Asteroseismic Tests Of Heavy Element Diffusion In Low-mass Stars

Element diffusion is a basic physical element transport mechanism. This process occur when multicomponent plasmas in stellar interior are subject to variational physical conditions (such as pressure, temperature or density) or are subject to external forces (such as gravitational settling or radiative force). Element diffusion occurs on a long time scale, typically more than 10¹³yr to diffuse a solar radius under solar conditions, which shows that effects on stellar structure are small and usually neglected in standard stellar models. Nevertheless, the precision required for evaluating helioseismological frequencies have demonstrated the importance of including element diffusion in solar models. Solar models including diffusion can reproduce much better than standard models the solar pulsation spectrum and the helioseismic values of helium surface abundance and depth of the convective envelope. Later, a large number of studies have been made of the effect of diffusion on the evolution of the Sun and other stars. For example, models and isochrones appropriate for globular cluster stars have shown that the inclusion of diffusion reduces the main sequence lifetime and effective temperatures of the stars. Because of the long time scale, the effects of diffusion are more obvious and efficient for the older main sequence stars than the age of the Sun. But we don't know whether or not considering the effects of element diffusion in the early-stage of main sequence stars.In order to answer this question, we choose the active late type starεEridani as the target to study it.εEridani is one of the nearest star from the Sun. The observational data is abundant. Many evidences from observations seem to indicate thatεEridani is a young main sequence star with age less than 1Gyr. Its mass lower than 1M_? and age smaller than the Sun. Following the success of helioseismology, asteroseismology is now becoming a fundamental tool for penetrating the internal structure of stars. Considering the heavy element (including helium and metal element) diffusion, we construct the stellar structure models ofεEridani. Using the asteroseismic method, we study the effects of helium and metal element diffusion on the stellar evolution, internal structure and the frequencies of stellar p-mode respectively.From studying, we conclude that heavy element diffusion could change the chemical composition, increase the heavy element and decrease the hydrogen element abundance in the stellar interior. So it could affect the nuclear reaction, speed up the stellar evolution and reduce the age of the star. We use the "second differences"â–³₂v to test the change of the position of the base of the convection zone. It is found that the heavy element diffusion could increase the depth of the convective envelope. At the same time, we use the small spacings d_ll+2 and the frequency separation ratio r_ll+2 to test the internal structure. We found that the effect of pure helium diffusion on the internal structure is sight, but the metal diffusion influence is obvious. The metal abundance affects stellar structure by affecting radiative opacities. The metal diffusion could change the temperature profile in the interior, and therefor affect the sound-speed and the oscillation frequency.At last, the results show that the heavy element diffusion could not be neglected in the low-mass stars and that the effects of metal diffusion are more obvious than that of helium diffusion in the early-stage of main sequence. Considering the heavy element diffusion, it could perfect the stellar structure models. It is important to study the physical problems about the stellar structure and evolution not only for low-mass stars but also for all the stars.