Type Ia supernovae (SNe Ia) are excellent tools in cosmology. Their intrinsic luminosities are found to vary systematically with the light-curve widths, providing an empirical calibration. This property, called the width-luminosity relation (WLR), is the basis of modern SN Ia cosmology and led to the unexpected discovery of the current accelerated rate of cosmic expansion. By examining the spectroscopic diversity of SNe Ia, this thesis aims to improve both the use of SNe Ia in cosmology and the physical understanding of the observed properties. Spectra of SNe Ia contain a wealth of information, but are difficult to organize. In this thesis, new methods are developed to consistently quantify and analyze the spectral features of supernovae. The efficacy of the methods is tested on a large library of observed spectra encompassing a wide range of properties. The spectroscopic diversity of SNe Ia enters cosmology through K-correction calculations. Before this work, K-correction was a major contributor of the systematic errors in cosmology. It is shown here that the systematic errors can be largely diminished by carefully quantifying the mean spectroscopic properties of SNe Ia. The remaining statistical errors are also quantified and shown to be redshift dependent. With the aid of principal component analysis (PCA), the multidimensional spectral information is reduced to a few components describing the largest variations in the spectral library. Using this tool, it is shown here that SN Ia intrinsic luminosity is the main driver of the spectroscopic diversity at maximum light, for every spectral feature from the ultraviolet to the near-infrared. These spectroscopic sequences can potentially account for a large fraction of the K-correction statistical errors and even enable the use of SN Ia spectra as independent indicators of intrinsic luminosity and colors. The established relations will also disentangle the effects of demographic shift and true evolution in high-redshift SN Ia spectra. The temporal evolution of the spectral features is shown to exhibit the persistence of the spectroscopic sequences throughout other epochs. The effect is attributed to the more rapid spectroscopic temporal evolution of fainter SNe Ia. This conclusion supports the theory that WLR is primarily a spectroscopic effect, rather than a bolometric one.

L'expérience H.E.S.S. est un réseau de télescope Tcherenkov, situé en Namibie, qui observe le rayonnement de sources astrophysiques de rayons gamma de très haute énergie. Depuis sa mise en service il y a plus de dix ans, H.E.S.S. a permis d'améliorer considérablement notre vision de l'univers à ces énergies. Depuis 2012, le démarrage de la deuxième phase de l'expérience avec la mise en service d'un cinquième télescope permet de baisser le seuil en énergie de l'expérience.L'optimisation d'une analyse multivariée au sein de l'expérience H.E.S.S. est présentée dans cette thèse. L'analyse Xeff a été amélioré en prenant en compte les conditions d'observations, et en introduisant de nouvelles variables dans l'estimateur. Un gain de sensibilité de l'analyse par rapport aux analyses standards est observé. Cette méthode a été ensuite utilisée pour diverses analyses de sources de rayons gamma.La prise en compte des effets de la focalisation de l'analyse de données de H.E.S.S. II est détaillée. L'ajustement de la distance entre la caméra et les miroir du cinquième télescope permet d'améliorer l'image dans le plan focal. L'amélioration de la reconstruction et le gain en sensibilité qui en découle est présenté.L'étude du rayonnement gamma de quatre candidats d'associations de vestiges de supernova et de nuages moléculaires est présentée : G349.7+0.2, W51, la nébuleuse de la Tornade, et HESS J1745-303. Une interprétation de ce rayonnement utilisant également les données de Fermi-LAT permet d'estimer l'origine dominante des mécanismes d'émission et d'appréhender l'efficacité d'accélération de particules par ces systèmes.Les travaux présentés dans cette thèse ont fait l'objet de deux notes internes à la collaboration H.E.S.S., de multiples présentations dans des conférences internationales et de publications scientifiques. / The H.E.S.S. experiment is an array of Cherenkov telescopes, located in Namibia, observing the very high energy gamma rays from astrophysical sources. H.E.S.S. greatly increased our understanding of the very high energy non thermal universe. Since 2012, a fifth telescope was installed at the center of the array. This improvement increase the energy range and the sensitivity of the detector.The optimization of a multivariate analysis method, within the H.E.S.S. framework, is presented in this thesis. The Xeff analysis is improved by taking into account the observation conditions and by increasing the number of discriminating variables. The sensitivity of this analysis compared to standard analyses is demonstrated. Xeff is then used to analyze several sources of gamma rays.The effects of the focus impacting the H.E.S.S. II data are described. They are taken into account in the analysis and an optimization of the distance between the mirrors and the camera is presented.The study of the gamma emission from four candidates of molecular clouds and supernova remnant associations is presented. The H.E.S.S. data from G349.7+0.2, W51, the Tornado nebula and HESS J1745-303 is performed. The interpretation of their emission, using also Fermi-LAT data, leads to the estimation of the particles acceleration efficiency in these objects.The work described in this thesis led to the production of two internal notes in the H.E.S.S. collaboration, to several presentations in international conferences, and scientific publications.

A supernova (SN) is the explosive destruction of a star. Via a luminous outpouring of radiation, the SN can rival the brightness of its SN host galaxy for months or years. In the past decade, astronomical surveys regularly observing the sky to deep limiting magnitudes have revealed that core collapse SNe (the demises of massive stars) are sometimes preceded by eruptive episodes by the progenitor stars during the years before the eventual SN explosion. Such SNe tend to show strong signatures of interaction between the SN ejecta and the circumstellar medium (CSM) deposited by the star before the SN explosion, likely by mass-loss episodes like the ones we have started to observe regularly. The complex CSM resolved around certain giant stars in our own galaxy and the eruptions of giant stars like η Car in the 19th century can be seen in this context. As the SN ejecta of an interacting SN sweep up the CSM of the progenitor, radiation from this process offers observers opportunity to scan the late mass loss history of the progenitor. In this thesis, interacting SNe and eruptive mass loss of their progenitors is discussed. The SN iPTF13z (discovered by the intermediate Palomar Transient Factory, iPTF) is presented. This transient was followed with optical photometry and spectroscopy during 1000 days and displayed a light curve with several conspicuous re-brigthenings ("bumps"), likely arising from SN ejecta interacting with denser regions in the CSM. Around 200 days before discovery, in archival data we found a clear precursor outburst lasting >~ 50 days. A well-observed (but not necessarily well understood) event like SN 2009ip, which showed both precursor outbursts and a light curve bump, makes an interesting comparison object. The embedding of the (possible) SN in a CSM makes it hard to tell if a destructive SN explosion actually happened. In this respect, iPTF13z is compared to e.g. SN 2009ip but also to long-lived interacting SNe like SN 1988Z. Some suggestions for future investigations are offered, to tie light curve bumps to precursor events and to clarify the question of core collapse in the ambiguous cases of some interacting SNe.

Stripped-envelope supernovae were thought to be the explosions of very massive stars (& 20 M) that lost their outer layers of hydrogen and/or helium in strong stellar winds. However, recent studies have highlighted that most stripped-envelope supernovae seem to be arising from rela- tively lower-mass progenitor stars in the 12 20 M(sun) range, creating a mystery about the fate of the higher-mass stars. In this licentiate thesis, we review our knowledge of stripped-envelope supernovae, and present the astrophysical problem of their missing high-mass progenitors. The thesis focuses on observations of unique and rare stripped-envelope supernovae classified with modern optical surveys such as the intermediate Palomar Transient Factory (iPTF) and the Public European Southern Observatory Spectroscopic Survey of Transient Objects (PESSTO). In these surveys we have discovered stripped-envelope supernovae with long-lasting broad lightcurves, which are thought to be a marker for highly massive (& 20 M[sun]) progenitor stars. Despite this exciting association, there are only a handful of existing examples of stripped- envelope supernovae with broad lightcurves published in the literature, not numerous enough to account for the missing high-mass stars. During our efforts, the first object we focused on was OGLE-2014-SN-131, a long-lasting supernova in the southern sky initially classified by PESSTO. We re-classified it as a supernova Type Ibn interacting with a helium-rich circumstellar environment. Unlike all other Type Ibn’s in the literature, OGLE-2014-SN-131 was found to have a long rise-time and large lightcurve broadness. By modeling its bolometric lightcurve, we concluded that OGLE-2014-SN-131 must have had an unusually massive progenitor star. Furthermore, since an ordinary radioactive- decay model could not reproduce the lightcurve, we investigated both a magnetar and circum- stellar interaction as potential powering scenarios and favored the latter due to the signatures of interaction present in the spectra. Next, we looked for similar objects in the supernova dataset of the iPTF, which contains over 200 stripped-envelope supernovae. Searching in a sub-sample of 100 well-observed supernovae, we identified 11 to have unusually broad lightcurves. We also constrained the distribution of lightcurve broadness for iPTF stripped-envelope supernovae. The 11 with broad lightcurves will be studied carefully in a forthcoming paper. The first part of this forthcoming paper, which describes the careful statistical identification of these super-novae, is included in this thesis. In it we identify that 10% of the iPTF stripped-envelope supernova sample have broad lightcurves, which a surprisingly high fraction given their rarity in the published literature. Finally, we evaluate whether our estimate of the fraction of broad stripped-envelope supernovae could help explain the missing high-mass progenitors, and con- clude that they can only be a small fraction of the missing high-mass progenitors.

<p>The effect of stellar feedback on the interstellar medium is investigated using numerical simulation. In particular, the roles of supernova feedback and ionization feedback on the star formation rate and structure of the interstellar medium are compared. We use Enzo, an adaptive mesh code, and employ the MUSCL-Hancock hydrodynamics scheme to run simulations of a section of a stratified galactic disk. A turbulent velocity field is imposed in the central region of the disk and self-gravity is applied. Star clusters are formed when density and temperature conditions are met, which, in turn, provide ionization and supernova feedback into the interstellar medium. Simulations were run with and without supernova and ionization feedback and the runs are compared. Ionization feedback is found to dominate over supernova feedback in regulating star formation rates. With no feedback, all the gas is converted to stars by 200 Myr. When supernova feedback is added, 98% of the gas is used to create stars by 300 Myr. With ionization feedback instead, at 1 Gyr into the run, only 30% of the gas is in stars. Even with supernova feedback added to ionization feedback, the gas converted to stars is just 29% at 1 Gyr. Very strong supernovae take this fraction down to 25%. The star formation rates in the runs with supernova feedback are consistent with the low end of the Kennicutt-Schmidt relation, while the runs without ionization feedback have star formation rates that are an order of magnitude larger. Gas phase masses and volumes produced in the ionization runs are broadly consistent with observations.</p> / Master of Science (MSc)

Some of the brightest and most energetic events in the Universe are associated with the death of stars. These stellar deaths power transient electromagnetic emission which are routinely observed on Earth. This dissertation presents our research on various such transients. Its topics includes, supernova remnants, kilonovae, gamma-ray bursts (GRBs): The "long'' type produced from core-collapse supernovae and the "short'' type associated with neutron star merger events. It also focuses on the disruption of stars by the tidal forces of supermassive black holes i.e., tidal disruption events (TDEs). We model the emission from these transients and compare them to observations in order to draw a number of conclusions and make predictions for future detections. For example, we find that the non-thermal emission from supernovae and kilonovae associated with GRBs can produce long term emission which may be detected as a re-brightening in the overall emission. The sharp cut off observed in some TDE flares can be caused by a pre-existing accretion disk present around a supermassive black hole, which is expected in active galactic nuclei. Our work successfully predicted the nature of the very first electromagnetic detection from a neutron star merger, and was able to reproduce the emission that had been observed for more than one hundred days after the merger. This dissertation also provides frameworks on how the observable features of these transients can be leveraged to probe the properties of the progenitor system and their environment. <br>

NIL

Cette thèse comprend deux parties relativement distinctes. Dans la première, nous présentons l'analyse de 16 millions de courbes de lumière collectées par EROS II vers différentes longitudes dans le plan Galactique durant 7 saisons d'observation. Après avoir crée des images de référence et des courbes de lumière, nous avons recherché des effets de microlentille gravitationnelle parmi les courbes de lumière produites en appliquant une analyse discriminante, tout en calculant notre efficacité de détection sur un lot de courbes de lumière simulées. Nous avons mis en évidence 24 candidats, ce qui nous a permis de mesurer la profondeur optique vers nos cibles. La profondeur optique moyenne vers les bras spiraux est $<\tau_(GSA)> = 0.38\pm0.08$. Notre résultat est compatible avec les prédictions de modèles Galactiques tirés de la littérature, dans lesquels le bulbe central est modélisé par une barre, et favorise ceux sans matière cachée dans le disque. Outre la statistique, la principale limi tation dans notre analyse provient du fait que les distances où sont situées les étoiles cibles sont mal connues.<br> Dans la seconde partie, nous présentons l'analyse des données spectroscopiques, collectées durant une campagne internationale de recherche de supernovae Ia menée au printemps 1999, à laquelle EROS II a participé. Une vingtaine de supernovae Ia ont été découvertes puis suivies pendant cette campagne. Nous avons réduit les données spectroscopiques les concernant, qui consistent en une centaine de spectres au total. Nous avons developpé pour celà un programme original permettant de séparer le flux de la supernovae de celui de sa galaxie hôte. Nous avons ensuite procédé à une étude de stan dardisations sur un lot regroupant des objets de la campagne 1999 et des objets découverts antérieuement. L'étude sur ces 17 supernovae Ia a permis de confirmer l'interêt des spectres pour la standardis ation. Elle montre en particulier que la standardisation à partir de spectres est aussi efficace que celle basée sur le taux de décroissance des courbes de lumière.

Le vide, de part sa nature et le rôle qu'il joue dans la description des phénomènes de infiniment petit comme ceux de infiniment grand, sert de fil conducteur pour introduire les différents travaux expérimentaux présentés dans ce manuscrit. En partant de infiniment petit, la préparation à la recherche de nouvelle physique au-delà du modèle standard de physique des particules est exposée avec le prototypage d'un calorimètre à argon liquide pour l'expérience ATLAS. Puis la recherche de supersymétrie avec violation de la R-parité auprès du détecteur ALEPH est présentée. Le modèle standard de la cosmologie est ensuite détaillé avec l'apparition de la mystérieuse énergie noire qui est à l'origine de l'accélération actuelle de notre univers. Cette accélération peut être mesurée grâce aux observations de supernovæ du sondage SNLS qui est présenté avec ses différentes phases : recherche des supernovæs puis mesure des paramètres cosmologiques et du taux d'explosion des supernovæ de type Ia.

Un pic de 60Fe récemment observé dans une croûte ferromanganésienne a été interprété comme la trace de l'explosion d'une supernova qui aurait explosé, suffisamment proche du système solaire pour qu'elle puisse y déposer son empreinte sur la Terre, il y a 2.8 ° 0.4 millions d'années (Myr). Pour confirmer cette interprétation, d'une part avec une meilleure résolution temporelle, d'autre part en ayant accès à la mesure simultanée d'autres isotopes dans différentes phases géochimiques, les sédiments marins sont un outil particulièrement adapté. Ce travail a pour objectif de rechercher dans les sédiments marins les résidus de cette supernova sous forme d'anomalies isotopiques. Les trois principaux noyaux auxquels nous nous sommes intéressés sont l'129I, le 60Fe, et l'26Al, tous trois mesurés sous forme de rapports isotopiques par Spectrométrie de Masse par Accélérateur (SMA). Quantifier le flux des noyaux correspondant à la signature de cette supernova permettrait de mieux contraindre les modèles théoriques de nucléosynthèse stellaire. Ces résidus sont des isotopes produits par une séquence de combustions hydrostatiques pendant l'évolution stellaire et/ou par nucléosynthèse explosive lors de l'explosion de la supernova. Les conditions qui règnent au moment de l'explosion (température, densité de neutrons) permettent de penser que les supernovae pourraient être le site astrophysique du processus r.<br />L'étude en 129I a montré que la mesure de rapports 129I/127I pré-anthropogéniques nécessite un contrôle très strict des différentes sources possibles de contaminations en 129I, et ce d'autant plus lorsque l'on travaille avec de petites quantités d'iode (quelques microgrammes). Cette étude a permis de révéler un écart de plusieurs ordres de grandeurs entre la valeur pré-anthropogénique théorique et expérimentale du rapport 129I/127I dans le milieu marin. Les mesures SMA en 60Fe et 26Al ont permis d'établir, dans la phase authigène des sédiments marins, l'absence de signal en 60Fe sur la zone temporelle correspondant au signal observé dans la croûte Fe-Mn (2.4-3.2 Myr), ainsi qu'en 26Al de 2.6 à 3.2 Myr.