Research Articles

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https://dir.muni.ac.ug/handle/20.500.12260/320

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Browsing Research Articles by Subject "Accretion"

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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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    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.