Searching for Hydrogen in Type Ib Supernovae

We present synthetic spectral fits of the typical Type Ib SN 1999dn and the Hydrogen Rich Ib SN 2000H using the generalized non-local thermodynamic equilibrium stellar atmospheres code \phx. We fit model spectra to five epochs of SN 1999dn ranging from ten days pre-maximum light to 17 days post-maximum light and the two earliest epochs of SN 2000H available, maximum light and six days post-maximum. Our goal is to investigate the possibility of hydrogen in Type Ib Supernovae (SNe Ib), specifically a feature around 6200\AA\ which has previously been attributed to high velocity H-alpha. In earlier work on SN 1999dn we found the most plausible alternative to H-alpha to be a blend of Si II and Fe II lines which can be adjusted to fit by increasing the metallicity. Our models are simple; they assume a powerlaw density profile with radius, homologous expansion, and solar compositions. The helium core is produced by burning 4H --> He in order to conserve nucleon number. For models with hydrogen the outer skin of the model consists of a shell of solar composition. The hydrogen mass of the standard solar composition shell is M_H less than about 0.001 times the mass of the sun in SN 1999dn and M_H less than about 0.2 times the mass of the sun for SN 2000H. Our models fit the observed spectra reasonably well, successfully reproducing most features including the characteristic He I absorptions. The hydrogen feature in SN 1999dn is clear, but much more pronounced in SN 2000H. We discuss a possible evolutionary scenario that accounts for the dichotomy in the hydrogen shell mass between these two supernovae.Comment: 20 pages, 11 figures, ApJ, in pres

Reliability, replicability and reproducibility in PET imaging

Positron Emission Tomography (PET) is a non-invasive biomedical imaging method which can quantify biochemical markers as well as functional and metabolic activity in vivo. Following image analysis, quantification can be performed using various pharmacokinetic models, or using simplified semi-quantitative methods. There are numerous methods by which PET data can be analysed, and outcomes which can be reported, which differ in their accuracy, stability and specificity. The quantification of PET data, as well as the statistical procedures used to test clinical hypotheses, leads to conclusions which may differ in their degree of correctness. This thesis explores themes of reliability, replicability and reproducibility for PET research. Reliability concerns the consistency and accuracy of an outcome for distinguishing between individuals. Replicability concerns the accuracy of research conclusions, and whether they can be obtained again using the same procedures in new studies. Reproducibility concerns steps towards increasing the transparency of data analysis, by recording and sharing the exact procedures used to arrive at the conclusions. Across the studies in this thesis, these themes are detailed, expanded upon, and used in a series of methodological and applied clinical PET studies. In Study I, the performance of surface-based methods for normalisation and smoothing of PET data were compared with volumetric methods for exploratory parametric analysis using PET test-retest data measuring [11C]SCH23390 BPND in cortical regions. We replicated previous results of decreased spread, and showed that these methods also show improved test-retest repeatability. In Study II, using the same data we evaluated the performance of post-reconstruction movement correction as well as automatic and manual methods for delineation of regions of interest. We showed that motion correction improves the reliability and repeatability of binding estimates, and that automatic methods for delineation do not perform less well than manual methods, and appear to be more consistent. Study III evaluated the test-retest performance of simplified ratio-based outcome measures for quantification of translocator protein (TSPO) binding using [11C]PBR28. We showed that these methods exhibit poor reliability, and little to no association with the gold-standard outcome measure VT, suggesting that caution is warranted for interpretation of studies making use of these measures. In Study IV, diurnal and seasonal changes in the availability of the serotonin 1A receptor and the serotonin transporter were measured across the day and year in a large sample of healthy controls. We replicated previous findings of seasonal changes in the availability of the serotonin 1A receptor, failed to replicate findings of seasonal changes in the availability of the serotonin transporter, and additionally showed diurnal changes in both targets. In Study V, the importance of reliability is discussed with reference to study design, and a new method is presented for making approximations of the reliability for new samples. This approach allows researchers to more effectively gauge the feasibility of new between-individual studies before collection of any data, and to focus their efforts on research questions which can be expected to yield more interpretable outcomes. In Study VI, we perform a direction replication of a previous finding of a strong association between the Self-Transcendence scale of the Temperament and Character Inventory using a much larger sample to assess the veracity of the original findings. We showed moderate to strong evidence for no effect relative to the the previous results, suggesting that the original results were more likely to be either a false positive or greatly overestimated. In Study VII, we carried out an individual-participant data meta-analysis of TSPO binding measured using second-generation tracers in healthy controls compared with schizophrenia and psychotic disorder patients. Contrary to the original hypothesis of increases in TSPO binding, we showed strong evidence for decreases in TSPO in patients compared to controls in both cortical and subcortical regions. In Studies VIII and IX, we hypothesised that D1 receptor binding would be higher with increasing proneness to develop psychosis and in the early stages of the disease prior to medication exposure, respectively. In Study VIII, we showed convergent evidence of no association between D1 receptor availability and delusional ideation in healthy controls. In Study IX, contrary to our hypotheses, we found moderate evidence in favour of lower levels of D1 receptor availability in the dorsolateral prefrontal cortex of first-episode drug-naive psychosis patients compared to healthy controls. While reliability and replicablity of previous findings were directly assessed, the theme of reproducibility concerned our sharing the analysis code, and the data where possible, such that all analysis steps including those which could not be adequately described in the papers were recorded to ensure transparency, and demonstrate the correctness of our conclusions. All of the three themes of the thesis concern efforts to improve the quality, robustness, and utility of scientific research. For a field such as PET imaging, which is not only resource-intensive, but also requires exposing participants to harmful radiation, it is especially important both from a scientific as well as an ethical perspective that data are processed and analysed in a manner which is transparent, generalisable and optimal such that they are made use of to their full potential

Institutional capacity and multiple conditionality in ACP-EU development cooperation.

The negotiations for the Mid-Term Review of the Fourth Lome Convention marked the introduction of multiple conditionality - economic adjustment and good governance - into the Lome relationship. It placed additional demands on the two parties, giving rise to the essential concern of this thesis: do the two sides possess the requisite institutional capacity to meet those demands. The introduction was not a sudden development. The origins of multiple conditionality lay in the Pisani Memorandum and its proposal of policy dialogue. The path from the proposal to multiple conditionality was assisted by developments within the Lome relationship, including the unintended effects of ACP initiatives. This thesis is thus, in one sense, the history of the Memorandum's legacy of inverted conditionality through policy dialogue. It is also an analysis of the capacities generated by the Convention and their applicability to multiple conditionality. My analysis of bargaining, operational and instrumental capacities demonstrates a weak ACP capacity and an asymmetrically greater EU capacity. My initial conclusion is that the EU is much more capable of meeting the demands of multiple conditionality. However, it too faces limits on its capacity, especially in dealing with the sociopolitical aspects of governance. This recognition highlights an ignored factor: there is a second legacy of the Pisani Memorandum. In addition to the instrument of policy dialogue, the Memorandum identified institutional capacity as the means to help overcome the problems of development. The new tale of two legacies illustrates an EU emphasis on policy control at the expense of capacity building. It has failed to perceive the importance of the link, in the Pisani Memorandum, between the instrument and the means. It forces me to amend my initial conclusion: neither side is adequately prepared for the demands of multiple conditionality

Comparisons of the radial distributions of core-collapse supernovae with those of young and old stellar populations

We present observational constraints on the nature of core-collapse supernovae through an investigation into their radial distributions with respect to those of young and old stellar populations within their host galaxies, as traced by H-alpha emission and R-band light respectively. We discuss results and the implications they have on the nature of supernova progenitors, for a sample of 177 core-collapse supernovae. We find that the radial positions of the overall core-collapse population closely follow the radial distribution of H-alpha emission, implying that both are excellent tracers of star formation within galaxies. Within this overall distribution we find that there is a central deficit of SNII which is offset by a central excess of SNIb/c. This implies a strong metallicity dependence on the relative production of the two types, with SNIb/c arising from higher metallicity progenitors than SNII. Separating the SNIb/c into individual classes we find that a trend emerges in terms of progenitor metallicity going from SNII through SNIb to SNIc, with the latter arising from the highest metallicity progenitors.Comment: Accepted for publication in MNRA

Charge pair dissociation and recombination dynamics in a P3HT–PC60BM bulk heterojunction

The authors thank the EPSRC, SUPA, the Condensed Matter Doctoral Training Centre and the European Union Seventh Framework programme (grant agreement 321305) for financial support.The mechanism by which Coulombically bound charge pairs dissociate into free carriers in photovoltaic donor–acceptor blends is of great interest. Here, we use polarization-sensitive transient absorption (TA) to study the diffusion of photogenerated holes in a polythiophene (P3HT)–fullerene (PC60BM) blend. We observe an initial anisotropy value of 0.4 for the absorption of photogenerated holes, indicating that holes generated on a 100 fs time scale are localized on the same polymer chain as their precursor excitons. Depolarization dynamics indicate fast initial hole motion on a 0.3 ps time scale and slower migration up to 100 ps. Charge pair recombination is found to occur on a much longer time scale of 10–1000 ns via a purely bimolecular process independent of excess energy. Our results show that nearly all charge pairs get separated by at least 7 nm in the absence of an external field and indicate that high charge mobility is crucial for charge separation.PostprintPeer reviewe