The nature of the edge-on eclipsing binary Epsilon Aurigae remains perplexing, despite notable progress since the recent 2009-2011 eclipse. The binary involves an early F supergiant with a still unknown companion enshrouded in a disk. Although the eclipse geometry produces a significant broad band polarization signature, semiregular pulsations of the F supergiant are also a source of variable polarization, with an amplitude that is commensurate with the effect of the eclipse. This fact makes use of the polarization for studying the disk of the companion far more challenging. In an effort to better understand the pulsation nature of the supergiant, we explore a simple model for the stellar contribution to the polarization signal. The model does reasonably well in characterizing the gross properties of the time-variable polarization.

Co-rotating Interaction Regions (CIRs) are a well-known phenomenon in the solar wind, and is a favored culprit for certain cyclical behavior observed in the spectra of some massive stars. A prime example are the discrete absorption components (DACs) seen in the UV wind lines of many O stars. Here we report on modeling for the variable continuum polarization that could arise from the presence of CIR structures. Considerations are limited to optically thin scattering. Using a core-halo approach for winds that are thick to electron scattering, an application to observed variable polarization of WR6 (EZ CMa; HD 50896) is presented.

Among the different types of massive stars in advanced evolutionary stages is the enigmatic WN8h type. There are only a few Wolf–Rayet (WR) stars with this spectral type in our Galaxy. It has long been suggested that WN8h-type stars are the products of binary evolution that may harbor neutron stars (NS). One of the most intriguing WN8h stars is the runaway WR 124 surrounded by its magnificent nebula M1-67. We test the presence of an accreting NS companion in WR 124 using ~100 ks long observations by the Chandra X-ray observatory. The hard X-ray emission from WR 124 with a luminosity of L X ~ 1031 erg s−1 is marginally detected. We use the non-local thermodynamic equilibrium stellar atmosphere code PoWR to estimate the WR wind opacity to the X-rays. The wind of a WN8-type star is effectively opaque for X-rays, hence the low X-ray luminosity of WR 124 does not rule out the presence of an embedded compact object. We suggest that, in general, high-opacity WR winds could prevent X-ray detections of embedded NS, and be an explanation for the apparent lack of WR+NS systems.

We utilize a Poisson likelihood in a maximum likelihood statistical analysis to analyze X-ray spectragraphic data. Specifically, we examine four extragalactic supernova remnants (SNR). IKT 5 (SNR 0047-73.5), IKT 25 (SNR 0104-72.3), and DEM S 128 (SNR 0103-72.4) which are designated as Type Ia in the literature due to their spectra and morphology. This is troublesome because of their asymmetry, a trait not usually associated with young Type Ia remnants. We present \emph{Chandra X-ray Observatory} data on these three remnants, and perform a maximum likelihood analysis on their spectra. We find that the X-ray emission is dominated by interactions with the interstellar medium. In spite of this, we find a significant Fe overabundance in all three remnants. Through examination of radio, optical, and infrared data, we conclude that these three remnants are likely not "classical" Type Ia SNR, but may be examples of so-called "prompt" Type Ia SNR. We detect potential point sources that may be members of the progenitor systems of both DEM S 128 and IKT 5, which could suggest a new subclass of prompt Type Ia SNR, Fe-rich CC remnants. In addition, we examine IKT 18. This remnant is positionally coincident with the X-ray point source HD 5980. Due to an outburst in 1994, in which its brightness changed by 3 magnitudes (corrsponding to an increase in luminosity by a factor of 16) HD 5980 was classified as a luminous blue variable star. We examine this point source and the remnant IKT 18 in the X-ray, and find that its non-thermal photon index has decreased from 2002 to 2013, corresponding to a larger proportion of more energetic X-rays, which is unexpected.

La formation des étoiles massives reste, à ce jour, encore mal connue à cause de l'extrême quantité d'énergie que ces étoiles dégagent, limitant en conséquence leurs masses théoriques et contredisant les observations de ce type d'étoile. Les observatoires du futur (en particulier l'observatoire spatial Herschel) vont tenter de répondre à cette problématique grâce notamment aux émissions moléculaires de l'eau. L'analyse précise et correcte de ces données, dans l'avenir, nécessite donc dès aujourd'hui un travail associant des observations et des modélisations des objets concernés. C'est dans ce but que cette thèse a consisté en l'élaboration d'une méthode de modélisation dite « globale » d'objets proto-stellaires massifs (proto-amas ou cœurs denses massifs). Celle-ci a permis une description physique et une étude chimique des multiples cœurs denses massifs étudiées, et a ouvert de nombreuses voies vers des aspects évolutifs. Elle a également donné des indices pour affiner le programme d'observation en temps garanti WISH des raies moléculaires de l'eau et confirmé le rôle clef de cette molécule pour la compréhension de la formation des étoiles massives.

Je présente et j'analyse dans ma thèse des observations de l'émission dans l'infrarouge proche de transitions rovibrationelles de H2 dans des régions de formation stellaire. Le sujet principal de ce travail concerne de nouvelles observations du nuage moléculaire d'Orion (OMC1) et en particulier de la région BN-KL. Les données sont constituées d'images des raies individuelles de H2 obtenues à haute résolution spatiale avec le Telescope Canada-France-Hawaii et avec l'ESO VLT. Grâce à la haute résolution spatiale du VLT il est possible d'analyser en détail (jusq'à 60 UA ~0.''13) des objets individuels dans cette région. De plus, j'ai analysé l'émission de H2 et [FeII] dans des écoulements (« outflows ») présents dans deux nuages sombres (les globules de Bok BHR71 et BHR137) ainsi que dans un « blob » à haute excitation dans le grand nuage de Magellan (N159-5). Ici les données sont constituées de spectres en fente longue obtenus à l'ESO-VLT. Pour réaliser ce travail j'ai tout d'abord calculé une grille complète de modèles de chocs composée de ~25 000 simulations (correspondant à 400 Go, environ). Ces modèles qui sont les plus récents comportent un grand nombre de paramètres libres qui peuvent être ajustés. Une grande partie demon travail a été d'analyser les résultats de cette grille avant de les mettre en ligne. En effet les résultats ne sont pas tous crédibles, et il m'a donc fallu de développer des méthodes pour les vérifier. Mais avec une bonne compréhension du modèle et un solide sens de la physique des chocs, il est maintenant assez facile d'interpréter les données sur H2 et [FeII]. Les modèles me permettent ensuite de prédire les conditions physiques à grande échelle dans OMC1, par exemple la densité, la vitesse des chocs, l'intensité du champ magnétique, etc. En général la densité du milieu avant le choc est ~105-107 cm-3 et la vitesse de choc est dans la gamme 10-40 km.s-1. Un autre résultat très intéressant de mon travail est le développement d'une nouvelle méthode pour analyser les chocs en arc (« bow shocks ») observés à une haute résolution spatiale. Pour un choc en arc isolé je prédis une vitesse de choc de ~50 km.s-1 et une densité avant le choc de 5×105 cm-3. La vitesse 3D a été mesurée très récemment à 55 km.s-1. Cela donne une confirmation indépendante de nos résultats.

Spectroscopic observations of cold and dense interstellar clouds show the presence of "dirty ice" mantles on dust grains, mainly composed by water molecules. These ices are enriched by the presence of other simple species that are either formed by surface reactions or accreted from the gas phase. While there is quite a general consensus that interstellar water ice is mainly amorphous, its morphology (porous or compact) still remains poorly known. Morphology is important due to its influence both on the catalytic efficiency of grain surfaces and on the release to the grain of the fraction of the formation energy of species, as shown by laboratory simulations of molecular hydrogen formation. Ice porosity may be identified through the weak infrared absorption features (~ 2.7 μm) showing the presence of dangling bonds on the pore surface. To our knowledge, there has been to date no detection of such absorptions in the infrared spectra of interstellar ices, perhaps suggesting that they may have a compact nature. It has been already investigated that interstellar porous ice may be compacted by the transient heating of stellar radiation and cosmic ray bombardment. In this thesis I report an experimental work, performed using FORMOLISM (the experimental apparatus at the University of Cergy-Pontoise - France), that shows relevant changes in the ice morphology following atomic hydrogen exposure. In particular, it is shown that a thin highly porous ice film is gradually changed into a more compact structure. This is probably due to the transient heating caused by the energy released to the ice during H2 formation. Such a process may also produce in the interstellar space compact amorphous ice mantles concurrently with the other envisaged processes. Moreover, I have experimentally analyzed the morphology of the just formed water ice. Analysing one of the possible mechanism of water formation (the pathway H + O2) under conditions mimicking those found in a molecular cloud, we have found that the water just synthesized has a non-porous structure. Indeed, the layers of water formed in this way show the kinetic characteristics typical of a compact (non-porous) ice, as for instance the D2 TPD peak position.

Thesis (M.Sc. (Physics))--North-West University, Potchefstroom Campus, 2008.

Active Galactic Nuclei (AGN) are powered by gas accretion onto supermassive Black Holes (BH). The luminosity of AGN can exceed the integrated luminosity of their host galaxies by orders of magnitude, which are then classified as Quasi-Stellar Objects (QSOs). Some mechanisms are needed to trigger the nuclear activity in galaxies and to feed the nuclei with gas. Among several possibilities, such as gravitational interactions, bar instabilities, and smooth gas accretion from the environment, the dominant process has yet to be identified. Feedback from AGN may be important an important ingredient of the evolution of galaxies. However, the details of this coupling between AGN and their host galaxies remain unclear. In this work we aim to investigate the connection between the AGN and their host galaxies by studying the properties of the extendend ionised gas around AGN. Our study is based on observations of ~50 luminous, low-redshift (z<0.3) QSOs using the novel technique of integral field spectroscopy that combines imaging and spectroscopy. After spatially separating the emission of AGN-ionised gas from HII regions, ionised solely by recently formed massive stars, we demonstrate that the specific star formation rates in several disc-dominated AGN hosts are consistent with those of normal star forming galaxies, while others display no detectable star formation activity. Whether the star formation has been actively suppressed in those particular host galaxies by the AGN, or their gas content is intrinsically low, remains an open question. By studying the kinematics of the ionised gas, we find evidence for non-gravitational motions and outflows on kpc scales only in a few objects. The gas kinematics in the majority of objects however indicate a gravitational origin. It suggests that the importance of AGN feedback may have been overrated in theoretical works, at least at low redshifts. The [OIII] line is the strongest optical emission line for AGN-ionised gas, which can be extended over several kpc scales, usually called the Narrow-Line Region (NLR). We perform a systematic investigation of the NLR size and determine a NLR size-luminosity relation that is consistent with the scenario of a constant ionisation parameter throughout the NLR. We show that previous narrow-band imaging with the Hubble Space Telescope underestimated the NLR size by a factor of >2 and that the continuum AGN luminosity is better correlated with the NLR size than the [OIII] luminosity. These affects may account for the different NLR size-luminosity relations reported in previous studies. On the other hand, we do not detect extended NLRs around all QSOs, and demonstrate that the detection of extended NLRs goes along with radio emission. We employ emission line ratios as a diagnostic for the abundance of heavy elements in the gas, i.e. its metallicity, and find that the radial metallicity gradients are always flatter than in inactive disc-dominated galaxies. This can be interpreted as evidence for radial gas flows from the outskirts of these galaxies to the nucleus. Recent or ongoing galaxy interactions are likely responsible for this effect and may turn out to be a common prerequisite for QSO activity. The metallicity of bulge-dominated hosts are systematically lower than their disc-dominated counterparts, which we interpret as evidence for minor mergers, supported by our detailed study of the bulge-dominated host of the luminous QSO HE 1029-1401, or smooth gas accretion from the environment. In this line another new discovery is that HE 2158-0107 at z=0.218 is the most metal poor luminous QSO ever observed. Together with a large (30kpc) extended structure of low metallicity ionised gas, we propose smooth cold gas accretion as the most likely scenario. Theoretical studies suggested that this process is much more important at earlier epochs of the universe, so that HE 2158-0107 might be an ideal laboratory to study this mechanism of galaxy and BH growth at low redshift more detailed in the furture. / Aktive Galaxienkerne (AGN) entstehen durch die Akkretion von Gas auf massive Schwarze Löcher, welche im Zentrum jeder Galaxie mit einer spherodialen Komponente vermutet werden. Die Leuchtkraft eines AGN kann die seiner gesamten Muttergalaxie um Größenordnungen übersteigen. In diesem Fall werden AGN oft als Quasi-Stellare Objekte (Quasare) bezeichnet. Spezielle Mechanismen müssen für das Auslösen dieser Kernaktivität in Galaxien verantwortlich sein. Verschiedene Prozesse wurden bereits identifiziert, aber der entscheidende Mechanismus wurde bisher noch nicht entdeckt. Die Wechselwirkung mit einem AGN könnte außerdem einen entscheidenden Einfluss auf die Entwicklung von Galaxien haben. Es ist noch unklar wie diese Wechselwirkung genau abläuft und ob es die Sternentstehung in Galaxien beeinflusst. In dieser Arbeit studieren wir die Eigenschaften des ausgedehnten ionisierten Gases in AGN-Muttergalaxien, um mögliche Wechselwirkungen zu untersuchen. Wir benutzen dazu eine Stichprobe von ~50 Quasaren bei geringer Rotverschiebung (z<0.3), die mit der neuartigen Technik der Integralfeld-Spektroskopie beobachtet wurden. Diese Technik kombiniert bildgebende und spektroskopische Verfahren. Wir können mit unserer Analyse zeigen, dass die spezifische Sternentstehungsrate in einigen Scheiben-dominierten AGN-Muttergalaxien vergleichbar mit denen von normalen Galaxien ohne Kernaktivität ist. Allerdings können wir in einigen AGN-Muttergalaxien keine Anzeichen von Sternentstehung feststellen. Ob Sternentstehung in diesen Galaxien momentan durch die Wechselwirkung mit dem AGN unterdrückt wird, ist daher nicht eindeutig. Hinweise auf Gasausflüsse liefert die Kinematik des ionisierten Gases für einige wenige Objekte, doch die Kinematik für die meisten AGN-Muttergalaxien kann allein durch das Wirken der Gravitation erklärt werden. Daraus schließen wir, dass der Einfluss von AGN auf ihre Muttergalaxien geringer sein könnte als theoretisch angenommen wird. Die [OIII] Emissionslinie ist die stärkste optische Linie für AGN-ionisiertes Gas und kann sich über eine Region von mehreren kpc vom Kern erstrecken, die als "Narrow-Line Region" (NLR) bezeichnet wird. Durch eine systematische Untersuchung der NLR-Ausdehnung können wir eine Beziehung zwischen NLR-Radius und AGN-Leuchtkraft bestimmen. Diese Relation ist konsistent mit einem konstanten Ionisationsparameter über die gesamte Ausdehnung der NLR. Frühere Studien mit dem Hubble Weltraumteleskop unterschätzten die Größe der NLR um mehr als einen Faktor 2. Andererseits können wir nicht für alle Quasare eine ausgedehnte NLR nachweisen, wobei eine NLR-Detektion bei einer höheren Radioleuchtkraft des Quasars wahrscheinlicher ist. Dies deutet auf eine Wechselwirkung eines Radio-Jets mit dem kernumgebenden Gas hin. Wir benutzen Emissionslinien des ionisierten Gases, um den Anteil von schweren Elementen im Gas, die so genannte Metallizität, zu bestimmen. Dabei finden wir, dass die radialen Metallizitätsgradienten in Scheiben-dominierten AGN-Muttergalaxien deutlich flacher sind als in vergleichbaren Galaxien ohne Kernaktivität, was wir als Anzeichen für radialen Gastransport vom Rand der Galaxien zum Kern interpretieren. Dies könnte durch kürzliche oder immer noch andauernde gravitative Wechselwirkungen zwischen Nachbargalaxien entstanden sein und stellt eventuell eine Voraussetzung für Kernaktivität dar. Sehr interessant ist unser Ergebnis, dass die ellptischen AGN-Muttergalaxien eine geringere Metallizität aufweisen als die Spiralgalaxien. Dies könnte z.B. durch das Verschmelzen mit kleinen Nachbargalaxien induziert werden, welche eine intrinsisch geringe Metallizität aufweisen. Am Beispiel der elliptischen Muttergalaxie des Quasars HE 1029-1401 können wir durch eine detaillierte Analyse des ionisierten Gases verschiedene Indizien für einen solchen Prozess nachweisen. Eine weiteres Resultat dieser Arbeit ist die Entdeckung eines leuchtkräftigen Quasars mit der geringsten Metallizität, die bisher für solche Objekte nachgewiesen werden konnte. Wir interpretieren die geringe Metallizität und die Ausdehnung des ionisierten Gases über 30kpc als deutliche Indizien für die Akkretion von intergalaktischem Gas. Dieser Prozess findet viel häufiger im frühen Universum statt. HE 2158-0107 könnte daher ein ideales Objekt sein, um diesen Prozess im nahen Universum detaillierter studieren zu können.

The Antennae (22 Mpc) is one of the most well-known mergers in the nearby Universe. Its distance allow us to observe and study the gas at the scales of stellar cluster formation. It is an ideal source to understand how the galaxy dynamics in mergers trigger the formation of stars. Most of the stars in the Antennae are formed in compact and massive stellar clusters, dubbed super-star clusters (SSCs). The most massive (>106 M⊙) and youngest (<6 Myr) SSCs are located in the overlap region, where the two galaxies collide, and are associated with massive (several 108 M⊙) and super-giant (few hundred of pc) molecular complexes (SGMCs). The formation of SSCs must involve a complex interplay of merger-driven gas dynamics, turbulence fed by the galaxy interaction, and dissipation of the kinetic energy of the gas. Within SGMCs, a hierarchy of structures must be produced, including dense and compact concentrations of molecular gas massive enough to form SSCs, pre-cluster clouds (PCCs). For star formation to occur, the mechanical energy of PCCs must be radiated away to allow their self-gravity to locally win over their turbulent gas pressure. Specific tracers of turbulent dissipation are therefore key inputs to test the validity of this theoretical scenario. In my thesis, I studied the Antennae overlap region. My work is based on observations with the SINFONI spectro-imager at the VLT, which includes H2 rovibrational and Brγ line emission, and with ALMA, which includes the CO(3-2) line and dust continuum emission. Both data-sets have the needed sub-arcsecond angular resolution to resolve the scales of SSC formation. The spectral resolutions are enough to resolve motions within SGMCs. Combining CO and H2 line emission is key in my PhD work. I use CO as a tracer of the distribution and kinematics of the molecular gas, and H2 as a tracer of the rate at which the gas mechanical energy is dissipated.My thesis focuses on diverse sources in the Antennae overlap region which trace different stages of star formation: the gathering of mass necessary to form SGMCs, the formation of PCCs within SGMCs and the disruption of a parent cloud by a newly formed SSC. I show that at each stage turbulence plays a key role. I found that the kinetic energy of the galaxies is not thermalized in large scale shocks, it drives the turbulence in the molecular ISM at a much higher level than what is observed in the Milky Way. Near-IR spectral diagnostics show that, outside of SSCs embedded in their parent clouds, the H2 line emission is powered by shocks and traces the dissipation of the gas turbulent kinetic energy. I relate the H2 emission to the loss of kinetic energy required to form gravitationally bound clouds. This interpretation is supported by the discovery of a compact, bright H2 source not associated with any known SSC. It has the largest H2/CO emission ratio and is located where the data show the largest velocity gradient in the interaction region. To our knowledge, this is the first time that an extragalactic source with such characteristics is identified. We would be witnessing the formation of a cloud massive enough to form a SSC. The data also allow us to study the disruption of a parent molecular cloud by an embedded SSC. Its matter is loosely bound and its gravity would be supported by turbulence, which makes it easier for feedback to disrupt the parent cloud. I end my manuscript presenting two projects. I propose to establish additional energy dissipation tracers observable with ALMA, which gives us the high spatial and spectral resolution needed to isolate scales at which clusters form. This is a Cycle 1 proposal accepted in first priority. I also plan to expand my work to other nearby extragalactic sources by investigating the turbulence-driven formation of stars in different extragalactic sources by combining near-IR and submillimeter observations.