通过星震学分析探索HD 49385大气中的小尺度磁场
Exploring the Small-scale Magnetic Fields in the Atmosphere of HD 49385 by Asteroseismic Analysis
AI总结：
该研究通过星震学分析探索HD 49385大气中小尺度磁场，纳入改进方程计算p模式频率，经与观测对比选最佳拟合模型，估计磁场强度与分布，确定氦核相关参数，进而准确得出恒星质量与年龄。
AI中文摘要：
近期星震学研究表明一些脉动红巨星内部可能存在磁场。受此启发，研究HD 49385中小尺度磁场对p模式振荡的影响。纳入修改的Eddington T-τ方程模拟磁场效应，计算l = 0、1和2的p模式频率。通过与观测频率比较选最佳拟合模型，不仅拟合频率，还重现有效温度和log g等参数。估计小尺度磁场强度约80 G，集中分布在大气中约1850 km高度处。通过选特定避免交叉模式的最佳拟合模型，确认其频率与氦核紧密相关，确定氦核质量为0.117M⊙，半径为0.078R⊙。基于这些改进，准确确定HD 49385质量为1.25 ± 0.02M⊙，GS98成分时年龄为4.1 Gyr，A09成分时年龄为4.5 Gyr。
英文摘要：
Recent asteroseismic studies have shown convincing evidences that magnetic fields may exist in the interior of some pulsating red giants. Inspired by this breakthrough, we explored the effect of small-scale magnetic fields on the p-mode oscillations in an evolved star, HD 49385. {\bf We incorporate a modified Eddington $T$-$τ$ equation that phenomenologically mimics the effect of the magnetic fields in the atmosphere of HD 49385,} and calculate the frequencies of p-modes with $l=0$, 1, and 2. By comparing the calculated frequencies with the observed ones, we select two best-fit models with either GS98 or A09 chemical composition. Our best-fit models not only fit satisfactorily the observed frequencies, but also well reproduce some spectroscopically observed stellar parameters such as effective temperature and log\,$g$. Based on the two best-fit models, we have estimated that the small-scale magnetic fields possess a strength of approximately 80\,G and spread concentratively at approximately a height of 1850 km in the atmosphere. By selecting the best-fit models with special requirement on the avoided-crossing mode, we have confirmed that the frequency of the avoided-crossing mode is tightly related to the helium core of the star, and determined the size of the helium core as 0.117${\rm M}_\odot$ in mass and 0.078${\rm R}_\odot$ in radius. Based on the improvements of previous two sides, we can accurately determine the mass of HD 49385 to be $1.25\pm 0.02\,{\rm M}_\odot$ with an age of 4.1\,Gyr for GS98 composition and 4.5\,Gyr for A09 composition.
