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

Abstract

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 formationand dynamical evolution. The current picture of how a planet forms involves a multistage processconsisting of planetary embryo formation, the accretion of pebbles and planetesimals, and finally gasaccretion.Numerous previous works have investigated the processes involved in planet formation and one wayof performing this analysis is to use hydrodynamic simulations ([6], [3]). This approach has led to adeeper understanding of the processes that likely lead to the formation of circumbinary planets suchas 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 discin 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 embeddedin 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 aimto understand the differences that might arise between both formation scenarios and to understand itsconsequences for the growth of the planets and the final masses of circumbinary planets versus planetsaround single stars.Methods. In this work we use a modified version of the FARGO3D ([1]) that treats the dust as afluid consisting of particles with a given internal density and a fixed size, and includes pebble accretiononto the planet.We simulate pebble accretion onto small planets around single and binary star systems with thismulti-fluid routine, using Kepler-16 as a template. The evolution of a low mass core embedded in agas 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, planetmass and initial orbital location. In our preliminary runs we have observed the opening of gaps inthe dust disc and in the gas disc while the planet’s mass is increasing due to pebble accretion. In ourongoing 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 theircicumbinary discs, and this is expected to influence the orbital evolution of the planet and its efficiencyin 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 planetformation and the results are relevant to understanding the different evolutionary paths the sameinitial setup can produce because of the presence of the binary. The pebble accretion efficiency willdefine which of the scenarios will grow a more massive core, and this will depend on the initial systemparameters. We expect our results to show that compact circumbinary planets will be more massivethan the ones around the single stars, due to the eccentric disc and planet leading to more efficientpebble accretion. References[1] Benı́tez-Llambay, P., and Masset, F. S. Fargo3d: A new gpu-oriented mhd code. TheAstrophysical Journal Supplement Series 223 (2016).[2] Coleman, G. A., Nelson, R. P., and Triaud, A. H. Dusty circumbinary discs: inner cavitystructures and stopping locations of migrating planets. Monthly Notices of the Royal AstronomicalSociety 513 (2022).[3] Kley, W., and Nelson, R. P. Planet-disk interaction and orbital evolution, 2012.[4] Lambrechts, M., and Johansen, A. Rapid growth of gas-giant cores by pebble accretion.Astronomy and Astrophysics 544 (2012).[5] Lambrechts, M., Johansen, A., and Morbidelli, A. Separating gas-giant and ice-giantplanets by halting pebble accretion. Astronomy and Astrophysics 572 (2014).[6] Nelson, R. P. On the evolution of giant protoplanets forming in circumbinary discs. MonthlyNotices of the Royal Astronomical Society 345 (2003).[7] Paardekooper, S. J., and Mellema, G. Planets opening dust gaps in gas disks. Astronomyand Astrophysics 425 (2004).[8] Pierens, A., and Nelson, R. P. Migration and gas accretion scenarios for the kepler 16, 34,and 35 circumbinary planets. Astronomy and Astrophysics 556 (2013).