Shaolan Bi

THE CONTRIBUTIONS OF INTERACTIVE BINARY STARS TO DOUBLE MAIN-SEQUENCE TURNOFFS AND DUAL RED CLUMP OF INTERMEDIATE-AGE STAR CLUSTERS

Double or extended main-sequence turnoffs (DMSTOs) and dual red clump (RC) were observed in intermediate-age clusters, such as in NGC 1846 and 419. The DMSTOs are interpreted as that the cluster has two distinct stellar populations with differences in age of about 200-300 Myr but with the same metallicity. The dual RC is interpreted as a result of a prolonged star formation. Using a stellar population-synthesis method, we calculated the evolution of a binary-star stellar population. We found that binary interactions and merging can reproduce the dual RC in the color-magnitude diagrams of an intermediate-age cluster, whereas in actuality only a single population exists. Moreover, the binary interactions can lead to an extended main-sequence turnoff (MSTO) rather than DMSTOs. However, the rest of the main sequence, subgiant branch, and first giant branch are hardly spread by the binary interactions. Part of the observed dual RC and extended MSTO may be the results of binary interactions and mergers.

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 in which 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 levels, 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 sigma and 3 sigma, 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.

Solar Models with Revised Abundance

We present new solar models in which we use the latest low abundances and further include the effects of rotation, magnetic fields, and extra-mixing processes. We assume that the extra-element mixing can be treated as a diffusion process, with the diffusion coefficient depending mainly on the solar internal configuration of rotation and magnetic fields. We find that such models can well reproduce the observed solar rotation profile in the radiative region. Furthermore, the proposed models can match the seismic constraints better than the standard solar models, also when these include the latest abundances, but neglect the effects of rotation and magnetic fields.

Stellar parameters and seismological analysis of the star 18 Scorpii

Aims. We constructed models of the structure and evolution of the stars including diffusion and extra-mixing caused by rotation to estimate stellar parameters of the solar twin 18 Scorpii. Methods. Based on the classical observed features, we considered three additional constraints, i.e., lithium abundance log N(Li), rotational period P eq, and average large frequency separation �� ν� , by combing stellar models with observations to determine stellar parameters and the possible evolutionary status of 18 Scorpii. Results. More accurate results of mass and age were found by our model than in previous studies. We estimate that the mass and

LITHIUM ABUNDANCE AS A PREDICTOR OF MASS AND AGE IN SOLAR-ANALOG STARS

In order to estimate the mass and age of stars, we construct a grid of stellar models for eight solar-analog stars including diffusion and rotation-induced mixing for the given ranges of stellar mass, metallicity, and rotational rate. By combining stellar models with observational data including lithium abundance, we obtain more accurate estimations of mass and age for solar-analog stars. The results indicate that stars HIP 56948, HIP 73815, and HIP 78399 are three possible solar twins. Furthermore, we find that lithium depletion due to extra-mixing in solar analogs strongly depends on mass, metallicity, and rotational history. Therefore, lithium abundance can be used as a good constraint in stellar modeling.

Seismological Analysis of the Stars γ Serpentis and ι Leonis: Stellar Parameters and Evolution

In order to assess the information that will be available from targets of the Microvariability and Oscillations of Stars (MOST) satellite, we explore the possible evolutionary status and perform preliminary seismological analysis of two targets: the stars gamma Ser and iota Leo, which show solar-like oscillations. Taking into account the astrometric observational constraints on the stars, we find that gamma Ser is in the main-sequence phase of evolution, while iota Leo has two distinct solutions that match well with observations: main-sequence models and post-main-sequence models. The evolutionary tracks cover the following ranges of mass and initial metallicity: gamma Ser, 1.15-1.20 M-circle dot and Z = 0.0135 +/- 0.003; iota Leo, 1.62-1.70 M-circle dot and Z = 0.024 +/- 0.002 (for values of the Galactic helium enrichment ratio Delta Y/Delta Z in the range 0 - 2). By combining stellar models with observational constraints, we determine age ranges of 3.2 +/- 0.6 Gyr for gamma Ser and 1.7 +/- 0.2 Gyr for iota Leo. The results show that the masses and ages of the stars are very sensitive to metallicity and evolutionary phase. Furthermore, we find that models with overshooting from a convective core predict a larger age for a given mass. Finally, we discuss future prospects for constraining stellar models with the help of asteroseismic observations.

The frequency separations of stellar p-modes

Aims. The purpose of this work is to investigate the characteristics of a new frequency separation of stellar p-modes. 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 sigma(l-1l+ 1)( n), which is similar to the scaled small separation d(ll+ 2)( n)/(2l + 3), is obtained from the asymptotic formula of stellar p-modes. The separations sigma(l-1l+ 1)( n) and d(ll+ 2)( n)/(2l+ 3) have the same order. And like the small separation, sigma(l)-(1l+ 1)( n) is mainly sensitive to the conditions in the stellar core. However, with the decrease in the central hydrogen abundance of stars, the separations sigma(02) and rho(13) deviate more and more from the scaled small separation. This characteristic could be used to extract the information on the central hydrogen abundance of stars.

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

Aims. The purpose of this work was to obtain diffusion coefficient for the magnetic angular momentum transport and material transport in a rotating solar model. Methods. We assumed that the transport of both angular momentum and chemical elements caused by magnetic fields could be treated as a diffusion process. Results. 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-circle dot 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.

Asteroseismology of 1523 misclassified red giants using Kepler data

We analysed solar-like oscillations in 1523

ASTEROSEISMIC ANALYSIS OF THE CoRoT TARGET HD 49933

\textit{Kepler}