20 research outputs found
Non-axisymmetric oscillations of differentially rotating relativistic stars
No full textNon-axisymmetric oscillations of differentially rotating stars are studied using both slow rotation and Cowling approximation. The equilibrium stellar models are relativistic polytropes where differential rotation is described by the relativistic j-constant rotation law. The oscillation spectrum is studied versus three main parameters: the stellar compactness M/R, the degree of differential rotation A and the number of maximun couplings [script-I]max. It is shown that the rotational splitting of the non-axisymmetric modes is strongly enhached by increasing the compactness of the star and the degree of differential rotation. Finally, we investigate the relation between the fundamental quadrupole mode and the corotation band of differentially rotating stars
Rotational effects on the oscillation frequencies of newly born proto-neutron stars
No full textIn this paper we study the effects of rotation on the frequencies of the quasi-normal modes of a proto-neutron star (PNS) born in a gravitational collapse during the first minute of life. Our analysis starts a few tenths of a second after the PNS formation, when the stellar evolution can be described by a sequence of equilibrium configurations. We use the evolutionary models which describe how a non-rotating star cools down and contracts while neutrino diffusion and thermalization processes dominate the stellar dynamics. For assigned values of the evolution time, we set the star into slow rotation and integrate the equations of stellar perturbations in the Cowling approximation, both in the time domain and in the frequency domain, to find the quasi-normal mode frequencies. We study the secular instability of the g modes, which are present in the oscillation spectrum due to the intense entropy and composition gradients that develop in the stellar interior, and we provide an estimate of the growth time of the unstable modes based on a post-Newtonian formula
The Slavic Homer: From Grigorios Stavridis to Grigor Prličev
No full textIn 1860 Stavridis/Prličev’s poem Armatol unexpectedly won him the University of Athens poetry competition, which met with opposition from part of the Greek community and a smear campaign in the press. Despite his declarations of having a “Hellenic heart”, the author from Ohrid was not well received by Athenian society. He took part in the competition once more two years later, this time trying to match Homer himself and presenting an extensive though unfinished epic poem with Gjergj Kastrioti Skanderbeg as the protagonist. The paper, which serves as an introduction to the Polish translation of Skanderbeg, contains extensive excerpts from the reports of both competition committees, chaired by Alexandros Rangavis, especially the second one, when the judges justified why they could not award Stavridis the prize despite being impressed with his poem. His disappointment at his cool reception and his failure in the competition most likely contributed to a radical change in the attitude of the Slavic Homer, who not only stopped “serving Greece” but began vigorously eliminating any Greek influences in his native Ohrid
Non radial oscillations of slowly ratating relativistic stars
No full textThis PhD thesis is devoted to the study of the non-radial oscillations of slowly rotating neutron stars, in the framework of General Relativity. We studied these oscillations using the linear perturbation theory. At first we constructed the background stellar model. Then we introduced small perturbations, linearized the Einstein equations and studied the response of the perturbed system by solving the equations describing it, as an initial value problem or as it is usually said by many authors, in the time domain. The basic assumptions that we made in this study are the following, ² The star is a perfect fluid and has zero temperature, ² The star is rotating uniformly with angular velocity and this rotation is a considered as perturbation of the non-rotating stellar model, ² The magnetic field of the star is negligible. All the above assumptions follow from observational facts. Subsequently we introduced small perturbations on this stellar model both on the fluid of the star and the spacetime around it. The perturbation functions are assumed to be of small order ± and in general are function of all the variables of the problem, i.e. t, r, µ, A. We then splitted the perturbation functions into radial and angular parts using spherical harmonics. This is allowed because of the spherical symmetry of the background. We also introduced a dimensionless parameter " = =K, that is the ratio of the angular velocity of the star, over the angular velocity at the mass shedding limit. By assuming that this parameter is small " << 1, i.e. the star is rotating slowly, we calculated the Einstein equations (3.33) and the equations of motion of the fluid (3.38) and linearized them to both parameters " and ±. By integrating the above equations over solid angles we eliminated the angular dependence of the perturbation functions. This way we arrived to a system of partial differential equations (PDEs) of time t and space r that describes the small non-radial perturbations of a slowly rotating neutron star. Having the above system of equations in hand, we tried to solve it numerically and calculate the eigenfrequencies of the system. In order to understand them better we have splitted them into two basic parts, as is common in the bibliography. The part that describes the fluid perturbations and the part that describes the spacetime perturbations. In the literature is common to use the term “Cowling Approximation” when the spacetime perturbations are neglected, and the term “Inverse Cowling Approximation (ICA)” when the perturbations of the stellar fluid are neglected. The first step was to study the part that describes the fluid perturbations, and extract the stellar oscillation modes. By studying this part of the problem we have gained useful information about f, p and r modes of slowly rotating neutron stars. We also studied a interesting phenomenon that appears in this level of approximation, i.e. existence of a continuous spectrum. The continuous spectrum has significant influence on the appearance and the life of the normal modes of the star, for different spherical harmonic indices l. As a second step we have re-written the equations that describe the perturbations of a slowly rotating neutron star, in a new gauge, that has been used up to now only for non-rotating stars. The motivation was that the already existing equations were not very well posed for numerical evolution, due to the existence of mixed second order spatial and temporal derivatives of the perturbation functions. Indeed the equations that we produced in the new gauge seemed more appropriate for numerical evolutions, and they could be rather easily transformed into a first order system. Subsequently we have turned to the old system of equations in the widely used Regge-Wheeler gauge. By redefinition of new variables and lengthy calculations we have managed to re-write it in first order form of evolution equations. As a test for the numerical stability of this system we evolved the part that describes the spacetime perturbations and showed that is numerically stable. For the first time we also calculated frequencies of w-modes for both polytropic and uniform density equations of state. Finally, in order to check the limits of our linear slow rotation approximation for the fluid modes, we added to the perturbed equations of motion of the fluid (3.38) the second order terms in rotation O("2). We then studied the improvement of the accuracy in the calculation of the background model i.e. the mass and the radius. We concluded our study by examining the way the eigenfrequencies of the various oscillation modes is influenced by the inclusion of the second order terms
Bounding the mass of the graviton with gravitational waves: Effect of spin precessions in massive black hole binaries
No full textInternational audienceObservations of gravitational waves from massive binary black-hole systems at cosmological distances can be used to search for a dependence of the speed of propagation of the waves on wavelength, and thereby to bound the mass of a hypothetical graviton. We study the effects of precessions of the spins of the black holes and of the orbital angular momentum on the process of parameter estimation based on the method of matched filtering of gravitational-wave signals vs theoretical template waveforms. For the proposed Laser Interferometer Space Antenna, we show that precessions, and the accompanying modulations of the gravitational waveforms, are effective in breaking degeneracies among the parameters being estimated, and effectively restore the achievable graviton-mass bounds to levels obtainable from binary inspirals without spin. For spinning, precessing binary black-hole systems of equal masses 106M⊙ at 3 Gpc, the lower bounds on the graviton Compton wavelength achievable are of the order of 5×1016km
Precessing supermassive black hole binaries and dark energy measurements with LISA
No full textInternational audienceSpin induced precessional modulations of gravitational wave signals from supermassive black hole binaries can improve the estimation of luminosity distance to the source by space based gravitational wave missions like the Laser Interferometer Space Antenna (LISA). We study how this impacts the ability of LISA to do cosmology, specifically, to measure the dark energy equation of state (EOS) parameter w. Using the ΛCDM model of cosmology, we show that observations of precessing binaries with mass ratio 10∶1 by LISA, combined with a redshift measurement, can improve the determination of w up to an order of magnitude with respect to the nonprecessing case depending on the total mass and the redshift
Partnership for the Americas: Western Hemisphere strategy and U. S. Southern Command
No full textEn el texto se exploran los enlaces geográficos, culturales, económicos y geopolíticos, que Estados Unidos comparte con América Latina y el Caribe. Se describe asimismo algunas de las difíciles condiciones que afectan a la región – pobreza y desigual distribución de las riquezas – y como estas influyen en desafíos específicos como: el crimen, la violencia y el tráfico ilícito de drogas, gente y armamento. The text explores the links geographic, cultural, economic and geopolitical, which the U.S. shares with Latin America and the Caribbean. It describes some of the difficult conditions facing the region - poverty and unequal distribution of wealth - and how are you influencing specific challenges such as crime, violence and drug trafficking, people and weapons.National Defense University PressÍNDICE | Foreword xi | Preface xiii | Introduction xvii | Chapter 1 We're All in This Together 1 | Chapter 2 Have a Plan 23 | Chapter 3 Pulling the Oar Together 49 | Chapter 4 Trafficking 73 | Chapter 5 People First, Human Rights Always 103 | Chapter 6 Health Engagement and Humanitarianism 135 | Chapter 7 Innovation 171 | Chapter 8 Youth Matters 205 | Chapter 9 Looking to the Future 229 | About the Author 263
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Counterterrorism and cybersecurity ::total information awareness /
No full textImagine James Bond meets Sherlock Holmes: Counterterrorism and Cybersecurity is the sequel to Facebook Nation in the Total Information Awareness book series by Newton Lee. The book examines U.S. counterterrorism history, technologies, and strategies from a unique and thought-provoking approach that encompasses personal experiences, investigative journalism, historical and current events, ideas from great thought leaders, and even the make-believe of Hollywood. Demystifying Total Information Awareness, the author expounds on the U.S. intelligence community, artificial intelligence in data mining, social media and privacy, cyber attacks and prevention, causes and cures for terrorism, and longstanding issues of war and peace. The book offers practical advice for businesses, governments, and individuals to better secure the world and protect cyberspace. It quotes U.S. Navy Admiral and NATO's Supreme Allied Commander James Stavridis: "Instead of building walls to create security, we need to build bridges." The book also provides a glimpse into the future of Plan X and Generation Z, along with an ominous prediction from security advisor Marc Goodman at TEDGlobal 2012: "If you control the code, you control the world." Counterterrorism and Cybersecurity: Total Information Awareness will keep you up at night but at the same time give you some peace of mind knowing that "our problems are manmade -- therefore they can be solved by man [or woman]," as President John F. Kennedy said at the American University commencement in June 1963
Current surgical treatment of uterine isthmocele: an update of existing literature
No full textThe prevalence of uterine isthmocele, also known as a uterine niche, has risen in parallel with increasing cesarean section (CS) rates, affecting approximately 60% of women depending on their history of cesarean deliveries. This condition, now categorized as cesarean scar disorder (CSD) by the “Delphi consensus,” is characterized by one primary or two secondary symptoms. Diagnosis can be made through transvaginal ultrasound, sonohysterography, hysteroscopy, or magnetic resonance imaging (MRI). Management of isthmocele may involve pharmacological or surgical interventions. This review aims to provide a thorough analysis of the surgical management options, focusing on postoperative symptom relief, intraoperative and postoperative complications, length of hospital stay, and impact on secondary infertility. PubMed was comprehensively searched for observational studies from inception to 07.08.2024. Surgical treatments include hysteroscopic resection, laparoscopic procedures, and vaginal approaches, all of which offer comparable symptom relief. However, the vaginal approach is associated with a longer hospital stay. The robotic-assisted approach shows promising results but lacks extensive data. Among surgical options, hysteroscopic treatment has the fewest complications but is generally avoided when residual myometrial thickness (RMT) is less than 3 mm. While many CSDs remain asymptomatic, and some women with uterine isthmocele may not wish to conceive, symptomatic patients or those desiring to conceive may benefit from surgical intervention. The choice of procedure should be based on individual patient characteristics, particularly RMT, to define the most appropriate surgical approach. © The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature 2024
Author's personal copy Mid-term results of internal fixation of proximal humeral fractures with the Philos plate
No full textThe proximal humeral fractures account for 5-9% of all fractures and mostly affect elderly osteoporotic patients with highest incidence among women in the ninth decade. The purpose of this study is to analyse our experience from the use of the Philos 1 plate (Synthes, Paoli, PA, USA), for the treatment of three-and four-part proximal humeral fractures. In addition, we aim to investigate the exact indications and contraindications of this implant and recognise the effect various patient factors, fracture patterns and specific technical details have on the final clinical outcome. Results: Healing of the fracture occurred uneventfully within 6 months. In three patients, humeral head collapsed due to aseptic necrosis after fracture healing and the plate had to be removed in two cases. In one patient, fracture healing occurred in >108 varus displacement. The clinical result according to the Constant-Murley score was 86 points (range: 58-112). Conclusions: Internal fixation with the Philos plate seems to be a reliable option in the operative treatment of upper end humeral fractures, especially in osteoporotic bone. It allows secure fracture fixation and quick shoulder mobilisation, while quick and uneventful fracture healing and very satisfactory clinical results are achieved. ß Patients and methods Betwee
