Thèses sur le sujet « Stars: supernovae »

Massive stars are thought to end their lives with spectacular explosions triggered by the gravitational collapse of their cores. Interacting supernovae are generally attributed to supernova explosions occurring in dense circumstellar media, generated through mass-loss which characterisie the late phases of the life of their progenitors. In the last two decades, several observational evidences revealed that mass-loss in massive stars may be related to violent eruptions involving their outer layers, such as the luminous blue variables. Giant eruptions of extragalactic luminous blue variables, similar to that observed in $\eta$~Car in the 19th century, are usually labelled `SN impostors', since they mimic the behaviour of genuine SNe, but are not the final act of the life of the progenitor stars. The mechanisms producing these outbursts are still not understood, although the increasing number of observed cases triggered the efforts of the astronomical community to find possible theoretical interpretations. More recently, a number of observational evidences suggested that also lower-mass stars can experience pre-supernova outbursts, hence becoming supernova impostors. Even more interestingly, there is growing evidence of a connection among massive stars, their outbursts and interacting supernovae. All of this inspired this research, which has been focused in particular on the characterisation of supernova impostors and the observational criteria that may allow us to safely discriminate them from interacting supernovae. Moreover, the discovery of peculiar transients, motivated us to explore the lowest range of stellar masses that may experience violent outbursts. Finally, the quest for the link among massive stars, their giant eruptions and interacting supernovae, led us to study the interacting supernova LSQ13zm, which possibly exploded a very short time after an LBV-like major outburst.
Le stelle massive terminano la loro vita con spettacolari esplosioni innescate dal collasso gravitazionale del loro nucleo. Le supernove interagenti sono tipicamente associate a questo tipo di esplosioni in mezzi circumstellari densi, generati da episodi di perdita di massa durante le fasi finali dell'evoluzione dei loro progenitori. Negli ultimi due decenni, diverse evidenze osservative hanno rivelato come questi episodi siano spesso correlati ad eruzioni violente che coinvolgono gli strati esterni degli inviluppi di stelle massive, come le `luminous blue variables'. In qualche caso questi eventi sono stati osservati come `supernova impostors' (impostori), transienti che imitano il comportamento di supernove reali, ma non costituiscono l'atto finale della vita dei loro progenitori. Questi sono spesso associati alle eruzioni giganti di luminous blue variable extragalattiche. I meccanismi che innescano queste eruzioni non sono ancora del tutto compresi, nonostante il crescente numero di casi osservati abbia attirato l'attenzione della comunit\'a astronomica nel tentativo di trovare delle spiegazioni teoriche a questi fenomeni. P\'iu di recente, un certo di numero di evidenze osservative ha portato ad ipotizzare che anche stelle meno massive possano mostrare eruzioni giganti che imitano gli osservabili delle supernove interagenti. Tutto questo, unitamente alla possibile connessione recentemente proposta tra le stelle massive, le loro eruzioni giganti e alcune supernove interagenti, ha ispirato questo lavoro di ricerca, che si \'e focalizzato in particolare sulla caratterizzazione del fenomeno degli impostori e sui possibili criteri osservativi che permettono di distinguere con sicurezza le supernove interagenti dagli impostori. Inoltre, la scoperta di oggetti peculiari ci ha motivato nell'analisi dell'estremo inferiore dell'intervallo di masse interessate da episodi eruttivi violenti. La caccia al collegamento tra le stelle massive, le loro eruzioni giganti e le supernove interagenti, ha portato alla scoperta della supernova peculiare LSQ13zm, che potrebbe essere esplosa poco tempo dopo un'eruzione gigante simile a quelle osservate nelle luminous blue variables.

In the last fifteen years, searches for the progenitors of core-collapse supernovae (SNe) in archival images have provided the crucial "missing link" between the final stages in the evolution of a massive star, and its explosion as a core-collapse SN. In this thesis, I present new detections and limits for core-collapse SN progenitors, together with a critical re-analysis of those already published. I present detailed studies of the progenitors of three nearby SNe. For the sub- luminous Type IIP SN 2009md, I find a coincident red supergiant progenitor with a mass of ~8 M in archival images, which is consistent with the low mass progenitors found for other faint Type IIP SNe. Such events appear to come from red supergiants at the lower extremum of the mass range for core-collapse. I discuss the intermediate Type IIP /L SN 2009kr, for which I find what appears to be a massive yellow supergiant progenitor. I consider possible explanations for the observed colour of the progenitor candidate at explosion, and the consequences for stellar evolution. I also analyse archival pre-explosion images of the site of the Type Ib SN 2009jf, for which a progenitor was not detected. I attempt to constrain the age of the stellar population in the locale of the SN, and use this to set a limit on the progenitor mass. I also extend the time- and volume-limited sample of core-collapse SNe presented by Smartt et al. (2009). I use this new data, together with an improved re-analysis of the original limits of Smartt et al., to set an upper limit on the mass of Type IIP and Type IIL SN progenitors of 20M, at a 95 per cent confidence level. This limit is lower than the most massive observed red supergiants, providing strong support for suggestions that some of the most massive red supergiants do not produce bright SNe, but rather collapse to form black holes either directly or via fall-back. Finally, I discuss some alternative approaches for understanding core-collapse SNe and their progenitors, including searching for progenitors in X-ray data, a survey for optically faint failed SNe, and a deeper search for a surviving companion to the progenitor of the Galactic SN Cas A.

In this thesis I present a detailed study of two type IIn supernovae: PTFll iqb and SN 2011jb. I find that PTFll iqb shows complex Ha and H~ profiles, which indicate that the progenitor underwent multiple periods of mass loss . The shells formed by these periods of mass loss are interacting with the ejecta . The measured widths of the narrow components are consistent with a Red Supergiant progenitor. Asymmetric line profiles at late times suggest dust formation. Contrastingly, SN 2011jb appears to be a type Ic supernova which is interacting with a CSM, similarly to SNe 2002ic and SN 2005gj. In these cases the spectra can be modeled as ejecta interacting with a CSM superimposed upon a type la or le spectrum, which appears weak due to the attenuation caused by the dust. I endeavor to conduct a search of all core-collapse supernovae which have exploded within the cryogenic lifetime of Spitzer, within 25Mpc. It is found that type Un SNe were twice as likely to be recovered as other type II supernovae, but due to the low numbers of type Iblc supernovae for which data exists it is not possible. to draw any conclusions about their likelihood of being recovered. The dust which is found in type II supernovae was likely to be caused by a light echo as the ejecta heated the surrounding circumstellar medium. However, dust formation is found to be an equally likely explanation for the presence of dust in other type II supernovae. Lastly, I discuss approaches for furthering our understanding of core-collapse supernovae. These include plans for discovering supernovae more quickly, and studying them more effectively, using surveys such as PESSTO, and pushing the boundaries for the faintest and furthest supernovae we can see using larger telescopes such as the E- ELT.

This thesis considers the various morphologies of radio supernova remnants (SNRs), and attempts to determine whether their appearance results from the properties of the progenitor star and its supernova explosion, or from the structure of the interstellar medium (ISM) and ambient magnetic field into which a SNR consequently expands. High-resolution observations of Supernova 1987A show a young remnant whose appearance and evolution are completely dominated by the structure of its progenitor wind. A statistical study of the Galactic population of bilateral SNRs demonstrates that the symmetry axes of these remnants run parallel to the Galactic Plane. This result can be explained by the interaction of main sequence stellar wind-bubbles with the ambient magnetic field; expansion of SNRs into the resulting elongated cavities results in a bilateral appearance with the observed alignment. Radio observations of SNR G296.8-00.3 show a double-ringed morphology which is best explained by expansion either into an anisotropic main-sequence progenitor wind or into multiple cavities in the ISM. Data on SNRs G309.2-00.6 and G320.4-01.2 (MSH 15-52) make a strong case that the appearance of both remnants is significantly affected by collimated outflows from a central source; for G309.2-00.6 the source itself is not detected, but for G320.4-01.2 there is now compelling evidence that the remnant is associated with and is interacting with the young pulsar PSR B1509-58. I conclude that, while the youngest SNRs are shaped by their progenitor's circumstellar material, the appearance of most SNRs reflects the properties of the local ISM and magnetic field. Remnants which interact with an associated pulsar or binary system appear to be rare, and are easily distinguished by their unusual and distorted morphologies.

This thesis considers the various morphologies of radio supernova remnants (SNRs), and attempts to determine whether their appearance results from the properties of the progenitor star and its supernova explosion, or from the structure of the interstellar medium (ISM) and ambient magnetic field into which a SNR consequently expands. High-resolution observations of Supernova 1987A show a young remnant whose appearance and evolution are completely dominated by the structure of its progenitor wind. A statistical study of the Galactic population of bilateral SNRs demonstrates that the symmetry axes of these remnants run parallel to the Galactic Plane. This result can be explained by the interaction of main sequence stellar wind-bubbles with the ambient magnetic field; expansion of SNRs into the resulting elongated cavities results in a bilateral appearance with the observed alignment. Radio observations of SNR G296.8-00.3 show a double-ringed morphology which is best explained by expansion either into an anisotropic main-sequence progenitor wind or into multiple cavities in the ISM. Data on SNRs G309.2-00.6 and G320.4-01.2 (MSH 15-52) make a strong case that the appearance of both remnants is significantly affected by collimated outflows from a central source; for G309.2-00.6 the source itself is not detected, but for G320.4-01.2 there is now compelling evidence that the remnant is associated with and is interacting with the young pulsar PSR B1509-58. I conclude that, while the youngest SNRs are shaped by their progenitor's circumstellar material, the appearance of most SNRs reflects the properties of the local ISM and magnetic field. Remnants which interact with an associated pulsar or binary system appear to be rare, and are easily distinguished by their unusual and distorted morphologies.

We present late-time Hubble Space Telescope (HST) images of the site of supernova (SN) 2009ip taken almost 3 yr after its bright 2012 luminosity peak. SN 2009ip is now slightly fainter in broad filters than the progenitor candidate detected by HST in 1999. The current source continues to be dominated by ongoing late-time circumstellar material interaction that produces strong Ha emission and a weak pseudo-continuum, as found previously for 1-2 yr after explosion. The intent of these observations was to search for evidence of recent star formation in the local (similar to 1 kpc; 10 arcsec) environment around SN 2009ip, in the remote outskirts of its host spiral galaxy NGC 7259. We can rule out the presence of any massive star-forming complexes like 30 Dor or the Carina nebula at the SN site or within a few kpc. If the progenitor of SN 2009ip was really a 50-80 M-circle dot star as archival HST images suggested, then it is strange that there is no sign of this type of massive star formation anywhere in the vicinity. A possible explanation is that the progenitor was the product of a merger or binary mass transfer, rejuvenated after a lifetime that was much longer than 4-5 Myr, allowing its natal H II region to have faded. A smaller region like the Orion nebula would be an unresolved but easily detected point source. This is ruled out within similar to 1.5 kpc around SN 2009ip, but a small H II region could be hiding in the glare of SN 2009ip itself. Later images after a few more years have passed are needed to confirm that the progenitor candidate is truly gone and to test for the possibility of a small H II region or cluster at the SN position.

Core-collapse supernovae (CCSNe) have very distinct observational properties that depend on the composition of the progenitor star, the dynamics of the explosion mechanism, and the surrounding stellar wind environment. In recent years, due to the uncertainty behind the type of massive star that evolves into different types of core-collapse events, there has been an increase in core-collapse supernova surveys aiding the advancement of numerical supernova simulations that explore the properties of the star before the explosion. Observationally, the unpredictable nature of these events makes it difficult to identify the type of star from which the CCSNe subtype evolves, but the issue from a theoretical standpoint relies on a gap in our current understanding of the explosion mechanism. The general light curve properties of CCSNe (rise, peak, and decay) by subtype are diverse, but appear to be homogeneous within each subtype, with the exception of Type IIn.. Simplified SN models can be processed quickly in order to explore the properties of the progenitor star along with the explosion mechanism and circumstellar medium. Here, we present a suite of SN light curve models presented using a 1-temperature, homologous outflow light curve code. The SN explosion is modeled from shock breakout through the ultimate uncovering of the nickel core. We are able to rapidly explore the diversity of the SN light curves by studying the effects of various explosion and progenitor star parameters, including ejecta mass, explosion energy, shock temperature, and stellar radii using this "simple" calculation technique. Furthermore, we compare UV and optical modeled light curves to Swift UVOT IIn observations to identify the general initial conditions that enable the difference between SN 2009ip and SN 2011ht light curve properties. Our results indicate that the peak light curve is dominated by the shock temperature and explosion energy, whereas the shape depends on the mass of the ejecta and the explosion energy. Based on this modeling approach, the comparison SN light curves are a product of processes occurring after shock breakout, but before Ni-56 decay. Therefore, the energy from nickel decay does not play a major role in the light curves of these explosions. In general, the diversity between SN 2009ip and SN 2011ht can be explained by the differences in the outer ejecta mass and the explosion energy.

We consider the effect of free -streaming axion emission on numerical models for the cooling of the newly born neutron star associated with SN1987A. We find that for an axion mass of greater than -10-3 eV, axion emission shortens the duration of the expected neutrino burst so significantly that it would be inconsistent with the neutrino observations made by the Kamiokande II (KII) and Irvine -Michigan- Brookhaven (IMB) detectors. However, we have not investigated the possibility that axion trapping (which should occur for masses 20.02 eV) sufficiently reduces axion emission so that axion masses greater than -2 eV would be consistent with the neutrino observations.

Supernova (SN) 2005ip was a Type IIn event notable for its sustained strong interaction with circumstellar material (CSM), coronal emission lines and infrared (IR) excess, interpreted as shock interaction with the very dense and clumpy wind of an extreme red supergiant. We present a series of late- time spectra of SN 2005ip and a first radio detection of this SN, plus late-time X-rays, all of which indicate that its CSM interaction is still strong a decade post- explosion. We also present and discuss new spectra of geriatric SNe with continued CSM interaction: SN 1988Z, SN 1993J and SN 1998S. From 3 to 10 yr post- explosion, SN 2005ip's Ha luminosity and other observed characteristics were nearly identical to those of the radio- luminous SN 1988Z, and much more luminous than SNe 1993J and 1998S. At 10 yr after explosion, SN 2005ip showed a drop in Ha luminosity, followed by a quick resurgence over several months. We interpret this Ha variability as ejecta crashing into a dense shell located less than or similar to 0.05 pc from the star, which may be the same shell that caused the IR echo at earlier epochs. The extreme Ha luminosities in SN 2005ip and SN 1988Z are still dominated by the forward shock at 10 yr post- explosion, whereas SN 1993J and SN 1998S are dominated by the reverse shock at a similar age. Continuous strong CSM interaction in SNe 2005ip and 1988Z is indicative of enhanced mass- loss for similar to 10(3) yr before core collapse, longer than Ne, O or Si burning phases. Instead, the episodic mass- loss must extend back through C burning and perhaps even part of He burning.

La physique des supernovae requiert la connaissance soit des phénomènes complexes hydrodynamiques dans la matière dense (comme le transport d'énergie et des neutrinos, le traitement du choc) soit de la microphysique liée à la physique des noyaux et de la matière nucléaire dans la matière dense et chaude. Dans le cadre de la théorie des supernovae de type II, la plus part des simulations numériques qui simulent l'effondrement du coeur de supernova jusqu'à la formation et la propagation de l'onde du choc n'arrive pas à reproduire l'explosion des couches extérieures des étoiles massives. La raison pour cela pourrait être due soit aux phénomènes hydrodynamiques comme la rotation, la convection, ou bien la relativité générale, soit aux processus microphysiques qui ne sont pas très bien connus dans ce domaine de densités, températures et asymétries. Le but de ce travail de thèse est d'étudier l'effet de certaines processus microphysiques, en particulier les processus électro-faibles, qui jouent un rôle fondamental pendant l'effondrement gravitationnel, et d'analyser leur impact avec une simulation hydrodynamique. Parmi les processus microphysiques qui interviennent lors d'un effondrement de supernova, le plus important processus électro-faible est la capture électronique sur les protons libres et sur les noyaux. La capture est essentielle pour déterminer l'évolution de la fraction leptonique dans le coeur pendant la phase de neutronisation. Elle a un impact sur l'efficacité du rebond et, par conséquent, sur l'énergie de l'onde du choc. De plus, l'équation d'état de la matière et les taux de capture électronique sur les noyaux sont modifiés par la masse effective des nucléons dans les noyaux, due aux correlations à multi-corps dans le milieu dense, et à sa dépendence de la température. Après une introduction générale qui contient une revue de la phénoménologie des supernovae en appuyant sur la nécessité de la connaissance des données nucléaires pour les simulations numériques, dans la première partie de la thèse les aspects nucléaires abordés dans ce travail sont présentés. Le Chapitre 2 est constitué par une courte introduction sur les concepts importantes qui sont développés dans la Partie I et utilisés dans la Partie II de la thèse; en particulier: la théorie du champ moyen, de l'appariement en approximation BCS, la définition de masse effective en connexion avec la densité des niveaux et l'énergie de symétrie. Dans le Chapitre 3, un modèle nucléaire dont le but est d'améliorer la densité d'états autours du niveau de Fermi dans les noyaux est présenté. On a inclu dans l'approche de la fonctionnelle de la densité une masse effective piquée en surface qui simule certains effets au delà de Hartree-Fock. Cela a été possible en ajoutant un terme à la fonctionnelle de Skyrme qui puisse reproduire l'augmentation de la masse effective et de la densité d'états à la surface de Fermi, comme attendu par les données expérimentales. On a étudié l'impact de ce nouveau terme sur les propriétés de champ moyen dans les noyaux 40Ca et 208Pb, et sur les propriétés d'appariement à température nulle et à température finie dans le noyau 120Sn. On a aussi commencé des nouveaux calculs pour évaluer la dépendance en température de la masse effective dans l'approche microphysique de la RPA, dont les résultats préliminaires sont montrés dans l'Appendice D. Cette partie nucléaire est complétée par une appendice (Appendice B), qui donne les détails des paramétrisations de Skyrme utilisées dans le texte, et par l'Appendice C qui analyse la dépendence de la température de la masse effective en connection avec le paramètre de densité des niveaux qui peut être extrait par les expériences de physique nucléaire. La deuxième partie de la thèse est dediée aux modèles de supernova sur lequels j'ai travaillé. On présente les résultats obtenus avec un approche à une zone, et deux modèles monodimensionnels en symétrie sphérique: newtonien et en relativité générale. Bien que un modèle en symétrie sphérique n'est pas capable de saisir tous les aspects complexes du phénomène de supernova, et les observations des vitesses des étoiles à neutrons ou des inhomogéneitées des éjecta requièrent l'inclusion dans les simulations des effets multidimensionnels, un modèle monodimensionnel permet un premier étude détaillé de l'impact des différentes données microphysiques en focalisant l'analyse sur l'incertitude des données de physique nucléaire. Après une introduction générale faite dans le Chapitre 4 qui décrit les principals ingrédients des différentes simulations numériques (comme le traitement du choc et le transport de neutrinos), les codes sur lequels j'ai travaillé sont illustrés en détail. Le Chapitre 5 présente un modèle à une zone, où le coeur de supernova a été approximé par une sphère de densité homogène. Bien que ceci est un modèle simple, il est capable de reproduire de façon qualitative (et quantitative dans ses ordres de grandeur) la "trajectoire" d'effondrement (i.e. l'évolution des grandeurs thérmodynamiques le long de l'effondrement). Dans ce cadre, on a évalué l'impact de la dépendance en température de l'énergie de symétrie (via la dépendance en température de la masse effective) dans la dymanique du collapse, et on a montré que, en incluant cette dépendance en température, la deleptonisation dans le coeur est systématiquement réduite et l'effet sur l'énergie du choc est non-négligeable. Ces résultats nous ont conduit à effectuer des simulations plus réalistes, en employant un code monodimensionnel newtonien en symétrie sphérique, avec transport des neutrinos. La description de ce code, développé par P. Blottiau et Ph. Mellor au CEA,DAM,DIF, est l'object du Chapitre 6. On a inclu dans l'équation d'état dérivée par Bethe et al.(BBAL), aussi utilisée dans le code à une zone, la même paramétrisation de la masse effective, qui agit à la fois sur les Q-valeurs des taux de capture et sur l'équation d'état du système. Les résultats de ces simulations ont confirmés ceux qui avaient été obtenus avec le code one-zone, c'est à dire la reduction systématique de la deleptonisation dans le coeur si on inclue la dépendance en température de l'énergie de symétrie. De plus, on en a estimé l'impact sur la position de la formation de l'onde du choc, qui est déplacée vers l'extérieur d'une quantité non-négligeable. On a aussi travaillé pour inclure dans le code l'équation d'état plus récente de Lattimer et Swesty. Enfin, le Chapitre 7 décrit un code, à l'origine développé par le groupe de Valence, écrit en rélativité générale et qui utilise un approche moderne pour le traitment du choc (la "capture du choc"). Bien que ce modèle ne contient pas le transport des neutrinos, l'équation de l'évolution de la fraction neutrinique est déjà écrite avec un schema multi-groupe qui permet une première analyse spectrale des neutrinos. On étudie l'effet de l'équation d'état dans la dynamique d'effondrement ainsi que l'impact de la capture électronique. Une versione newtonienne a été aussi implémentée et les résultats obtenus sont en accord avec la littérature. Cette partie est complétée par plusieurs appendices. Dans l'Appendice A, les différentes unités de mesure employées dans les codes sont listées. Les Appendices E et F sont dédiées à deux équations d'état: la prémière est celle d'un gas de neutrons, protons et électrons; la deuxième décrit l'équations d'état de Lattimer et Swesty et les modifications qu'on a apportés pour corriger une erreur dans la définition de l'énergie de liaison des particules alpha et pour étendre l'équation d'état à des densités plus basses. Enfin, l'Appendice G détaille les processus des neutrinos implémentés dans les simulations. Le développement des codes numériques pour simuler l'effondrement gravitationnel de supernova effectué dans ce travail de thèse est apte pour tester les propriétés de la matière et peux constituer un outil pour des projets de recherche futurs.

Many young, massive stars are found in close binaries. Using population synthesis simulations. we predict the likelihood of a companion star being present when these massive stars end their lives as core-collapse supernovae (SNe). We focus on stripped-envelope SNe, whose progenitors have lost their outer hydrogen and possibly helium layers before explosion. We use these results to interpret new Hubble Space Telescope observations of the site of the broad-lined Type. Ic SN 2002ap, 14 years post-explosion. For a subsolar metallicity consistent with SN 2002ap, we expect a main-sequence (MS) companion present in about two thirds of all stripped-envelope SNe and a compact companion (likely a stripped helium star or a white dwarf/neutron star/black hole) in about 5% of cases. About a quarter of progenitors are single at explosion (originating from initially single stars, mergers, or disrupted systems). All of the latter scenarios require a massive progenitor, inconsistent with earlier studies of SN 2002ap. Our new, deeper upper limits exclude the presence of an MS companion star > 8-10 M., ruling out about 40% of all stripped-envelope SN channels. The most likely scenario for SN 2002ap includes nonconservative binary interaction of a primary star initially. 23 M.. Although unlikely (< 1% of the scenarios), we also discuss the possibility of an exotic reverse merger channel for broadlined Type. Ic events. Finally, we explore how our results depend on the metallicity and the model assumptions and discuss how additional searches for companions can constrain the physics that govern. the evolution of SN progenitors.

Massive stars live fast and die young. They shine furiously for a few million years, during which time they synthesize most of the heavy elements in the universe in their cores. They end by blowing themselves up in a powerful explosion known as a supernova (SN). During this process, the core collapses to a neutron star or a black hole, while the outer layers are expelled with velocities of thousands of kilometers per second. The resulting fireworks often outshine the entire host galaxy for many weeks. The explosion energy is eventually radiated away, but powering of the newborn nebula continues by radioactive isotopes synthesized in the explosion. The ejecta are now quite transparent, and we can see the material produced in the deep interiors of the star. To interpret the observations, detailed spectral modeling is needed. This thesis aims to develop and apply state-of-the-art computational tools for interpreting and modeling SN observations in the nebular phase. This requires calculation of the physical conditions throughout the nebula, including non-thermal processes from the radioactivity, thermal and statistical equilibrium, as well as radiative transport. The inclusion of multiline radiative transfer, which we compute with a Monte Carlo technique, represents one of the major advancements presented in this thesis. On February 23 1987, the first SN observable by the naked eye since 1604 exploded, SN 1987A. Its proximity has allowed unprecedented observations, which in turn have lead to significant advancements in our understanding of SN explosions. As a first application of our model, we analyze the 44Tipowered phase (t & 5 years) of SN 1987A. We find that a magnetic field is present in the nebula, trapping the positrons that provide the energy input, and resulting in strong iron lines in the spectrum. We determine the 44Ti mass to 1.5(+0.5−0.5)*10−4 M⊙. From the near-infrared spectrum at an age of 19 years, we identify strong emission lines from explosively synthesized metals such as silicon, calcium, and iron. We use integral-field spectroscopy to construct three-dimensional maps of the ejecta, showing a morphology suggesting an asymmetric explosion. The model is then applied to the close-by and well-observed Type IIP SN 2004et, analyzing its ultraviolet to mid-infrared evolution. Based on its Mg I] 4571 Å, Na I 5890, 5896 Å, [O I] 6300, 6364 Å, and [Ne II] 12.81 mm nebular emission lines, we determine its progenitor mass to be around 15 M⊙. We confirm that silicate dust, SiO, and CO have formed in the ejecta. Finally, the major optical emission lines in a sample of Type IIP SNe areanalyzed.We find that most spectral regions in Type IIP SNe are dominated by emission from the massive hydrogen envelope, which explains the relatively small variation seen in the sample. We also show that the similar line profiles seen from all elements suggest extensive mixing occurring in most hydrogenrich SNe.
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Accepted.

We explore the effects of rotation on convective carbon, oxygen, and silicon shell burning during the late stages of evolution in a 20 M-circle dot star. Using the Modules for Experiments in Stellar Astrophysics we construct one-dimensional (1D) stellar models both with no rotation and with an initial rigid rotation of 50% of critical. At different points during the evolution, we map the 1D models into 2D and follow the multidimensional evolution using the FLASH compressible hydrodynamics code for many convective turnover times until a quasi-steady state is reached. We characterize the strength and scale of convective motions via decomposition of the momentum density into vector spherical harmonics. We find that rotation influences the total power in solenoidal modes, with a slightly larger impact for carbon and oxygen shell burning than for silicon shell burning. Including rotation in 1D stellar evolution models alters the structure of the star in a manner that has a significant impact on the character of multidimensional convection. Adding modest amounts of rotation to a stellar model that ignores rotation during the evolutionary stage, however, has little impact on the character of the resulting convection. Since the spatial scale and strength of convection present at the point of core collapse directly influence the supernova mechanism, our results suggest that rotation could play an important role in setting the stage for massive stellar explosions.

Dans le cadre de la théorie des Supernovae de type II, la plus part des simulations numériques échouent de reproduire l'explosion observée, à cause de phénomènes hydrodynamiques et des processus nucléaires pas encore bien connus. Le but de ce travail est d'étudier certains processus microphysiques et d'évaluer leur impact parmi des simulations hydrodynamiques. Parmi les processus électro-faibles intervenant pendant l'effondrement, le plus important est la capture électronique, crucial pour déterminer l'évolution de la fraction leptonique dans la phase de neutronization. Elle a un impact sur l'efficacité du rebond et l'énergie de l'onde du choc. De plus, l'équation d'état de la matière et les taux de capture électronique dans les noyaux sont modifiés par la masse effective des nucléons dans les noyaux, dûe aux corrélations à multi-corps, et à sa dépendance de la température. On présente un modèle nucléaire avec le but d'étudier la masse effective nucléaire. On a inclus dans une approche de la fonctionnelle de la densité une masse effective piquée en surface pour reproduire des effets au delà de Hartree-Fock. On présente aussi les modèles de supernova sur lesquels j'ai travaillé, dans une approximation à une zone et à une dimension en symétrie sphérique, newtonienne et en relativité générale. On montre que, en introduisant une masse effective dépendante de la température dans un code à une zone et newtonien en symétrie sphérique avec transport des neutrinos, la deleptonization est réduite : cela a un impact non-negligeable sur la formation de l'onde du choc. On présente aussi les résultats obtenus avec un code en relativité générale avec un traitement muIti-groupe des neutrinos
Ln the framework of type II Supernovae theory, most of numerical simulations of the supernova core collapse and shock wave propagation fail to reproduce the observed explosion, because of both hydrodynamical phenomena and to some microphysical processes involved in the picture and not yet completely understood. The aim of this work is to investigate some microphysical aspects and to analyze their effects through hydrodynamical simulations. Among electro-weak processes occuring in core-collapse supernova, the most important one is the electron capture, crucial to determine the evolution of lepton fraction during the neutronization phase. It affects the efficiency of the bounce and the strength of the shock wave. Moreover, both the equation of state of supernova matter and electron capture rates in nuclei are modified by the nuclear effective mass in nuclei, induced by many-body correlations, and its temperature dependence. I will present a nuclear model aimed at studying the nuclear effective mass. We have included in a energy density functional approach a surface-peaked nuclear effective mass to mimic some effects beyond Hartree-Fock. I will then present the supernova models I have worked on, in a one-zone approximation, and in spherically symmetric one-dimensional approximation, Newtonian and General Relativistic. We will show that, introducing a temperature dependent effective mass into a one-zone and a one dimensional Newtonian code with neutrino transport, the deleptonization is reduced and has a non-negligible effect on the shock wave energetics. We will also present results obtained with the General Relativistic code with a multi-group treatment of neutrinos

Supernova (SN) iPTF13bvn in NGC 5806 was the first Type Ib SN to have been tentatively associated with a progenitor in pre-explosion images. We performed deep ultraviolet (UV) and optical Hubble Space Telescope observations of the SN site similar to 740 days after explosion. We detect an object in the optical bands that is fainter than the pre-explosion object. This dimming is likely not produced by dust absorption in the ejecta; thus, our finding confirms the connection of the progenitor candidate with the SN. The object in our data is likely dominated by the fading SN, implying that the pre-SN flux is mostly due to the progenitor. We compare our revised pre-SN photometry with previously proposed models. Although binary progenitors are favored, models need to be refined. In particular, to comply with our deep UV detection limit, any companion star must be less luminous than a late-O star or substantially obscured by newly formed dust. A definitive progenitor characterization will require further observations to disentangle the contribution of a much fainter SN and its environment.

We present optical and near-IR photometry and spectroscopy of SN 2013L for the first 4 yr post-explosion. SN 2013L was a moderately luminous (M-r = -19.0) Type IIn supernova (SN) that showed signs of strong shock interaction with the circumstellar medium (CSM). The CSM interaction was equal to or stronger to SN 1988Z for the first 200 d and is observed at all epochs after explosion. Optical spectra revealed multicomponent hydrogen lines appearing by day 33 and persisting and slowly evolving over the next few years. By day 1509, the Ha emission was still strong and exhibiting multiple peaks, hinting that the CSM was in a disc or torus around the SN. SN 2013L is part of a growing subset of SNe IIn that shows both strong CSM interaction signatures and the underlying broad lines from the SN ejecta photosphere. The presence of a blue Ha emission bump and a lack of a red peak does not appear to be due to dust obscuration since an identical profile is seen in Pa beta. Instead this suggests a high concentration of material on the near-side of the SN or a disc inclination of roughly edge-on and hints that SN 2013L was part of a massive interactive binary system. Narrow Ha P-Cygni lines that persist through the entirety of the observations measure a progenitor outflow speed of 80-130 km s(-1), speeds normally associated with extreme red supergiants, yellow hypergiants, or luminous blue variable winds. This progenitor scenario is also consistent with an inferred progenitor mass-loss rate of 0.3-8.0 x 10(-3) M-circle dot yr(-1).

We present the results of an extensive observational campaign on the nearby Type Ibn SN 2015G, including data from radio through ultravioletwavelengths. SN2015Gwas asymmetric, showing late-time nebular lines redshifted by similar to 1000 km s(-1). It shared many features with the prototypical SN Ibn 2006jc, including extremely strong He I emission lines and a late-time blue pseudo-continuum. The young SN 2015G showed narrow P-Cygni profiles of He I, but never in its evolution did it showany signature of hydrogen -arguing for a dense, ionized and hydrogenfree circumstellar medium moving outward with a velocity of similar to 1000 km s(-1) and created by relatively recent mass-loss from the progenitor star. Ultraviolet through infrared observations show that the fading SN 2015G (which was probably discovered some 20 d post-peak) had a spectral energy distribution that was well described by a simple, single-component blackbody. Archival HST images provide upper limits on the luminosity of SN 2015G's progenitor, while non-detections of any luminous radio afterglow and optical non-detections of outbursts over the past two decades provide constraints upon its mass-loss history.

Les supernovæ à effondrement de cœur sont l’un des phénomènes connus les plus puissants de l’univers. Elles résultent de l’explosion d’étoiles très massives, ayant brûlé tout leur combustible. Le résidu chaud et compact, appelée proto-étoile à neutrons, se refroidit pour devenir une étoile à neutrons, objet inerte. La dynamique et la structure des étoiles compactes, c’est-à-dire les supernovæ à effondrement de cœur, les proto-étoiles à neutrons et les étoiles à neutrons, ne sont pas encore complètement connues, et sont aujourd’hui au cœur d’intenses recherches, en association avec les observations astrophysiques et les expériences nucléaires. L’un des ingrédients clés de la modélisation d’étoile compacte concerne l’équation d’état. La difficulté de l’obtention d’une équation d’état réaliste et consistante pour tous ces objets stellaires réside dans le fait que l’on doit considérer une large variété de conditions thermodynamiques, c’est-à-dire des valeurs de densités, de fractions de protons et de températures, très différentes. Le travail de cette thèse consiste à modéliser, à partir des degrés de libertés nucléoniques, la structure microscopique ainsi que la composition interne de la matière baryonique des étoiles compactes, afin d’obtenir une équation d’état réaliste et unifiée. En particulier, on est intéressé à utiliser un formalisme qui peut s’appliquer à des densités aussi bien sous-saturées que sur-saturées, et qui, à la limite thermodynamique de température nulle, est compatible avec les interactions effectives modernes et réalistes données par la théorie microscopique d’Hartree-Fock-Bogoliubov et contraintes par les expériences nucléaires. Pour atteindre cet objectif, on présente, pour la matière sous-saturée, un modèle en équilibre statistique nucléaire, qui correspond à une superposition statistique de configurations finies, appelées cellules de Wigner-Seitz. Chaque cellule contient un noyau, ou agrégat, baignant dans un gaz homogène d’électrons ainsi que dans un gaz homogène de neutrons et de protons. Au sein de chaque cellule, on étudie les différentes composantes de l’énergie nucléaire des agrégats en interaction avec les gaz. L’utilisation de la théorie nucléaire de champ moyen pour la description des agrégats ainsi que du gaz de nucléons permet de traiter de façon consistante la matière sous-saturée et la matière sur-saturée. À des densités de plus de deux-trois fois la densité de saturation, l’apparition de degrés de liberté supplémentaires pose de nouveau des problèmes de consitance théorique qui ne sont pas traités dans cette thèse. La thèse est organisée selon trois parties. Dans la partie I, on présente le modèle en équilibre statistique nucléaire, basé sur l’ensemble grand canonique et sur les interactions non relativistes de Skyrme. Des résultats en équilibre β sont présentés, et l’importance de la distribution en masse d’agrégats d’une part, et d’un traitement réaliste de l’énergie libre d’autre part, est discutée. Dans la partie II, on étudie le comportement fonctionnel de l’énergie baryonique des cellules de Wigner-Seitz, en utilisant l’approximation de Thomas-Fermi étendue. En particulier, les effets de volume et de surface dus au milieu stellaire sont étudiés, et leur dépendance en termes de taille et d’asymétrie du noyau, ainsi que de densité et d’asymétrie du gaz de nucléons est analysée. Des résultats préliminaires de l’effet de l’interaction de surface du milieu sont présentés, sous hypothèse de certaines approximations et dans le cas de l’équilibre β. Dans la partie III, on développe des approximations afin d’obtenir une expression analytique fiable de formule de masse, directement reliée à la forme fonctionnelle et aux paramètres de l’interaction de Skyrme. Dans cette partie, on se concentre principalement sur les noyaux dans le vide, et l’on analyse les différentes composantes de l’énergie de liaison en termes de propriétés de volume et de surface, ainsi que de propriétés isoscalaire et isovecteur
The core collapse supernova is one of the most powerful known phenomena in the universe. It results from the explosion of very massive stars after they have burnt all their fuel. The hot compact remnant, the so-called proto-neutron star, cools down to become an inert catalyzed neutron star. The dynamics and structure of compact stars, that is core collapse supernovae, proto-neutron stars and neutron stars, are still not fully understood and are currently under active research, in association with astrophysical observations and nuclear experiments. One of the key components for modelling compact stars concerns the Equation of State. The task of computing a complete realistic consistent Equation of State for all such stars is challenging because a wide range of densities, proton fractions and temperatures is spanned. This thesis deals with the icroscopic modelling of the structure and internal composition of baryonic matter with nucleonic degrees of freedom in compact stars, in order to obtain a realistic unified Equation of State. In particular, we are interesting in a formalism which can be applied both at sub-saturation and super-saturation densities, and which gives in the zero temperature limit results compatible with the microscopic Hartree-Fock-Bogoliubov theory with modern realistic effective in- teractions constrained on experimental nuclear data. For this purpose, we present, for sub-saturated matter, a Nuclear Statistical Equilibrium model which corresponds to a statistical superposition of finite configurations, the so-called Wigner-Seitz cells. Each cell contains a nucleus, or cluster, embedded in a homogeneous electron gas as well as a homogeneous neutron and proton gas. Within each cell, we investigate the different components of the nuclear energy of clusters in interaction with gases. The use of the nuclear mean-field theory for the description of both the clusters and the nucleon gas allows a theoretical consistency with the treatment at saturation and beyond. At densities above two-three times saturation, other degrees of freedom are expected to appear, which potentially lead to other consistency problems but this issue will not be treated in this thesis. The thesis is divided into three parts. In part I, we present the Nuclear Statistical Equilibrium model based on the grand canonical statistics and non-relativistic Skyrme interactions. Results at β-equilibrium are shown and the importance of the clusters distribution as well as a realistic treatment for the free energy model is discussed. Part II investigates the functional behavior of the baryonic energy in the Wigner-Seitz cell within the Extended-Thomas-Fermi approximation. In particular, both bulk and surface in-medium effects are studied, and their dependence on cluster size and asymmetry as well as gas densities and asymmetry is investigated. A preliminary result of in-medium surface effects is presented within some approximations in the case of β-equilibrated matter

In my PhD work I carried out a detailed investigation on the final fates and chemical ejecta produced by intermediate-mass and massive stars. The first part of the thesis is focused on massive and very massive stars. We derive the ejecta for a large number of elemental species (H, He, C, N, O, F, Ne, Na, Mg, Al, Si, S Ar, K, Ca, Sc, Ti, Cr, Mn, Fe, Ni, Zn) during the pre-supernova evolution and after the explosion or collapse event. We use a set of stellar tracks computed with PAdova and TRieste Stellar Evolution Code (PARSEC), with initial masses in the range between 8 M to 350 M , for thirteen different initial metallicities from Z = 0.0001 to Z = 0.02. Adopting suitable explodability criteria available in the recent literature, for each stellar model we derive the final fate and remnant mass, which critically depend on the initial mass and metallicity. Three main classes of explosion events are considered. Massive stars with initial masses from 8 Msun to 100 Msun , build a degenerate iron core which eventually collapses either generating a successful explosion and a neutron star, or experiencing an inexorable infall with consequent black hole formation (failed supernovae). Very massive objects (VMOs), with initial mass ∼ 100 M , can end their life either as pulsation pair instability supernovae (PPISN), pair instability supernovae (PISN), or directly collapsing to black hole (DBH). For these objects, the fate is mainly determined by the mass of helium-core. From our analysis we derive a general scenario on the fate of massive and very massive stars emerges. It is evident that both the pre-SN evolution and the subsequent SN channel are significantly affected by the initial metallicity, as a consequence of its impact on the efficiency of mass loss and the growth of the stellar core. In particular, we find that suitable conditions for the occurrence of PPISN and PISN events are not limited to extremely low metallicities, as invoked in early studies. Rather, such energetic events may take place already at Z > Zsun /3, hence in the local Universe, in agreement with recent findings in the literature. Once final fates and remnant masses are known, we compute the elemental ejecta for all stars in the grid, accounting for both wind and explosion contributions. The wind ejecta are directly derived from PARSEC stellar evolution models, for all isotopes from 1 H to 28 Si and heavier elements up to Zn. The explosion ejecta are obtained from supernova nucleosynthesis calculations available in the literature, for the three classes here considered(CCSN, PISN or PPISN). Suitable parameters (masses of the CO and He cores) are adopted to link the explosion models to our PARSEC tracks. We also calculate the integrated yields ejected by a simple stellar population with a specified initial mass function in view of comparing the chemical contributions of both winds and explosions from the three classes of stars (CCSNe, PISNe and PPISNe). As a final result of this work, we aim at releasing a large database of chemical ejecta and compact remnants produced by massive and very massive stars over a large range of initial masses and metallicites. These will be a key relevance in the framework of the galaxy chemical evolution studies. In the second part of the thesis we investigate the chemical ejecta of intermediate-mass stars, with particular focus on the thermally-pulsing asymptotic giant branch (TP-AGB) stars that experience both the third dredge-up and hot-bottom burning. This study was performed in the context of the LUNA (Laboratory Underground Nuclear Astrophysics) collaboration. Nucleosynthesis calculations were carried out adopting the new rate for the key reaction 22 Ne(p, γ) 23 Na, which plays a major role in determining the abundance of sodium. To this aim we used the PARSEC and COLIBRI codes to compute the complete evolution, from the pre-main sequence up to the termination of the TP-AGB phase, of a set of stellar models with initial masses in the range 3.0 Msun 6.0 Msun , and metallicities Z=0.0005, Z=0.006, and Z = 0.014. We find that the new LUNA measurements have much reduced the nuclear uncertainties of the tors of 22Ne and 23Na AGB ejecta, which drop from fac-10 to only a factor of few for the lowest metallicity models. Relying on the most recent estimations for the destruction rate of 23Na, the uncertainties that still affect the 22Ne and 23Na AGB ejecta are mainly dominated by evolutionary aspects (efficiency of mass-loss, third dredge- up, convection). Finally, we discuss how the LUNA results impact on the hypothesis that invokes massive AGB stars as the main agents of the observed O-Na anti-correlation in Galactic globular clusters. We derive quantitative indications on the efficiencies of key physical processes (mass loss, third dredge-up, sodium destruction) in order to simultaneously reproduce both the Na-rich, O-poor extreme of the anti-correlation, and the observational constraints on the CNO abundance. While best-fitting AGB models can be singled out, the AGB hypothesis still needs to be validated, as various issues still remain.
Il mio lavoro si occupa dell'analisi degli ejecta chimici espulsi dalle stelle di massa intermedia e massiccia. E' strutturato in due macro-argomenti relativi, rispettivamente, alle stelle massicce e alle stelle di massa intermedia. Nella prima parte, questo lavoro si concentra sullo studio dei final fates e degli ejecta chimici prodotti da stelle massicce e molto massicce. Abbiamo ottenuto il materiale espulso per un gran numero di elementi (H, He, C, N, O, F, Ne, Na, Mg, Al, Si, S Ar, K, Ca, Sc, Ti, Cr, Mn, Fe, Ni, Zn) sia durante l'evoluzione pre-supernova che durante l'esplosione o il collasso. A questo scopo abbiamo usato un set di tracce evolutive calcolate con il codice di evoluzione stellare Padova and Trieste stellar Evolution Code (PARSEC), con masse iniziali nel range tra 8 M a 350 M , per tredici diverse metallicità iniziali da Z = 0.0001 a Z = 0.02. Abbiamo ottenuto il final fate e il resto di supernova per ciascuna delle tracce PARSEC. Abbiamo quindi considerato separatamente due sottoclassi: le stelle massicce, che vanno da 8 Msun a 100 Msun e si evolvono come core-collapse supernovae; i very massive objects (VMOS), che sono in generale piu' massicci di 100 Msun e, a seconda della massa del core di helio, possono evolvere come pair instability supernovae (PISN), pulsation instability supernovae (PPISN) o collassare direttamente al buco nero (DBH). Dalla nostra analisi si ricava un quadro generale sui final fates di stelle massicce e molto massicce. e' evidente che l'evoluzione pre-supernova e il verificarsi dell'esplosione sono significativamente influenzati dalla metallicità iniziale, conseguentemente al suo impatto sull'efficienza della perdita di massa e sulla crescita del nucleo stellare. In particolare, abbiamo ottenuto che le condizioni nelle quali si verificano eventi di PPISN e PISN non sono limitati a bassissime metallicita', come invocato nei primi studi. Piuttosto, tali eventi energetici possono aver luogo gia' a Z > Z/3, quindi nell'universo locale, in accordo con le recenti scoperte presenti in letteratura. Una volta noti i final fates e i resti di supernova, abbiamo calcolato gli elementi del materiale espulso per tutte le stelle nella griglia, dividendoli in contributi di vento e di esplosione. Gli elementi espulsi nel vento stellare sono derivati direttamente dai modelli di evoluzione stellare PARSEC, per tutti gli isotopi dall'H al Si-28 e gli elementi piu' pesanti fino a Zn. Il materiale espulso e' stato ottenuto da calcoli di nucleosintesi di supernova disponibili in letteratura, per le tre classi qui considerate (CCSN, PISN o PPISN). Sono stati inoltre adottati alcuni parametri (come la massa del core di CO e di He) per adattare gli ejecta di altri modelli di esplosione alle nostre tracce PARSEC. Abbiamo anche calcolato gli ejecta integrati - ottenuti da una semplice popolazione stellare e da una funzione di massa iniziale specificata - in vista di un successivo confronto del contributo all'inquinamento chimico in termini di vento ed ejecta esplosivi dovuto alle CCSNe, PISNe e PPISNe. Come risultato finale di questo lavoro, ci proponiamo di fornire un ampio database di ejecta chimici e resti di supernova prodotti da stelle massicce e molto massicce in un ampio intervallo di masse iniziali e metallicita' . Questi potreanno essere utilizzati nell'ambito dell' evoluzione chimica delle galassie. La seconda parte di questo lavoro si occupa dell'analisi del materiale espulso da stelle di massa intermedia, con particolare attenzione alle stelle nella fase di "thermally-pulsing asymptotic giant branch" (TP-AGB), in cui ha luogo il processo di "hot-bottom burning". Questo lavoro e' stato svolto in collaborazione con LUNA (Linear Underground National Laboratory), che ha fornito una nuova misura della sezione d'urto per la reazione 22Ne(p,gamma)23Na. A questo scopo sono stati utlilizzati i codici di evoluzione stellare PARSEC e COLIBRI per completare l'evoluzione stellare dalla pre-main sequence alla fine della fase TP-AGB, per un set di modelli con massa iniziale nell'intervallo 3.0 Msun-6.0 Msun e metallicità iniziali Z = 0.0005, Z = 0.006, and Z = 0.014. Grazie alla misura di sezione d'urto fornita dalla collaborazione LUNA abbiamo ridotto l'incertezza sugli ejecta di 22Ne e 23Na, abbassandola da un fattore 10 a poche unita'  per le metellicita'  piu' basse. Basandosi sulle piu' recenti stime della sezone d'urto della reazione siamo affermare che le incertezze influenti sulle quantita' di 22Ne e 23Na espulse sono perlopiu' dominate da aspetti evolutivi (come l'efficienza della mass loss, il terzo dredge-up e la convezione). Infine, abbiamo discusso il modo in cui i risultati di LUNA impattano sull'impotesi che pone le stelle AGB come principali responabili dell'anticorrelazione O-Na osservata negli ammassi globulari Galattici. Abbiamo deriveato quantitativamente l'efficienza dei processi fisici principali (mass loss, terzo dredge-up, distruzione del Na) al fine di riprodurre le situazioni estreme dell'anticorrelazione O-Na, e i vincoli dati dalle osservazioni sull'abbondanza deli elementi C,N e O. Nonostante siano stati individuate prescrizioni fisiche ragionevoli che consentono di soddisfare tali vincoli, l'ipotesi che attribuisce alle stelle AGB la causa dell'anticorrelazione O-Na deve essere ancora convalidata, a causa di problematiche non ancora risolte.

While the progenitors of Type Ia supernovae (SNe Ia) have long been thought to be thermonuclear explosions of white dwarf stars, what triggers the explosion are still a topic of debate. This thesis considers constraints on single-degenerate progenitors of SNe Ia based on the presence of a Roche-lobe filling companion. The ejecta strips material from the companion, that maybe detectable via Hα emission during the nebular phase. Using the full structure from simulations produces line widths are larger than those produced in simple models. The structure formed by the ejecta-companion interaction produce a broken reverse shock that may be visible in X-rays via the Fe Kα line at the age of Tycho's supernova remnant (SNR). If the similar structures in Tycho’s SNR are produced this way then the companion star must have been massive, with M ~ 2 M_⊙. Detections of circumstellar material within the supernova provides another way to indirectly probe the companion star. Mass loss through winds or novae are expected to shape the circumsteller medium for single-degenerate progenitors and the velocities, v ~ 100 km s^-1 appear to be consistent with recurrent nova shells, a model that is tested by analysing simulations of RS Ophiuchi. Models of RS Ophiuchi can explain the absorption lines seen around the 2006 outburst if the mass loss is 10⁻⁶ M_⊙ yr^{-1,/sup>. The circumsteller medium is shown to produce in the velocity and relative strengths of the features seen in SN 2006X. However, whether density in the shells is high enough to produce the required recombination timescale and to overcome ionization by γ-rays for shells at 5 × 1016 cm remains uncertain.}

Beyond supernovae (SNe), very few cosmic explosions can release an amount of kinetic energy of the order of 10ˆ{51} erg (1 foe). In past years, modern all-sky surveys discovered numerous peculiar transients releasing much lower energies. With the label of Intermediate Luminosity Optical Transients or Gap Transients, we refer to objects fainter than typical SNe but brighter than classical novae (i.e., they lay in the magnitude range -10< Mv < -15 mag). In this poorly populated luminosity range (the “gap”), we find several types of stellar transients, including faint supernovae, giant eruptions of massive stars including luminous blue variables (LBVs), intermediate-luminosity red transients (ILRTs), and luminous red novae (LRNe). These gap transients may originate from various physical mechanisms, and sometimes the classification is a tricky task. A major goal of this research work is characterising the observational properties and correlating the physical parameters of gap transients, shedding light on their nature. In this thesis project, I studied in detail a recent faint and red transient, AT 2017be, classified as an ILRT, and show that an electron-capture supernova (EC SN) is the most likely scenario to explain the observed outburst. In-depth analysis on a large ILRT sample favours the same explosion mechanism for all of them. Finally, I studied an unprecedented object, AT 2018hso, that reveals transitional observational properties between ILRTs and LRNe, making its precise classification dubious. However, follow-up observations support it to be a LRN, hence most likely a transient produced by a stellar merging event.
Oltre alle supernove (SNe), pochissime esplosioni cosmiche sono in grado di rilasciare una quantità di energia cinetica dell’ordine di 1051 erg (1 foe). Negli anni passati, moderne “all-sky surveys” hanno permesso di scoprire numerosi transienti peculiari che rilasciavano energie molto più modeste. Con il nome di transienti ottici di luminosità intermedia o transienti nel “gap”, ci riferiamo ad oggetti che sono più deboli delle tipiche SNe ma più luminosi delle classiche novae (cio`e hanno magnitudine nell’intervallo -10< MV < -15 mag). In questo intervallo di luminosità scarsamente popolato (il “gap”), troviamo diversi tipi di transienti stellari, tra cui supernove deboli, eruzioni giganti di stelle massicce incluse le Variabili Luminose Blu (LBV), i transienti rossi di luminosità intermedia (ILRTs) e le novae rosse luminose (LRNe). Questi transienti di “gap” possono essere prodotti da diversi meccanismi fisici, e talvolta la loro classificazione è un compito arduo. Uno degli obiettivi principali di questo studio è caratterizzare le proprietà osservative e correlare i parametri fisici dei transienti di “gap”, svelandone la natura. In questo lavoro di tesi, ho studiato in dettaglio un recente evento transiente debole e rosso, AT 2017be, classificato come ILRT, e ho mostrato come una supernova prodotta da cattura elettronica (EC SN) sia lo scenario più probabile per spiegare l’evento eruttivo osservato. Un’analisi approfondita su un grande campione di ILRTs favorisce lo stesso meccanismo di esplosione per tutti questi transienti. Infine, ho studiato un oggetto senza precedenti, AT 2018hso, che rivela proprietà osservative intermedie tra quelle degli ILRT e le LRNe, e che rendono una sua precisa classificazione incerta. Tuttavia, le osservazioni di follow-up supportano la tesi che sia un LRN, quindi probabilmente un transiente prodotto da un evento di coalescenza stellare.

We present an ongoing, five-year systematic search for extragalactic infrared transients, dubbed SPIRITS-SPitzer InfraRed Intensive Transients Survey. In the first year, using Spitzer/IRAC, we searched 190 nearby galaxies with cadence baselines of one month and six months. We discovered over 1958 variables and 43 transients. Here, we describe the survey design and highlight 14 unusual infrared transients with no optical counterparts to deep limits, which we refer to as SPRITEs (eSPecially Red Intermediate-luminosity Transient Events). SPRITEs are in the infrared luminosity gap between novae and supernovae, with [4.5] absolute magnitudes between -11 and -14 (Vega-mag) and [3.6]-[4.5] colors between 0.3 mag and 1.6 mag. The photometric evolution of SPRITEs is diverse, ranging from < 0.1 mag yr(-1) to > 7 mag yr(-1). SPRITEs occur in star-forming galaxies. We present an indepth study of one of them, SPIRITS 14ajc in Messier 83, which shows shock-excited molecular hydrogen emission. This shock may have been triggered by the dynamic decay of a non-hierarchical system of massive stars that led to either the formation of a binary or a protostellar merger.

We present multi-epoch mid-infrared (IR) photometry and the optical discovery observations of the "impostor" supernova (SN) 2010da in NGC. 300 using new and archival Spitzer Space Telescope images and ground-based observatories. The mid-infrared counterpart of SN. 2010da was detected as Spitzer Infrared Intensive Transient Survey (SPIRITS). 14bme in the SPIRITS, an ongoing systematic search for IR transients. Before erupting on 2010 May 24, the SN. 2010da progenitor exhibited a constant mid-IR flux at 3.6 and only a slight similar to 10% decrease at 4.5 mu m between 2003 November and 2007 December. A sharp increase in the 3.6 mu m flux followed by a rapid decrease measured similar to 150 days before and similar to 80 days after the initial outburst, respectively, reveal a mid-IR counterpart to the coincident optical and high luminosity X-ray outbursts. At late times, after the outburst (similar to 2000 days), the 3.6 and 4.5 mu m emission increased to over a factor of two. times the progenitor flux and is currently observed (as of 2016 Feb) to be fading, but still above the progenitor flux. We attribute the re-brightening mid-IR emission to continued dust production and increasing luminosity of the surviving system associated with SN. 2010da. We analyze the evolution of the dust temperature (T-d similar to 700-1000 K), mass (Md similar to 0.5-3.8 x. 10(-7) M circle dot), luminosity (L-IR similar to 1.3-3.5 x 10(4) L circle dot), and the equilibrium temperature radius (R-eq similar to 6.4-12.2 au) in order to resolve the nature of SN. 2010da. We address the leading interpretation of SN. 2010da as an eruption from a luminous blue variable high-mass X-ray binary (HMXB) system. We propose that SN. 2010da is instead a supergiant (sg)B[e]-HMXB based on similar luminosities and dust masses exhibited by two other known sgB[e]-HMXB systems. Additionally, the SN. 2010da progenitor occupies a similar region on a mid-IR color-magnitude diagram (CMD) with known sgB[e] stars in the Large Magellanic Cloud. The lower limit estimated for the orbital eccentricity of the sgB[e]-HMXB (e > 0.82) from X-ray luminosity measurements is high compared to known sgHMXBs and supports the claim that SN. 2010da may be associated with a newly formed HMXB system.

In this project we investigate the feasibility of detecting the signatures of Pop III stars in metal poor second generation stars and in gas clouds at high redshifts. First, the nucleosynthetic yields of Pair Instability Supernova and how they are manifested in gas clouds are presented. Next, some basic quantities of radio astronomy are explained and the requirements of SKA are shown. Then, the minimum detectable hydrogen column density of SKA for gas clouds at high redhsift is calculated and after that the basic principles of spectroscopy and the requirements of the HiReS instrument of E-ELT are demonstrated. Finally, suggestions about where the observations with HiReS should focus are made.

Oxygen sequence Wolf-Rayet stars (WO) are thought to be the final evolution phase of some high-mass stars, as such they may be the progenitors of Type Ic SNe as well as potential progenitors of broad-lined Ic and long gamma-ray bursts. We present the first spectropolarimetric observations of the Galactic WO stars WR93b and WR102 obtained with FORS1 on the Very Large Telescope. We find no sign of a line effect, which could be expected if these stars were rapid rotators. We also place constraints on the amplitude of a potentially undetected line effect. This allows us to derive upper limits on the possible intrinsic continuum polarization and find Pcont < 0.077 per cent and Pcont < 0.057 per cent for WR93b and WR102, respectively. Furthermore, we derive upper limits on the rotation of our WO stars by considering our results in the context of the wind compression effect. We estimate that for an edge-on case the rotational velocity of WR93b is vrot < 324 km s−1 while for WR102 vrot < 234 km s−1. These correspond to values of vrot/vcrit < 19 per cent and j) < 18.0 cm2 s−1 for WR93b and 2 s−1 for WR102. The upper limits found on vrot/vcrit and log(j) for our WO stars are therefore similar to the estimates calculated for Galactic Wolf-Rayet (WR) stars that do show a line effect. Therefore, although the presence of a line effect in a single WR star is indicative of fast rotation, the absence of a line effect does not rule out significant rotation, even when considering the edge-on scenario.

26Al is an important radionuclide in astrophysics. Its decay to 26Mg results in the emission of a 1.8 MeV gamma-ray which is detected and mapped across the galaxy, providing evidence of ongoing nucleosynthesis in the universe. Its origin is still not understood, however observations suggest massive stars as a possible main production site. A post processing network calculation study modelled nucleosynthesis in the C/Ne convective-shell before the core collapse of a massive star and found that the 23Na(α,p)26Mg reaction is important for the synthesis of 26Al in this environment. Due to large uncertainties in previous experimental measurements of this reaction, theoretically calculated Hauser-Feshbach cross sections were used to calculate the 23Na(α,p)26Mg reaction rate for the post processing calculations. This theoretical rate has large uncertainties as the statistical model used to calculate the cross sections is not thought to be applicable for the level density of the compound nucleus 27Al. The 23Na(α,p)26Mg reaction is also found to play an important role in the nucleosynthesis of several nuclei in type Ia supernovae explosions by several sensitivity studies. Again these studies used the reaction rate from Hauser-Feshbach statistical model cross-section calculations. A measurement has been made of the 23Na(α,p)26Mg reaction cross section in inverse kinematics using the TUDA scattering chamber at TRIUMF laboratory in Canada. The cross sections were calculated in the energy range Ec.m. = 1.28 - 3.15 MeV and found to be in reasonable agreement with the Hauser Feshbach model calculations. A new reaction rate has been calculated providing tight constraints on the uncertainty in the production of 26Al in the C/Ne convective shell of massive stars due to the 23Na(α,p)26Mg reaction.

When modelling the collapse of massive stars leading to supernova explosions and the cooling of neutron stars, understanding the microphysical processes, such as the interaction of neutrinos within a dense medium are of vital importance. The interaction of neutrinos with nucleons (neutrons and protons) is altered by the presence of the medium, compared to the same process with free nucleons. Neutrino scattering and production processes may be characterized in terms of the excitations that are created or destroyed in the nuclear medium. One way to analyse the effects of the medium is by using Landau's theory of normal Fermi liquids. This theory gives simple relationships between physical quantities such as the spin susceptibility or the response to a weak interaction probe in terms of Landau parameters, that are measures of the interaction between quasiparticles. One problem when using Landau Fermi liquid theory for nucleon matter is that the interaction has a tensor component. The tensor interaction does not conserve the total spin and, as a consequence, there are generally contributions to long-wavelength response functions from states that have more than one quasiparticle-quasihole pair in the intermediate state. Such contributions cannot be calculated in terms of Landau parameters alone, since in the usual formulation of Landau theory, only singlepair excitations are considered. In this thesis three problems are addressed. First, we obtain bounds on the contributions from more than one quasiparticle-quasihole pair by using sum-rule arguments. Second, we derive expressions for static response functions allowing for the tensor components of the interaction. We analyse which the most important effects are on the static response of nucleon matter, and find that the major contributions comes from renormalization of coupling constants and transitions to states with more than one quasiparticle-quasihole pair. Third, we show how contributions to the dynamical response coming from states containing two quasiparticle-quasihole pairs may be evaluated in terms of Landau theory if one allows for the effect of collisions in the Landau kinetic equation. We consider the case of asymmetric nuclear matter, and our work goes beyond earlier works in that they contain the effects of collisions in addition to those of the mean field.

La formation des premières étoiles, quelques centaines de millions d'années après le Big Bang, marque la fin de l’âge sombre. Actuellement, nous n’avons aucune observation de la formation de ces étoiles, appelée popIII, mais d’après des simulations numériques de différents groupes, il semblerait que ces étoiles primordiales étaient très massives: plusieurs centaines de masses solaires. Ces premières étoiles, ont produits aussi des sursauts gamma (GRBs). Ainsi, l’étude des GRBs produits à partir des popIII, pourraient permette d’étudier directement le stade final des étoiles primordiales. Les télescopes d'aujourd'hui ne peuvent pas regarder assez loin dans le passé cosmique pour observer la formation des premières étoiles, mais la nouvelle génération de télescopes permettra de tester des idées théoriques sur la formation des premières étoiles.Les GRBs sont liés à la mort d’étoiles massives et qu'ils sont connectés avec des supernovae. En ce sens, les GRBs sont l'une des classes de processus explosifs en physique stellaire et devraient suivre les mêmes lois physiques que l'explosion des supernovae. Ce travail tente d'aborder le problème des GRBs comme un problème d'explosion stellaire et utilise les données d’observation sur les spectres et les courbes de lumières notamment.Dans le cadre de cette thèse, des outils spécifiques ont été développés pour étudier les explosions stellaires: un code numérique pour résoudre les réactions nucléaires a été incorporé dans le code hydrodynamique existant. Ces outils ont été utilisés dans les simulations de supernovae afin d’étudier les connections avec les sursauts gamma: analyse spectrale et étude statistique en fonction du redshift
The formation of the first stars hundreds of millions of years after the Big-Bang marks the end of the Dark Ages. Currently, we have no direct observations on how the primordial stars formed, but according to modern theory of stellar evolution these stars should be very massive (about 100 Msun) Population III stars have a potential to produce probably most energetic flashes in the Universe - gamma-ray bursts. GRBs may provide one of the most promising methods to probe directly final stage of life of primordial stars. Today's telescopes cannot look far enough into the cosmic past to observe the formation of the first stars, but the new generation of telescopes will test theoretical ideas about the formation of the first stars.Thanks to many years of observations we have good GRB's data -statistics of occurrence, spectrum, lightcurves. But there are still a lot of questions in the theory of GRBs. We know that GRBs are related to the death of stars and that they are connected with supernovae. So gamma-ray bursts are one of the classes of explosive processes in stellar physics that should have a lot of common with supernovae explosions. In that case GRBs should follow the same physical laws of explosion as supernovae. This work tries to approach the problem of GRBs as a problem of stellar explosion.Necessary instruments of studying stellar explosion were developed as a part of doctoral research: code for solving systems of nuclear reaction equations was incorporated into hydrodynamical code. These tools were applied for supernovae simulations in order to find possible connection with GRBs. Basing on analysis of supernovae simulations spectral analysis of GRBs was performed

Chemical evolution addresses the problem of the formation of the chemical elements and their evolution throughout the history of the universe. This thesis discusses in particular the chemical evolution in the young universe and what we may learn from the observations of the oldest stars. The present day production of carbon in the Galaxy is also discussed. Interstellar media of young, metal-poor, star-forming systems are expected to show large chemical abundance inhomogeneities due to local supernova explosions. These inhomogeneities are reflected in the surface abundances of the population of longlived, low-mass stars. A stochastic model of the chemical evolution in such systems is presented and used to study the metallicity distribution and the scatter in chemical abundance ratios. The model takes into account mixing of the enriched material by turbulent motions and cloud collisions in the interstellar medium as well as infall of pristine matter. The predicted metallicity distribution shows, in accordance with observations of extreme Pop II strars in the Galactic halo, a distinct cut-off at [Fe/H]~-4. However, the fraction of stars below [Fe/H]=-4 agrees with observatrion only if a population of metal-free stars (Pop III) was never able to form. The predicted scatter in abundance ratios is demonstrated to be crucially dependent on the as yet uncertain supernova yields and the relatively small star-to-star scatter is tentatively explained by the averaging of a large number of contributing supernovae and by the selection effects favouring contributions from supernovae in a certain mass range for the most metal-poor stars. Furthermore, stars enriched by one single supernova are predicted to be found in very narrow sequences in the abundance ratio diagrams (so called A/A diagrams). Verification of the existence of such features, called single supernova sequences, is observationally challenging. Abundance analysis of carbon was performed in a large sample of solar-type stars in the Galactic disk using the forbidden [C I] line at 8727 Å. A comparison between the relation of [C/O] with metallicity for the Galactic stars and that of dwarf irregular galaxies suggests that large amounts of carbon are produced today by massive, so called Wolf-Rayet stars. Low-mass stars are less important. This was also demonstrated by modelling the chemical evolution of carbon.

Galaxies, and clusters of galaxies, can act as gravitational lenses and magnify the light of objects behind them. The effect enables observations of very distant supernovae, that otherwise would be too faint to be detected by existing telescopes, and allows studies of the frequency and properties of these rare phenomena when the universe was young. Under the right circumstances, multiple images of the lensed supernovae can be observed, and due to the variable nature of the objects, the difference between the arrival times of the images can be measured. Since the images have taken different paths through space before reaching us, the time-differences are sensitive to the expansion rate of the universe. One class of supernovae, Type Ia, are of particular interest to detect. Their well known brightness can be used to determine the magnification, which can be used to understand the lensing systems. In this thesis, galaxy clusters are used as gravitational telescopes to search for lensed supernovae at high redshift. Ground-based, near-infrared and optical search campaigns are described of the massive clusters Abell 1689 and 370, which are among the most powerful gravitational telescopes known. The search resulted in the discovery of five photometrically classified, core-collapse supernovae at redshifts of 0.671<z<1.703 with significant magnification from the cluster. Owing to the power of the lensing cluster, the volumetric core-collapse supernova rates for 0.4 ≤ z < 2.9 were calculated, and found to be in good agreement with previous estimates and predictions from cosmic star formation history. During the survey, two Type Ia supernovae in A1689 cluster members were also discovered, which allowed the Type Ia explosion rate in galaxy clusters to be estimated. Furthermore, the expectations of finding lensed supernovae at high redshift in simulated search campaigns that can be conducted with upcoming ground- and space-based telescopes, are discussed. Magnification from a galaxy lens also allows for detailed studies of the supernova properties at high redshift that otherwise would not be possible. Spectroscopic observations of lensed high-redshift supernovae Type Ia are of special interest since they can be used to test for evolution of the standard candle nature of these objects. If systematic redshift-dependent properties are found, their utility for future surveys could be challenged. In the thesis it is shown that the strongly lensed and very distant supernova Type Ia PS1-10afx at z=1.4, does not deviate from the well-studied nearby and intermediate populations of normal supernovae Type Ia. In a different study, the discovery of the first resolved multiply-imaged gravitationally lensed supernova Type Ia is also reported.

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

2008/2009
In this thesis we study the effect of galactic fountains, namely gas and flows from the disk of galaxies produced by multiple supernova explosions, on the chemical evolution of galaxies. Sequential supernova explosions create a superbubble, whereas the swept up interstellar medium is concentrated in a supershell which can break out a stratified medium, producing bipolar outflows. The gas of the supershells can fragment into clouds which eventually fall toward the disk producing so-called galactic fountains. Many works in literature have dealt with superbubble expansion in stratified media. However, very few papers in the past have taken into account the chemical evolution of the superbubble and how the supershell get polluted from the metals produced by supernova explosions. With this thesis for the first time the effect of galactic fountains we consider in a detailed chemical evolution model for the Milky Way. In the first part of our work we study the expansion law and chemical enrichment of a supershell powered by the energetic feedback of a typical Galactic OB association at various galactocentric radii. We follow the orbits of the fragments created when the supershell breaks out and we compare their kinetic and chemical properties with the available observations of high - and intermediate - velocity clouds. We use the Kompaneets (1960) approximation for the evolution of the superbubble driven by sequential supernova explosions and we compute the abundances of oxygen and iron residing in the thin cold supershell. Due to Rayleigh-Taylor instabilities we assume that supershells are fragmented and we follow the orbit of the clouds either ballistically or by means of a hybrid model considering viscous interaction between the clouds and the extra-planar gas. We find that if the initial metallicity is solar, the pollution from the dying stars of the OB association has a negligible effect on the chemical composition of the clouds. The maximum height reached by the clouds above the plane seldom exceeds 2 kpc and when averaging over different throwing angles, the landing coordinate differs from the throwing coordinate by only 1 kpc. Therefore, it is unlikely that galactic fountains can affect abundance gradients on large scales. The range of heights and [O/Fe] ratios spanned by our clouds suggest that the high velocity clouds cannot have a Galactic origin, whereas intermediate velocity clouds have kinematic properties similar to our predicted clouds but have observed overabundances of the [O/Fe] ratios that can be reproduced only with initial metallicities which are too low compared to those of the Galaxy disk. Even if it is unlikely that galactic fountains can affect abundance gradients on large scales, they can still affect the chemical enrichment of the interstellar medium (ISM) because of the time-delay due to the non-negligible time taken by fountains to orbit around and fall back into the Galaxy. This implies a delay in the mixing of metals in ISM which conflicts with the instantaneous mixing approximation usually assumed in all models in literature. We test whether relaxing this approximation in a detailed chemical evolution model can improve or worsen the agreement with observations. To do that, we investigate two possible causes for relaxing of the instantaneous mixing: i) the ``galactic fountain time delay effect'' and ii) the ``metal cooling time delay effect''. We find that the effect of galactic fountains is negligible if an average time delay of 0.1 Gyr, as suggested by our model, is assumed. Longer time delays produce differences in the results but they are not realistic. The metal cooling time delays produce strong effects on the evolution of the chemical abundances only if we adopt stellar yields depending on metallicity. If, instead, the yields computed for the solar chemical composition are adopted, negligible effects are produced, as in the case of the galactic fountain delay. The relaxation of the IMA by means of the galactic fountain model, where the delay is considered only for massive stars and only in the disk, does not affect the chemical evolution results. The combination of metal dependent yields and time delay in the chemical enrichment from all stars starting from the halo phase, instead, produces results at variance with observations.
XXII Ciclo
1979

Understanding the evolution of massive stars and their deaths in core-collapse supernovae is of fundamental importance not only to stellar astrophysics, but has implications for other broader areas of astronomy. Massive stars are thought to be the main drivers of the chemical and dynamical evolution of galaxies through their strong stellar winds and their explosive deaths. Supernovae are principally separated into two categories, those without hydrogen (Type I) and those with (Type II). The plateau subclass of Type II supernovae (SNe II-P) are thought to arise from the explosions of red supergiants (RSGs), which have initial masses greater than 8-10 M° and have retained their hydrogen envelopes before core collapse. Until the discovery of the 8 M° red supergiant (RSG) that exploded as SN 2003gd, there had been no direct confirmation that SNe II-P did indeed arise from the explosion of RSGs. Before this detection there had been only two other unambiguous detections of Type II progenitors, neither of which fitted the evolutionary scenario that is commonly accepted. These were the progenitors of the peculiar Type II SN 1987A (Sk —69°202), which was a blue supergiant (BSG), and the Type IIb SN 1993J that arose in a massive interacting binary system. There is at the moment only a handful of well studied and documented SNe II-P. The best observed supernova of this class to date is SN 1999em, which was followed for over 600 days at many different wavelength ranges. Because of the questions that still surround the progenitors of supernovae, it is of vital importance to understand the nature of the supernovae themselves as well as to compare them with the observed progenitor masses. This thesis tells the tales of two SNe II-P, 2003gd and 2004A, which have unambiguous detections of their progenitors with masses of 8 and 9M°, respectively. SN 2003gd is of particular importance because of the question of its nature. It was found to have a lower tail luminosity than is normal for SNe II-P, indicative of a lower ejected nickel mass. However, the supernova does not belong to the proposed group of low-luminosity supernovae, which could either be the result of the low-energy explosion of massive or low-mass stars. In the high-mass model the collapsing core forms a black hole and a significant amount of fallback of material occurs, giving the low-luminosity and low-nickel mass. Instead, SN 2003gd appears to be a member of a continuous heterogeneous group of SNe II-P. This thesis also includes a study of Sher 25, a curious BSG in NGC 3603, which has an impressive emission line nebula in the form of an apparent circumstellar ring and bipolar nebulae. The appearance of the nebula of Sher 25 bears a striking similarity to the rings around the remnant of SN 1987A, which were almost certainly there before the supernova exploded.

The high FIR luminosity of starburst (SB) galaxies is a direct measure of their SFR and implies an high SN rate. This seems in contradiction with the fact that very few SNe have been discovered in SB. Likely, this is related to the presence of dust that introduce a bias due to extinction . A search in the IR, because of the reduced extinction, is deemed to detected the hidden SN population. Early attempts of NIR searches (e.g Mannucci et al 2003, Mattila et al 2007) show no evidence of enhanced SN rate in starbursts, but statistics is still very low. For this reason we started a new complementary search to measure the SN rate using an excellent instrument as HAWK-I@VLT in a sample of 30 nearby starburst galaxies. We obtained about 270 visits during 3 different runs, during which we found 6 SNe: 4 with spectroscopic confirmation and 2 without spectroscopic confirmation, due to their faintness. Through 500 MonteCarlo simulations we extimated, from SFRs based on FIR luminosity, the expected number of events in our search and, with the adopted parameters (IMF, extinction distribution, SF distribution...) and the measured search detection efficiency, we predict, on average, the discovery of 5.4+/-2.3 SNe. The expected number agrees very well with the observed number. Indeed we found that the expected number of SNe is <= to 6 in 55% of the experiments. Even if we consider only the SNe with spectroscopic classification (4) their occurrence is significant (22%). We also discussed the effects of different assumptions and choices of the model parameters. The main conclusion is that the number of expected SNe is consistent with the observations, so not confirming the claim of previous infrared SN search that estimated a number of expected SNe higher than observed. In another part of the PhD project we used LBC@LBT to estimate the SN rate at high redshift, in order to have an independent check of the claim of a decline of the Ia SNe rates at z > 0.8. If confirmed, this may have important consequences for the use of Ia SNe as cosmological probes. We obtained only 2 of the 6 epochs planned. However, a sample of 14 candidates have been found, a number consisent with expectations (18). Unfortunately, although this is a very interesting and promising work, the chances of success were limited by bad weather and scheduling problems. The first chapter introduce the phenomenon of supernovae, with thier classification and thier use as cosmological probes. In the second chapter is described the star formation history, with the main indicators of star formation. Is shown also the supernova rate problem introduce in the work of Horiuchi et a. (2011). The third chapter addresses the fundamental relation between star formation rate and supernova rate, with the different implications of the CC SN rate and type Ia rate. Then i show the measurements of the SN rate during the years at different redshift. In this context, a description of our work with LBT is shown. To introduce the next chpater, i describe the importance to analyze the SN rate in starburst galaxies, showing at the same time previous results of optical and IR SN searches in starburst galaxies. Th fourth chapter is a detailed description of our search strategy and data reduction. In the second part of the chapter is shown the SN sample, with description and light curves of all SN discovered. The fifth chapter is focused on the estimation of the expected SN rate with the MonteCarlo simulation. A detailed description of the simulation tool qith the main input parameter is given. In the second part i describe the results of simulation, comparing to the observed number. In the last chapter i summarized the entire work, with the scientific conclusion and the future perspective of the IR searches and the SN searches in starburst galaxies.
L'elevata luminosità nel lontano infrarosso delle galassie starburst (SB) è una misura diretta del loro tasso di formazione stellare che a sua volta implica un'elevata frequenza di supernovae. Questo tuttavia sembra in contraddizione con il fatto che sono state scoperte poche supernovae in queste galassie. Molto probabilmente questo è legato alla presenza di grandi quantità di polvere che introducono un bias dovuto al fenomeno dell'estinzione. Quindi, una ricerca di supernovae condotta alle lunghezze d'onda infrarosse, per via della ridotta estinzione, è più indicata per rilevare la poplazione di supernovae oscurata dalla polvere. I primi tentativi di osservare queste seprnovae nel vicino infrarosso non hanno tuttavia mostrato un aumento evidente della frequenza di supernovae in galassie starburst, anche se fino ad ora la statistica rimane piuttosto bassa. Per questa ragione abbiamo iniziato un nuovo e complementare progetto che consiste nella ricerca di supernovae in un campione di 30 galassie starburst con lo scopo di misurare la frequenza di supernovae, utilizzando un eccellente strumento quale HAWK-I@VLT. Abbiamo ottenuto un totale di 270 epoche durante 3 differenti periodi di osservazioni, durante i quali abbiamo trovato 6 supernovae: 4 con conferma spettroscopica, 2 senza a causa della loro luminosità troppo debole. Attraverso 500 esperimenti MonteCarlo abbiamo stimato, partendo dal tasso di formazione stellare basato sulla luminosità nel lontano infrarosso, il numero atteso di supernovae nella nostra campagna osservativa, e, con i parametri adottati (concernenti la funzione iniziale di massa, l'estinzioni, la distribuzione della formazione stellare...) e la misura della magnitudine limite di tutte le epoche, ci aspettiamo, in media, la scoperta di 5.4+/-2.3 supernovae. Il numero atteso è quindi in ottimo accordo con quello delle supernovae osservate. Infatti il numero di supernovae attese è <= a 6 nel 55% degli esperimenti. Anche se consideriamo solo il numero di supernovae confermate spettroscopicamente (4), il loro verificarsi è comunque significativo (22%). Abbiamo inoltre discusso gli effetti delle differenti assunzioni e scelte dei parametri della simulazione. La principale conclusione è che il numero di supernovae è consistente con le osservazioni, non confermando in tal modo i risultati dei precedenti lavori di ricerca di supernovae nell'infrarosso, ovvero che la frequenza di supernovae aspettata risultava essere più alta di quello osservata. In un'altra parte del progetto di dottorato abbiamo utilizzato LBC@LBT per stimare la frequenza di supernovae ad alti redshift, con lo scopo di verificare il declino della frequenza di supernovae di tipo Ia a redshift >0.8. Se confermato, questo potrebbe avere importanti conseguenze sull'utilizzo delle supernovae Ia in ambito cosmologico. Abbiamo ottenuto solo 2 epoche delle 6 previste, riuscendo comunque a scoprire 14 robusti candidati (un numero consistente con le attese, 18). Sfortunatamente, sebbene questo fosse un progetto molto interessante e promettente, le chances di successo sono state severamente limitate dal cattivo tempo e dai numerosi problemi sofferti dallo strumento. Il primo capitolo introduce il fenomeno delle supernovae, con la relativa classificazione e la loro importanza in ambito cosmologico. Il secondo capitolo è dedicato all'importanza dello studio della formazione stellare in cosmologia, con la descrizione dei relativi indicatori. Viene anche illustrato il problema relativo alla frequenza di supernovae così come descritto in Horiuchi et al. (2011). Il terzo capitolo è dedicato alla fondamentale relazione tra tasso di formazione stellare e frequanza di supernovae. Vengono quindi illustrate le diverse misure della frequenza di supernovae negli anni a differenti redshift. viene poi descritto brevemente il nostro lavoro sulla frequenza di supernovae ad alti redshift con l'utilizzo del telescopio LBT. In seguito, vengono introdotti i concetti base per i futuri capitoli: cosa sono e perchè sono importanti le galassie starburst e i precedenti lavori di ricerca di supernovae nell'ottico e nell'infrarosso. Nel quarto capitolo viene descritta la strategia osservativa ed il procedimento di riduzione dei dati. La seconda parte è dedicata alla desfrizione del campione di supernovae trovate. Il quinto capitolo introduce i dettagli della simulazione MonteCarlo per stimare il numero di supernovae aspettate, e confronta questo numero con quello osservato. Il quinto capitolo è dedicato al riassunto del lavoro con relativo conclusioni scientifiche e prospettive future della ricerca infrarossa di supernovae e della ricerca in generale in galassie starburst.

SPitzer InfraRed Intensive Transients Survey-SPIRITS-is an ongoing survey of nearby galaxies searching for infrared (IR) transients with Spitzer/IRAC. We present the discovery and follow-up observations of one of our most luminous (M-[4.5] = -17.1 +/- 0.4 mag, Vega) and reddest ([3.6] - [4.5] = 3.0 +/- 0.2 mag) transients, SPIRITS 15c. The transient was detected in a dusty spiral arm of IC. 2163 (D approximate to 35.5 Mpc). Pre-discovery ground-based imaging revealed an associated, shorter-duration transient in the optical and near-IR (NIR). NIR spectroscopy showed a broad (approximate to 8400 km s(-1)), double-peaked emission line of He I at 1.083 mu m, indicating an explosive origin. The NIR spectrum of SPIRITS 15c is similar to that of the Type IIb SN 2011dh at a phase of approximate to 200 days. Assuming an A(V) = 2.2 mag of extinction in SPIRITS 15c provides a good match between their optical light curves. The NIR light curves, however, show some minor discrepancies when compared with SN 2011dh, and the extreme [3.6]-[4.5] color has not been previously observed for any SN IIb. Another luminous (M-4.5 = -16.1 +/- 0.4 mag) event, SPIRITS 14buu, was serendipitously discovered in the same galaxy. The source displays an optical plateau lasting greater than or similar to 80 days, and we suggest a scenario similar to the low-luminosity Type. IIP SN 2005cs obscured by A(V) approximate to 1.5 mag. Other classes of IR-luminous transients can likely be ruled out in both cases. If both events are indeed SNe, this may suggest that greater than or similar to 18% of nearby core-collapse SNe are missed by currently operating optical surveys.