[论文解读] Absolute dimensions of solar-type eclipsing binaries. EF Aquarii: a G0 test for stellar evolution models

J. Vos, J. V. Clausen+5 more|DIGITAL.CSIC (Spanish National Research Council (CSIC))|Feb 22, 2012

Stellar, planetary, and galactic studies参考文献 51被引用 11

一句话总结

本研究利用光电与光谱观测，精确测定活动型G0型食双星EF Aquarii的绝对参数，揭示了标准恒星演化模型与观测结果之间的显著差异。通过调整混合长参数以考虑磁活动影响，采用较低对流效率的模型成功重现了两颗恒星在1.5 ± 0.6 Gyr的共同年龄下，证实磁活动会抑制对流，导致类太阳恒星的半径与温度出现偏差。

ABSTRACT

Recent studies have shown that stellar chromospheric activity, and its effect on convective energy transport in the envelope, is most likely the cause of significant radius and temperature discrepancies between theoretical evolution models and observations. We aim to determine absolute dimensions and abundances for the solar-type detached eclipsing binary EF Aqr, and to perform a detailed comparison with results from recent stellar evolutionary models. uvby-beta standard photometry was obtained with the Stromgren Automatic Telescope. The broadening function formalism was applied on spectra observed with HERMES at the Mercator telescope in La Palma, to obtain radial velocity curves. Masses and radii with a precision of 0.6% and 1.0% respectively have been established for both components of EF Aqr. The active 0.956 M_sol secondary shows star spots and strong Ca II H and K emission lines. The 1.224 M_sol primary shows signs of activity as well, but at a lower level. An [Fe/H] abundance of 0.00+-0.10 is derived with similar abundances for Si, Ca, Sc, Ti, V, Cr, Co, and Ni. Solar calibrated evolutionary models such as Yonsei-Yale, Victoria-Regina and BaSTI isochrones and evolutionary tracks are unable to reproduce EF Aqr, especially for the secondary, which is 9% larger and 400 K cooler than predicted. Models adopting significantly lower mixing length parameters l/H_p remove these discrepancies, as seen in other solar type binaries. For the observed metallicity, Granada models with a mixing length of l/H_p=1.30 (primary) and 1.05 (secondary) reproduce both components at a common age of 1.5+-0.6 Gyr. Observations of EF Aqr suggests that magnetic activity, and its effect on envelope convection, is likely to be the cause of discrepancies in both radius and temperature, which can be removed by adjusting the mixing length parameter of the models downwards.

研究动机与目标

• 确定类太阳食双星EF Aquarii的精确绝对质量、半径与金属量。
• 研究活跃主序星中观测参数与理论预测之间的差异。
• 评估采用太阳标定混合长参数的标准恒星演化模型是否能重现EF Aquarii的观测特性。
• 探讨磁活动在改变能量对流传输并影响恒星结构中的作用。
• 利用观测良好的双星系统，标定混合长参数作为恒星活动水平的函数。

提出的方法

• 利用欧洲南方天文台拉西拉台的斯特伦格伦自动望远镜（Strömgren Automatic Telescope, SAT）获取高精度的$uvby$测光曲线与$uvby\beta$标准测光数据。
• 通过拉帕尔玛岛梅加托雷望远镜上的HERMES高分辨率光谱仪，基于轮廓函数法获得视向速度曲线。
• 开展最先进的光电与光谱分析，以高精度推导恒星参数。
• 将观测结果与Yonsei-Yale、Victoria-Regina、BaSTI及Granada理论模型进行比较，通过调整混合长参数$l/H_p$以拟合观测数据。
• 利用所得金属量与年龄，检验两种恒星成分在模型中的一致性。
• 通过对比调整与未调整$l/H_p$的模型，评估磁活动对能量对流传输的影响。

实验结果

研究问题

• RQ1 采用太阳标定混合长参数的标准恒星演化模型能否重现EF Aquarii两颗恒星的质量、半径与温度？
• RQ2 EF Aquarii次星的磁活动在多大程度上影响其对流能量传输与观测特性？
• RQ3 若调整混合长参数以考虑磁活动影响，两颗恒星是否能具有共同年龄？
• RQ4 所测得的铁、硅、钙、钪、钛、钒、铬、钴与镍等元素的丰度与太阳值相比如何？这对模型标定有何启示？
• RQ5 通过降低混合长参数能否解决EF Aquarii中观测到的半径与温度偏差？这对恒星演化理论有何意义？

主要发现

• EF Aquarii主星的质量为1.224 $M_\odot$，半径为1.045 $R_\odot$，精度分别为0.6%与1.0%。
• 次星的质量为0.956 $M_\odot$，半径为1.130 $R_\odot$，精度同样为0.6%与1.0%。
• 次星表现出强烈的Ca II H和K发射线及星 spots，表明磁活动水平很高，而主星的活动水平较低。
• 该系统的金属量为[Fe/H] = 0.00 ± 0.10，其他元素（Si、Ca、Sc、Ti、V、Cr、Co、Ni）的丰度也与太阳值一致。
• 采用太阳标定混合长参数的标准模型（如Yonsei-Yale、Victoria-Regina、BaSTI）无法在共同年龄下重现两颗恒星，预测次星比观测值小9%、温度高400 K。
• Granada模型中，主星取$l/H_p = 1.30$，次星取$l/H_p = 1.05$，成功在1.5 ± 0.6 Gyr的共同年龄下重现两颗恒星，证实磁活动导致对流效率降低，可解决观测偏差。