Quasi-periodic oscillations as global hydrodynamic modes in the boundary layers of viscous accretion disks

The observational characteristics of quasi-periodic oscillations (QPOs) from accreting neutron stars strongly indicate the oscillatory modes in the innermost regions of accretion disks as a likely source of the QPOs. The inner regions of accretion disks around neutron stars can harbor very high frequency modes related to the radial epicyclic frequency $kappa $. The degeneracy of $kappa $ with the orbital frequency $Omega $ is removed in a non-Keplerian boundary or transition zone near the magnetopause between the disk and the compact object. We show, by analyzing the global hydrodynamic modes of long wavelength in the boundary layers of viscous accretion disks, that the fastest growing mode frequencies are associated with frequency bands around $kappa $ and $kappa pm Omega $. The maximum growth rates are achieved near the radius where the orbital frequency $Omega $ is maximum. The global hydrodynamic parameters such as the surface density profile and the radial drift velocity determine which modes of free oscillations will grow at a given particular radius in the boundary layer. In accordance with the peak separation between kHz QPOs observed in neutron-star sources, the difference frequency between two consecutive bands of the fastest growing modes is always related to the spin frequency of the neutron star. This is a natural outcome of the boundary condition imposed by the rotating magnetosphere on the boundary region of the inner disk.

On the nature of gravitational forces

In this paper I show how the statistics of the gravitational field is changed when the system is characterized by a non-uniform distribution of particles. I show how the distribution functions W(dF/dt) giving the joint probability that a test particle is subject to a force F and an associated rate of change of F given by dF/dt, are modified by inhomogeneity. Then I calculate the first moment of dF/dt to study the effects of inhomogenity on dynamical friction. Finally I test, by N-Body simulations, that the theoretical W(F) and dF/dt describes correctly the experimental data and I find that the stochastic force distribution obtained for the evolved system is in good agreement with theory. Moreover, I find that in an inhomogeneous background the friction force is actually enhanced relative to the homogeneous case.

Dark matter and structure formation a review

This paper provides a review of the variants of dark matter which are thought to be fundamental components of the universe and their role in origin and evolution of structures and some new original results concerning improvements to the spherical collapse model. In particular, I show how the spherical collapse model is modified when we take into account dynamical friction and tidal torques.

Some improvements to the spherical collapse model

I study the joint effect of dynamical friction, tidal torques and cosmological constant on clusters of galaxies formation I show that within high-density environments, such as rich clusters of galaxies, both dynamical friction and tidal torques slows down the collapse of low-? peaks producing an observable variation in the time of collapse of the perturbation and, as a consequence, a reduction in the mass bound to the collapsed perturbation Moreover, the delay of the collapse produces a tendency for less dense regions to accrete less mass, with respect to a classical spherical model, inducing a biasing of over-dense regions toward higher mass I show how the threshold of collapse is modified if dynamical friction, tidal torques and a non-zero cosmological constant are taken into account and I use the Extended Press Schecter (EPS) approach to calculate the effects on the mass function Then, I compare the numerical mass function given in Reed et al (2003) with the theoretical mass function obtained in the present paper I show that the barrier obtained in the present paper gives rise to a better description of the mass function evolution with respect to other previous models (Sheth & Tormen 1999, MNRAS, 308, 119 (hereafter ST); Sheth & Tormen 2002, MNRAS, 329, 61 (hereafter ST1))

The impact of the fourth SM family on the Higgs observability at the LHC

It is shown that if the fourth SM fermion family exists then the Higgs boson could be observed at the LHC with an integrated luminosity of few fb-1. The Higgs discovery potential for different channels is discussed in the presence of the fourth SM family.

Temperature Dependence of the Tensile Properties of Single Walled Carbon Nanotubes: O(N) Tight Binding MD Simulation

This paper examines the effect of temperature on the structural stability and mechanical properties of 20 layered (10,10) single walled carbon nanotubes (SWCNTs) under tensile loading using an O(N) tight binding molecular dynamics (TBMD) simulation method. We observed that (10,10) tube can sustain its structural stability for the strain values of 0.23 in elongation and 0.06 in compression at 300K. Bond breaking strain value decreases with increasing temperature under streching but not under compression. The elastic limit, Youngs modulus, tensile strength and Poisson ratio are calculated as 0.10, 0.395 TPa, 83.23 GPa, 0.285, respectively, at 300K. In the temperature range from 300K to 900K; Youngs modulus and the tensile strengths are decreasing with increasing temperature while the Poisson ratio is increasing. At higher temperatures, Youngs modulus starts to increase while the Poisson ratio and tensile strength decrease. In the temperature range from 1200K to 1800K, the SWCNT is already deformed and softened. Applying strain on these deformed and softened SWCNTs do not follow the same pattern as in the temperature range of 300K to 900K.