Barycentric Reference Frame Research Articles

A pulsation zoo in the hot subdwarf B star KIC 10139564 observed by Kepler

We present our analyses of 15 months of Kepler data on KIC 10139564. We detected 57 periodicities with a variety of properties not previously observed all together in one pulsating subdwarf B (sdB) star. Ten of the periodicities were found in the low-frequency region, and we associate them with nonradial g modes. The other periodicities were found in the high-frequency region, which are likely p modes. We discovered that most of the periodicities are components of multiplets with a common spacing. Assuming that multiplets are caused by rotation, we derive a rotation period of 25.6 ± 1.8 d. The multiplets also allow us to identify the pulsations to an unprecedented extent for this class of pulsator. We also detect l ≥ 2 multiplets, which are sensitive to the pulsation inclination and can constrain limb darkening via geometric cancellation factors. While most periodicities are stable, we detected several regions that show complex patterns. Detailed analyses showed that these regions are complicated by several factors. Two are combination frequencies that originate in the super-Nyquist region and were found to be reflected below the Nyquist frequency. The Fourier peaks are clear in the super-Nyquist region, but the orbital motion of Kepler smears the Nyquist frequency in the barycentric reference frame and this effect is passed on to the sub-Nyquist reflections. Others are likely multiplets but unstable in amplitudes and/or frequencies. The density of periodicities also makes KIC 10139564 challenging to explain using published models. This menagerie of properties should provide tight constraints on structural models, making this sdB star the most promising for applying asteroseismology. To support our photometric analysis, we have obtained spectroscopic radial-velocity measurements of KIC 10139564 using low-resolution spectra in the Balmer-line region. We did not find any radial-velocity variation. We used our high signal-to-noise average spectrum to improve the atmospheric parameters of the sdB star, deriving Teff = 31 859 K and log g = 5.673 dex.

How Is Charge Transport Different in Ionic Liquids and Electrolyte Solutions?

In this article we show that, analyzed in a barycentric reference frame, the deviation in conductivity measured directly from impedance experiments with respect to that estimated indirectly from NMR diffusion experiments has different origins in electrolyte solutions and pure salts. In the case of electrolyte solutions, the momentum conservation law is satisfied by solvent + ions. Instead, in a molten salt or ionic liquid momentum conservation must be satisfied solely by the ions. This has significant implications. While positively correlated motion of ions of opposite charge is a well justified explanation for the reduction in impedance conductivity in the case of electrolyte solutions, it is not so in the case of ionic liquids and molten salts. This work presents a set of equations that in the case of ionic liquids and molten salts can be used to obtain from direct measurements of impedance and NMR the distinct part of the diffusion coefficient matrix in the barycentric reference frame. In other words, by using experimentally measurable quantities, these equations allow us to access the motional coupling between ions for which there is no single direct experimental measurement technique. While equations of this type have been proposed before, the ones presented here can be easily derived from the momentum conservation law and linear response theory. Our results indicate that the decrease in the impedance conductivity with respect to NMR conductivity in ionic liquids and molten salts is due to anticorrelated motion of ions of same charge. This scenario is different in electrolyte solutions, where the positively correlated motion of ions of opposite charge makes a significant contribution to the decrease in the impedance conductivity. In contrast, in a system comprising a single binary salt (a room temperature ionic liquid or a molten salt), the cation-anion distinct diffusion coefficient is negative definite and opposes the contribution from the cation-cation and anion-anion distinct diffusion coefficients. This property of the cation-anion distinct diffusion coefficient in systems comprising just two ion-constituents holds true not just in the barycentric reference frame but also in any of the internal reference frames of nonequilibrium thermodynamics.

Distinct Diffusion in Binary Mixtures Confined in Slit Graphite Pores

Distinct diffusion coefficients in the barycentric reference frame for equimolar binary mixtures confined in slit-shaped graphite pores have been evaluated using equilibrium molecular dynamics simulations. In all mixtures the ratio of the interaction energies and mass ratios are similar to those of a Xe-Ar mixture with $\sigma_{22}/\sigma_{11}$ varied between 1 and 1.4. When $\sigma_{22}/\sigma_{11}$ = 1.0 both species adsorb in similar layers in the pore and the distinct diffusion is positive. As $\sigma_{22}/\sigma_{11}$ is increased, the distinct diffusion is found to become progressively more negative. This increasing associative tendency as $\sigma_{22}/\sigma_{11}$ is increased is predominantly due to the formation of distinct layers by individual species. In-plane pair correlation functions also reveal the increased association between unlike species with increasing $\sigma_{22}/\sigma_{11}.$ The Xe-Ar mixture is observed to be associative, with negative values of the distinct diffusion when the fluid forms two layers in the pore. At smaller pore widths, where only a single layer is accommodated, the mixture is weakly dissociative. Simulations with a non-Lorentz- Berthelot mixture were carried out to illustrate the effect of the interaction between unlike species.

Aberration in proper motions

Approved space astrometric missions (SIM, GAIA) are aiming at a few microarcseconds per year precision in yearly proper motions and even less than a microarcsecond in the definition of an extragalactic reference frame. At such a level of accuracy, the curvature of stellar orbits around the center of the Galaxy cannot be neglected. The curvature of the Solar system barycentric motion induces a time-dependent component of the aberration, which has the properties of an apparent proper motion of the galaxies. This eect reaches 4as per year in some regions of the sky. In the case of stars, it is combined with a similar eect due to the curvature of the circular galactocentric orbit of the star, which may reach 60 as per year for a star situated at 500 parsecs from the center of the Galaxy, and much larger closer to it. The paper gives the proofs and the formulae permitting one to compute this aberration in proper motions. The conclusion is that, at this high level of accuracy, one should present the astrometric data in a galacto-centric rather than in a barycentric reference frame.

Mutual diffusion in a binary Ar-Kr mixture confined within zeolite NaY.

Molecular dynamics investigations of the mutual diffusion coefficients in an Ar-Kr mixture confined in the zeolite NaY are reported. Velocity auto- and cross correlations were computed at two different temperatures (200 and 600 K). The importance of the appropriate choice of reference frame while evaluating the time correlation functions is illustrated for argon in the zeolite NaY. Mutual diffusivities in the mixture were obtained in the barycentric reference frame. Recently, Zhou and Miller showed that the distinct diffusivity D(d) is zero for the Ar-Kr mixture in bulk. On confinement, it is seen that at 200 K the ratio R=D(11)/D(s)=0.77, where D11 is the mutual diffusivity and D(s) is the mixture self-diffusivity. However, at 600 K, R=0.97, implying that the contribution from distinct diffusion is only slightly negative. The large negative D(d) at 200 K could be attributed to strong localization of Ar and Kr in the physisorption sites within the zeolite cages. Analysis of error bars and an efficient computational algorithm for evaluation of the velocity cross correlation function are also presented. The results have implications in biology, chemistry, and other situations where transport of confined mixtures is encountered.

Green−Kubo Formulas for Mutual Diffusion Coefficients in Multicomponent Systems

Green−Kubo relations for the mutual diffusion coefficients of an n-component system are derived from hydrodynamic equations, linear response theory, and the statistical theory of fluctuations. The mutual diffusion coefficients, which form an (n − 1)-dimensional matrix, are formulated as a product of a kinetic matrix and a thermodynamic matrix. The kinetic matrix consists of time integrals of diffusion flux correlation functions, i.e., the Onsager phenomenological coefficients. The thermodynamic matrix is determined by spatial integrals of radial distribution functions. The formulas are derived initially in the barycentric reference frame and then transformed to the volume-fixed reference frame, which is more appropriate to laboratory studies. Explicit expressions for binary and ternary mixtures are presented, from which various empirical relations between the mutual diffusion coefficients and self-diffusion coefficients are obtained using particular assumptions regarding the velocity cross-correlation functions. Assuming zero values for certain combinations of the cross-correlation functions yields the Darken or Hartley−Crank equation for binary systems and Cooper's diffusion model for ternary systems, while assigning negative values to these combinations results in Lasaga's relation, which is a special case of the so-called common force diffusion model.

Influence of Ephemeris Uncertainties and Relativity Modeling on Lunar Laser Ranging Data Analysis

In this paper authors pointed out that the root mean square of post-residual is two or three times larger than that of normal points. It is mainly due to the ephemeris uncertainties, i.e. the uncertainties of obliquity of the ecliptic and equinox and the inaccurary of lunar love numbers adopted. At present, with exception of the corrections of ocean tide and pole tide the theoretical modeling such as earth solid tide, lunar tide, plate motion and the relativity modeling, especially the space curvature caused by the earth, have been considered to match the present observing precision in the magnitude of centimeter. The equivalent of relativity modeling used in geocentric or barycentric reference frame is further described.

Velocity correlations in the molecular dynamics ensemble: Computation of the distinct diffusion coefficients

The theory of the correction for the difference in the fluctuations among the different ensembles is applied to derive the time correlation function formula required to compute the distinct diffusion coefficients in the barycentric reference frame by molecular dynamics. In the process it is found that the dynamical variables in the time correlation function of any mass transport coefficient of any reference frame are the same in the canonical as in the molecular dynamics ensemble. The result agrees with the accepted formulas for computing the kinetic part of the mutual diffusion coefficient in a binary mixture of nonelectrolytes and for computing the electrical conductivity in molten salts and electrolyte solutions. The static velocity correlations in the molecular dynamics ensemble are analyzed without recourse to the other ensembles. In sharp contrast with the ensembles which are not constrained with respect to the total momentum, the laboratory-frame velocities of two different particles at the same time are found to be correlated. The result obtained is equivalent to the initial value of the canonical time correlation function of the barycentric distinct diffusion coefficients derived in the preceding paper.