Atomic Diffusion and Mixing in Old Stars. III. Analysis of NGC 6397 Stars under New Constraints

We have previously reported on chemical abundance trends with evolutionary state in the globular cluster NGC 6397 discovered in analyses of spectra taken with FLAMES at the VLT. Here, we reinvestigate the FLAMES-UVES sample of 18 stars, ranging from just above the turnoff point (TOP) to the red giant branch below the bump. Inspired by new calibrations of the infrared flux method, we adopt a set of hotter temperature scales. Chemical abundances are determined for six elements (Li, Mg, Ca, Ti, Cr, and Fe). Signatures of cluster-internal pollution are identified and corrected for in the analysis of Mg. On the modified temperature scales, evolutionary trends in the abundances of Mg and Fe are found to be significant at the 2{\sigma} and 3{\sigma} levels, respectively. The detailed evolution of abundances for all six elements agrees with theoretical isochrones, calculated with effects of atomic diffusion and a weak to moderately strong efficiency of turbulent mixing. The age of these models is compatible with the external determination from the white dwarf cooling sequence. We find that the abundance analysis cannot be reconciled with the strong turbulent-mixing efficiency inferred elsewhere for halo field stars. A weak mixing efficiency reproduces observations best, indicating a diffusion-corrected primordial lithium abundance of log {\epsilon}(Li) = 2.57 +- 0.10. At 1.2{\sigma}, this value agrees well with WMAP-calibrated Big-Bang nucleosynthesis predictions.Comment: 14 pages, 5 figures, accepted by Ap

Design of helicopter rotor blades for optimum dynamic characteristics

The mass and stiffness distributions for helicopter rotor blades are tailored in such a way to give a predetermined placement of blade natural frequencies. The optimal design is pursued with respect of minimum weight, sufficient inertia, and reasonable dynamic characteristics. Finite element techniques are used as a tool. Rotor types include hingeless, articulated, and teetering

Reversable heat flow through the carbon nanotube junctions

Microscopic mechanisms of externally controlled reversable heat flow through the carbon nanotube junctions (NJ) are studied theoretically. Our model suggests that the heat is transfered along the tube section ${\cal T}$ by electrons ($e$) and holes ($h$) moving ballistically in either in parallel or in opposite directions and accelerated by the bias source-drain voltage $V_{\rm SD}$ (Peltier effect). We compute the Seebeck coefficient $\alpha$, electric $\sigma$ and thermal $\kappa$ conductivities and find that their magnitudes strongly depend on $V_{\rm SD}$ and $V_{\rm G}$. The sign reversal of $\alpha$ versus the sign of $V_{\rm G}$ formerly observed experimentally is interpreted in this work in terms of so-called chiral tunneling phenomena (Klein paradox)

A non-LTE study of neutral and singly-ionized iron line spectra in 1D models of the Sun and selected late-type stars

A comprehensive model atom for Fe with more than 3000 energy levels is presented. As a test and first application of this model atom, Fe abundances are determined for the Sun and five stars with well determined stellar parameters and high-quality observed spectra. Non-LTE leads to systematically depleted total absorption in the Fe I lines and to positive abundance corrections in agreement with the previous studies, however, the magnitude of non-LTE effect is smaller compared to the earlier results. Non-LTE corrections do not exceed 0.1 dex for the solar metallicity and mildly metal-deficient stars, and they vary within 0.21 dex and 0.35 dex in the very metal-poor stars HD 84937 and HD 122563, respectively, depending on the assumed efficiency of collisions with hydrogen atoms. Based on the analysis of the Fe I/Fe II ionization equilibrium in these two stars, we recommend to apply the Drawin formalism in non-LTE studies of Fe with a scaling factor of 0.1. For the Fe II lines, non-LTE corrections do not exceed 0.01 dex in absolute value. The solar non-LTE abundance obtained from 54 Fe I lines is 7.56+-0.09 and the abundance from 18 Fe II lines varies between 7.41+-0.11 and 7.56+-0.05 depending on the source of the gf-values. Thus, gf-values available for the iron lines are not accurate enough to pursue high-accuracy absolute abundance determinations. Lines of Fe I give, on average, a 0.1 dex lower abundance compared to those of Fe II lines for HD 61421 and HD 102870, even when applying a differential analysis relative to the Sun. A disparity between Fe I and Fe II points to problems of stellar atmosphere modelling or/and effective temperature determination.Comment: 19 pages, 8 figures, online material, accepted by A&

Electron spin relaxation in paramagnetic Ga(Mn)As quantum wells

Electron spin relaxation in paramagnetic Ga(Mn)As quantum wells is studied via the fully microscopic kinetic spin Bloch equation approach where all the scatterings, such as the electron-impurity, electron-phonon, electron-electron Coulomb, electron-hole Coulomb, electron-hole exchange (the Bir-Aronov-Pikus mechanism) and the $s$-$d$ exchange scatterings, are explicitly included. The Elliot-Yafet mechanism is also incorporated. From this approach, we study the spin relaxation in both $n$-type and $p$-type Ga(Mn)As quantum wells. For $n$-type Ga(Mn)As quantum wells where most Mn ions take the interstitial positions, we find that the spin relaxation is always dominated by the DP mechanism in metallic region. Interestingly, the Mn concentration dependence of the spin relaxation time is nonmonotonic and exhibits a peak. This behavior is because that the momentum scattering and the inhomogeneous broadening have different density dependences in the non-degenerate and degenerate regimes. For $p$-type Ga(Mn)As quantum wells, we find that Mn concentration dependence of the spin relaxation time is also nonmonotonic and shows a peak. Differently, this behavior is because that the $s$-$d$ exchange scattering (or the Bir-Aronov-Pikus) mechanism dominates the spin relaxation in the high Mn concentration regime at low (or high) temperature, whereas the DP mechanism determines the spin relaxation in the low Mn concentration regime. The Elliot-Yafet mechanism also contributes the spin relaxation at intermediate temperature. The spin relaxation time due to the DP mechanism increases with Mn concentration due to motional narrowing, whereas those due to the spin-flip mechanisms decrease with Mn concentration, which thus leads to the formation of the peak.... (The remaining is omitted due to the space limit)Comment: 12 pages, 8 figures, Phys. Rev. B 79, 2009, in pres

Design of helicopter rotor blades for optimum dynamic characteristics

The possibilities and limitations of tailoring blade mass and stiffness distributions to give an optimum blade design in terms of weight, inertia, and dynamic characteristics are discussed. The extent that changes in mass of stiffness distribution can be used to place rotor frequencies at desired locations is determined. Theoretical limits to the amount of frequency shift are established. Realistic constraints on blade properties based on weight, mass, moment of inertia, size, strength, and stability are formulated. The extent that the hub loads can be minimized by proper choice of E1 distribution, and the minimum hub loads which can be approximated by a design for a given set of natural frequencies are determined. Aerodynamic couplings that might affect the optimum blade design, and the relative effectiveness of mass and stiffness distribution on the optimization procedure are investigated