Browsing by Author "Ndugu, Nelson"

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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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    Projected land use change in an oil-rich landscape in Uganda: A participatory modelling approach
    (Elsevier, 2022-03-13) Twongyirwe, Ronald; Fisher, Eleanor; Karungi, Christine; Ndugu, Nelson
    The discovery of oil in the North Albertine Rift Landscape of Uganda has increased pressure on land and heightened the potential for resource use conflict. In this article, we focus on changing land use dynamics as oil extraction unfolds in a new resource frontier. We ask how the development of the nascent oil industry will affect land use dynamics, including land use conflicts. This leads us to identify the land use change already arising and to use this as the basis for participatory modelling of projected change. Given they are dominant forms of land use, agriculture and forestry are central to our analysis. Design of the methodology combined remote sensing with innovative modelling incorporating participatory development methods. This facilitated insight into projected land use patterns, and specifically relationships between small-scale food production, commercial sugarcane production, and forestry conservation adjacent to settlement areas. Our data show that ill–defined land boundaries and an aggressive sugarcane out-grower scheme are avenues for so-called land grabbing. Modelling scenarios under both the status quo and under oil extraction suggest the land area covered by sugarcane production will increase at the expense of food crop farming. Given a context where forestry conservation is an important form of land use, we also consider the implications of local agricultural change on land reserved for conservation. Overall, our modelling indicates that in accounting for land use change within the resource frontier associated with oil extraction, there needs to be insight into the intricate interconnections between different forms of rural land use as future change unfolds. Understanding how oil extraction effects rural land use patterns holds relevance for planning in contexts of the Global South where new oil industries are emerging. Innovative methodologies for teasing out these complex land use dynamics can aid planning that seeks to anticipate and reduce land use conflict and support agricultural livelihoods.
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    The role of density perturbation on planet formation by pebble accretion
    (arXiv preprint, 2022-01-28) Andama, Geoffrey; Ndugu, Nelson; Anguma, Simon Katrini; Jurua, Edward
    Protoplanetary discs exhibit a diversity of gaps and rings of dust material, believed to be a manifestation of pressure maxima commonly associated with an ongoing planet formation and several other physical processes. Hydrodynamic disc simulations further suggest that multiple dust ring-like structures may be ubiquitous in discs. In the recent past, it has been shown that dust rings may provide a suitable avenue for planet formation.We study how a globally perturbed disc affects dust evolution and core growth by pebble accretion. We performed global disc simulations featuring a Gaussian pressure profile, in tandem with global perturbations of the gas density, mimicking wave-like structures, and simulated planetary core formation at pressure minima and maxima. With Gaussian pressure profiles, grains in the inside disc regions were extremely depleted in the first 0.1 Myrs of disc lifetime. The global pressure bumps confined dust material for several million years, depending on the strength of perturbations. A variety of cores formed in bumpy discs, with massive cores at locations where core growth was not feasible in a smooth disc, and small cores at locations where massive cores could form in a smooth disc. We conclude that pressure bumps generated by a planet and/or other physical phenomena can completely thwart planet formation from the inside parts of the disc. While inner disc parts are most favourable for pebble accretion in a smooth disc, multiple wave-like pressure bumps can promote rapid planet formation by pebble accretion in broad areas of the disc.