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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Spectral Matching for Elemental Abundances of Evolved Stars of Globular Clusters

Simpson, Jeffrey January 2013 (has links)
In order to understand the origin of globular clusters, large samples of their stars need to be observed and analyzed for their chemical composition. This is especially true for the complex, multimetallic cluster ω Centauri, with its large range of iron, carbon, nitrogen, oxygen, sodium and barium abundances. In order to accomplish this, an automated spectral matching pipeline was developed to determine these abundances. This thesis made use of photometry and low resolution spectroscopy to analyze the chemical composition of evolved stars in three clusters: ω Cen, 47 Tuc and NGC 6752. The latter two clusters are monometallic and selected due to their similar metallicities to the metal-rich and metal-poor stars in ω Cen. This allowed them to be used as test-cases for the spectral matching pipeline. For ω Cen, two analyses were performed. In the first, 221 giant branch stars were selected that had known [O/Fe]. These stars showed the expected anticorrelation in [C/Fe]to [N/Fe]. In the second, spectral indices were used to estimate the oxygen abundance of the stars, leading to a determination of whether a particular star was oxygen-rich or oxygen-poor. From this a catalogue of abundances of iron, carbon and barium of 848 giant branch stars were determined, of which 557 also had well-defined nitrogen abundances. k-means clustering analysis was used to group the stars in ω Cen into four homogeneous groups based upon these abundances. These groups suggest that there were at least four main periods of star formation in the cluster. The exact order of these star formation events is not yet understood, with some models predicting the groups formed from iron-poorest to iron-richest, while others suggest the potential for iron-poorer groups to form after iron-rich groups. These results compare well with those found from higher resolution studies and show the value of more extensive lower resolution spectral surveys. They also highlight the need for large samples of stars when working with a complex object like ω Cen.
2

Probabilistic Constraints on the Mass and Composition of Proxima b

Bixel, Alex, Apai, Dániel 21 February 2017 (has links)
Recent studies regarding the habitability, observability, and possible orbital evolution of the indirectly detected exoplanet Proxima b have mostly assumed a planet with M similar to 1.3 M-circle plus, a rocky composition, and an Earth-like atmosphere or none at all. In order to assess these assumptions, we use previous studies of the radii, masses, and compositions of super-Earth exoplanets to probabilistically constrain the mass and radius of Proxima. b, assuming an isotropic inclination probability distribution. We find it is similar to 90% likely that the planet's density is consistent with a rocky composition; conversely, it is at least 10% likely that the planet has a significant amount of ice or an H/He envelope. If the planet does have a rocky composition, then we find expectation values and 95% confidence intervals of < M >(rocky) = 1.63(-0.72)(+1.66) M-circle plus for its mass and < R >(rocky) = 1.07(-0.31)(+0.38) R-circle plus for its radius.
3

Analysis of three close eclipsing binary systems : BP Velorum, V392 Carinae and V752 Centauri : a thesis submitted in partial fulfilment of the requirements for the degree of Master of Science, University of Canterbury /

Schumacher, Hana Josephine. January 2008 (has links)
Thesis (M. Sc.)--University of Canterbury, 2008. / Typescript (photocopy). Includes bibliographical references (p. 66-68).
4

Estimates of Fractional Habitability for Proxima Centauri b using a 3D GCM

Sparrman, Viktor January 2020 (has links)
Exoplanet discovery has grown more quickly in recent years. However, the nature of their discovery leaves many unanswered in questions regarding exoplanetary habitability. Proxima Centauri b, an exoplanet which orbits the Sun's closest stellar neighbour, Proxima Centauri, was recently discovered with a subzero equilibrium temperature. Although not considered habitable based on the classical definition of the liquid water range, there may be fractions of Proxima Centauri b which are habitable. A prior study simulated the climate conditions of Proxima Centauri b until equilibrium was reached, using a variety of initial conditions. In this project, various metrics for calculating the fractional habitability of Proxima Centauri b are presented and applied to the results of the prior study's simulations. Colormaps are used to show the ice and temperature distributions that produce the calculated values of fractional habitability. The fractional habitabilities calculated show that while the value is both case and metric dependent, for the vast majority of all cases and metrics the value is nonzero implying that Proxima Centauri b is likely to have habitable regions. / Upptäckandet av exoplaneter har ökat i takt över de senaste åren. Samtidigt, på grund av sättet som de upptäcks finns många obesvarade frågor angående planeternas beboelighet. Proxima Centauri b är en exoplanet som kretsar kring solens närmsta granne, Proxima Centauri. Exoplaneten upptäcktes nyligen med en jämviktstemperatur under $0\degree$C. Trots att exoplaneten inte anses beboelig enligt klassisk definition kan det finnas delar av Proxima Centauri b som är beboeliga. En tidigare studie simulerade klimatförhållandena av Proxima Centarui b till jämvikt nåddes, med varierade begynnelsetillstånd. I detta projekt beräknas andelen av Proxima Centauri b som är beboelig genom flera olika mått för "fractional habitability". Måtten jämförs med den tidigare studien och dess simuleringar. Grafiskt åsikdligörs resultaten via färgkartor över planeten för istjocklek och yttemperatur. De beräknade värdena på Proxima Centauri b's "fractional habitability" påvisar beroende på mått och begynnelsetillstånd. Däremot, för en majoritet av både fall och mått är värdet nollskilt vilket antyder att Proxima Centauri b är delvist beboelig.
5

The HST large programme on omega Centauri - I. Multiple stellar populations at the bottom of the main sequence probed in NIR-Optical

Milone, A. P., Marino, A. F., Bedin, L. R., Anderson, J., Apai, D., Bellini, A., Bergeron, P., Burgasser, A. J., Dotter, A., Rees, J. M. 07 1900 (has links)
As part of a large investigation with Hubble Space Telescope to study the faintest stars within the globular cluster omega Centauri, in this work we present early results on the multiplicity of its main sequence (MS) stars, based on deep optical and near-infrared observations. By using appropriate colour-magnitude diagrams, we have identified, for the first time, the two main stellar Populations I and II along the entire MS, from the turn-off towards the hydrogen-burning limit. We have compared the observations with suitable synthetic spectra of MS stars and conclude that the two main sequences (MSs) are consistent with stellar populations with different metallicity, helium and light-element abundance. Specifically, MS-I corresponds to a metal-poor stellar population ([Fe/H] similar to -1.7) with Y similar to 0.25 and [O/Fe] similar to 0.30. The MS-II hosts helium-rich (Y similar to 0.37-0.40) stars with metallicity ranging from [Fe/H] similar to -1.7 to -1.4. Below the MS knee (m(F160W) similar to 19.5), our photometry reveals that each of the two main MSs hosts stellar subpopulations with different oxygen abundances, with very O-poor stars ([O/Fe] similar to -0.5) populating the MS-II. Such a complexity has never been observed in previous studies of M-dwarfs in globular clusters. A few months before the launch of the James Webb Space Telescope, these results demonstrate the power of optical and near-infrared photometry in the study of multiple stellar populations in globular clusters.
6

Modèles d'atmosphères hors-ETL avec métaux : applications aux étoiles sous-naines chaudes

Latour, Marilyn 04 1900 (has links)
No description available.
7

Premiers résultats de l'expérience HESS et étude du potentiel de détection de matière noire supersymétrique

Guy, Julien 20 May 2003 (has links) (PDF)
Le réseau d'imageurs Cherenkov atmosphériques HESS (High Energy Stereoscopic System) est présenté. La simulation des gerbes atmosphériques et de l'instrument est détaillée et comparée aux données obtenues pendant les six premiers mois de fonctionnement. Après la présentation des méthodes d'étalonnage et d'extraction du signal gamma, ces techniques sont appliquées aux données prises sur la nébuleuse du Crabe. On obtient un signal significatif permettant d'estimer un flux différentiel à 1 TeV en accord avec les résultats obtenus par d'autres expériences dans le même domaine en énergie. Ces résultats sont exploités pour estimer le potentiel de détection de HESS au signal gamma d'annihilation de neutralinos en provenance du centre galactique et de l'amas globulaire Omega du Centaure. Dans cette étude, une modélisation des halos de matière noire est confrontée aux observations astrophysiques.

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