Multi-fluid hydrodynamical simulations of circumbinary planet formation via pebble accretion

Context. Since the detection of the first known transiting circumbinary planet (CBP), Kepler-16b,
by the Kepler mission, a total pf 14 CBPs have been detected, raising questions about their formation
and dynamical evolution. The current picture of how a planet forms involves a multistage process
consisting of planetary embryo formation, the accretion of pebbles and planetesimals, and finally gas
accretion.
Numerous previous works have investigated the processes involved in planet formation and one way
of performing this analysis is to use hydrodynamic simulations ([6], [3]). This approach has led to a
deeper understanding of the processes that likely lead to the formation of circumbinary planets such
as the Kepler-16, -34 and -35 systems ([8], [2]).
Pebble accretion has been explored also in the formation of planets around single star systems ([4],
[5]). An important consideration is the ability of a planet to open gaps in the dust and gas in the disc
in the vicinity of the planet, depending on the mass of the planet, as presented in [7], for example.
Aims. In this work, we explore how circumbinary planets undergo pebble accretion while embedded
in circumbinary discs close to the vicinity of the central binary system. To calibrate our simulations,
we compare the evolution and results to similar planets accreting in discs around single stars. We aim
to understand the differences that might arise between both formation scenarios and to understand its
consequences for the growth of the planets and the final masses of circumbinary planets versus planets
around single stars.
Methods. In this work we use a modified version of the FARGO3D ([1]) that treats the dust as a
fluid consisting of particles with a given internal density and a fixed size, and includes pebble accretion
onto the planet.
We simulate pebble accretion onto small planets around single and binary star systems with this
multi-fluid routine, using Kepler-16 as a template. The evolution of a low mass core embedded in a
gas disc with a continuous flux of pebbles passing through the system is carefully analyzed.
Results. Pebble accretion efficiency depends mostly on the size of the dust, dust-to-gas ratio, planet
mass and initial orbital location. In our preliminary runs we have observed the opening of gaps in
the dust disc and in the gas disc while the planet’s mass is increasing due to pebble accretion. In our
ongoing simulations, we are evolving both single star and binary systems with an embedded planet.
In line with previous work, we expect the binary systems to form an eccentric inner cavity in their
cicumbinary discs, and this is expected to influence the orbital evolution of the planet and its efficiency
in accreting pebbles compared to planets orbiting a single star.
Conclusions. This work compares a single star with a binary star system in the context of planet
formation and the results are relevant to understanding the different evolutionary paths the same
initial setup can produce because of the presence of the binary. The pebble accretion efficiency will
define which of the scenarios will grow a more massive core, and this will depend on the initial system
parameters. We expect our results to show that compact circumbinary planets will be more massive
than the ones around the single stars, due to the eccentric disc and planet leading to more efficient
pebble accretion.

 

References
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