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(abridged) AIMS. We investigate the dynamics and stability of post-shock plasma streaming along nonuniform stellar magnetic fields at the impact region of accretion columns. We study how the magnetic field configuration and strength determine the structure, geometry, and location of the shock-heated plasma. METHODS. We model the impact of an accretion stream onto the chromosphere of a CTTS by 2D axisymmetric magnetohydrodynamic simulations. Our model takes into account the gravity, the radiative cooling, and the magnetic-field-oriented thermal conduction. RESULTS. The structure, stability, and location of the shocked plasma strongly depend on the configuration and strength of the magnetic field. For weak magnetic fields, a large component of B may develop perpendicular to the stream at the base of the accretion column, limiting the sinking of the shocked plasma into the chromosphere. An envelope of dense and cold chromospheric material may also develop around the shocked column. For strong magnetic fields, the field configuration determines the position of the shock and its stand-off height. If the field is strongly tapered close to the chromosphere, an oblique shock may form well above the stellar surface. In general, a nonuniform magnetic field makes the distribution of emission measure vs. temperature of the shocked plasma lower than in the case of uniform magnetic field. CONCLUSIONS. The initial strength and configuration of the magnetic field in the impact region of the stream are expected to influence the chromospheric absorption and, therefore, the observability of the shock-heated plasma in the X-ray band. The field strength and configuration influence also the energy balance of the shocked plasma, its emission measure at T > 1 MK being lower than expected for a uniform field. The above effects contribute in underestimating the mass accretion rates derived in the X-ray band., 11 pages, 11 Figures; accepted for publication on A&A. Version with full resolution images can be found at http://www.astropa.unipa.it/~orlando/PREPRINTS/sorlando_accretion_shocks.pdf

Context. Theoretical arguments and numerical simulations of radiative shocks produced by the impact of the accreting gas onto young stars predict quasi-periodic oscillations in the emitted radiation. However, observational data do not show evidence of such periodicity. Aims. We investigate whether physically plausible perturbations in the accretion column or in the chromosphere could disrupt the shock structure influencing the observability of the oscillatory behavior. Methods. We performed local 2D magneto-hydrodynamical simulations of an accretion shock impacting a chromosphere, taking optically thin radiation losses and thermal conduction into account. We investigated the effects of several perturbation types, such as clumps in the accretion stream or chromospheric fluctuations, and also explored a wide range of plasma-\beta values. Results. In the case of a weak magnetic field, the post-shock region shows chaotic motion and mixing, smoothing out the perturbations and retaining a global periodic signature. On the other hand, a strong magnetic field confines the plasma in flux tubes, which leads to the formation of fibrils that oscillate independently. Realistic values for the amplitude, length, and time scales of the perturbation are capable of bringing the fibril oscillations out of phase, suppressing the periodicity of the emission. Conclusions. The strength of a locally uniform magnetic field in YSO accretion shocks determines the structure of the post-shock region, namely, whether it will be somewhat homogeneous or if it will split up to form a collection of fibrils. In the second case, the size and shape of the fibrils is found to depend strongly on the plasma-\beta value but not on the perturbation type. Therefore, the actual value of the protostellar magnetic field is expected to play a critical role in the time dependence of the observable emission., Comment: Accepted for publication in A&A

Context. UV observations of some massive globular clusters have revealed a significant population of stars hotter and fainter than the hot end of the horizontal branch (HB), the so-called blue hook stars. This feature might be explained either by the late hot flasher scenario where stars experience the helium flash while on the white dwarf cooling curve or by the progeny of the helium-enriched sub-population postulated to exist in some clusters. Previous spectroscopic analyses of blue hook stars in ωCen and NGC2808 support the late hot flasher scenario, but the stars contain much less helium than expected and the predicted C and N enrichment cannot be verified. Aims. We compare the observed effective temperatures, surface gravities, helium abundances, and carbon line strengths (where detectable) of our targets stars with the predictions of the two aforementioned scenarios. Methods. Moderately high resolution spectra of hot HB stars in the globular cluster ωCen were analysed for radial velocity variations, atmospheric parameters, and abundances using LTE and non-LTE model atmospheres. Results. We find no evidence of close binaries among our target stars. All stars below 30 000 K are helium-poor and very similar to HB stars observed in that temperature range in other globular clusters. In the temperature range 30 000 K to 50 000 K, we find that 28% of our stars are helium-poor (log nHe nH < −1.6), while 72% have roughly solar or super-solar helium abundance (log nHe nH ≥ −1.5). We also find that carbon enrichment is strongly correlated with helium enrichment, with a maximum carbon enrichment of 3% by mass. Conclusions. A strong carbon enrichment in tandem with helium enrichment is predicted by the late hot flasher scenario, but not by the helium-enrichment scenario. We conclude that the helium-rich HB stars in ωCen cannot be explained solely by the heliumenrichment scenario invoked to explain the blue main sequence. peerReviewed

Advances in laser and Z-pinch technology, coupled with the development of plasma diagnostics and the availability of high-performance computers, have recently stimulated the growth of high-energy density laboratory astrophysics. In particular a number of experiments have been designed to study radiative shocks and jets with the aim of shedding new light on physical processes linked to the ejection and accretion of mass by newly born stars. Although general scaling laws are a powerful tools to link laboratory experiments with astrophysical plasmas, the phenomena modelled are often too complicated for simple scaling to remain relevant. Nevertheless, the experiments can still give important insights into the physics of astrophysical systems and can be used to provide the basic experimental validation of numerical simulations in regimes of interest to astrophysics. We will illustrate the possible links between laboratory experiments, numerical simulations and astrophysics in the context of stellar jets. First we will discuss the propagation of stellar jets in a cross-moving interstellar medium and the scaling to Z-pinch produced jets. Our second example focuses on slab-jets produced at the PALS (Prague Asterix Laser System) laser installation and their practical applications to astrophysics. Finally, we illustrate the limitations of scaling for radiative shocks, which are found at the head of the most rapid stellar jets., 30 pages, 9 figures

We have investigated the stellar and wind properties of a sample of five late-type O dwarfs in order to address the weak wind problem. A grid of TLUSTY models was used to obtain the stellar parameters, and the wind parameters were determined by using the CMFGEN code. We found that the spectra have mainly a photospheric origin. A weak wind signature is seen in CIV 1549, from where mass-loss rates consistent with previous CMFGEN results regarding O8-9V stars were obtained. A discrepancy of roughly 2 orders of magnitude is found between these mass-loss rates and the values predicted by theory (Mdot(Vink)), confirming a breakdown or a steepening of the modified wind momentum-luminosity relation at log L/Lsun < 5.2. We have estimated the carbon abundance for the stars of our sample and concluded that its uncertainty cannot cause the weak wind problem. Upper limits on Mdot were established for all objects using lines of different ions, namely, PV 1118,28, CIII 1176, NV 1239,43, Si IV 1394,03, and NIV 1718. All the values obtained are also in disagreement with theoretical predictions, bringing support to the reality of weak winds. Together with CIV 1549, the use of NV 1239,43 results in the lowest mass-loss rates: the upper limits indicate that Mdot must be less than about -1.0 dex Mdot(Vink). Regarding the other transitions, the upper limits still point to low rates: Mdot must be less than about $(-0.5 \pm 0.2)$ dex Mdot(Vink). We have studied the behavior of the Halpha line with different mass-loss rates. We have also explored ways to fit the observed spectra with Mdot(Vink). By using large amounts of X-rays, we verified that few wind emissions take place, as in weak winds. However, unrealistic X-rays luminosities had to be used (log Lx/Lbol > -3.5) (abridged)., A&A, accepted

UV observations of some massive globular clusters uncovered a significant population of very hot stars below the hot end of the horizontal branch (HB), the so-called blue hook stars. This feature might be explained either as results of the late hot flasher scenario where stars experience the helium flash while on the white dwarf cooling curve or by the progeny of the helium-enriched sub-population recently postulated to exist in some clusters. Moderately high resolution spectra of stars at the hot end of the blue HB in omega Cen were analysed for atmospheric parameters and abundances using LTE and Non-LTE model atmospheres. In the temperature range 30,000K to 50,000K we find that 35% of our stars are helium-poor (log(n_He/n_H) < -2), 51% have solar helium abundance within a factor of 3 (-1.5 -0.4). We also find carbon enrichment in step with helium enrichment, with a maximum carbon enrichment of 3% by mass. At least 14% of the hottest HB stars in omega Cen show helium abundances well above the highest predictions from the helium enrichment scenario (Y = 0.42 corresponding to log(n_He/n_H) ~ -0.74). In addition, the most helium-rich stars show strong carbon enrichment as predicted by the late hot flasher scenario. We conclude that the helium-rich HB stars in omega Cen cannot be explained solely by the helium-enrichment scenario invoked to explain the blue main sequence. (Abridged), 4 pages, 3 figures, uses aa.cls (enclosed), accepted as A&A Letter

We have constructed a comprehensive grid of 1540 metal line-blanketed, NLTE, plane-parallel, hydrostatic model atmospheres for the basic parameters appropriate to early B-type stars. The BSTAR2006 grid considers 16 values of effective temperatures, 15,000 K, 50 pages; to appear in ApJS; models available at http://nova.astro.umd.edu

International audience; We investigate the structure of the radiation field generated in high Mach number shock waves (supercritical shock waves [1]) which have a finite extension R in the direction perpendicular to the shock wave propagation. A simple analytical model allows us to calculate how the radiation field decreases with R for a medium with an homogeneous radiation emission in volume. We then perform a 3D radiative transfer calculation of the radiation field created by a stationary shock wave, and compare the predicted radiative energy density and flux with 1D calculations. We find that the variation of the radiation field with R follows the trend given by the analytical calculation. Finally, we perform radiative-hydrodynamical calculations to investigate further the variation of the radiation field with R and to estimate the decrease of the velocity of the radiative precursor withR.

An analysis of high-resolution VLT/UVES spectra of two B-type main sequence stars, NGC 346-11 and AV 304, in the Small Magellanic Cloud (SMC), has been undertaken, using the non-LTE TLUSTY model atmospheres to derive the stellar parameters and chemical compositions of each star. The chemical compositions of the two stars are in reasonable agreement. Moreover, our stellar analysis agrees well with earlier analyses of H II regions. The results derived here should be representative of the current base-line chemical composition of the SMC interstellar medium as derived from B-type stars., 10 pages, 4 figures, 1 online table

High-resolution UCLES/AAT spectra are presented for nine B-type supergiants in the SMC, chosen on the basis that they may show varying amounts of nuclear-synthetically processed material mixed to their surface. These spectra have been analysed using a new grid of approximately 12 000 non-LTE line blanketed {\sc tlusty} model atmospheres to estimate atmospheric parameters and chemical composition. The abundance estimates for O, Mg and Si are in excellent agreement with those deduced from other studies, whilst the low estimate for C may reflect the use of the \ion{C}{ii} doublet at 4267\AA. The N estimates are approximately an order of magnitude greater than those found in unevolved B-type stars or \ion{H}{ii} regions but are consistent with the other estimates in AB-type supergiants. These results have been combined with results from a unified model atmosphere analysis of UVES/VLT spectra of B-type supergiants (Trundle et al. \cite{Tru04}) to discuss the evolutionary status of these objects. For two stars that are in common with those discussed by Trundle et al., we have undertaken a careful comparison in order to try to understand the relative importance of the different uncertainties present in such analyses, including observational errors and the use of static or unified models. We find that even for these relatively luminous supergiants, {\sc tlusty} models yield atmospheric parameters and chemical compositions similar to those deduced from the unified code {\sc fastwind}., Comment: Submitted to astronomy and astrophysics

We present FUSE spectra of three He-rich sdB stars. Two of these stars, PG1544+488 and JL87, reveal extremely strong C III lines, suggesting that they have mixed triple-alpha carbon from the deep interior out to their surfaces. Using TLUSTY NLTE line-blanketed model atmospheres, we find that PG1544+488 has a surface composition of 96% He, 2% C, and 1% N. JL87 shows a similar surface enrichment of C and N but still retains a significant amount of hydrogen. In contrast, the third star, LB1766, is devoid of hydrogen and strongly depleted of carbon, indicating that its surface material has undergone CN-cycle processing. We interpret these observations with new evolutionary calculations which suggest that He-rich sdB stars with C-rich compositions arise from a delayed helium-core flash on the white-dwarf cooling curve. During such a flash the interior convection zone will penetrate into the stellar envelope, thereby mixing the envelope with the He- and C-rich core. Such "flash-mixed" stars will arrive on the extreme horizontal branch (EHB) with He- and C-rich surface compositions and will be hotter than the hottest canonical EHB stars. Two types of flash mixing are possible: "deep" and "shallow", depending on whether the hydrogen envelope is mixed deeply into the site of the helium flash or only with the outer layers of the core. Based on both their stellar parameters and surface compositions, we suggest that PG1544+488 and JL87 are examples of "deep" and "shallow" flash mixing, respectively., 4 pages, 2 figures, to appear in "Extreme Horizontal Branch Stars and Related Objects", Astrophysics and Space Science, Kluwer Academic Publishers, edited by P. Maxted

International audience; We investigate a chromospheric scenario to explain the characteristics of the far-UV emission lines of beta Pictoris, revealed by FUSE spectra of this famous circumstellar disk system. The model assumes a thin region heated up to a few 105 K located close to the stellar photosphere. The resonance lines of C III at 977 Å and O VI at 1032-1037 Å, seen in emission, are produced in this chromosphere-transition region complex. Using complementary data in the mid and near UV (specifically, a resonance doublet of C IV at 1548-1550 Å and Mg II h & k), we show that the whole dataset is remarquably well reproduced by the model. In addition, we investigate the properties of the C III* multiplet at 1176 Å and conclude that this line likely forms in a weak warm wind, originating from the prominent circumstellar disk of beta Pictoris. Finally, radiative losses have been calculated and have provided estimates of the amount of non radiative energy dissipated in the external atmosphere of beta Pictoris, which is a measure of the star's activity level. Such behavior for an A5 V star presents a challenge for both evolution and activity models since the former predict that main-sequence A stars should not be active, while the latter are unable to quantitatively account for the characteristics of the chromospheric heating of beta Pictoris.

We present a UV CMD spanning the hot horizontal branch (HB), blue straggler, and white dwarf populations of the globular cluster NGC 2808. These data were obtained with the Space Telescope Imaging Spectrograph (STIS). Although previous optical CMDs of NGC 2808 show a high temperature gap within the hot HB population, no such gap is evident in our UV CMD. Instead, we find a population of hot subluminous HB stars, an anomaly only previously reported for the globular cluster omega Cen. Our modeling indicates that the location of these subluminous stars in the UV CMD, as well as the high temperature gap along the HB in optical CMDs, can be explained if these stars underwent a late He-core flash while descending the white dwarf cooling curve. We show that the convection zone produced by such a late He flash will penetrate into the H envelope, thereby mixing H into the hot He-burning interior, where it is rapidly consumed. This phenomenon is analogous to the "born again" scenario for producing H-deficient stars following a late He-shell flash. The flash mixing of the envelope greatly enhances the envelope He and C abundances, and leads, in turn, to a discontinuous increase in the HB effective temperatures at the transition between canonical and flash-mixed stars. We argue that the hot HB gap is associated with this theoretically predicted dichotomy in the HB properties. Moreover, the changes in the emergent spectral energy distribution caused by these abundance changes are primarily responsible for explaining the hot subluminous HB stars. Although further evidence is needed to confirm that a late He-core flash can account for the subluminous HB stars and the hot HB gap, we demonstrate that an understanding of these stars requires the use of appropriate theoretical models for their evolution, atmospheres, and spectra., 26 pages, Latex. 17 figures. Uses corrected version of emulateapj.sty and apjfonts.sty (included). Accepted for publication in ApJ. T.M. Brown currently at STScI

Dedicated SN searches started over 100 years ago. Over that time, astronomers have collected large sets of observational data. They have developed detailed classification and achieved general agreement on the nature of these events. Nevertheless, a lot of questions remain unanswered. In short, most Type II SNe (hydrogen-rich SNe) are terminal explosions of red supergiant (RSG) stars. The main source of knowledge about these objects are the way their luminosity changes with time (`light curves') and how their radiation is distributed in wavelength. One of the widely used methods to extract the information from the observational data is computer modelling. The largest part of our work lays in the numerical simulations. In Chapter 1 (Introduction), we present succinctly the necessary theory which includes stellar evolution, explosion physics and ejecta evolution. We discuss different types of SNe; the modern knowledge on CCSNe and its problems. We discuss stellar evolution theory. We describe the nucleosynthesis that takes place in the cores of massive stars and gives rise to their final chemical stratification. We describe our numerical approach in Chapter 2 (Supernova modelling). It includes three major steps: stellar evolution modelling, explosion of the resulting RSG star, and ejecta evolution modelling. We present modelled structure of RSG stars; these models and computational techniques are similar to models used in subsequent chapters. We then discuss our numerical methods of exploding a SN once its degenerate core starts collapsing. We discuss explosive nucleosynthesis and its impact on the progenitor composition, production of unstable isotopes and the basic physics of radioactive decay. In Chapter 3 (Observational properties), we discuss the photometric and spectral observational properties of core-collapse SNe. We extract statistical properties of the existing sample. Using the presented technique, we performed a detailed study of the well observed object SN 2008bk (Chapter 4). We are able to constrain its progenitor and explosion properties. Our modelling allows us to compare not only the basic properties such as luminosity, but also to analyze in detail the spectral features, such as line identification and morphology. We show that a 12 M⊙ star on the main sequence is a good candidate for the progenitor of SN 2008bk. Also we discuss the asymmetric shape of the Hα line and conclude that it stems from the overlap with the strong Ba II 6596.9 Å line. SN 2008bk, together with about 20 objects, form a subclass of low-luminosity CCSNe Type II. We devoted a particular attention to this class of objects, whose uniformity and observational data allows us to draw important conclusions. In Chapter 5 (Low-luminosity Type II-P SNe), we study the sample of 17 low-luminosity SNe and analyze the possibility that these events are the result of explosions of low- and high-mass progenitors. There is no solid agreement in the astronomical community on the possible progenitors of the low-luminosity explosions of Type II SNe. Our analysis shows that lower-mass massive stars (~12 M⊙) are good candidates for the progenitors of this subclass of SNe. Moreover, our simulations of high-mass stars (25 and 27 M⊙) show that such low brightness of the explosion of such a massive object would have notable observational properties which are not present in the data. In Chapter 6 (Kinetic energy variation), we extend our study further on the whole class of hydrogen-rich core-collapse SN, using more energetic models than in Chapters 4 and 5. We provide evidences that what differentiates the diversity of SNe II is the explosion energy rather than the ejecta mass (or H-rich envelope mass of the progenitor).; Les recherches de SNe ont commencé il y a plus de 100 ans. Depuis, il a été possible de collecter beaucoup de données d'observations astronomiques. Les astronomes ont développé une classification détaillée et ont abouti un relatif consensus sur la nature physique de ces événements très différents. Néanmoins, beaucoup de questions restent sans réponse. En résumé, les supernovæ de type II (riche en hydrogène) résultent de l'éjection l'enveloppe des supergéantes rouges (SGR). Les principales sources de connaissance sur ces objets sont l'évolution de leur luminosité en fonction du temps (`courbes de lumière') et leurs spectres observés à différentes époques. La méthode la plus utilisée pour extraire les informations des données d'observation est la modélisation des courbes de lumières et des spectres des supernovæ. Dans le Chapitre 1 (Introduction), nous présentons successivement l’évolution stellaire, la physique des explosions et l’évolution des éjectas. Nous décrivons aussi les différents types de supernova ; l’état actuel des connaissances sur les CCSNe ainsi que ces limitations. Nous discutons de la théorie de l'évolution stellaire. Nous décrivons notre approche numérique au Chapitre 2 (Supernova modelling). Elle consiste en trois étapes principales : la modélisation de l'évolution stellaire, l'explosion de l'étoile SGR résultante, et la modélisation de l'évolution des éjectas. Nous présentons la structure modélisée des étoiles SGR ; ces modèles et techniques de calcul sont similaires aux modèles utilisés dans les chapitres suivants. Nous discutons notre méthode d'explosion d'un progéniteur quand son noyau dégénéré commence à s'effondrer. Dans le Chapitre 3 (Observational properties), nous discutons les propriétés observées en photométrie et spectroscopie des CCSNe. Nous extrayons les propriétés statistiques de l'échantillon existant. En utilisant la technique présentée, nous avons effectué une étude détaillée de SN 2008bk, une supernova bien observée (Chapitre 4). Nous pouvons contraindre les propriétés du progéniteur et des paramètres d'explosion. Notre modélisation nous permet de comparer non seulement les propriétés de base telles que la luminosité, mais aussi à analyser en détail les caractéristiques spectrales, telles que la présence de certaines raies spectrales ainsi que leur morphologie. Nous montrons qu'une étoile de 12M⊙ sur la séquence principale est un bon candidat au progéniteur de SN 2008bk. Aussi, nous discutons de la forme asymétrique de la raie Hα et concluons qu'elle provient du chevauchement avec la raie forte du Ba II 6596.9 Å. SN 2008bk, avec quelques dizaines d'autres objets, forme une sous-classe importante de CCSNe — supernovae à faible luminosité. Nous avons consacré une attention particulière à cette classe d'objets, dont l'uniformité et les données d'observation nous permet de tirer des conclusions importantes. Au Chapitre 5, nous étudions l'échantillon de 17 SNe de faible luminosité et analysons la possibilité que ces événements résultent d'explosions de progéniteurs de petite ou de grande masse. Il n'y a pas d'accord solide dans la communauté astronomique sur les progéniteurs possibles des explosions de SNe à faible luminosité. Notre analyse montre que les étoiles massives de masse inférieure (~12 M⊙) sont de bons candidats pour les progéniteurs de cette sous-classe de SNe. De plus, nos simulations d'étoiles de masse élevée (25 et 27 M⊙) montrent qu'une explosion ayant une luminosité aussi faible aurait des propriétés d’observation remarquables qui ne sont pas présentes dans les données. Dans le Chapitre 6, nous étendons notre étude sur toutes les CCSNe, en utilisant des modèles plus énergétiques que dans les Chapitres 4 et 5. Nous fournissons des preuves que ce qui différencie la diversité de SNe II est l'énergie d'explosion plutôt que la masse des éjectas (ou plus précisément la masse de l'enveloppe riche en H de progéniteur).

Dedicated SN searches started over 100 years ago. Over that time, astronomers have collected large sets of observational data. They have developed detailed classification and achieved general agreement on the nature of these events. Nevertheless, a lot of questions remain unanswered. In short, most Type II SNe (hydrogen-rich SNe) are terminal explosions of red supergiant (RSG) stars. The main source of knowledge about these objects are the way their luminosity changes with time (`light curves') and how their radiation is distributed in wavelength. One of the widely used methods to extract the information from the observational data is computer modelling. The largest part of our work lays in the numerical simulations. In Chapter 1 (Introduction), we present succinctly the necessary theory which includes stellar evolution, explosion physics and ejecta evolution. We discuss different types of SNe; the modern knowledge on CCSNe and its problems. We discuss stellar evolution theory. We describe the nucleosynthesis that takes place in the cores of massive stars and gives rise to their final chemical stratification. We describe our numerical approach in Chapter 2 (Supernova modelling). It includes three major steps: stellar evolution modelling, explosion of the resulting RSG star, and ejecta evolution modelling. We present modelled structure of RSG stars; these models and computational techniques are similar to models used in subsequent chapters. We then discuss our numerical methods of exploding a SN once its degenerate core starts collapsing. We discuss explosive nucleosynthesis and its impact on the progenitor composition, production of unstable isotopes and the basic physics of radioactive decay. In Chapter 3 (Observational properties), we discuss the photometric and spectral observational properties of core-collapse SNe. We extract statistical properties of the existing sample. Using the presented technique, we performed a detailed study of the well observed object SN 2008bk (Chapter 4). We are able to constrain its progenitor and explosion properties. Our modelling allows us to compare not only the basic properties such as luminosity, but also to analyze in detail the spectral features, such as line identification and morphology. We show that a 12 M⊙ star on the main sequence is a good candidate for the progenitor of SN 2008bk. Also we discuss the asymmetric shape of the Hα line and conclude that it stems from the overlap with the strong Ba II 6596.9 Å line. SN 2008bk, together with about 20 objects, form a subclass of low-luminosity CCSNe Type II. We devoted a particular attention to this class of objects, whose uniformity and observational data allows us to draw important conclusions. In Chapter 5 (Low-luminosity Type II-P SNe), we study the sample of 17 low-luminosity SNe and analyze the possibility that these events are the result of explosions of low- and high-mass progenitors. There is no solid agreement in the astronomical community on the possible progenitors of the low-luminosity explosions of Type II SNe. Our analysis shows that lower-mass massive stars (~12 M⊙) are good candidates for the progenitors of this subclass of SNe. Moreover, our simulations of high-mass stars (25 and 27 M⊙) show that such low brightness of the explosion of such a massive object would have notable observational properties which are not present in the data. In Chapter 6 (Kinetic energy variation), we extend our study further on the whole class of hydrogen-rich core-collapse SN, using more energetic models than in Chapters 4 and 5. We provide evidences that what differentiates the diversity of SNe II is the explosion energy rather than the ejecta mass (or H-rich envelope mass of the progenitor)., Les recherches de SNe ont commencé il y a plus de 100 ans. Depuis, il a été possible de collecter beaucoup de données d'observations astronomiques. Les astronomes ont développé une classification détaillée et ont abouti un relatif consensus sur la nature physique de ces événements très différents. Néanmoins, beaucoup de questions restent sans réponse. En résumé, les supernovæ de type II (riche en hydrogène) résultent de l'éjection l'enveloppe des supergéantes rouges (SGR). Les principales sources de connaissance sur ces objets sont l'évolution de leur luminosité en fonction du temps (`courbes de lumière') et leurs spectres observés à différentes époques. La méthode la plus utilisée pour extraire les informations des données d'observation est la modélisation des courbes de lumières et des spectres des supernovæ. Dans le Chapitre 1 (Introduction), nous présentons successivement l’évolution stellaire, la physique des explosions et l’évolution des éjectas. Nous décrivons aussi les différents types de supernova ; l’état actuel des connaissances sur les CCSNe ainsi que ces limitations. Nous discutons de la théorie de l'évolution stellaire. Nous décrivons notre approche numérique au Chapitre 2 (Supernova modelling). Elle consiste en trois étapes principales : la modélisation de l'évolution stellaire, l'explosion de l'étoile SGR résultante, et la modélisation de l'évolution des éjectas. Nous présentons la structure modélisée des étoiles SGR ; ces modèles et techniques de calcul sont similaires aux modèles utilisés dans les chapitres suivants. Nous discutons notre méthode d'explosion d'un progéniteur quand son noyau dégénéré commence à s'effondrer. Dans le Chapitre 3 (Observational properties), nous discutons les propriétés observées en photométrie et spectroscopie des CCSNe. Nous extrayons les propriétés statistiques de l'échantillon existant. En utilisant la technique présentée, nous avons effectué une étude détaillée de SN 2008bk, une supernova bien observée (Chapitre 4). Nous pouvons contraindre les propriétés du progéniteur et des paramètres d'explosion. Notre modélisation nous permet de comparer non seulement les propriétés de base telles que la luminosité, mais aussi à analyser en détail les caractéristiques spectrales, telles que la présence de certaines raies spectrales ainsi que leur morphologie. Nous montrons qu'une étoile de 12M⊙ sur la séquence principale est un bon candidat au progéniteur de SN 2008bk. Aussi, nous discutons de la forme asymétrique de la raie Hα et concluons qu'elle provient du chevauchement avec la raie forte du Ba II 6596.9 Å. SN 2008bk, avec quelques dizaines d'autres objets, forme une sous-classe importante de CCSNe — supernovae à faible luminosité. Nous avons consacré une attention particulière à cette classe d'objets, dont l'uniformité et les données d'observation nous permet de tirer des conclusions importantes. Au Chapitre 5, nous étudions l'échantillon de 17 SNe de faible luminosité et analysons la possibilité que ces événements résultent d'explosions de progéniteurs de petite ou de grande masse. Il n'y a pas d'accord solide dans la communauté astronomique sur les progéniteurs possibles des explosions de SNe à faible luminosité. Notre analyse montre que les étoiles massives de masse inférieure (~12 M⊙) sont de bons candidats pour les progéniteurs de cette sous-classe de SNe. De plus, nos simulations d'étoiles de masse élevée (25 et 27 M⊙) montrent qu'une explosion ayant une luminosité aussi faible aurait des propriétés d’observation remarquables qui ne sont pas présentes dans les données. Dans le Chapitre 6, nous étendons notre étude sur toutes les CCSNe, en utilisant des modèles plus énergétiques que dans les Chapitres 4 et 5. Nous fournissons des preuves que ce qui différencie la diversité de SNe II est l'énergie d'explosion plutôt que la masse des éjectas (ou plus précisément la masse de l'enveloppe riche en H de progéniteur).

This thesis aims at studying 2 massive clusters NGC3603 and R136, and the mechanisms that govern their physics, These clusters host the most massive stars known in the local universe so far and are important clues to understand the formation and fate of very massive star clusters. The manuscript outlines the photometric analysis of the core of R136 and NGC3603 on the basis of HST data in the visible and the VLT high dynamic imaging that I obtained in the infrared thanks to the SPHERE focal instrument operated since 2015 and its extreme Adaptive Optics, In an extensive photometric study of these data I discovered a significantly larger number of faint low-mass stars in the core of both these clusters compared to previous works. These stars are often detected in the vicinity of known massive bright objects. By comparing HST and SPHERE measures, NGC3603 does not show any signature of mass segregation in its core since the MF slope of the very core and the next radial bin are similarly flat and agree well with the MF found in previous works of the outer regions. On the other hand R136 is partially resolved using the SPHERE/IRDIS mode with most of the massive stars having visual companions. Considering the spectroscopic and photometric errors on the extinction and the age of cluster members, I estimate a mass range for each detected star. The MF is plotted at different ages with given errors on stellar masses. Finally I demonstrate that we need more resolution to go further on studying R136 which is 7-8 times further than NGC3603.; Cette thèse a pour objectif de comprendre les différents aspects de l'évolution des amas d’étoiles massives NGC3603 et R136 qui possèdent les étoiles les plus massives connues de l'univers local. L'analyse photométrique des noyaux de R136 et NGC3603 utilisant l’imagerie infrarouge de l’instrument SPHERE sur VLT et son système d’optique adaptative extrême de SPHERE, m’a permis de détecter pour la 1ière fois un grand nombre d’étoiles de faibles masse et luminosité au coeur de ces amas et pour la plupart au voisinage des étoiles les plus lumineuses et massives. La comparaison des données de SPHERE de NGC3603 à celles du HST montre l’absence de ségrégation de masse dans le noyau de cet amas. De plus la pente de la fonction de masse de cette région est la même que celle de la région suivante et similaire aux valeurs de la MF correspondant aux régions extérieures de l’amas connues jusqu’ici. L’amas R136 est partiellement résolu par SPHERE/IRDIS dans l’IR. La majorité de ses étoiles massives ont des compagnons visuels. En prenant compte des mesures spectroscopiques et photométriques et leurs erreurs sur l'extinction et l'âge des membres de l’amas, j’ai estimé une gamme de masse pour chaque étoile identifiée. La MF a été calculée pour différents âges ainsi que les erreurs sur les masses stellaires. J’ai simulé des séries d'images de R136 grâce au code Nbody6, et les ai comparées aux observations du HST/WFPC2. Ces simulations permettent de vérifier l'effet de la binarité initiale des étoiles de l’amas, la ségrégation de masse et l'évolution des étoiles sur l'évolution dynamique propre à R136.