The YNEV stellar evolution and oscillation code

We have developed a new stellar evolution and oscillation code YNEV, which calculates the structures and evolutions of stars, taking into account hydrogen and helium burning. A nonlocal turbulent convection theory and an updated overshoot mixing model are optional in this code. The YNEV code can evolve low- and intermediate-mass stars from pre-main sequence (PMS) to thermal pulsing asymptotic branch giant (TP-AGB) or white dwarf. The YNEV oscillation code calculates the eigenfrequencies and eigenfunctions of the adiabatic oscillations of given stellar structure. The input physics and the numerical scheme adopted in the code are introduced in this paper. The examples of solar models, stellar evolutionary tracks of low- and intermediate-mass stars with different convection theory (i.e., mixing-length theory (MLT) and the nonlocal turbulent convection theory), and stellar oscillations are shown.Comment: 18 pages, 14 figures, accepted for publication in the Research in Astronomy and Astrophysics (RAA

A simple scheme to implement a nonlocal turbulent convection model for the convective overshoot mixing

The classical 'ballistic' overshoot models show some contradictions and are not consistence with numerical simulations and asteroseismic studies. Asteroseismic studies imply that overshoot is a weak mixing process. Diffusion model is suitable to deal with it. The form of diffusion coefficient in a diffusion model is crucial. Because the overshoot mixing is related to the convective heat transport (i.e., entropy mixing), there should be a similarity between them. A recent overshoot mixing model shows consistence between composition mixing and entropy mixing in overshoot region. A prerequisite to apply the model is to know the dissipation rate of turbulent kinetic energy. The dissipation rate can be worked out by solving turbulent convection models (TCMs). But it is difficult to apply TCMs because of some numerical problems and the enormous time cost. In order to find a convenient way, we have used the asymptotical solution and simplified the TCM to be a single linear equation for turbulent kinetic energy. This linear model is easy to be implemented in the calculations of stellar evolution with ignorable extra time cost. We have tested the linear model in stellar evolution, and have found that the linear model can well reproduce the turbulent kinetic energy profile of full TCM, as well as the diffusion coefficient, abundance profile and the stellar evolutionary tracks. We have also studied the effects of different values of the model parameters and have found that the effect due to the modification of temperature gradient in the overshoot region is slight.Comment: 20 pages, 10 figures, accepted for publication in Ap

The solar abundance problem: the effect of the turbulent kinetic flux on the solar envelope model

Recent 3D-simulations have shown that the turbulent kinetic flux (TKF) is significant. We discuss the effects of TKF on the size of convection zone and find that the TKF may help to solve the solar abundance problem. The solar abundance problem is that, with new abundances, the solar convection zone depth, sound speed in the radiative interior, the helium abundance and density in the convective envelope are not in agreement with helioseismic inversions. We have done Monte Carlo simulations on solar convective envelope models with different profile of TKF to test the effects. The solar abundance problem is revealed in the standard solar convective envelope model with AGSS09 composition, which shows significant differences (\rm{\sim 10 %}) on density from the helioseicmic inversions, but the differences in the model with old composition GN93 is small (\rm{\sim 0.5 %}). In the testing models with different imposed TKF, it is found that the density profile is sensitive to the value of TKF at the base of convective envelope and insensitive to the structure of TKF in the convection zone. Required value of turbulent kinetic luminosity at the base is about \rm{-13%\sim-19%L_{\odot}}. Comparing with the 3D-simulations, this value is plausible. This study is for the solar convective envelope only. The evolutionary solar models with TKF are required for investigating its effects on the solar interior structure below the convection zone and the whole solar abundance problem, but the profile of TKF in the overshoot region is needed.Comment: 5 pages, 2 figures, accepted for publication in the ApJ Letter

Multi-pion Bose-Einstein correlations in high energy heavy-ion collisions

Multi-pion correlations and wavepacket size effects on the pion multiplicity distribution, pion momentum distribution and two-pion interferometry are studied. It is shown that multi-pion Bose-Einstein correlations and the wavepacket size cause an abundance of pions at low momentum, increase the mean pion multiplicity and decrease both the apparent radius of the source and the coherent source parameter derived from two-pion interferometry.Comment: 9 pages, 7 figure

Constraints on the size of the quark gluon plasma

We use simple entropy arguments to estimate the possible size of the QGP at the AGS and the SPS. We find that the possibility to form a large volume of QGP at the AGS or the SPS is very small. The size of the QGP at RHIC and the LHC is also predicted.Comment: new references of finite size effects on the observables are adde

Testing the core overshooting mixing described by the turbulent convection model on the eclipsing binary star HY Vir

Helioseismic investigation has suggested to apply the turbulent convection models (TCMs) to the convective overshooting. Using the turbulent velocity in the overshooting region determined by the TCM, one can deal with the overshooting mixing as a diffusion process, which leads to incomplete mixing. It has been found that this treatment can improve the solar sound speed and the Li depletion in open clusters. In order to investigate whether the TCM can be applied to the overshooting mixing outside the stellar convective core, new observations of the eclipsing binary star HY Vir are adopted to calibrate the overshooting mixing parameter. The main conclusions are as follows: (i) the TCM parameters and the overshooting mixing parameter in the solar case are also suitable for the eclipsing binary system HY Vir; (ii) the incomplete mixing results in a continuous profile of the hydrogen abundance; (iii) the e-folding length of the region, in which the hydrogen abundance changes due to the overshooting mixing, increases during the stellar evolution.Comment: 9 pages, 7 figures, ApJ accepte

Automorphisms of K3 surfaces

In this note, we report some progress we made recently on the automorphisms groups of K3 surfaces. A short and straightforward proof of the impossibility of Z/(60) acting purely non-symplectically on a K3 surface, is also given, by using Lefschetz fixed point formula for vector bundles.Comment: submitted to Proc. Conf. for 10th anniversary of Math. Inst. East China Normal U. Contemporary Math. Amer. Math. So

Quotients of K3 Surfaces Modulo Involutions

Let X be a K3 surface with an involution g which has non-empty fixed locus X^g and acts non-trivially on a non-zero holomorphic 2-form. We shall construct all such pairs (X, g) in a canonical way, from some better known double coverings of log del Pezzo surfaces of index at most 2 or rational elliptic surfaces, and construct the only family of each of the three extremal cases where X^g contains 10 (maximum possible) curves. We also classify rational log Enriques surfaces of index 2. Our approach is more geometrical rather than lattice-theoretical (see Nikulin's paper for the latter approach).Comment: 30 pages. Japanese J. Mathematics, to appea

Convective overshoot mixing in stellar interior models

The convective overshoot mixing plays an important role in stellar structure and evolution. However, the overshoot mixing is a long standing problem. The uncertainty of the overshoot mixing is one of the most uncertain factors in stellar physics. As it is well known, the convective and overshoot mixing is determined by the radial chemical component flux. In this paper, a local model of the radial chemical component flux is established based on the hydrodynamic equations and some model assumptions. The model is tested in stellar models. The main conclusions are as follows. (i) The local model shows that the convective and overshoot mixing could be regarded as a diffusion process, and the diffusion coefficient for different chemical element is the same. However, if the non-local terms, i.e., the turbulent convective transport of radial chemical component flux, are taken into account, the diffusion coefficient for each chemical element should be in general different. (ii) The diffusion coefficient of convective / overshoot mixing shows different behaviors in convection zone and in overshoot region because the characteristic length scale of the mixing is large in the convection zone and small in the overshoot region. The overshoot mixing should be regarded as a weak mixing process. (iii) The result of the diffusion coefficient of mixing is tested in stellar models. It is found that a single choice of our central mixing parameter leads to consistent results for a solar convective envelope model as well as for core convection models of stars with mass from 2M to 10M.Comment: 9 pages, 2 figures, accepted for publication in ApJ

Multipion symmetrization effects on the source distribution

Any-order pion interferometry formulas for fixed pion multiplicity events and for mixed events are given. Multipion Bose-Einstein correlation effects on the two-pion interferometry and source distribution are studied. It is shown that generalized pion interferometry formula should depneds on pion multiplicity distribution. Pion condensate is also discussed in the paper.Comment: invited Talk given at the 8-th international workshop on multiparticle production(correlation and fluctuation'98), June 14-21, 1998 Hungary. To be published in the proccedin