arXiv subjects

W. M. Yang

Publications and source records attributed to W. M. Yang.

8 recordsLinked to original sources

arXiv2016

Asteroseismic analysis of two alpha-enhanced stars KIC 7976303 and KIC 8694723

We intent to use stellar models with and without alpha-enhancement, as well as asteroseismic analysis, to study two alpha-enhanced stars, KIC 7976303 and KIC 8694723. For the alpha-enhanced models, we adopt [alpha/Fe] = 0.2, and 0.4. For the evolved sub-giant KIC 7976303 with mixed-modes, alpha-enhanced models fit the observations better than the models without alpha-enhancement, and point to a star with M = 1.20 $\pm$ 0.02 M_odot, t = 4.88 $\pm$ 0.08 Gyr, R = 2.04 $\pm$ 0.01 R_odot, and L = 5.07 $\pm$ 0.05 L_odot. For the post turnoff star KIC 8694723, we find that the models fit the observations well in all three cases ([alpha/Fe] = 0.0, 0.2, 0.4). According to the observed metal abundances, only alpha- enhanced models are used to estimate the stellar parameters, which are M = 1.13 $\pm$ 0.06 M_odot, t = 5.47 $\pm$ 0.43 Gyr, R = 1.55 $\pm$ 0.13 R_odot, and L = 3.18 $\pm$ 0.26 L_odot. Our alpha-enhanced models indicate KIC 7976303 has a larger mass and younger age than in previous works based on standard models. Furthermore, the differences of estimated mass and age between the three cases are $\sim$ 0.1 M_odot and $\sim$ 0.5 - 1.3 Gyr. These results suggest that we include alpha-enhancement in the modeling of alpha-enhanced stars, such as members of GCs (Globular Clusters), metal-poor stars in the disc and in the halo.

astro-ph.SR↗

arXiv2013

Parallel field magnetoresistance in topological insulator thin films

We report that the finite thickness of three-dimensional topological insulator (TI) thin films produces an observable magnetoresistance (MR) in phase coherent transport in parallel magnetic fields. The MR data of Bi2Se3 and (Bi,Sb)2Te3 thin films are compared with existing theoretical models of parallel field magnetotransport. We conclude that the TI thin films bring parallel field transport into a unique regime in which the coupling of surface states to bulk and to opposite surfaces is indispensable for understanding the observed MR. The β parameter extracted from parallel field MR can in principle provide a figure of merit for searching TI compounds with more insulating bulk than existing materials.

cond-mat.mes-hall↗

arXiv2008

Asteroseismic constraints on the OPAL opacity interpolation

The frequency difference between a model used only two-point interpolation of opacity and a model used piecewise linear interpolation of opacity is of the order of several microHertz at a certain stage, which is almost 10 times worse than the observational precision of p-modes of solar-like stars. Therefore, the two-point interpolation of opacity is unsuitable in modelling of solar-like stars with element diffusion.

astro-ph↗

arXiv2008

Angular momentum transport and element mixing in the stellar interior I. Application to the rotating Sun

The purpose of this work was to obtain diffusion coefficient for the magnetic angular momentum transport and material transport in a rotating solar model. We assumed that the transport of both angular momentum and chemical elements caused by magnetic fields could be treated as a diffusion process. The diffusion coefficient depends on the stellar radius, angular velocity, and the configuration of magnetic fields. By using of this coefficient, it is found that our model becomes more consistent with the helioseismic results of total angular momentum, angular momentum density, and the rotation rate in a radiative region than the one without magnetic fields. Not only can the magnetic fields redistribute angular momentum efficiently, but they can also strengthen the coupling between the radiative and convective zones. As a result, the sharp gradient of the rotation rate is reduced at the bottom of the convective zone. The thickness of the layer of sharp radial change in the rotation rate is about 0.036 $R_{\odot}$ in our model. Furthermore, the difference of the sound-speed square between the seismic Sun and the model is improved by mixing the material that is associated with angular momentum transport.

astro-ph↗

arXiv2008

Solar Models with Revised Abundances and Opacities

Using reconstructed opacities, we construct solar models with low heavy-element abundance. Rotational mixing and enhanced diffusion of helium and heavy elements are used to reconcile the recently observed abundances with helioseismology. The sound speed and density of models where the relative and absolute diffusion coefficients for helium and heavy elements have been increased agree with seismically inferred values at better than the 0.005 and 0.02 fractional level respectively. However, the surface helium abundance of the enhanced diffusion model is too low. The low helium problem in the enhanced diffusion model can be solved to a great extent by rotational mixing. The surface helium and the convection zone depth of rotating model M04R3, which has a surface Z of 0.0154, agree with the seismic results at the levels of 1 $σ$ and 3 $σ$ respectively. M04R3 is almost as good as the standard model M98. Some discrepancies between the models constructed in accord with the new element abundances and seismic constraints can be solved individually, but it seems difficult to resolve them as a whole scenario.

astro-ph↗

arXiv2007

The frequency separations of stellar p-modes

Aims. The purpose of this work is to investigate a new frequency separation of stellar p-modes and its characteristics. Methods. Frequency separations are deduced from the asymptotic formula of stellar p-modes. Then, using the theoretical adiabatic frequencies of stellar model, we compute the frequency separations. Results. A new separation $σ_{l-1 l+1}(n)$, which is similar to the scaled small separation $d_{l l+2}(n)/(2l+3)$, is obtained from the asymptotic formula of stellar p-modes. The separations $σ_{l-1 l+1}(n)$ and $d_{l l+2}(n)/(2l+3)$ have the same order. And like the small separation, $σ_{l-1 l+1}(n)$ is mainly sensitive to the conditions in the stellar core. However, with the decrease of the central hydrogen abundance of stars, the $σ_{02}$ and $σ_{13}$ more and more deviate from the scaled small separation. This characteristic could be used to extract the information on the central hydrogen abundance of stars.

gr-qc↗

arXiv2006

Ground state mass spectrum for scalar diquarks with Bethe-Salpeter equation

In this article, we study the structures of the pseudoscalar mesons $π$, $K$ and the scalar diquarks $U^a$, $D^a$, $S^a$ in the framework of the coupled rainbow Schwinger-Dyson equation and ladder Bethe-Salpeter equation with the confining effective potential. The $u$, $d$, $s$ quarks have small current masses, and the renormalization is very large, the mass poles in the timelike region are absent which implements confinement naturally. The Bethe-Salpeter wavefunctions of the pseudoscalar mesons $π$, $K$ and the scalar diquarks $U^a$, $D^a$, $S^a$ have the same type (Gaussian type) momentum dependence, center around zero momentum and extend to the energy scale about $q^2=1GeV^2$ which happen to be the energy scale for the chiral symmetry breaking, the strong interactions in the infrared region result in bound (or quasi-bound) states. The numerical results for the masses and decay constants of the $π$, $K$ mesons can reproduce the experimental values, the ground state masses of the scalar diquarks $U^a$, $D^a$, $S^a$ are consistent with the existing theoretical calculations. We suggest a new Lagrangian which may explain the uncertainty of the masses of the scalar diquarks .

hep-ph↗

arXiv2005

A new approach for calculating the Nambu-Gorkov propagator in color superconductivity theory

In this article, we propose a new approach to calculate the Nambu-Gorkov propagator intuitively with some linear algebra techniques in presence of the scalar diquark condensates. With the help of energy projective operators, we can obtain relatively simple expressions for the quark propagators, which greatly facilitate the calculations in solving the Schwinger-Dyson equation to obtain the gap parameters.

hep-ph↗