分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： Convective-core overshoot mixing is a significant uncertainty in stellar evolution. Because numerical simulations and turbulent convection models predict exponentially decreasing radial rms turbulent velocity, a popular treatment of the overshoot mixing is to apply a diffusion process with exponentially decreasing diffusion coefficient. It is important to investigate the parameters of the diffusion coefficient because they determine the efficiency of the mixing in the overshoot region. In this paper, we have investigated the effects of the core overshoot mixing on the properties of the core in solar models and have constrained the parameters of the overshoot model by using helioseismic inferences and the observation of the solar 8B neutrino flux. For solar-mass stars, the core overshoot mixing helps to prolong the lifetime of the convective core developed at the ZAMS. If the strength of the mixing is sufficiently high, the convective core in a solar model could survive till the present solar age, leading to large deviations of the sound-speed and density profiles comparing with the helioseismic inferences. The 8B neutrino flux also favours a radiative solar core. Those provide a constraint on the parameters of the exponential diffusion model of the convective overshoot mixing. A limited asteroseismic investigation of 13 Kepler low-mass stars with 1.0 < M < 1.5 shows a mass-dependent range of the overshoot parameter. The overshoot mixing processes for different elements are analyzed in detail. It is found that the exponential diffusion overshoot model leads to different effective overshoot mixing lengths for elements with different nuclear equilibrium timescale.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Jian Ge
Hui Zhang
Weicheng Zang
Hongping Deng
Shude Mao
Ji-Wei Xie
Hui-Gen Liu
Ji-Lin Zhou
Kevin Willis
Chelsea Huang
Steve B. Howell
Fabo Feng
Jiapeng Zhu
Xinyu Yao
Beibei Liu
Masataka Aizawa
Wei Zhu
Ya-Ping Li
Bo Ma
Quanzhi Ye
摘要： We propose to develop a wide-field and ultra-high-precision photometric survey mission, temporarily named "Earth 2.0 (ET)". This mission is designed to measure, for the first time, the occurrence rate and the orbital distributions of Earth-sized planets. ET consists of seven 30cm telescopes, to be launched to the Earth-Sun's L2 point. Six of these are transit telescopes with a field of view of 500 square degrees. Staring in the direction that encompasses the original Kepler field for four continuous years, this monitoring will return tens of thousands of transiting planets, including the elusive Earth twins orbiting solar-type stars. The seventh telescope is a 30cm microlensing telescope that will monitor an area of 4 square degrees toward the galactic bulge. This, combined with simultaneous ground-based KMTNet observations, will measure masses for hundreds of long-period and free-floating planets. Together, the transit and the microlensing telescopes will revolutionize our understandings of terrestrial planets across a large swath of orbital distances and free space. In addition, the survey data will also facilitate studies in the fields of asteroseismology, Galactic archeology, time-domain sciences, and black holes in binaries.
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