Search results for "Dimensionless quantity"
showing 10 items of 96 documents
Structure and stability of traversable thin-shell wormholes in Palatini f(R) gravity
2020
We study the structure and stability of traversable wormholes built as (spherically symmetric) thin shells in the context of Palatini f(R) gravity. Using a suitable junction formalism for these theories we find that the effective number of degrees of freedom on the shell is reduced to a single one, which fixes the equation of state to be that of massless stress-energy fields, contrary to the general relativistic and metric f(R) cases. Another major difference is that the surface energy density threading the thin shell, needed in order to sustain the wormhole, can take any sign and may even vanish, depending on the desired features of the corresponding solutions. We illustrate our results by…
Expressions of Effective Hamiltonian Parameters of XY4 Molecules in the Tetrahedral Formalism
1998
We have derived expressions of second-order effective Hamiltonian parameters of XY4 molecules in the tetrahedral formalism (1992, J. P. Champion et al., "Spectroscopy of the Earth's Atmosphere and Interstellar Medium: Spherical Top Spectra," Academic Press, San Diego). They are written as a function of the force constants of the potential expanded in terms of the dimensionless normal coordinates. These expressions can be used in the isolated band scheme as well as in the polyad one. The ambiguity of the effective Hamiltonian parameters is treated. Relations between the parameters for q2 and q4 terms and Hecht's anharmonicity constants (1960, K. T. Hecht, J. Mol. Spectrosc. 5, 355-389) in th…
Gravitational Waves from Disks Around Spinning Black Holes: Simulations in Full General Relativity
2020
We present fully general-relativistic numerical evolutions of self-gravitating tori around spinning black holes with dimensionless spin $a/M = 0.7$ parallel or anti-parallel to the disk angular momentum. The initial disks are unstable to the hydrodynamic Papaloizou-Pringle Instability which causes them to grow persistent orbiting matter clumps. The effect of black hole spin on the growth and saturation of the instability is assessed. We find that the instability behaves similarly to prior simulations with non-spinning black holes, with a shift in frequency due to spin-induced changes in disk orbital period. Copious gravitational waves are generated by these systems, and we analyze their det…
Anti-phase wave patterns in a ring of electrically coupled oscillatory neurons
2013
International audience; Space-time dynamics of the network system modeling collective behavior of electrically coupled nonlinear cells is investigated. The dynamics of a local cell is described by the dimensionless Morris-Lecar system. It is shown that such a system yields a special class of traveling localized collective activity so called "anti-phase wave patterns". The mechanisms of formation of the patterns are discussed and the region of their existence is obtained by using the weakly coupled oscillators theory.
Effects of nonlinear sweep in the Landau-Zener-Stueckelberg effect
2002
We study the Landau-Zener-Stueckelberg (LZS) effect for a two-level system with a time-dependent nonlinear bias field (the sweep function) W(t). Our main concern is to investigate the influence of the nonlinearity of W(t) on the probability P to remain in the initial state. The dimensionless quantity epsilon = pi Delta ^2/(2 hbar v) depends on the coupling Delta of both levels and on the sweep rate v. For fast sweep rates, i.e., epsilon << l and monotonic, analytic sweep functions linearizable in the vicinity of the resonance we find the transition probability 1-P ~= epsilon (1+a), where a>0 is the correction to the LSZ result due to the nonlinearity of the sweep. Further increase …
A study of free convection in air around horizontal cylinders of different diameters based on holographic interferometry. Temperature field equations…
2002
Abstract Holographic interferometry is used to study free convection in air around horizontal cylinders of different diameters and equal length, involving different surface temperatures, with the aim of defining the corresponding temperature fields. Interferograms were obtained to determine the temperature (T) of each point as well as its distance (x) from the surface of the cylinder. These values in turn made it possible to define functions (of an exponential nature in our case), T=f(x), that satisfactorily reproduced the temperature fields. Posteriorly, these functions were used to calculate the local convection coefficients, determining their dependency upon temperature and the direction…
Thermal conduction by dark matter with velocity and momentum-dependent cross-sections
2014
We use the formalism of Gould and Raffelt to compute the dimensionless thermal conduction coefficients for scattering of dark matter particles with standard model nucleons via cross-sections that depend on the relative velocity or momentum exchanged between particles. Motivated by models invoked to reconcile various recent results in direct detection, we explicitly compute the conduction coefficients $\alpha$ and $\kappa$ for cross-sections that go as $v_{\rm rel}^2$, $v_{\rm rel}^4$, $v_{\rm rel}^{-2}$, $q^2$, $q^4$ and $q^{-2}$, where $v_{\rm rel}$ is the relative DM-nucleus velocity and $q$ is the momentum transferred in the collision. We find that a $v_{\rm rel}^{-2}$ dependence can sig…
Dark sectors with dynamical coupling
2019
Coupled dark matter-dark energy scenarios are modeled via a dimensionless parameter $��$, which controls the strength of their interaction. While this coupling is commonly assumed to be constant, there is no underlying physical law or symmetry that forbids a time-dependent $��$ parameter. The most general and complete interacting scenarios between the two dark sectors should therefore allow for such a possibility, and it is the main purpose of this study to constrain two possible and well-motivated coupled cosmologies by means of the most recent and accurate early and late-time universe observations. We find that CMB data alone prefers $��(z) >0$ and therefore a smaller amount of dark ma…
Effect of spin on the inspiral of binary neutron stars
2019
We perform long-term simulations of spinning binary neutron stars, with our highest dimensionless spin being $\chi \sim 0.32$. To assess the importance of spin during the inspiral we vary the spin, and also use two equations of state, one that consists of plain nuclear matter and produces compact stars (SLy), and a hybrid one that contains both nuclear and quark matter and leads to larger stars (ALF2). Using high resolution that has grid spacing $\Delta x\sim 98$ m on the finest refinement level, we find that the effects of spin in the phase evolution of a binary system can be larger than the one that comes from tidal forces. Our calculations demonstrate explicitly that although tidal effec…
Empirical determination of Einstein A-coefficient ratios of bright [Fe II] lines
2014
The Einstein spontaneous rates (A-coefficients) of Fe+ lines have been computed by several authors with results that differ from each other by up to 40%. Consequently, models for line emissivities suffer from uncertainties that in turn affect the determination of the physical conditions at the base of line excitation. We provide an empirical determination of the A-coefficient ratios of bright [Fe II] lines that would represent both a valid benchmark for theoretical computations and a reference for the physical interpretation of the observed lines. With the ESO-Very Large Telescope X-shooter instrument between 3000 Å and 24700 Å, we obtained a spectrum of the bright Herbig-Haro object HH 1. …