Browsing by Author "Andama, Geoffrey"

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    Planet population synthesis: The role of stellar encounters
    (Royal Astronomical Society, 2022-02-28) Ndugu, Nelson; Oyirwoth, Patrick Abedigamba; Andama, Geoffrey
    Depending on the stellar densities, protoplanetary discs in stellar clusters undergo: background heating; disc truncation-driven by stellar encounter; and photo-evaporation. Disc truncation leads to reduced characteristic sizes and disc masses that eventually halts gas giant planet formation. We investigate how disc truncation impacts planet formation via pebble-based core accretion paradigm, where pebble sizes were derived from the full grain-size distribution within the disc lifetimes. We make the best-case assumption of one embryo and one stellar encounter per disc. Using planet population syntheses techniques, we find that disc truncation shifts the disc mass distributions to the lower margins. This consequently lowered the gas giant occurrence rates. Despite the reduced gas giant formation rates in clustered discs, the encounter models mostly show as in the isolated field; the cold Jupiters are more frequent than the hot Jupiters, consistent with observation. Moreover, the ratio of hot to cold Jupiters depend on the periastron distribution of the perturbers with linear distribution in periastron ratio showing enhanced hot to cold Jupiters ratio in comparison to the remaining models. Our results are valid in the best-case scenario corresponding to our assumptions of: only one disc encounter with a perturber, ambient background heating and less rampant photo-evaporation. It is not known exactly of how much gas giant planet formation would be affected should disc encounter, background heating and photo-evaporation act in a concert. Thus, our study will hopefully serve as motivation for quantitative investigations of the detailed impact of stellar cluster environments on planet formations.
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    Planetary core formation via multispecies pebble accretion
    (Oxford University Press, 2021-12-02) Andama, Geoffrey; Ndugu, Nelson; Anguma, Simon Katrini; Jurua, Edward
    In the general classical picture of pebble-based core growth, planetary cores grow by accretion of single pebble species. The growing planet may reach the so-called pebble isolation mass, at which it induces a pressure bump that blocks inward drifting pebbles exterior to its orbit, thereby stalling core growth by pebble accretion. In recent hydrodynamic simulations, pebble filtration by the pressure bump depends on several parameters including core mass, disc structure, turbulent viscosity and pebble size. We have investigated how accretion of multiple, instead of single, pebble species affects core growth rates, and how the dependence of pebble isolation mass on turbulent viscosity and pebble size sets the final core masses. We performed numerical simulations in a viscous one-dimensional disc, where maximal grain sizes were regulated by grain growth, fragmentation and drift limits. We confirm that core growth rates and final core masses are sensitive to three key parameters: the threshold velocity at which pebbles fragment on collision, the turbulent viscosity and the distribution of pebble species, which yield a diversity of planetary cores. With accretion of multiple pebble species, planetary cores can grow very fast, reaching over 30–40 ME in mass. Potential cores of cold gas giants were able to form from embryos initially implanted as far as 50 au. Our results suggest that accretion of multispecies pebbles could explain: the estimated 25–45 ME heavy element abundance inside Jupiter’s core; the massive cores of extrasolar planets; the disc rings and gaps at wider orbits; and the early and rapid formation of planetary bodies.
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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.