In this work, the effect of rotation on the oscillation frequencies of some radially and nonradially oscillating Delta-Scuti stars have been explored. Rotation has been considered as a perturbation and treated up to the second order. Series of evolutionary models have been calculated for the oscillating stars in question and compared with the observational parameters. Three stars are considered: V350 Peg with no rotation, CC And with a rotational velocity Vsini=20 km/s, and BS Tuc with Vsini=130 km/s. We find that splitting in the oscillation frequencies are conspicuous especially in fast rotating stars, with a considerable contribution from the related terms due to second order effect.

We search for planets and brown dwarves around white dwarves (WDs). Finding extra-solar planets is the first step toward establishing the existence and abundance of life in the Universe. The low mass and luminosity of WDs make them ideal stars to search for low mass companion objects. Theoretical predictions generally agree that a star will consume and destroy close-in, low mass planets as it ascends the red giant and asymptotic giant branch evolutionary tracks, but larger mass objects and those further out will survive. The matter ejected from the star as it evolves into a white dwarf may also be accreted onto daughter planets, or may coalesce into a disk from which planets can form. We employ two techniques to search for planets and brown dwarves (BDs) around WDs. A subset of pulsating white dwarf stars have a pulsational stability that rivals pulsars and atomic clocks. When a planet is in orbit around a such a star the orbital motion of the star around the centre of mass is detectable as a change in arrival times of the otherwise stable pulsations. We search for, and find, a sample of suitable pulsators, monitor them for between three and four years, and place limits on companions by constraining the variation consistent with a 2.4M[subscript J] planet in a 4.6 year orbit. We also observe a large sample of WDs to search for a mid-infrared excess caused by the presence of sub-stellar companions. We present evidence for a potential binary system consisting of a WD and a BD on the basis of an observed excess flux at near and mind-infrared wavelengths. We also place limits on the presence of planetary mass companions around those stars and compare our results to predictions of planetary survival theories. Our findings do not support suggestions of planet formation or accretion of extra mass during stellar death.

In this study, vibrational properties and stability of delta-Scuti like pulsating pre-main sequence stars have been investigated. Studies were held in the mass range 2-4 Mo and limited to radial linear adiabatic pulsations. Numerical computations were performed by the oscillation program written by Kirbiyik &amp / Al-Murad (1993). The models were selected to be at the latest phases of the pre-main sequence evolution where the luminosity starts to increase. We have limited our calculations upto the end of the radiative inner regions, since at the surface of the star, our adiabatic perturbation computation does not perfectly fit to the relatively thin non-adiabatic convective envelope of the star. The results of the stability analysis showed that the PMS models undergo an instability whose time period is a function of mass. Instability Strip of pulsating PMS stars was re-drawn with comparison to M. Marconi &amp / F.Palla (1998). The effect of gravitational contraction on stability was also investigated.

Thesis (Ph. D.)--University of Canterbury, 2008. / Typescript (photocopy). Includes bibliographical references (p. 190-195). Also available via the World Wide Web.

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

No description available.

Environ 10% des étoiles de type spectral O, B ou A ont un champ magnétique fort, détectable, stable et à grande échelle à leur surface, qui ressemble le plus souvent à un dipôle. Des modèles théoriques et des simulations numériques prédisent ces champs magnétiques vus en surface pénètrent aussi dans les zones radiatives et influencent la structure interne. Les modèles prédisent que ces champs magnétiques imposent une rotation uniforme dans les zones radiatives et peuvent supprimer la pénétration convective autour du cœur. Cela a des conséquences sur l’évolution de ces étoiles chaudes magnétiques. Pour ce faire, l’astérosismologie est la meilleure méthode car les paramètres des pulsations stellaires sont directement liés aux conditions physiques internes. Plusieurs types de pulsations stellaires sont connus et classés en fonction de leur force de rappel. Parmi eux, les plus à même de sonder les régions proches du cœur des étoiles, sur lequel se concentre notre intérêt dans cette thèse sont les modes de gravité, qui sont gouvernés par la force d’Archimède.Notre premier objectif était d’identifier des étoiles chaudes, pulsantes et magnétiques et de caractériser leurs propriétés magnétiques et sismiques. Des étoiles ont été sélectionnées grâce à des diagnostics observationnels indirects de la présence d’un champ magnétique qou nous confirmons grâce à de la spectropolarimétrie optique à haute résolution obtenue avec ESPaDOnS, Narval et ESPaDOnS. Pour deux étoiles magnétiques connues, HD43317 et o Lup, nous avons caractérisé la géométrie et l’intensité du champ magnétique aves des séries temporelles spectropolarimétriques. Pour toutes les étoiles de notre échantillon, nous avons également obtenu des séries temporelles photométriques très précises grâce aux télescopes spatiaux BRITE, CoRoT ou K2 pour étudier leur variabilité (périodique) cohérente. Seulement HD43317 a révélé des dizaines de fréquences de pulsations stellaires, pointant plutôt vers des modes de gravité.Nous nous sommes ensuite concentrés sur HD43317 dans pour déterminer observationellement la structure interne de cette étoile magnétique chaude. Nous avons fait usage de modélisation sismique: les fréquences des modes de pulsations observées dans les données CoRoT, couvrant 150j, ont été ajustées à celles des modes gravito-inertiels calculés avec le code de pulsations GYRE couplé aux modèles MESA. Nous avons pu associer les fréquences des modes de pulsations à des séries de modes (l,m) = (1,−1) et (2,−1) se chevauchant. La petite zone de pénétration convective dans la zone radiative telle que déduite du modèle MESA optimal s’avère cohérente avec les prédictions théoriques. Néanmoins, les intervalles de confiance sur certains paramètres physiques issus des modèles sont très larges et compatibles avec les valeurs de la littérature pour des étoiles chaudes et pulsantes mais non-magnétiques. Nous en concluons que la série temporelle de 150j de données CoRoT est trop courte pour déterminer d’une manière non-équivoque la structure interne des étoiles magnétiques chaudes, et par conséquent pour distinguer leur structure interne de celle des étoiles chaudes non-magnétiques.Malgré nos efforts de modélisation détaillée de la meilleure étoile chaude pulsantemagnétique HD43317, nous n’avons pas pu corroborer observationnellement les prédictions théoriques d’une structure interne altérée pour les étoiles chaudes magnétiques. Des simplifications et des approximations ont dû être faites au cours de la modélisation sismique en raison de la résolution en fréquence limitée des données CoRoT. D’autres efforts pour inclure le magnétisme dans les codes de pulsations ou le magnétisme, la rotation et le transfert du moment cinétique dans les modèles d’évolution stellaire seront nécessaires afin de déterminer si les signatures magnétiques sont présentes pour les nombreux pulsateurs gravito-inertiels récemment découverts dans la base de données de Kepler. / About ten percent of stars with spectral type O, B or A have a detectable stable strong large-scale magnetic field at their surface, which most often resembles a magnetic dipole. These large-scale magnetic fields extend into the radiative layers of the OBA stars. Theory and simulations predict that they alter the internal structure and physical properties of these stars. In particular, it is expected that these large-scale magnetic fields enforce uniform rotation in the radiative layers and may suppress convective core overshooting. This has consequences for the evolution of these magnetic hot stars and it has implications for galactic evolution. Therefore, we observed and investigated the internal structure of magnetic hot stars. To do so, asteroseismology is the best method as the oscillation properties are directly related to the internal physical conditions. Various types of stellar oscillations are known and they are classified based on their dominant restoring force. Of these, gravity modes are governed by the buoyancy force and have their strongest probing power in the near core region, which is the domain of our interest.Our first objective was to identify pulsating magnetic hot stars and characterize their magnetic and seismic properties. We constructed a sample of magnetic candidate stars, by following indirect observational diagnostics for the presence of a large-scale magnetic field, to confirm with ground-based high-resolution optical spectropolarimetry taken with ESPaDOnS, Narval or HARPSpol. For two known magnetic stars, HD43317 and o Lup, we characterized the geometry and strength of the field in detail by analysing spectropolarimetric time series. For each star in our sample, we obtained high-cadence high-precision space-based photometry from BRITE, CoRoT, or K2 to study (periodic) variability. Only HD43317 revealed tens of stellar pulsations mode frequencies that pointed towards gravity modes. Only a few other stars studied showed a few pulsation mode frequencies, unsuitable for seismic modelling.We investigated the B3.5V star HD43317 in detail to determine the internal structure of a magnetic hot star. We did this by forward seismic modelling, where observed stellar pulsation mode frequencies in the CoRoT data covering ∼150d were fit to those of gravito-intertial modes computed with the pulsation code GYRE, coupled to MESA stellar structure models. We identified the pulsation mode frequencies as overlapping (l, m) = (1,-1) and (2,-1) mode series. The small convective core overshooting region derived from the seismic modelling was in line with the theoretical predictions. Yet, some of the parameters for the best fitted models were also compatible with literature values for non-magnetic pulsators within the derived uncertainties. We conclude that the CoRoT time series of ∼150d is too short to lead to stringent constraints and tests of the stellar interior to discriminate between magnetic and non-magnetic pulsating hot stars.From our detailed modelling efforts of the best studied pulsating magnetic hot star HD43317, we were unable to observationally corroborate the theoretical predictions of an altered internal structure for magnetic hot stars. Simplifications and approximations were made during the forward seismic modelling due to the limited frequency resolution of the CoRoT data in terms of its time base. Further efforts to include magnetism in the pulsation codes, or magnetism, rotation, and angular momentum transport in the evolutionary models, are worthwhile to test whether magnetic signatures are present in the numerous (non-magnetic) gravito-inertial pulsators recently found in the nominal Kepler database (which has a ten times better frequency resolution compared to CoRoT).

No description available.

The theory of stellar evolution and stellar structure relies on the observation of stars in di erent phases of their evolutionary cycle. The relation between observations and theory can be strengthened by obtaining observational data of a large sample of stars in a particular evolutionary phase. The search for Cephei stars, as conducted in this study, can contribute to the sample of known Cephei stars, where these interesting stars are massive non-supergiant early B-type stars, displaying pulsating behaviour which is not well understood. Stars tend to form in clusters where it can therefore be expected that young massive stars can be found in open clusters. For this reason two young southern open clusters were observed in order to search for B-type pulsating stars. The region of NGC 6204 and Hogg 22 was observed over a period of thirteen nights in Johnson B, V and I bands. NGC 6204 is believed to be the oldest cluster of the two at a distance of 0.8 kpc while the much younger Hogg 22 is more distant at 2.8 kpc. These two open clusters are located 6 arcminutes apart which made it possible to observe them simultaneously with a 12.8 12.8 arcminute eld of view. The observations were done with the newly installed 1600 telescope of the North-West University, South Africa. In order to do a variability search, periodic stars need to be identi ed from the cluster data, where a typical data set may contain thousands of scienti c images. In addition to the main motivation for this study, a pipeline was created in order to automate the photometry and data reduction processes. A Lomb-Scargle transform was applied to the stellar light curves in order to identify periodic sources. 354 signi cantly periodic stars were identi ed from the 3182 observed stars. Amongst them, two new possible Cephei stars were found together with a possible slowly pulsating B star (SPB), and numerous eclipsing binary systems. By using photometry of this region obtained by Forbes & Short (1996), instrumental magnitudes were transformed to a standard system in order to compare photometry results. From the constructed colour magnitude diagram of the two clusters, it could be seen that some stars, indicated by Forbes & Short (1996) to be cluster members, were in fact eld stars belonging to neither cluster. The reduction and photometry pipeline was implemented successfully on the data set, which also highlighted the importance of instrumentation and correct data analysis procedures. Possible improvements were identi ed in order to overcome di culties experienced during this study. / Thesis (MSc (Space Physics))--North-West University, Potchefstroom Campus, 2013

The theory of stellar evolution and stellar structure relies on the observation of stars in di erent phases of their evolutionary cycle. The relation between observations and theory can be strengthened by obtaining observational data of a large sample of stars in a particular evolutionary phase. The search for Cephei stars, as conducted in this study, can contribute to the sample of known Cephei stars, where these interesting stars are massive non-supergiant early B-type stars, displaying pulsating behaviour which is not well understood. Stars tend to form in clusters where it can therefore be expected that young massive stars can be found in open clusters. For this reason two young southern open clusters were observed in order to search for B-type pulsating stars. The region of NGC 6204 and Hogg 22 was observed over a period of thirteen nights in Johnson B, V and I bands. NGC 6204 is believed to be the oldest cluster of the two at a distance of 0.8 kpc while the much younger Hogg 22 is more distant at 2.8 kpc. These two open clusters are located 6 arcminutes apart which made it possible to observe them simultaneously with a 12.8 12.8 arcminute eld of view. The observations were done with the newly installed 1600 telescope of the North-West University, South Africa. In order to do a variability search, periodic stars need to be identi ed from the cluster data, where a typical data set may contain thousands of scienti c images. In addition to the main motivation for this study, a pipeline was created in order to automate the photometry and data reduction processes. A Lomb-Scargle transform was applied to the stellar light curves in order to identify periodic sources. 354 signi cantly periodic stars were identi ed from the 3182 observed stars. Amongst them, two new possible Cephei stars were found together with a possible slowly pulsating B star (SPB), and numerous eclipsing binary systems. By using photometry of this region obtained by Forbes & Short (1996), instrumental magnitudes were transformed to a standard system in order to compare photometry results. From the constructed colour magnitude diagram of the two clusters, it could be seen that some stars, indicated by Forbes & Short (1996) to be cluster members, were in fact eld stars belonging to neither cluster. The reduction and photometry pipeline was implemented successfully on the data set, which also highlighted the importance of instrumentation and correct data analysis procedures. Possible improvements were identi ed in order to overcome di culties experienced during this study. / Thesis (MSc (Space Physics))--North-West University, Potchefstroom Campus, 2013