Browsing by Author "Bitsch, Bertram"

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    How the chemical composition of solids influences the formation of planetesimals
    (EDP Sciences, 2025-07-11) Xenos, Konstantinos Odysseas; Bitsch, Bertram; Andama, Geoffrey
    The formation of planetesimals is a necessary step for the formation of planets. While several methods exist that can explain the formation of planetesimals, an increase in the local dust-to-gas ratio above unity is a strong requirement to trigger the collapse of the pebble cloud to form planetesimals. One prime location for this to happen is at the water-ice line, where large water-rich pebbles evaporate and release their smaller silicate cores, resulting in an increase in the local dust-to-gas ratio originating from the different inward velocities of the large and small pebbles. While previous work indicated that planetesimal formation becomes very challenging at overall dustto-gas ratios below 0.6%, in line with the occurrence of close-in super-Earths, it is unclear how the overall disc composition influences the formation of planetesimals. Observations of stellar abundances not only indicate a decrease in the overall C/O ratio for low metallicity stars, they also show a large spread in the C/O ratios. However, the C/O ratio sets the abundance of water ice within the disc. Using the 1D numerical disc evolution code chemcomp, we simulated protoplanetary discs with varying C/O ratios and dust-to-gas ratios over a 3 Myr timescale. Planetesimal formation is modelled by implementing conditions based on dust-gas dynamics and pebble fragmentation. Our results confirm that planetesimal formation is highly dependent on disc metallicity with lower metallicity discs forming significantly fewer planetesimals. We find that a decreased carbon fraction generally enhances planetesimal formation, while a higher carbon fraction suppresses it due to a reduced water abundance at the same dust-to-gas ratio. The opposite is the case with the oxygen ratio, where larger oxygen fractions allow a more efficient formation of planetesimals at the same overall dust-to-gas ratio. Consequently we make the prediction that planets around low metallicity stars should be more common if the stars have low C/O ratios, especially when their oxygen abundance is increased compared to other elements, testable through observations. Our simulations thus open a pathway to understanding whether the composition of the planet-forming material influences the growth of planets.
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    Which stars can form planets: Planetesimal formation at low metallicities
    (EDP Sciences, 2024-01-26) Andama, Geoffrey; Mah, Jingyi; Bitsch, Bertram
    The diversity of exoplanets has been linked to the disc environment in which they form, where the host star metallicity and the formation pathways play a crucial role. In the context of the core accretion paradigm, the initial stages of planet formation require the growth of dust material from micrometre-sized to planetesimal-sized bodies before core accretion can kick in. Although numerous studies have been conducted on planetesimal formation, it is still poorly understood how this process takes place in low-metallicity stellar environments. In this work, we explore how planetesimals are formed in stellar environments with primarily low metallicities. We performed global 1D viscous disc evolution simulations, including the growth of dust particles and the evaporation and condensation of chemical species at ice lines. We followed the formation of planetesimals during disc evolution and tested different metallicities, disc sizes, and turbulent viscosity strengths. We find that at solar and sub-solar metallicities, there is a significant increase in the midplane dust-to-gas mass ratios at the ice lines, but this leads to planetesimal formation only at the water–ice line. In our simulations, [Fe/H] = −0.6 is the lower limit of metallicity for planetesimal formation where a few Earth masses of planetesimals can form. Our results further show that for such extreme disc environments, large discs are more conducive than small discs for forming large amounts of planetesimals at a fixed metallicity because the pebble flux can be maintained for a longer time, resulting in a longer and more efficient planetesimal formation phase. At lower metallicities, planetesimal formation is less supported in quiescent discs compared to turbulent discs, which produce larger amounts of planetesimals, because the pebble flux can be maintained for a longer time. The amount of planetesimals formed at sub-solar metallicities in our simulations places a limit on core sizes that could potentially result only in the formation of super-Earths.