21 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
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
On the Proper Characterization of 'Nonconcatenative' Languages
No full textNonconcatenative languages have been claimed to employ a special type of phonological spreading of a consonant over a vowel, which assumes a representation that segregates vowels and consonants on different planes. I argue that this type of spreading can and must be eliminated from the theory, by reducing it to segmental copying as in reduplication. Crucial to this reduction is the notion of gradient violation of constraints in Optimality Theory (Prince & Smolensky 1993), and the notion of Correspondence with its particular application to reduplicative morphology (McCarthy & Prince 1995a). The reduction is demonstrated in detail for Temiar, one of the main indigenous languages of Malaysia, notorious for the complexity of its copying patterns. Extensions of the proposal to Semitic languages are also discussed. Two main theoretical implications of this reduction are then developed. First, the distinction between concatenative and nonconcatenative languages need not and should not be encoded in terms of the special phonological mechanisms of consonantal spreading over a vowel, applying under planar segregation. Second, the locus of the distinction is found to be, instead, in the mode of affixation employed in nonconcatenative languages, namely, a-templatic reduplicative affixation. This type of affixation is predicted, though heretofore undocumented in the typology of word formation.This is the author's revised version of a chapter that was published in The Articulatory Basis of Locality in Phonology (1999)Gafos, A. (1999). On the proper characterization of 'nonconcatenative' languages. In The articulatory basis of locality in phonology (pp. 73-130). New York, NY: Garland.ISBN: 0815332866 (Published book
Models and Theories of Speech Production
No full textThis eBook is a collection of articles from a Frontiers Research Topic. Frontiers Research Topics are very popular trademarks of the Frontiers Journals Series: they are collections of at least ten articles, all centered on a particular subject. With their unique mix of varied contributions from Original Research to Review Articles, Frontiers Research Topics unify the most influential researchers, the latest key findings and historical advances in a hot research area! Find out more on how to host your own Frontiers Research Topic or contribute to one as an author by contacting the Frontiers Editorial Office: frontiersin.org/about/contac
Alexandros Papadiamantis: Easter chanter
No full textTitle: Λαμπριάτικος Ψάλτης (Easter chanter) Originally published: newspaper ’Aκρόπολις, 1893 Language: Greek The excerpt used is from Panayotis Moullas, Α.Παπαδιαμάντης Αυτοβιογραφούμενος (Athens: Εστία 1999), pp. 100–103. About the author Alexandros Papadiamantis: [Skiathos (central Greece) 1851 – Skiathos 1911]: short story writer and translator. He was the third son of the priest Adamantios, hence the family name (papa-Diamantis). His mother was the offspring of a well-off family from the ..
Alexandros Papadiamantis: Easter chanter
No full textTitle: Λαμπριάτικος Ψάλτης (Easter chanter) Originally published: newspaper ’Aκρόπολις, 1893 Language: Greek The excerpt used is from Panayotis Moullas, Α.Παπαδιαμάντης Αυτοβιογραφούμενος (Athens: Εστία 1999), pp. 100–103. About the author Alexandros Papadiamantis: [Skiathos (central Greece) 1851 – Skiathos 1911]: short story writer and translator. He was the third son of the priest Adamantios, hence the family name (papa-Diamantis). His mother was the offspring of a well-off family from the ..
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
The Missing Link: How Lack of Branding Can Drive to Failures in e-Business and e-Commerce Ventures
No full textIn the paper we present first hand and personal experiences from the lack of branding and how much it can influence the failure of a project or a venture. We do so in terms of present a personal failure story, namely this of the PACE toolkit, a methodology that the author has co-developed at ALTEC Research Programmes Division for valuating intellectual and intangible assets, in parallel with the success story of the business model canvas developed by Alex Osterwalder. We emphasise on the need for e-business and e-commerce professionals and practitioners to more seriously and enthusiastically invest in branding related activities. Involving others such as future potential users or customers as well as investors is a key in the adoption and uptake of our offerings
