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Dust grains are a crucial component of disks around young stellar systems where current observations and theory show that planets form. Dust grains must grow 10 orders of magnitude in size to become planets. However, one of the early steps in this growth phase faces stringent theoretical constraints. The metre barrier relates to two well-studied physical mechanisms which inhibit grain growth beyond centimetre sizes. We report on numerical studies which probe these early stages of planet formation including instabilities that promote dust concentration such as the streaming instability (SI). We explore several different SPH models for dusty gas evolution. We find the linear SI is difficult for SPH to capture because it begins with perturbations below the 1% level. We also employ the Athena 3rd order Eulerian code which has been used to study the SI in the linear phase and the non-linear or saturated phase. We present numerical confirmations of recent analytical predictions of enhancements to the SI growth rates caused by the dust settling to the disk midplane in the earliest stages of the protoplanetary disk evolution. Symmetric analytical predictions for SI growth are not directly relevant to the non-axisymetric, planar geometry of the saturated, non-linear phase. We lay the ground work to explore this in future work. / Thesis / Master of Science (MSc)

Professional identity is defined as the internalized values of a profession as a representation of the self, and is formed through a process of socialization, or how a student learns to become a member of their profession. As medical students transition to residency, new social environments, clinical experiences, and curricular emphases can impact how they identify as professionals. The purpose of this thesis is to investigate how professional identity formation (PIF) occurs in surgical residents over their first year of postgraduate training. Twenty-four surgical residents were interviewed at the start of their postgraduate training. Questions explored participants’ understanding of what it means to be a medical professional. Six months later, residents completed a follow-up interview to investigate how their experiences in their training programs have influenced their professional identity. Thematic analysis was utilized to identify themes in responses. Central to participants’ understanding of their professional identity was their relationships with patients and the public, other healthcare professionals, their training program, and their own expectations for themselves. After six months, less emphasis was placed on the influence of the public and one’s own personal expectations. Factors such as patient encounters, a team-based work environment, time constraints and high volumes of work, and being prepared for all situations were all identified as crucial to professional development. It is important for medical education programs to consider their impact on the development of students, especially regarding the transition to competency-based medical education (CBME) that is currently occurring in medical education. This change could impact the social environment and formal curricula of these programs. The explicit study of PIF is important not only for students as they develop into independent health professionals, but also to ensure the proper care of the patients these surgeons will be working with. / Thesis / Master of Science (MSc)

no / In three experiments we investigated the dynamics of impression formation when perceivers encounter unsurprising (e.g. male mechanic) versus surprising (e.g. female mechanic) social category conjunctions. In Experiment 1, participants took longer to form an impression of targets described using a surprising versus an unsurprising conjunction of categorizations. In Experiment 2, we investigated the stages during which impressions of category conjunctions are formed. While unsurprising category combinations were characterized with reference to ‘constituent’ stereotypic traits, surprising combinations were characterized initially by stereotypic traits but later by ‘emergent’ impressions. In Experiment 3, we investigated motivational states that drive the dynamics of category conjunction. We found that higher Personal Need for Structure (PNS) predicted the use of more emergent and fewer constituent attributes in the impressions formed of surprising combinations. Across all three experiments, more ‘causal attributes’ were used in descriptions of the surprising combination. We discuss the implications of these findings for developing a model of the dynamics and composition of social category conjunctions.

In this dissertation the linearized dynamics and stability of a two-craft Coulomb tether formation are investigated. With a Coulomb tether the relative distance between two satellites is controlled using electrostatic Coulomb forces. A charge feedback law is introduced to stabilize the relative distance between the satellites to a constant value. Compared to previous Coulomb thrusting research, this is the first feedback control law that stabilizes a particular formation shape. The two craft are connected by an electrostatic virtual tether that essentially acts as a long, slender near-rigid body. Inter-spacecraft Coulomb forces cannot influence the inertial angular momentum of this formation. However, the differential gravitational attraction can be exploited to stabilize the attitude of this Coulomb tether formation about an orbit nadir direction. Stabilizing the separation distance will also stabilize the in-plane rotation angle, while the out-of-plane rotational motion remains unaffected. The other two relative equilibriums of the charged 2-craft problem are along the orbit-normal and the along-track direction. Unlike the charged 2-craft formation scenario aligned along the orbit radial direction, a feedback control law using inter-spacecraft electrostatic Coulomb forces and the differential gravitational accelerations is not sufficient to stabilize the Coulomb tether length and the formation attitude. Therefore, hybrid feedback control laws are presented which combine conventional thrusters and Coulomb forces. The Coulomb force feedback requires measurements of separation distance error and error rate, while the thruster feedback is in terms of Euler angles and their rates. This hybrid feedback control is designed to asymptotically stabilize the satellite formation shape and attitude while avoiding plume impingement issues. The relative distance between the two satellites can be increased or decreased using electrostatic Coulomb forces. The linear dynamics and stability analysis of such reconfiguration are studied for all the three equilibrium. The Coulomb tether expansion and contraction rates affect the stability of the structure and limits on these rates are discussed using the linearized time-varying dynamical models. These limits allow the reference length time histories to be designed while ensuring linear stability of the virtual structure. Throughout this dissertation the Coulomb tether is modeled as a massless, elastic component and, a point charge model is used to describe the charged craft. / Ph. D.

The fossiliferous Yorktown Formation (Pliocene) of Virginia was used as a natural laboratory for testing predictions of ecological and evolutionary theories. Specifically, coordinated stasis and ecological locking models have testable elements that can be analyzed using data from the Yorktown Formation. The ecological locking model requires that species within an ecosystem have strong interactions in order to stabilize morphologies of multiple lineages over millions of years. Species intereactions that are strong enough to do this should also be strong enough to be a major ordering force on the composition of paleocommunities. Single and replicate samples were taken from 30 cm stratigraphic intervals within the Rushmere and Morgart's Beach Members at several localities. A total of 142 samples were collected from 5 localites, which yielded 29,000 specimens belonging to 140 species of bivalves, gastropods, and other taxonomic groups. Principle components analysis, ANOVA, MANOVA, and other analyses were used to test the occurence and recurrence of local paleocommunities, paleocommunities, and paleocommunity types. Three paleocommunity types which occured under specific paleoenvironmental conditions were defined: rubbly bottom, transitional, and muddy bottom. Within a single locality samples from the same paleocommunity type yielded very similar faunal compositions, based on the relative abundance of the contained species. However, samples from the same paleocommunity type but different localities displayed low similarity values. This is consistant with local paleoenvironmental control of paleocommunity composition being more important than strong species interactions. The pattern predicted by the model of ecologic locking is absent from these Yorktown paleocommunities. A guild analysis was performed on the data to test whether the same types of organisms recurred in a predictable fashion under similar paleoenvironmental conditions. While the guild structure of the rubbly bottom paleocommunity type did recur at several localities, the guild structure of the other paleocommunity types varied greatly from place to place. / Ph. D.

Mémoire numérisé par la Direction des bibliothèques de l'Université de Montréal.