outer boundary). Hence, mass-loss may induce additional dissipation of tidal energy.

Towards a complete picture of the evolution of planetary systems around evolved stars

In order to investigate the orbital evolution of companions around AGB stars, both mass loss and tidal dissipation play crucial roles. Complex simulations are essential for understanding how companions impact the star’s mass loss rate, and the efficiency of accretion onto the companion. Tidal dissipation, relying on internal structure and boundary conditions, requires additional studies regarding mass loss. The interplay between winds, pulsations, and tides signifies a mutual influence on mass loss and tidal dissipation, presenting a complex problem demanding a dedicated investigation.

• Esseldeurs, M. (2023). Towards a complete picture of the evolution of planetary systems around evolved stars. Complex Planetary Systems II – Kavli-IAU Symposium 382.
  Poster BIB URL Abstract Additional Information Proceedings

  @conferencepres{Namur_2023,
    author = {Esseldeurs, Mats},
    title = {Towards a complete picture of the evolution of planetary systems around evolved stars},
    publisher = {Complex Planetary Systems II – Kavli-IAU Symposium 382},
    year = {2023},
    proceedings = {Esseldeurs+2023b.pdf}
  }
  

  About 95% of all stars in the galaxy have an initial mass lower than 8 solar masses. When these stars evolve off the Main Sequence, they will go through the Asymptotic Giant Branch (AGB) phase, just before turning into a White Dwarf (WD). This evolutionary phase is characterized by significant mass loss, large stellar radii, strong pulsations, and extreme luminosities. It must be studied to get a complete picture of the evolution of planetary systems from the birth to the end of their host stars. Indeed, population synthesis studies indicate that the majority of these stars have at least one (sub)stellar companion, where the abrupt changes in stellar characteristics may completely transform a planetary/binary system. In order to establish whether the planetary/(sub)stellar companion survives this evolutionary phase, and explain the presence of planets orbiting around WDs, one must study their orbital evolution for which both the stellar mass loss rate and mass accretion efficiency onto the companion and tidal interactions between the star and its companion play an important role. In current literature this is only addressed taking into account the simplest equilibrium tide, with mass-loss rate prescriptions and mass accretion efficiencies now considered outdated. For this reason, the treatment of these processes needs to be improved. First, we now study the dynamical tide, which can exceed the dissipation of the equilibrium tide by 3 orders of magnitude, which needs to be studied all along the evolution of stars. Second, we compute complex 3D hydrodynamic simulations that take into account all the complex components of the wind launching mechanism, as well as the gravitational perturbation of the companion. This makes it possible to account for the companion in simulating the mass loss rates, model for wind Roche Lobe overflow determining the mass accretion efficiency on the companion, as well as simulate the morphological structure of the AGB surroundings created by the companion. This allows us to build step by step coherent models of planetary systems orbiting evolved AGB stars.