68,900 research outputs found
Seismic analysis of 70 Ophiuchi A: A new quantity proposed
The basic intent of this paper is to model 70 Ophiuchi A using the latest
asteroseismic observations as complementary constraints and to determine the
fundamental parameters of the star. Additionally, we propose a new quantity to
lift the degeneracy between the initial chemical composition and stellar age.
Using the Yale stellar evolution code (YREC7), we construct a series of stellar
evolutionary tracks for the mass range = 0.85 -- 0.93 with
different composition (0.26 -- 0.30) and (0.017 -- 0.023).
Along these tracks, we select a grid of stellar model candidates that fall
within the error box in the HR diagram to calculate the theoretical
frequencies, the large- and small- frequency separations using the Guenther's
stellar pulsation code. Following the asymptotic formula of stellar -modes,
we define a quantity which is correlated with stellar age. Also, we
test it by theoretical adiabatic frequencies of many models. Many detailed
models of 70 Ophiuchi A have been listed in Table 3. By combining all
non-asteroseismic observations available for 70 Ophiuchi A with these
seismological data, we think that Model 60, Model 125 and Model 126, listed in
Table 3, are the optimum models presently. Meanwhile, we predict that the
radius of this star is about 0.860 -- 0.865 and the age is about
6.8 -- 7.0 Gyr with mass 0.89 -- 0.90 . Additionally, we prove that
the new quantity can be a useful indicator of stellar age.Comment: 23 pages, 5 figures, accepted by New Astronom
Optical properties of Si/Si0.87Ge0.13 multiple quantum well wires
Nanometer-scale wires cut into a Si/Si0.87Ge0.13 multiple quantum well structure were fabricated and characterized by using photoluminescence and photoreflectance at temperatures between 4 and 20 K. It was found that, in addition to a low-energy broadband emission at around 0.8 eV and other features normally observable in photoluminescence measurements, fabrication process induced strain relaxation and enhanced electron-hole droplets emission together with a new feature at 1.131 eV at 4 K were observed. The latter was further identified as a transition related to impurities located at the Si/Si0.87Ge0.13 heterointerfaces
Asteroseismic study of solar-like stars: A method of estimating stellar age
Asteroseismology, as a tool to use the indirect information contained in
stellar oscillations to probe the stellar interiors, is an active field of
research presently. Stellar age, as a fundamental property of star apart from
its mass, is most difficult to estimate. In addition, the estimating of stellar
age can provide the chance to study the time evolution of astronomical
phenomena. In our poster, we summarize our previous work and further present a
method to determine age of low-mass main-sequence star.Comment: 2 pages, 1 figures, submitted to IAUS25
DC Spin Current Generation in a Rashba-type Quantum Channel
We propose and demonstrate theoretically that resonant inelastic scattering
(RIS) can play an important role in dc spin current generation. The RIS makes
it possible to generate dc spin current via a simple gate configuration: a
single finger-gate that locates atop and orients transversely to a quantum
channel in the presence of Rashba spin-orbit interaction. The ac biased
finger-gate gives rise to a time-variation in the Rashba coupling parameter,
which causes spin-resolved RIS, and subsequently contributes to the dc spin
current. The spin current depends on both the static and the dynamic parts in
the Rashba coupling parameter, and , respectively, and is
proportional to . The proposed gate configuration has the
added advantage that no dc charge current is generated. Our study also shows
that the spin current generation can be enhanced significantly in a double
finger-gate configuration.Comment: 4 pages,4 figure
The extinction of subharmonic vibration under the effects of gravity on the equilibrium of motion.
The extinction of second order subharmonic vibration response of a single degree of freedom system is investigated for the case of damped centrifugal excitation. The degree of the restoring force asymmetry resulting from the effect of gravity is expressed in terms of the parameter of static deflection. The resonance under gravity effects is analysed theoretically for a wide range of physical conditions to determine the behavioural characteristics of the subharmonic components. The inherently coupled algebraic equations are obtained by the approximate energy method of Ritz-Galerkin and by the method of harmonic balance. These two methods are not bounded to any degree of non-linearity. As there is no exact solution for this investigation and because of the dissipative forces inevitably introducing the problem of stability, the actual existence of the approximate solution over the frequency band-width is ascertained. There are no real roots in the instability region. The algebraic polynomial expressions cannot be satisfied simultaneously because of the accumulative effect in an accompanying harmonic of the vibratory motion. The build-up oscillation occurs in the second order region, having a frequency the same as that of the main component of subharmonic motion. The stability criterion is derived from comparing the characteristic exponent of solution to the variational equation with damping coefficient of the system. The response characteristics are then investigated where the polynomial equations are simplified through justifying the approximation of the fundamental harmonic as the effective amplitude of the disturbing force. The results are of comparable accuracy for cases in which gravity effects do not increase the effective non-linearity with resonance. The approximation, however, is applicable whatever tha physical characteristic behaviour of the non-linearity in the region of the critical state of subharmonic extinction. The subharmonic motion in the process of analysis is shown to exist in two opposite phases, differing by pi radians. The resulting phase of periodic vibration depends upon initial conditions. The isocline graphical method is used to depict the transient motion. The effective non-linearity is determined to be governed by the influence of gravity effects on the equilibrium of moition. The pronounced subharmonics are of the order one half, and the extinction conditions for the resonance predominant over the higher orders ere investigated through expressing the limiting inequalities in terms of the system parameters. In the critical state, complete suppression of the subharmonics is achieved. The limiting condition is then examined where damping is fixed at a convenient minimum value and the corresponding optimum limit of gravity effect tolerable can be evaluated for which the amplitude of excitation has no influence on the effective non-linearity as regard to exciting the resonance. As inequality is also presented from which the limiting frequencies of the subharmonic vibration can be predicted with reasonable accuracy. In these investigations the limiting inequalities are not dependant on the resulting variables in the non linear phenomena. An experimental test-rig is designed to demonstrate the subharmonic response. The values recorded from it compare favourably with the approximated theoretical results and with the experimental results obtained from the electronic analogue computer
Solid state NMR and X-ray diffraction studies of α-d-galacturonic acid monohydrate
Crystalline a-d-galacturonic acid monohydrate has been studied by 13C CPMAS NMR and X-ray crystallography. The molecular dynamics were investigated by evaluating 13C spin-lattice relaxation in the rotating frame (T1?) and chemical-shift-anisotropy properties of each carbon. Only limited molecular motions can be detected in the low frequency
After heat distribution of a mobile nuclear power plant
A computer program was developed to analyze the transient afterheat temperature and pressure response of a mobile gas-cooled reactor power plant following impact. The program considers (in addition to the standard modes of heat transfer) fission product decay and transport, metal-water reactions, core and shield melting and displacement, and pressure and containment vessel stress response. Analyses were performed for eight cases (both deformed and undeformed models) to verify operability of the program options. The results indicated that for a 350 psi (241 n/sq cm) initial internal pressure, the containment vessel can survive over 100,000 seconds following impact before creep rupture occurs. Recommendations were developed as to directions for redesign to extend containment vessel life
Phase glass and zero-temperature phase transition in a randomly frustrated two-dimensional quantum rotor model
The ground state of the quantum rotor model in two dimensions with random
phase frustration is investigated. Extensive Monte Carlo simulations are
performed on the corresponding (2+1)-dimensional classical model under the
entropic sampling scheme. For weak quantum fluctuation, the system is found to
be in a phase glass phase characterized by a finite compressibility and a
finite value for the Edwards-Anderson order parameter, signifying long-ranged
phase rigidity in both spatial and imaginary time directions. Scaling
properties of the model near the transition to the gapped, Mott insulator state
with vanishing compressibility are analyzed. At the quantum critical point, the
dynamic exponent is greater than one. Correlation
length exponents in the spatial and imaginary time directions are given by
and , respectively, both assume values
greater than 0.6723 of the pure case. We speculate that the phase glass phase
is superconducting rather than metallic in the zero current limit.Comment: 14 pages, 4 figures, to appear in JSTA
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