The effect of thyroid hormones on purine biosynthesis was studied in vitro. With 100,000 x g supernatant fraction of rat liver homogenate, the results suggested that thyroid hormones stimulated total purine synthesis from labeled glycine-1-¹⁴C. Further studies indicated that these hormones stimulated AMP synthesis but inhibited GMP synthesis. The stimulatory effect on AMP synthesis was found to be due to the stimulation of adenylosuccinate synthetase. Adenylosuccinate synthetase was isolated and purified from rat liver. The maximum stimulatory effect of these hormones occurred with 2.5 x 10⁻⁵ M thyroxine (T₄) and 2.5 x 10⁻⁹ M triiodo-L-thyronine (T₃). A slight increase or decrease in concentration of these hormones caused a drastic decrease in their stimulatory effect. Some analogues of T₄ were also studied and results of such experiments agreed qualitatively with their effects in vivo. Those which are physiologically active are capable of stimulating this enzyme and those which are physiologically inactive had little or no effect on this enzyme. Based on these results, a hypothesis, that thyroid hormones regulate the levels of AMP and GMP synthesis, was proposed. The significance of this preferential stimulatory effect on AMP synthesis was discussed. / Ph. D.

M. S.

M.S.

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<p>The currently-favoured cosmological paradigm, Lamda-CDM, predicts that galaxies are built up from smaller galaxies in a bottom-up process known as hierarchical merging. Lambda-CDM is extremely successful for large-scale structures, but is less so for the detailed features of individual galaxies. We can study these features - the galaxies’ foundations and the remnants of the smaller components that built them - only in the closest galaxies in which we can resolve individual stars. In this thesis, we use data from the Canada-France-Hawaii Telescope (CFHT)/MegaCam as part of the Pan-Andromeda Archaeological Survey (PAndAS) to observe M33 (the Triangulum Galaxy) and the detailed features of its old stellar population. The study of these details is vital for our understanding of galaxy formation and evolution. We search for two types of components within the old stellar population: globular star clusters and the faint, diffuse stellar halo. We find only one new unambiguous outer halo star cluster, in addition to the five previously known in the M33 outer halo (r=10-50 kpc). A further 2440 cluster candidates are identified, which we analyse using two different types of simulated clusters. We are able to describe the type of clusters that are likely to remain hidden from our searches. Our study of a population of red giant branch (RGB) stars far from the M33 disk reveals a low-luminosity, centrally concentrated component which we interpret as the discovery of M33’s halo. It is everywhere in our data fainter than mu_V ~ 33 mag arcsec^−2, with scale length r_exp ~ 20 kpc, an overall luminosity not more than a few percent of the total luminosity of M33, and is possibly also not azimuthally symmetric. For M33 to have so few outer halo clusters compared to M31 and to have such a low-luminosity halo, with the possible asymmetry that we see, suggests tidal stripping of M33’s halo components by M31 - a view that is also favoured by the morphology of the disk substructure and recent modelling.</p> / Doctor of Philosophy (PhD)

Malgré leur rareté et leur vie éphémère, les étoiles massives, par l’entremise de leur forte émission de rayonnements ultraviolet et de leurs vents stellaires importants, ont un fort impact sur l’enrichissement du milieu interstellaire en éléments lourds, ainsi que sur son ionisation, sa structure et sa cinématique. L’étude des étoiles massives et de leur perte de masse est essentielle pour mieux comprendre l’évolution stellaire. En particulier en fin de vie, lors de la phase Wolf-Rayet, on constate parfois la présence d’une nébuleuse sous forme de bulle autour de ces étoiles. Dans le cadre de cette étude, nous analysons la présence et les caractéristiques de ces bulles autour d’étoiles Wolf-Rayet dans la galaxie M33, avec les données de SITELLE, un spectromètre imageur installé au télescope Canada-France-Hawaii. Sur les 211 étoiles Wolf-Rayet connues dans cette galaxie, nous avons pu en observer 178 et détecter la présence de bulles autour de 43 d’entre elles grâce à des critères que nous avons déterminés à partir de nos données. Nous avons analysé chacune d’entre elles et nous présentons notre analyse et les résultats obtenus. Nous remarquons notamment l’absence de biais favorisant la présence d’une bulle autour d’un type spectral par rapport aux autres, et la présence de bulles intriquées autour de certaines étoiles WR pouvant attester de l’évolution des vents stellaires durant les différentes phases de la vie de l’étoile. / Despite their scarcity and their short lifetime, massive stars have a strong impact on the enrichment of the interstellar medium via their heavy element production, ultraviolet radiation and stellar wind emission. Hence, the study of massive stars and their mass-loss history is essential for the understanding of stellar evolution. Especially at the end of their lives, during the Wolf-Rayet stage, we often observe bubble-like nebulae surrounding massive stars. In this study, we analyze the presence and characteristics of these Wolf-Rayet bubbles in M33, with data from SITELLE, an instrument mounted on the Canada-France-Hawaii telescope. Of the 211 known Wolf-Rayet stars in M33, we were able to observe 178 of them and we detected the presence of a bubble around 43, according to the criteria we adopted based on our data. We present here the analysis and the results of our study. We find notably that the presence of bubbles doesn’t seem to be correlated with the spectral type of the progenitor star, and we observe a structure of nested bubbles around some of the WR stars, enlightening us on the evolution of stellar winds during the different phases of a massive star’s life.

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