The gravitational radiation has been proposed a long time before, as an explanation for the observed relatively low spin frequencies of young neutron stars and of accreting neutron stars in low-mass X-ray binaries as well. In the present work we studied the effects of the neutron star equation of state on the r-mode instability window of rotating neutron stars. Firstly, we employed a set of analytical solution of the Tolman-Oppemheimer-Volkoff equations with special emphasis on the Tolman VII solution. In particular, we tried to clarify the effects of the bulk neutron star properties (mass, radius, density distribution, crust size and elasticity) on the r-mode instability window. We found that the critical angular velocity Ω c depends mainly on the neutron star radius. The effects of the gravitational mass and the mass distribution are almost negligible. Secondly, we studied the effect of the elasticity of the crust, via to the slippage factor S and also the effect of the nuclear equation of state, via the slope parameter L, on the instability window. We found that the crust effects are more pronounced, compared to those originated from the equation of state. Moreover, we proposed simple analytical expressions which relate the macroscopic quantity Ω c to the radius, the parameter L and the factor S. We also investigated the possibility to measure the radius of a neutron star and the factor S with the help of accurate measures of Ω c and the neutron star temperature. Finally, we studied the effects of the mutual friction on the instability window and discussed the results in comparison with previous similar studies.

No abstract

No abstract

Abstract:In this work, the structure and the oxidation resistance of Zn deposited Cu and brass metallic components are examined. The deposition was accomplished with pack cementation chemical deposition. The examination of the samples was performed with electron microscopy and X-ray diffraction analysis. It was found that coatings on Cu substrate consist of two layers with different Zn concentrations, while coatings on brass were single layered with almost constant Zn concentration. The presence of distinct Zn-Cu phases was revealed in both cases. The subjection of the as coated samples together with the uncoated substrates in air at 400ºC showed that both Zn coated samples have enhanced resistivity in such atmospheres, as most of the coating remained mostly unoxidised, and the substrates were fully protected. On the contrary, the bare substrates appear to have undergone severe damage as brittle oxides were formed on their surface.

We employ a variational method, in the framework of the Thomas-Fermi approximation, to study the effect of the symmetry energy on the neutron skin thickness and the symmetry energy coefficients of various neutron rich nuclei. We concentrate our interest on 208 Pb, 124 Sn, 90 Zr, and 48 Ca, although the method can be applied in the totality of medium and heavy neutron rich nuclei. Our approach has the advantage that the isospin asymmetry function α(r), which is the key quantity to calculate isovector properties of various nuclei, is directly related with the symmetry energy as a consequence of the variational principle. Moreover, the Coulomb interaction is included in a self-consistent way and its effects can be separated easily from the nucleon-nucleon interaction. We confirm, both qualitatively and quantitatively, the strong dependence of the symmetry energy on the various isovector properties for the relevant nuclei, using possible constraints between the slope and the value of the symmetry energy at the saturation density. PACS number(s): 21.65. Ef, 21.65.Mn, 21.65.Cd, 21.10.Gv