Academic literature on the topic 'Collapse'

Large waves cause a great number of collapsed-ship accidents, resulting in the loss of many lives and properties. It has been found that most of these collapses are caused by encountering oblique waves. As a result, the ship structure experiences a complex collapse under combined bending and torsion. This paper utilizes a numerical hydroelasto-plastic approach, coupling CFD (Computational Fluid Dynamics) with the nonlinear FEM (Finite-Element Method), to study the structural collapse of a containership in oblique waves. First, a 4600 TEU containership was selected to study its collapse mechanism under oblique waves. Second, a hydroelasto-plastic numerical coupling of CFD and nonlinear FEM is used to co-calculate the wave loads and structural collapse of containership. The hydrodynamic model is constructed and used to solve wave loads in the CFD solver, and a nonlinear FEM model of containership with finer meshes is also modeled to solve the structural collapses, including plasticity and buckling. Third, several oblique-wave cases involving heading angles of 120°, 135°, 150°, and 180° are determined and calculated. Typical cases are discussed for time-domain stress histories and collapsed courses. Finally, the influence of oblique-wave parameters on structural collapse is discussed, and the collapse mechanism of containerships under the action of oblique waves is obtained, which provides a new understanding of ship structure design.

The occurrence of earthquakes on the Korean Peninsula has been increasing in recent years. However, current emergency action plans for dam and reservoir failures only consider scenarios related to flooding but ignore dam collapses caused by earthquakes. In this study, we simulated dam collapses caused by earthquakes. We focused on the Chungju Dam watershed located in the upper reaches of the Namhan River and analyzed the dam collapse and downstream impacts using the DAMBRK model. We compared the downstream impacts of dam collapse caused by earthquakes with those caused by the PMF. When the dam collapsed because of an earthquake, the estimated peak flood discharge downstream was 44.9%-86.7% of that of the PMF scenario. Similarly, the simulated peak water level downstream when the dam collapsed due to an earthquake was 64.4%-96.9% of that of the PMF scenario. These results indicate that dam collapses caused by seismic events, even in the absence of rainfall, can lead to significant damage downstream. Therefore, future emergency action plans for dams should consider the potential for dam failures caused by earthquakes.

Buildings are vulnerable to collapse incidents. We adopt a workflow to detect unusual vertical surface motions before building collapses based on PS-InSAR time series analysis and spatiotemporal data mining techniques. Sentinel-1 ascending and descending data are integrated to decompose vertical deformation in the city of Alexandria, Egypt. Collapsed building data were collected from official sources, and overlayed on PS-InSAR vertical deformation results. Time series deformation residuals are used to create a space–time cube in the ArcGIS software environment and analyzed by emerging hot spot analysis to extract spatiotemporal patterns for vertical deformation around collapsed buildings. Our results show two spatiotemporal patterns of new cold spot or new hot spot before the incidents in 66 out of 68 collapsed buildings between May 2015 and December 2018. The method was validated in detail on four collapsed buildings between January and May 2019, proving the applicability of this workflow to create a temporal vulnerability map for building collapse monitoring. This study is a step forward to create a PS-InSAR based model for building collapse prediction in the city.

The tubing used in a gas well rarely collapses and fails during applying annulus pressure. In this study, the failure causes of tubing collapse were analyzed by means of data verification, macroscopic observation, magnetic particle inspection, physical and chemical inspection, optical microscopy, and tubing collapse test. Mechanical analysis of the string and full-scale physical simulation test simulating downhole working conditions. Finally, the verification analysis of the collapse test is carried out by the finite element analysis (FEA). The results showed that (1) the physical dimension, physical and chemical properties, and collapse resistance of this batch of tubing met the requirements of the tubing ordering technical standard. (2) Assuming that the well packer slip was unsealed and could slide freely, the mechanical theoretical analysis of collapsed tubing string and collapse test under simulated working condition load was carried out, which reproduced the load when the tubing collapsed. It can be seen from this that the packer did fail. (3) The FEA calculation results showed that when the external pressure was greater than 30.75 MPa, it would inevitably lead to collapse failure in case of packer unsealed. In conclusion, the root cause for the collapse failure of the 105th underground tubing string was that the packer lost its sealing function, resulting in an abnormal axial load. While under the action of external pressure, the tubing was overloaded and collapsed. It is recommended to carry out verification tests on the material performance of packer slip, the dimensional changes of packer tool outer diameter and inner diameter under actual well conditions, the creep behavior of packer seal, and the performance of shear pin under actual working conditions, especially in the well containing H2S, so as to prevent the pressure leakage of gas well annulus caused by packer unsealing and the reoccurrence of such downhole string collapse accidents. The first collapse test under simulated working condition load is conducted in this paper. Analyzing the collapse failure work and putting forward suggestions to effectively prevent similar failures from happening again are of great significance to the oilfield.

Bridge collapse risk can be evaluated more rigorously if the hydrologic characteristics of bridge collapse sites are demystified, particularly for peak flows. In this study, forty-two bridge collapse sites were analyzed to find any trend in the peak flows. Flood frequency and other statistical analyses were used to derive peak flow distribution parameters, identify trends linked to flood magnitude and flood behavior (how extreme), quantify the return periods of peak flows, and compare different approaches of flood frequency in deriving the return periods. The results indicate that most of the bridge collapse sites exhibit heavy tail distribution and flood magnitudes that are well consistent when regressed over the drainage area. A comparison of different flood frequency analyses reveals that there is no single approach that is best generally for the dataset studied. These results indicate a commonality in flood behavior (outliers are expected, not random; heavy-tail property) for the collapse dataset studied and provides some basis for extending the findings obtained for the 42 collapsed bridges to other sites to assess the risk of future collapses.

AbstractAs self-gravitating systems, dense star clusters exhibit a natural diffusion of energy from their innermost to outermost regions, leading to a slow and steady contraction of the core until it ultimately collapses under gravity. However, in spite of the natural tendency toward “core collapse,” the globular clusters (GCs) in the Milky Way exhibit a well-observed bimodal distribution in core radii separating the core-collapsed and non-core-collapsed clusters. This suggests an internal energy source is at work, delaying the onset of core collapse in many clusters. Over the past decade, a large amount of work has suggested that stellar black holes (BHs) play a dynamically-significant role in clusters throughout their entire lifetimes. Here we review our latest understanding of BH populations in GCs and demonstrate that, through their dynamical interaction with their host cluster, BHs can naturally explain the distinction between core-collapsed and non-core-collapsed clusters through a process we call “black hole burning.”

It is necessary to repair the cracks in the concrete structure to prevent further damage that can lead to the collapse of the structure. Repairs can restore and increase the strength of structural elements so that they are able to withstand the load in accordance with the design load, finally further decisions can be made for the next function. The results of the flexural strength test of K-150 collapsed concrete beams 100% able to withstand a load of 58 kN, K-250 collapsed 100% able to withstand a load of 64.33 kN, K-150 coated with CRFP tensile part collapsed 100% able to withstand a load of 82 kN, K -250 is coated with CRFP, the tensile part collapses 100% able to withstand a load of 91.67 kN, the quality of K-250 collapses 60% is able to withstand a load of 39.17 kN. K-250 strength concrete beam collapsed 60% maximum CRFP repair able to withstand a load of 87.67 kN. For testing of concrete beams with f'c quality of 20.71 MPa, 100% collapse with a maximum average of 29.00 MPa. The flexural strength of the concrete beam with f'c quality is 20.71 MPa, 60% collapse with an average load of 19.00 MPa. For the flexural strength of the f'c 20.71 MPa concrete beam, improvements were made to be able to withstand an average load of 52.86 MPa. Analysis of the test results by increasing the area and CRFP can increase the bearing capacity of concrete blocks. If in a construction work there is a doubt from the user about the portal being built, then CRFP can increase its flexural strength.

Abstract Highway tunnel collapse during the operation period is a serious threat to the stability of tunnel operation. Thus, it is important to investigate the disaster-causing mechanism to prevent the occurrence of disasters. In this paper, based on 155 cases of domestic and foreign highway tunnel collapses during operation, we statistically analyze the causes of the collapses and the year of occurrence, and we explore the disaster-causing mechanism of tunnel collapses through numerical simulation. Tunnel collapses in operation period are affected by geological factors, lining factors, and accidental factors. The primary causes of tunnel collapses are determined to be the post-lining cavity, lining crack expansion, and fire. A single factor cannot lead to tunnel collapse, and the probability of collapse increases under the coupling of multiple factors. Through a numerical simulation method, the evolution mechanism of tunnel collapse caused by the post-lining cavity is analyzed, which lays the foundation for establishing the tunnel collapse prediction model and qualitative risk assessment system of highway tunnel collapse.

Abstract. Ice shelf collapse reduces buttressing and enables grounded glaciers to contribute more rapidly to sea-level rise in a warming climate. The abrupt collapses of the Larsen A (1995) and B (2002) ice shelves on the Antarctic Peninsula (AP) occurred, at least for Larsen B, when long-period ocean swells damaged the calving front and the ice shelf was inundated with melt lakes that led to large-scale hydrofracture cascades. During collapse, field and satellite observations indicate föhn winds were present on both ice shelves. Here we use a regional climate model and machine learning analyses to evaluate the contributory roles of föhn winds and associated melt events prior to and during the collapses for ice shelves on the AP. Föhn winds caused about 25 % ± 3 % of the total annual melt in just 9 d on Larsen A prior to and during collapse and were present during the Larsen B collapse, which helped form extensive melt lakes. At the same time, the off-coast wind direction created by föhn winds helped melt and physically push sea ice away from the ice shelf calving fronts that allowed long-period ocean swells to reach and damage the front, which has been theorized to have ultimately triggered collapse. Collapsed ice shelves experienced enhanced surface melt driven by föhn winds over a large spatial extent and near the calving front, whereas SCAR inlet and the Larsen C ice shelves are affected less by föhn-wind-induced melt and do not experience large-scale melt ponds. These results suggest SCAR inlet and the Larsen C ice shelves may be less likely to experience rapid collapse due to föhn-driven melt so long as surface temperatures and föhn occurrence remain within historical bounds.

Abstract Mining explosions and collapses, in addition to earthquakes, may trigger the future Comprehensive Test Ban Treaty (CTBT) monitoring system. Many shallow, spontaneously occurring mine collapses have implosional source mechanisms that might provide a physical basis to discriminate them from explosions. In this study, an explosively induced mine collapse was investigated. The collapse occurred immediately after the support pillars of a 320-m-deep underground mine opening were destroyed by explosives. It had an Lg body-wave magnitude (mbLg) of 2.8. We analyzed free-surface ground-motion data (within 1200 m) from the collapse by waveform forward modeling and time-dependent source moment-tensor inversion. The results indicate that the source mechanism of the collapse can be represented by a horizontal opening and closing crack. The time functions of the diagonal source moment-tensor components are similar to that of a spall source accompanying an underground explosion. A unique source characteristic of the induced collapse is that, unlike spontaneous collapses, the induced collapse initiated as a tensile crack. Because of the initially expansive source characteristic, this kind of induced mine collapses may pose some difficulties to the seismic discrimination problem. Despite the similarities between the induced mine collapse and underground explosions, the collapse has a more band-limited source spectrum and seems to be more efficient in shear and surface-wave generation.

Through large sculptural works that are often caricatures of representational objects, my work explores the complicated moments and tangled histories of childhood Jewish schooling in Winnipeg and travels to Israel and Palestine as an adult. My thesis exhibition Collapse, as well as most of my graduate work, examines my investigation through manmade constructions that control and restrict or unite and connect the movement of others. Sculptures about a destroyed bridge’s imagined longing for exotic places, a giant onion serving as a resuscitation mechanism against tear gas or a construction crane to Armageddon are some examples of work that explore the poetry I find in dichotomies, and serve as a series of recollections that negotiate experiences beyond full understanding.

This project is an active exploration of subjectivities through the medium of weaving. In a narrative illustrated with woven works, the emotional trajectory of this person of mixed cultural background through Swedish society is described. First, the work is contextualized in terms of the larger politico-cultural-discursive context, entitled the outside. It is then positioned in terms of the individual context, the inside, largely dramatized as the developments in the art of western tapestry in the past century. This culminates in the final works, three of which are presented in the final exhibition. They are entitled monads, the etymology of which, in Greek, relates to the words one, alone, or singularity. Each is a conceptual microcosm proposing a different light distribution scheme. They are imprints of alternative actualizing tendencies. There could only be one; at the end of the experiment, Schrodinger’s cat is either dead or alive. The collapse involves the very structure of the ground, and is therefore embedded directly into the weave structure. But remember, each act of observation is an irreversible disturbance to the system. In the interpretation of the primary emanation arises difference, but in which direction will the pendulum swing, and on what forces does its motion hinge? Do subjectivities follow the locality assumption? Do you produce subjectivities or do they produce you?

This dissertation examines the collapse of masonry structures in response to large support displacements and horizontal ground accelerations. There are two main classes of masonry structure: arches that thrust, and supporting elements, such as walls and buttresses, which resist the thrust. This dissertation analyses the safety of arches and buttresses and identifies the resulting collapse mechanisms due to support displacements or horizontal accelerations. In particular, this research investigates the stability of a masonry arch supported on buttresses and the conditions necessary for collapse to occur. Engineers are frequently asked to determine the safety of masonry structures that have been severely distorted over the years, often due to subsidence or other long-term movements in the foundations, and this dissertation provides guidance in the assessment of such structures. The resistance of masonry buttresses to high-level horizontal loads is examined. In the case of failure due to overturning, a fracture will develop in the masonry, significantly reducing the resistance of the buttress. The capacity is further reduced by outward leaning of the buttresses, a common source of distress for masonry structures due to movements in the supporting foundations. Based on these considerations, new measures of safety are proposed for buttresses under horizontal loading. Outward leaning of the buttresses increases the span of the arch or vault. Spreading supports will cause large deformations in the arch, which increase the horizontal thrust of the arch and may lead to collapse. In addition, lateral ground accelerations can cause the collapse of arches. The influence of seismic action can be approximated to first order by equivalent static analysis to determine the initial collapse mechanism. These problems are analysed for circular masonry arches, and the collapse conditions are identified for various geometries. The findings are combined to investigate the stability of the masonry arch supported on buttresses. The safety of the system is examined by studying the influence of imposed displacements. As the buttresses lean, the thrust of the vault increases and the resistance of the buttress decreases. The collapse mechanisms are identified for both the static case of leaning buttresses and the dynamic case of horizontal acceleration. This analysis illustrates that the arch will collapse and the buttresses will remain standing in most cases. Based on these considerations, new methods are proposed for assessing the safety of masonry structures and determining the influence of future movements on the stability of existing masonry structures.

The thesis is concerned with the modelling of urinary motion during bladder collapse and is mathematically based. The bladder model as a collapsing vessel is developed as a step towards complementing use of nomograms. Urine motion inside is taken as unsteady flow of incompressible fluid, while the dimensions and collapse rate of the vessel are prescribed using data which is close to realistic biological values. Evolutions of velocities, volume ow rates and effects of the collapse rate are examined. An introduction is made which includes lower urinary tract urodynamics as well as the unique feature that the bladder changes shape and size substantially compared with other vessels. An investigation of simple two-dimensional shapes takes place in chapter two, along with limiting cases for thin vessels. Chapter three analyses simple axisymmetric shapes especially the sphere because of its relevance in addition to its fundamental nature. Development of a numerical scheme is addressed in the next chapter to tackle more complex shapes through the boundary element method and an iterative finite difference scheme with emphasis on flexibility of approach. An extension to more advanced structures of the vessel is constructed in chapter five by means of a concise boundary condition and shape definition. Chapter six takes the work a step further as the approach is applied to axisymmetric configurations. While in an appendix, an extension to implement full viscous effects is then inspected. Finally, further work is discussed in the conclusion.

A numerical approach to including electromagnetism with general relativity is developed using GRAXI as a starting point. We develop a mathematical model describing electromagnetism coupled to a scalar field in an evolving axisymmetric spacetime. As there are numerous formulations of electromagnetism, we evalute different formulations in a limited flat space case. The full curved space system is then developed, using the flat case as a guide to implementing electromagnetism. This model is then implemented using GRAXI as a code base.

2008/2009
We introduce the measurement problem in quantum mechanics and we briefly discuss the solutions proposed in literature. We then focus our attention on models of spontaneous wavefunction collapse. We describe the two most popular models (GRW, CSL) and list other proposals. We analyze in detail a third collapse model (QMUPL), which is particularly simple (but physically meaningful) to be studied in great mathematical detail. We discuss its main properties. We also describe a "finite temperature" version of this model, which includes dissipative terms. These models are Markovian, i.e. the collapse mechanism is driven by a white noise. Since the ultimate goal is to identify the noise responsible for the collapse with a random field in Nature, it becomes important to study non-Markovian generalizations of collapse models, where the collapsing field has a generic correlation function, likely with a cut off at high frequencies. Models of this kind have already been studied, as a generalization of the CSL model. In this thesis we describe in mathematical detail the generalization of the QMUPL model to non-Markovian noises. After having proved, under suitable conditions, the separation of the center-of-mass and relative motions for a generic ensemble of particles, we focus our analysis on the time evolution of the center of mass of an isolated system (free particle case). We compute the explicit expression of the Green's function via the path integral formalism, for a generic Gaussian noise. We analyze in detail the case of an exponential correlation function, providing the exact analytical solution. We next study the time evolution of average quantities, such as the mean position, momentum (which satisfy Ehrefest's theorem) and energy (which is not conserved like in the other collapse models). We also compute the non-Markovian master equation for an harmonic oscillator, according to this model, and compare its structure to the well-known Lindblad structure of Markovian open quantum systems. We eventually specialize to the case of Gaussian wave functions, and prove that all basic facts about collapse models (reduction process, amplification mechanism, etc.), which are known to be true in the white noise case, hold also in the more general case of non-Markovian dynamics. We further analyze the evolution of Gaussian wave function according to the three different realizations of the QMUPL model so far developed (Markovian, non-Markovian and "finite temperature"), comparing their fundamental features. Finally, by analyzing different localization criteria, we set new lower bounds on the parameters of these models, and we compare them with the upper bounds coming from known experimental data.
Nel primo capitolo si introduce il problema della misura in Meccanica Quantistica e si discutono brevemente le soluzioni proposte nella letteratura. Nel capitolo 2 si discutono i modelli di collasso spontaneo della funzione d'onda, con particolare attenzione per i modelli GRW e CSL; si elencano altri modelli. Si analizza in dettaglio anche il modello di riduzione QMUPL, il quale è particolarmente semplice (ma fisicamente significativo) da poter essere studiato dettagliatamente dal punto di vista matematico. Si discutono le sue proprietà principali. Si descrive inoltre una versione "a temperatura finita" di questo modello, che include termini dissipativi. Questi modelli sono Markoviani, ovvero il meccanismo di collasso è guidato da un rumore bianco. Poichè parte significativa della ricerca consiste nell'identificare il rumore responsabile del collasso con un campo stocastico esistente in Natura, diventa importante studiare le generalizzazioni non-Markoviane dei modelli di riduzione, in cui il campo di collasso ha una funzione di correlazione generale, probabilmente con un cutoff ad alte frequenze. Modelli di questo tipo, come la generalizzazione del modello CSL, sono già stati studiati. In questa tesi si descrive in dettaglio la generalizzazione a rumori non-Markoviani del modello QMUPL. Dopo aver provato, sotto particolari condizioni, la separazione del moto del centro di massa da quello relativo per un generico ensemble di particelle, si pone attenzione all'evoluzione temporale del centro di massa di un sistema isolato (particella libera). Si dà l'espressione esplicita per la funzione di Green attraverso il formalismo del path-integral, per un generico rumore Gaussiano. Si analizza in particolare il caso della funzione di correlazione esponenziale, fornendo la soluzione analitica esatta delle equazioni. Successivamente si studia l'evoluzione dei valori medi, in particolare della posizione, del momento (che soddsfa il teorema di Ehrenfest) e dell'energia (che non è conservata come negli altri modelli di riduzione). Si scrive inoltre la master equation non-Markoviana per un oscillatore armonico per questo modello, e si confronta la sua struttura con le ben nota struttura di Lindblad dei sistemi quantistici aperti Markoviani. Ci si specializza al caso di funzioni d'onda Gaussiane, e si prova che tutte le nozioni di base sui modelli di riduzione (processo di collasso, meccanismo di amplificazione, ecc.), che sono note essere vere nel caso Markoviano, valgono anche nel caso più generale di dinamiche non-Markoviane. Infine, si analizza l'evoluzione di funzioni d'onda Gaussiane secondo le tre differenti realizzazioni del modello QMUPL finora analizzate (Markoviana, non-Markoviana e "a temperatura finita"), confrontando le loro caratteristiche fondamentali. Inoltre, analizzando differenti criteri di localizzazione, si individano nuovi limiti inferiori per i parametri di questi modelli, e si confrontano con i limiti superiori che vengono da dati sperimentali noti.
XXII Ciclo
1982

This Thesis is focused on a photometric and spectroscopic study of four Type IIn supernovae (i.e. SN 20 06gy, 2007bt, 2007bw and 2008fz), which are among the brightest supernovae (SNe) ever detected. They belong to the sample of overluminous or Very Luminous SuperNovae (VLSNe) which currently includes other 3-4 well studied events. Their absolute luminosity at maximum, MV < -20 is much higher than any other previous supernovae, either core-collapse and or thermonuclear. Their huge brightness (> 10^(51)erg) are emitted in the first »200 days) link these events to massive or supermassive progenitors, which experi- enced extreme mass-losses during their last stages of evolution. However, other explosion mechanisms or sources of energy are being investigated; the debate on their nature is still open. The first object discussed in this Thesis is SN 2006gy, which is one the most debated supernovae ever. Contrary to typical IIn SNe, this event did dot show any strong x-rays or radio emission near the epoch of maximum. This has lead to consider other feasible non-standard sources of energy beyond interaction. In this thesis, the evolution of multiband light curves, the pseudo-bolometric (BVRI) light curve and an extended spectral sequence are presented and used to derive constraints on the origin and evolution of the nature of the SN. Its light curve is characterized by a broad, bright (MR = -21.7 at about 70 days) peak, followed by a rapid luminosity fading which turns into a slower decline by day 180. At late phases (> 237 days), because of the large lu- minosity drop (> 3 mag), only upper visibility limits are obtained in the B, R and I bands. In the near-infrared, two K-band detections on days 411 and 510 possibly indicate dust formation or IR echoes scenarios. At all epochs the spectra are characterized a multicomponent Halpha profile, without any P-Cygni. By means of a semi-analytical code, the light curve in the first 170 days is found to be consistent with the explosion of a compact progenitor (R (6 -8 x 10^(12) cm, Mej =5 -14Msol), whose ejecta collided with massive (6 - 10Msol¯), opaque clumps of previously ejected material. These clumps do not completely obscure the SN photosphere, so that at its peak the luminosity is due both to the decay of 56Ni and to interaction with the circumstellar medium (CSM). After 170 days spectroscopic and photometric similarities are found between SN 2006gy and bright, interaction-dominated SNe (e.g. SN 1997cy, SN 1999E and SN 2002ic). This suggests that ejecta-CSM interaction plays a key role in SN 2006gy about 6 to 8 months after maximum, sustaining the late-time-light curve. Alternatively, the late luminosity may be related to the radioactive decay of 3Msol of 56Ni. In this scenario, a supermassive star is not required to explain the observational data, nor is an extra-ordinarily large explosion energy. For the SNe 2007bt, 2007bw and 2008fz UBVRI light curves and an extended spectral sequence are also presented. Analogies and differences are highlighted, both among each other and with respect to the sample of VLSNe from the literature. Photometrically, it is shown that the light curves of SNe 2007bt and 2007bw are substantially dfferent from that of SN 2008fz, evolving more slowly, being redder at the earlier phases and decaying with a rate consistent with that pre- dicted by the radioactive decay of 56Co. On the contrary, the photometric evolution of SN 2008fz is reminiscent to the light curves of IIL SNe, showing a short peak followed by a steep decline. Spectroscopically the three events are characterized by high-velocity (up to 12000km/s), slowly-decelerating emission lines. The spectra of the SNe 2007bt and 2007bw are dominated by Balmer lines, overimposed on a relatively °at continuum (TBB = 6000 ¡ 7000 K); an asymmetry in the early profile of Halpha is observed, slowly disappearing with time. Measurements of the narrow components of Halpha in SN 2007bt indicate CSM speed of 320 km/s , which are only consistent with the winds surrounding luminous blue variable (LBV) stars. The early spectra of SN 2008fz are found to be similar to SN 2006gy; however, they show higher temperatures (TBB=14000 K) and a more rapid evolution. For the three events, the energetic, luminosity, initial radius (> 10^(15)cm) and the kinematic derived from the analysis of the light curves and spectra could be reproduced by the conversion of kinetic energy into radiation by a clumpy CSM which is hit by the energetic SN ejecta, similarly to what was proposed for SN 2006gy. For SNe 2007bt and 2007bw the asymmetry in the Halpha line can be explained if a massive (>10 Msol) clumpy CSM lies face-on in the direction of the observer. The asymmetry in the CSM distribution around the star could be due by a binarity fefect in the progenitor system, or asymmetric mass ejection of a single star. For SN 2008fz the rapid expansion of the black-body radius favor a less massive CSM ( 1Msol), which is efficiently warmed up and accelerated by the high-velocity SN ejecta. Because of the relatively small mass in the CSM/shell, the photon diffusion time is smaller than that calculated for SN 2006gy, and the radiated energy plummets rapidly as the light curve. As for the case of SN 2006gy, these scenarios have the advantage that they do not involve any exotic explosion mechanism for these VLSNe. However, other scenarios could be consistent with their photometric evolution. Among these, the possibility of a pair-instability explosion cannot be excluded. This and other likely hypothesis proposed by other authors are discussed.
Questa Tesi si incentra sullo studio fotometrico e spettroscopico di quattro su pernovae (SNe) di tipo IIn (cioµe SN 2006gy, 2007bt, 2007bw e 2008fz), che sono tra le supernovae più brillanti mai scoperte. Infatti appartengono alla classe delle SNe iperluminose o Very Luminous SuperNovae (VLSNe, supernovae molto brillanti), che al momento include altri 3-4 oggetti ben studiati. La loro luminosità assoluta all'epoca del massimo, MV < -20, è superiore rispetto a qualsiasi altro evento, sia di natura termonucleare che di collasso del core. L'enorme luminosità emessa (> 10^(51)erg nei primi 200 giorni) associa questi eventi a progenitori massicci o supermassicci, che hanno subito fenomeni di perdita di massa estremi durante le loro fasi evolutive finali. Comunque, al momento si stanno studiando anche altri meccanismi o possibili fonti di energia, e il dibattito sulla natura di questi eventi è tuttora aperto. Il primo oggetto discusso è la SN 2006gy, che è una delle supernovae più dibattute in assoluto. Contrariamente alle tipiche IIn, essa non mostrava alcuna emissione X o radio all'epoca del massimo di luminosità. Questo ha portato a considerare altre possibili sorgenti di energia oltre all'interazione. In questa Tesi, l'evoluzione delle curve di luce multibanda, la curva di luce pseudo-bolometrica e una sequenza di spettri vengono studiati per ricavare delle infor- mazioni sull'evoluzione e sulla natura della supernova e del progenitore. La curva di luce µe caratterizzata da un picco ampio e luminoso (MR = -21.7 a circa 70 giorni), seguito da un declino di luminosità veloce, il quale si assesta su un declino piµu lento, a circa 180 giorni. A fasi avanzate (>237 giorni), a causa del forte indebolimento della luminosità (>3 mag) vengono ricavati solo dei limiti ottici nelle bande B, R ed I. Nel vicino infrarosso, due detection nella banda K' indicano una possibile presenza di regioni di formazione di polvere, o eventualmente di echi infrarossi. A tutte le epoche gli spettri sono caratterizzati dalla presenza di pro¯li di righe a multi-componente, senza però alcun pro¯lo P-Cygni. Tramite un codice semi-analitico si trova che la curva di luce nei primi 170 giorni è consistente con l'esplosione di un progenitore compatto (R = 6-8 x 10^(12)cm, Mej = 5-14Msol), le cui ejecta collidono con dei clumps massicci (6-10 Msol) e opachi di materiale espulso precedentemente. Tali clumps non oscurano completamente la fotosfera della supernova, cosicché all'epoca del picco la luminosità è dovuta sia al decadimento radioattivo del 56Ni che all'interazione con il mezzo circumstellare. Vengono inoltre evidenziate, a partire da circa 170 giorni, delle analogie fotometriche e spettroscopiche tra la SN 2006gy e un gruppo di supernovae interagenti (cioè SN 1997cy, 1999E e 2002ic). Ciò suggerisce che l'interazione tra ejecta e CSM gioca un ruolo importante anche nella SN 2006gy a circa 6-8 mesi dal massimo, sostenendo la curva di luce a fasi avanzate. In alternativa, la luminositµa a queste fasi potrebbe essere dovuta al decadimento radioattivo di 3Msol di 56Ni. Questo scenario non richiede la presenza di una stella supermassiccia o di un'energia straordinariamente grande per spiegare i dati osservativi. Anche per le supernovae 2007bt, 2007bw e 2008fz vengono presentate delle curve di luce UBVRI e una sequenza di spettri estesa. Vengono messe in luce analogie e differenze tra tali supernovae e tra le VLSNe in letteratura. Dal punto di vista fotometrico si mostra che le curve di luce delle SNe 2007bt e 2007bw differiscono sostanzialmente da quella della SN 2008fz, poiché evolvono più lentamente, sono piµu rosse a fasi iniziali e decadono ad un tasso consistente con quello predetto dal decadimento del 56Co. Spettroscopicamente i tre eventi sono caratterizzati da righe di emissione ad alte velocità, ¯fino a 12000 km/s . Gli spettri delle supernovae 2007bt e 2007bw sono dominati dalle righe di Balmer su un continuo relativamente piatto (TBB = 6000-¡ 7000 K). Inoltre viene osservata un'asimmetria nel profilo iniziale di Halpha, che però si indebolisce col tempo. Dalla misura della componente strette di Halpha nella SN 2007bt si ricavano velocità di 320 km/s , le quali sono consistenti solo con i venti di stelle LBV (luminose, blu, variabili). Si trova inoltre che i primi spettri della SN 2008fz sono consistenti con quelli della SN 2006gy; tuttavia, essi indicano temperature maggiori (TBB = 14000 K) ed un'espansione piµu rapida. Per i tre eventi, l'energia in gioco, la luminositµa, il raggio iniziale (> 10^(15)cm) e la cinematica derivati dall'analisi delle curve di luce e degli spettri potrebbe essere riprodotta dalla conversione di energia cinetica in radiazione da parte di un mezzo circumstellare ricco di clumps, il quale viene raggiunto dalle ejecta energetiche della supernova, similmente a quanto supposto per SN 2006gy. Per le SNe 2007bt e 2007bw l'asimmetria del pro¯lo di Halpha può essere spie- gata se un mezzo massiccio (>10 Msol ) ricco di clumps si trova esattamente davanti all'osservatore, perpendicolarmente alla linea di vista. L'asimmetria nella distribuzione del mezzo circumstellare potrebbe essere dovuta ad effetti di binarietà del sistema del progenitore, o ad espulsioni di materiale asimmetriche in una stella singola. Per la SN 2008fz la rapida espansione del raggio iniziale di corpo nero tende a favorire un mezzo meno massiccio (> 10Msol), il quale viene riscaldato ed accelerato efficientemente dalle ejecta ad alta velocità. A causa della massa relativamente piccola del mezzo, il tempo di diffusione dei fotoni inferiore di quanto calcolato per la SN 2006gy, cosicché l'energia radiativa diminuisce rapidamente, come la curva di luce. Come nel caso della SN 2006gy, il vantaggio di questi scenari è che non involvono alcun meccanismo di esplosione esotico. Tuttavia, la loro evoluzione fotometrica può essere consistente anche con altri scenari. Tra questi, anche l'esplosione di una supernova di instabilità di coppia non può essere esclusa. Questi ed altri scenari vengono discussi nel capitolo conclusivo.

This thesis is organized as follows: In chapter 1 the theory of a massless scalar field in curved spacetime is introduced. Furthermore the Bogolubov transformation are discussed. In chapter 2 the Hawking effect is derived with the necessary approximations. Then it is briefly mentioned the problem of the information loss paradox In chapter 3 the contribution of the Hawking radiation to the bi-dimensional stress energy tensor is analyzed. In chapter 4 a generic model of gravitational collapse is analyzed with emphasis on the quantum effects. Then we restrict to the case of the collapse of a thin shell, and the accuracy of the Unruh state approximation is examined. In chapter 5 the proposals that the emission of pre-Hawking radiation could prevent the formation of an event horizon for a collapsing object are analyzed. In chapter 6 the arguments which aim to disprove the possibility that pre-Hawking radiation could prevent the formation of a black hole are discussed. Then is questioned the existence of pre-Hawking radiation.

This paper analyzes the long-run relationship between output collapsesdefined defined as GDP falling substantially below trendand total factor productivity (TFP), using a panel of 71 developed and developing countries during the period 1960-2003 to identify episodes of output collapse and estimate counterfactual post-collapse TFP trends. Collapses are concentrated in developing countries, especially African and Latin American, and were particularly widespread in the 1980s in Latin America. Overall, output collapses are systematically associated with long-lasting declines in TFP. The paper explores the conditions under which collapses are least or most damaging, as well as the type of shocks that make collapses more likely or severe, and additionally quantifies the welfare cost associated with output collapses.