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The UC Research Repository collects, stores and makes available original research from postgraduate students, researchers and academics based at the University of Canterbury.
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The frequency and mass-ratio distribution of binaries in young star clusters.
(2024) Alexander, Jason S.
Binary stars play a key role in the evolution of star clusters, therefore to develop our
understanding it is of great importance to determine the binary fractions of these clusters. In
this thesis, we focus on determining the frequency and mass-ratio distributions of binaries in
six young star clusters, specifically Collinder 69, 𝛼 Persei, Pleiades, NGC 6405, Trumpler
10, and UPK 640. Using data from Gaia DR3, the memberships of stars within these
clusters were determined through a selection and filtration procedure that resulted in our
final datasets.
We implemented an adapted version of the probabilistic generative model from Albrow
& Ulusele (2022) for colour-magnitude diagrams (CMDs) to ascertain the binary fractions
and mass-ratio distributions of each of the clusters. To account for overlapping populations
this method applies a Gaussian mixture model to represent stars as probabilities of being
single stars, binaries, or outliers. We used two different basis function sets to model the
mass-ratio distribution: Legendre polynomials and histograms. We found the results to be
consistent with each other.
We tested the sensitivity of our models to colour shifts of the isochrone. We ran our
model with incremental shifts in the colour up to a shift of ±0.01 mag. We found that the
effect of applying these colour shifts had a nominal effect on our results, and lie well within
quoted uncertainties. This shows that our model is extremely robust to the position of the
isochrone and that our results are well founded. We noticed that the small changes to the
results followed trends and we explain the cause of these effects.
The mass-ratio distributions of all our clusters exhibit a distinct preference to high-mass ratio binaries. Furthermore, we define 𝐹𝑄75 as the ratio of the number of binaries which
have 0.75 ≤ 𝑞 ≤ 1.0 to the number which have 0.5 ≤ 𝑞 ≤ 1.0, and find all of our clusters
have 𝐹𝑄75 > 0.5 and ranging between 0.598 and 0.852. The binary fraction for 𝑞 ≥ 𝑞𝑚𝑖𝑛
is determined by integrating the mass-ratio distribution from 1 to 𝑞𝑚𝑖𝑛. We find that the
binary fraction of these clusters ranges between 0.170 and 0.274.
We find a strong positive correlation between the binary fraction for 𝑞 ≥ 0.5 and 𝐹𝑄75,
a moderate negative correlation between the age of a cluster and the binary fraction for
𝑞 ≥ 0.5, and a moderate negative correlation between the age of a cluster and 𝐹𝑄75. To
account for the decreasing binary fraction with age, we propose a period of binary destruction
early in the lifetime of a star cluster (<∼ 100 Mya), where binaries are destroyed by stellar
disruptions at a higher rate than the escape of low-mass single stars. The proportion of equal mass
binaries may decrease with age because higher-order systems; which are unstable and
more likely to be disrupted over time, are blending into the binary region of the CMD,
or that high-mass-ratio binaries are more likely to form in a soft binary system and be
disrupted. Each of our clusters exhibited a high incidence of equal-mass binaries, and our
interpretations of the other trends directly imply that the frequency of equal-mass binaries
is more variable than at other mass-ratios, therefore it has the greatest effect on the binary
fraction.
Effects of methamphetamine on behavioural measures of impulsivity in rats.
(2023) Bodeker (Ryan), Rebecca Rose Hazel
This thesis investigated how rats would respond to methamphetamine in a novel,
rapid acquisition, concurrent chains task that measured choice for delayed or probabilistic
reinforcers when contingencies changed unpredictably across sessions. In the first
experiment, the ability of rats to forgo an immediate small reward in favour of larger but
delayed reward was investigated. Both delay discounting and an effect of overall magnitude
on discounting (‘magnitude effect’) were obtained in baseline. Increasing doses of acute
methamphetamine were given prior to the delay discounting and magnitude effect tasks
before a chronic phase where the highest methamphetamine dose was given several hours
before completion of the behavioural tasks. A re-establish baseline phase completed the
experimental procedure. In the second experiment, choice for a large risky reward over a
small certain option was investigated using a similar rapid acquisition concurrent chains
procedure. Probability discounting and a magnitude effect where choice for the probabilistic
reinforcer was reduced for the larger reward were obtained in baseline, before the same
methamphetamine regimen was administered. The final experiment considered an aspect of
impulsivity termed the signalling effect, where people and animals choose a riskier course of
action when the outcomes of that choice were signalled. Once again, the signalling procedure
was established before acute and chronic methamphetamine was administered to rats and a
re-establish baseline phase concluded the experiment. In Experiment 1, acute
methamphetamine decreased both sensitivity to delay and the magnitude effect. In
Experiment 2, acute methamphetamine decreased sensitivity to probability and magnitude,
and in Experiment 3, reduced sensitivity to probability but only in signalled sessions. The
effect of chronic methamphetamine was not as consistent across experiments. Overall, these
results show that methamphetamine administration can reduce control by reinforcement
contingencies in delay and probability discounting, thereby increasing the likelihood of
impulsive or maladaptive choices, but that the effects can depend on the contingencies that
change across sessions. The results of these three experiments establish the validity of the
rapid-acquisition concurrent chains task for investigating rats’ choices between delayed and
probabilistic rewards, and contribute to our understanding of how methamphetamine
administration can affect the likelihood of impulsive choice.
The burden of dysphagia in New Zealand : Exploring prevalence, economic costs, and risk-sharing arrangements.
(2024) Duncan, Shnece
This thesis explores the economic consequences of dysphagia, a debilitating disorder characterised by impaired swallowing function, in New Zealand. By analysing various dimensions of this disorder, including its prevalence, economic burden, and the effectiveness of performance-based arrangements to increase uptake of new interventions, I provide a comprehensive understanding of dysphagia in New Zealand from an economic perspective.
I draw on existing literature to estimate that 1.5% (1.1%–2.0%) of the total New Zealand population lived with the effects of dysphagia in 2020 and I project this to increase to 2.6% (2.0%–3.5%) by 2073. Using hospitalisation data from the Canterbury District Health Board (CDHB), I find that the presence of dysphagia in a hospital stay, relating to any primary condition, increases the total cost of hospitalisation on average by 56%. Through the REGIONS Care national research data, I estimate new onset stroke-related dysphagic patients in 2021 to present a total additional cost of $24,200 per patient, or $77.2 million overall, to New Zealand society. The increasing prevalence of dysphagia alongside the condition’s large cost will likely result in a major financial burden on the New Zealand healthcare system. Medical interventions designed to aid in the treatment or management of dysphagic patients could lead to significant future cost avoidance.
I then explore the potential of Performance-Based Risk-Sharing Arrangements as a mechanism to mitigate the financial risks associated with dysphagia. Through an insurance framework, I show how these arrangements present a potential solution to increase the chance of new healthcare interventions being adopted and funded by healthcare payers and/or providers. My research lays a solid groundwork for policymakers, healthcare providers, and researchers to develop targeted interventions, optimise resource allocation, and enhance the well-being of individuals impacted by dysphagia.
The problematic self : groundwork for a new existentialist approach to ethics.
(2023) McBride, Lance
The foundational premise of this work is that the concept of individual moral
agency that underlies contemporary ethical discourse rests on a flawed model of
selfhood inherited, in most cases unwittingly, from the Enlightenment; particularly
from the efforts of the German Idealists to rebuild moral theory upon the
cornerstone of human reason. The model of selfhood they provide carries with it a
number of conceptual difficulties that prove fatal to any attempt to construct a
unified and comprehensive theory of ethics – specifically the problems of relating
the subjective to an objective reality, the nature of temporal existence, the challenge
of nihilism, and a seemingly inevitable doctrine of conflict between self and Other.
However, these problems did not escape the notice of those philosophers of the
phenomenological tradition that we now identify as existentialists. Unfortunately,
none of the existentialists, either, met with any greater success in finding a way
from the existent self to a working theory of intersubjective ethics. Indeed, their
unique awareness of, and focus on, the specific difficulties confronting that project
has often resulted in a far clearer sense of failure than we are accustomed to
admitting to in contemporary ethics. My primary contention is that, by observing
the treatment of selfhood through the works of a selection of the most prominent
existentialists – Kierkegaard, Nietzsche, and Jean-Paul Sartre – and examining both
the manner in which each stumbles in reaching towards intersubjective ethics and
the solutions they offer to the failures that precede them, we might identify a set of
desiderata that would helpfully inform any new theory of existentialist ethics. For
the most part, the metaphysical commitments of each theorist are drawn directly
from their primary works, but, in the case of the infamously obscurantist Nietzsche,
no clear statement exists in the primary literature. We will, therefore, rely on a
promising contemporary analysis drawn principally from the discourse between
John Richardson and Paul Katsafanas. I will argue that, viewed together, these
desiderata may at least suggest a possible path forward in the form of a shift away
from the substance ontology that underlies the Idealist position towards a form of
process phenomenology.
Sustainability reporting assurance practice in New Zealand : an accountability enabler or a managerial supplement?
(2023) Ahmed, Heba Gaber
Purpose: This thesis aims to understand the nature and role of sustainability assurance (SA) in the context of corporate accountability in the New Zealand (NZ) market. This understanding is developed based on an extensive investigation of the perspectives of sustainability managers (SMs), sustainability assurance providers (SAPs), and stakeholders. The study gives particular attention to understanding the perspectives of multiple stakeholder groups, which is missing in the literature. Stakeholders are overwhelmed with various corporate sustainability (CS) disclosures that are susceptible to greenwashing and there is an urgent need to ensure their integrity and reliability. The Global Reporting Initiative (GRI) recommends the use of SA to ensure the credibility of sustainability reports (SRs), but the practice is growing slowly in NZ and the assurance engagement parties are still determining the nature of best practice. The study contributes to the relevant literature in SA practice, highlighting its weaknesses and suggesting possible solutions to reach an ideal model.
Approach: The study employed an interpretive qualitative approach, based on the philosophical assumptions of constructivism and interpretivism. A thorough understanding of SA practice was acquired through the application of the hermeneutic circle. Data was collected from 35 in-depth semi-structured interviews with three categories of participants: SMs from the NZX-listed companies and Sustainable Business Council (SBC) members, SAPs from accounting and non-accounting firms, and representatives from five different stakeholder groups: sustainability consultants, the External Reporting Board (XRB), the New Zealand Stock Exchange (NZX), non-governmental organisations (NGOs), and financial capital providers.
Findings: The practice of SA exists in the NZ market in the form of a weak model: an internal management practice. It is demanded by management and targeted at management, and focuses on the assessment of the quality of both internal control systems and SR presentations, without assessing sustainability performance per se. This weak model of SA achieves accountability to management, but fails to fulfil the promise of stakeholder accountability. Three fundamental factors are currently causing the practice of SA in NZ to diminish in importance. There is a high demand from internal stakeholders, coupled with a strong desire from accounting firms to gain SA market dominance against limited demand from external stakeholders. The study provides evidence that managerial and professional capture occur at different stages of the assurance engagement.
Originality: To my knowledge, this is the first NZ study to make an extensive investigation into the perspectives of stakeholders. This is also the first NZ study to consider SMs, SAPs, and stakeholders simultaneously in the practice of SA. Considering the three parties of the SA equation together enriched the findings. These findings elicited a list of country-specific factors that are unique to the NZ market, which help in the interpretation of the weak model of SA.
Practical implications: The findings of this study will prove beneficial to SMs, SAPs, and stakeholders. The proposed ideal model of SA practice offers detailed guidance for the three parties on specific mechanisms to achieve stakeholder accountability, instead of management accountability. It may assist them in developing new approaches before, during, and after the assurance engagement in order to avoid falling back into the weak model of SA. The depiction of the two contrasting models of SA practice together provides powerful insights for practitioners and researchers.
Theoretical implications: The study helps to clearly visualise the weak model of SA practice, identify its causes, and suggest future practical steps that should be taken to turn the practice into its ideal shape. Furthermore, new insight is added through the application of Michael Power’s work in the interpretation of the findings.