通过星震学探测对流核超射物质混合扩散系数

The convective overshoot is the most uncertainty in stellar evolution theory at present. Classical non-local mixing length overshoot models have been excluded by a recent helioseismic study (Christensen-Dalsgaard et al. 2011). The turbulent convection models and numerical simulations of stellar fluid have shown that the diffusion coefficient of the convective overshoot is an exponential decreasing function. For an exponential function, the initial value is crucial. According to a popular diffusion overshoot model developed by Herwig (2000), the initial value of diffusion coefficient is set as a typical value in the convective zone. However, a recent model of convective overshoot mixing developed by Zhang (2013) has concluded that the diffusion coefficient significantly decreases near convective boundary and thus the initial value of diffusion coefficient is much lower than the typical value in the convective zone. This distinction leads to significant difference of the efficient of overshoot mixing. Herwig's model results in a fully mixed overshoot region, but Zhang's model results in a partially mixed overshoot region. It is important for validating overshoot models and investigating the efficient of overshoot mixing to detect the initial value of the exponential decreasing diffusion coefficient in the overshoot region. This proposal plans to detect the diffusion coefficient in the overshoot region by using asteroseismology. We plan to use some SPB stars, such as KIC10526294 to do this. By using the numerical results of stellar evolutionary models and comparing their theoretical adiabatic oscillation frequencies with observations, we can find the ranges of initial value of diffusion coefficient. And we can judge an important thing that the overshoot region is fully mixed or partially mixed.

对流超射是目前恒星演化理论中最不确定的因素。经典的非局地混合长超射模型已经被日震学研究否定。目前较完善的湍动对流模型以及流体数值模拟结果显示，超射区中的物质混合扩散系数为指数衰减型。对于一个指数衰减型的扩散系数，其在对流边界附近的初始值至关重要。按照比较流行的Herwig扩散模型，初始值与对流区内的值一致；而最近的超射混合模型则表示扩散系数在对流边界附近会迅速下降，从而初始值远低于对流区内的值。两个模型的此种差异导致截然不同的效果：前者为完全混合，而后者为部分混合。测定扩散系数的初始值对于检验理论模型和认识对流超射混合效率有重要意义。本申请项目计划使用星震学的方法来测定超射混合扩散系数的初始值。拟使用KIC10526294这类的SPB星作为候选源，通过计算恒星理论模型及其振动频率，与观测频率对比，测定扩散系数的初始值，并对对流超射区混合究竟是完全混合还是部分混合这一重要问题做出可靠的判断。

对流超射是目前恒星演化理论中最不确定的因素。经典的非局地混合长超射模型已经被日震学研究否定。目前较完善的湍动对流模型以及流体数值模拟结果显示，超射区中的物质混合扩散系数为指数衰减型。对于一个指数衰减型的扩散系数，其在对流边界附近的初始值至关重要。按照比较流行的Herwig扩散模型，初始值与对流区内的值一致；而最近的超射混合模型则表示扩散系数在对流边界附近会迅速下降，从而初始值远低于对流区内的值。测定扩散系数在对流边界附近的迅速衰减，对于检验理论模型和调查对流超射混合效率有重要意义。..本项目对KIC10526294进行了星震学分析，结果支持扩散系数在对流边界附近存在明显的迅速衰减而后再按指数衰减。对流边界附近约衰减4个数量级时，KIC10526294的理论模型频率与观测符合的最好。这支持了我前期的对流超射混合理论模型，为恒星对流理论的发展提供了新线索。本项目对十余个Kepler类太阳星进行了的星震学分析，通过分析对恒星内核性质敏感的大小频率间隔比，测定了对流核超射扩散系数衰减因子的取值范围，表明小质量星的衰减因子与恒星质量相关，结论与通过食双星测定对流核超射的结果一致。本项目还深入研究了“太阳丰度问题”这一存在十余年的恒星物理难题，并取得重要进展。“太阳丰度问题”是指，采用了新测定的太阳化学组成以后，原本与日震学反演符合的较好的标准太阳模型出现较大的问题，其内部声速密度轮廓、对流区深度和对流区氦丰度都与日震学反演出现严重偏差。我们仔细分析了标准太阳模型的问题，将对流超射等物理过程应用在太阳演化模型上，获得的改进太阳模型的日震学性质、表面锂丰度和中微子流量都与观测符合，首次成功构建了全面符合观测的太阳模型。