AI 中文总结
本研究通过计算含原子扩散的主序恒星演化模型，揭示主序A型恒星表层铁、钙丰度演化变化的机制，指出Am恒星与慢旋转非磁A型主序恒星起源相同，化学异常由恒星质量与年龄决定。
AI 中文摘要
本文计算了主序恒星演化模型，并求解了从氢到镍共16种元素的原子扩散方程。演化轨迹网格包含恒星质量在1.4至2.5倍太阳质量（$M_\bigodot$）范围内的模型，初始氦丰度$Y=0.28$、金属丰度$Z=0.02$。计算针对质量损失率$10^{-15}M_\bigodot/\text{yr}\text{≤}\text{Ṁ}\text{≤}10^{-12}M_\bigodot/\text{yr}$以及$\text{Ṁ}=0$的情况开展。研究表明，Am恒星表层铁丰度偏高是由于辐射加速作用于铁原子；钙的吸收系数显著更小，导致其发生重力沉降，并在温度$T\text{~}10^6\textrm{K}$的不透明度极大值上方累积；表层钙丰度的恢复则是由于外部对流区底部下探至钙丰度过高的层位。化学元素表层丰度演化变化的重要作用由中间对流区承担，该对流区在最初约3亿年（$\text{≲}300$ Myr）内形成，原因是铁和镍原子在温度$T\text{~}2\times10^5\textrm{K}$的层位累积。对于质量$M\text{≤}1.9M_\bigodot$的恒星，外部对流区底部随演化下探，导致外部与中间对流区合并，过剩的铁和镍通过对流被输运至表层。对流区合并会使钙和铁的表层丰度发生显著变化，变化持续时间为恒星主序寿命的四分之一至二分之一，具体取决于恒星质量。因此，Am恒星与慢旋转非磁A型主序恒星具有共同起源，其化学异常的出现取决于恒星质量与年龄。
英文摘要
Main sequence stellar evolution models were computed together with solution of the equations of atomic diffusion for 16 elements from hydrogen to nickel. The grid of evolutionary tracks comprises the models with stellar masses ranged from 1.4 to $2.5M_\odot$ computed for initial helium and metal abundances $Y=0.28$ and $Z=0.02$, respectively. The calculations were done for the mass loss rates $10^{-15}M_\odot/\textrm{yr}\le\dot M\le 10^{-12}M_\odot/\textrm{yr}$ as well as for $\dot M=0$. The high superficial abundance of iron in Am stars is shown to be due to the radiative acceleration acting on the atoms of iron. The significantly smaller absorption coefficient of calcium is responsible for its gravitational settling and accumulation above its opacity maximum at $T\sim 10^6\:\textrm{K}$. Recover of the superficial calcium abundance is due to plunge of the outer convection zone bottom to layers with its excessive abundance. A significant role in the evolutionary variations of superficial abundances of chemical elements belongs to the intermediate convection zone arising for the first $\lesssim 300$ Myr due to accumulation of the atoms of iron and nickel in the layers with temperature $T\sim 2\times 10^5\:\textrm{K}$. In stars with mass $M\le 1.9M_\odot$ both the outer and intermediate convection zones merge due to evolutionary descend of the bottom of the outer convection zone so that overabundant iron and nickel are transported to the outer layers by convection. The merging of the convective zones is responsible for considerable variations of superficial abundances of calcium and iron with duration ranging from a quarter to a half of the main--sequence lifetime depending on the stellar mass. Therefore, Am stars as well as slowly rotating nonmagnetic A main--sequence stars have the common origin, whereas appearance of their chemical anomalies depend of the stellar mass and age.
Comments12 pages, 12 figures, accepted to Astronomy Letters
