Desorption From Interstellar Ices

The desorption of molecular species from ice mantles back into the gas phase in molecular clouds results from a variety of very poorly understood processes. We have investigated three mechanisms; desorption resulting from H_2 formation on grains, direct cosmic ray heating and cosmic ray induced photodesorption. Whilst qualitative differences exist between these processes (essentially deriving from the assumptions concerning the species-selectivity of the desorption and the assumed threshold adsorption energies, E_t) all three processes are found to be potentially very significant in dark cloud conditions. It is therefore important that all three mechanisms should be considered in studies of molecular clouds in which freeze-out and desorption are believed to be important. Employing a chemical model of a typical static molecular core and using likely estimates for the quantum yields of the three processes we find that desorption by H_2 formation probably dominates over the other two mechanisms. However, the physics of the desorption processes and the nature of the dust grains and ice mantles are very poorly constrained. We therefore conclude that the best approach is to set empirical constraints on the desorption, based on observed molecular depletions - rather than try to establish the desorption efficiencies from purely theoretical considerations. Applying this method to one such object (L1689B) yields upper limits to the desorption efficiencies that are consistent with our understanding of these mechanisms.Comment: 11 pages, 5 figures, accepted by MNRAS subject to minor revision which has been carried ou

Laboratory studies of the formation of molecular hydrogen on surfaces at cryogenic temperatures.

The interstellar medium (ISM) is the region of space between the stars, where star and planet formation occurs. Molecular hydrogen in the ISM initiates all the chemistry that occurs in these regions and without it, no stars or planets could form. The molecules formed by the chemistry initiated by H2 provide cooling mechanisms for the huge molecular clouds which collapse to form stars. It is well known that the abundance of molecular hydrogen in the ISM is too high for it to form through gas phase processes alone and hence, the widely accepted theory is that H2 forms through heterogeneous catalysis on the surfaces of interstellar dust grains. These grains make up approximately 1% of the mass of the ISM and are thought to be carbonaceous or silicate in nature. Despite its importance, only recently have laboratory experiments been set-up to study the formation of molecular hydrogen on interstellar grain analogues in detail. This thesis presents results from an experiment designed to determine what happens to the energy released on the formation of H2, under conditions similar to those of the ISM. The experiment involves using an atom source, ultrahigh vacuum chamber and cryogenic cooling methods in order to reach the temperatures and pressures of interstellar space. The laser technique of Resonance Enhanced Multiphoton Ionisation (REMPI) is employed to look at the internal energy distribution of newly formed H2 and HD molecules from a graphite surface. Improvements to an existing experiment are described, including the introduction of a second atom source to study HD formation. New results are presented, including the first observations of molecular hydrogen formed ro-vibrationally excited in states v" = 1 and v" = 2. The results are placed in context with the results of other experiments to form H2 under ISM conditions, and the astrophysical implications are discussed

Cutting the threads of patchwork policy : the impact of decentralization on pandemic containment in Nova Scotia and British Columbia

1 online resource (41 pages)Includes abstract.Includes bibliographical references (pages 37-41).This thesis explores the topic of Canadian federalism and decentralization in the context of the COVID-19 pandemic. Federalism has shaped Canadian healthcare and over time has led to discrepancies in health policy and administration in terms of the distribution of federal, provincial, and territorial powers, and the institutional design of healthcare that varies across jurisdictions. The emergence of the COVID-19 pandemic has thrust public health to the forefront of policy at all levels, placing tension on Canada's already fragmented healthcare system. These tensions are analyzed further through a comparative case study of the provinces of Nova Scotia and British Columbia to demonstrate how the historic federal, provincial, territorial divide has impacted provincial containment of COVID-19 during the first wave of the pandemic (January 2020 - September 2020). A brief history of Canadian federalism is given in section one, followed by an assessment of the strict public health measures that are necessary to effectively contain the virus in section two, and lastly section three contains a case study of the provinces of Nova Scotia and British Columbia to analyze how these provinces were able to effectively manage the spread of the virus in the first wave. As their shared success began to diminish in the second wave of the pandemic, this thesis argues that a bottom-up, pan-Canadian health strategy could foster continued collaboration between the federal, provincial, and territorial governments, through the establishment of documented best practices to encourage the implementation of the public health measures needed to contain the virus. For it is in times like these; when entire healthcare systems across the country are called to action, that our "patchwork" model of healthcare governance manifests its true weakness and highlights the need for change

Investigating the Effects of the MathemAntics Number Line Activity on Children's Number Sense

Number sense, which can broadly thought of as the ability to quickly understand, approximate, and manipulate numerical quantities, can be a difficult construct for researchers to operationally define for empirical study. Regardless, many researchers agree it plays an important role in the development of the symbolic number system, which requires children to master many tasks such as counting, indentifying numerals, comparing magnitudes, transforming numbers and performing operations, estimating, and detecting number patterns, skills which are predictive of later math achievement. The number line is a powerful model of symbolic number consistent with researchers' hypotheses concerning the mental representation of number. The MathemAntics Number Line Activity (MANL) transforms the number line into a virtual manipulative, encourages estimation, provides multiple attempts, feedback, and scaffolding, and introduces a novel features where the user can define his own level of risk on the number line. The aim of the present study was to examine how these key features of MANL are best implemented to promote number sense in low-income second-graders. Sixty-six students from three schools were randomly assigned to one of three conditions; MANL User-Defined Range (UDR), and MANL Fixed Range (FR), and a Reading comparison condition and underwent a pretest session, four computer sessions, and a posttest session. During the computer sessions, researchers coded a child's observed strategy in placing targets on the number line. The results showed that children with higher number sense ability at pretest performed better on a posttest number line estimation measure when they were in the UDR condition than in the FR condition. Conversely, children with low number sense ability at pretest performed better on the number line estimation posttest measure when they were in the FR condition than UDR. Although in general, all children improved over time, children with low number sense ability at pretest were more likely to use the UDR tool ineffectively, thus negatively impacting performance. When children were not coded as responding quickly, target number significantly impacted performance in the computer sessions. Finally, children in the UDR condition utilized better expressed strategies on the number line estimation posttest than children in the Reading comparison group. These findings indicate that prior number sense ability plays a role in how children engage with MANL, which in turn affects the learning benefits the child receives. Implications for researchers, software designers, and math educators, as well as limitations are discussed

Laboratory evidence for the non-detection of excited nascent H2 in dark clouds

There has always been a great deal of interest in the formation of H2 as well as in the binding energy released upon its formation on the surface of dust grains. The present work aims at collecting experimental evidence for how the bond energy budget of H2 is distributed between the reaction site and the internal energy of the molecule. So far, the non-detection of excited nascent H2 in dense quiescent clouds could be a sign that either predictions of emission line intensities are not correct or the de-excitation of the newly formed molecules proceeds rapidly on the grain surface itself. In this letter we present experimental evidence that interstellar molecular hydrogen is formed and then rapidly de-excited on the surface of porous water ice mantles. In addition, although we detect ro-vibrationally excited nascent molecules desorbing from a bare non-porous (compact) water ice film, we demonstrate that the amount of excited nascent hydrogen molecules is significantly reduced no matter the morphology of the water ice substrate at 10 K (both on non-porous and on porous water ice) in a regime of high molecular coverage as is the case in dark molecular clouds.Comment: 15 pages, 3 figures, to be published in MNRA

Molecular excitation in the Interstellar Medium: recent advances in collisional, radiative and chemical processes

We review the different excitation processes in the interstellar mediumComment: Accepted in Chem. Re

Investigating the Effects of the MathemAntics Number Line Activity on Children's Number Sense

Number sense, which can broadly thought of as the ability to quickly understand, approximate, and manipulate numerical quantities, can be a difficult construct for researchers to operationally define for empirical study. Regardless, many researchers agree it plays an important role in the development of the symbolic number system, which requires children to master many tasks such as counting, indentifying numerals, comparing magnitudes, transforming numbers and performing operations, estimating, and detecting number patterns, skills which are predictive of later math achievement. The number line is a powerful model of symbolic number consistent with researchers' hypotheses concerning the mental representation of number. The MathemAntics Number Line Activity (MANL) transforms the number line into a virtual manipulative, encourages estimation, provides multiple attempts, feedback, and scaffolding, and introduces a novel features where the user can define his own level of risk on the number line. The aim of the present study was to examine how these key features of MANL are best implemented to promote number sense in low-income second-graders. Sixty-six students from three schools were randomly assigned to one of three conditions; MANL User-Defined Range (UDR), and MANL Fixed Range (FR), and a Reading comparison condition and underwent a pretest session, four computer sessions, and a posttest session. During the computer sessions, researchers coded a child's observed strategy in placing targets on the number line. The results showed that children with higher number sense ability at pretest performed better on a posttest number line estimation measure when they were in the UDR condition than in the FR condition. Conversely, children with low number sense ability at pretest performed better on the number line estimation posttest measure when they were in the FR condition than UDR. Although in general, all children improved over time, children with low number sense ability at pretest were more likely to use the UDR tool ineffectively, thus negatively impacting performance. When children were not coded as responding quickly, target number significantly impacted performance in the computer sessions. Finally, children in the UDR condition utilized better expressed strategies on the number line estimation posttest than children in the Reading comparison group. These findings indicate that prior number sense ability plays a role in how children engage with MANL, which in turn affects the learning benefits the child receives. Implications for researchers, software designers, and math educators, as well as limitations are discussed

Color Me This: An Investigation of a Priming Manipulation in the DCCS in Pre-School Aged Children

The Dimensional Change Card Sorting (DCCS) task is commonly used to investigate children’s executive function abilities. In this task, participants are first asked to sort cards by one dimension (e.g., shape) and then switch to sorting by a different dimension (e.g., color). Three-year-old children can sort the cards by the first dimension, however cannot successfully switch to the new dimension. The current project aimed at reducing perseveration errors using a priming manipulation between phases of the task. Results indicated that engaging in a coloring activity between phases for as little as two minutes was enough to help children successfully switch to sorting by the dimension of color; however contrary to previous literature, a two minute break alone with no priming activity was also enough to eliminate perseveration