[论文解读] Searching for links between magnetic fields and stellar evolution. II. The evolution of magnetic fields as revealed by observations of Ap stars in open clusters and associations

J. D. Landstreet, S. Bagnulo+5 more|arXiv (Cornell University)|Jun 3, 2007

Stellar, planetary, and galactic studies参考文献 56被引用 97

一句话总结

本研究通过分析年龄确定良好的星协和星团成员的磁场强度，探讨了Ap星磁场的演化。结果表明，大质量Ap星（$M > 3M_\odot$）的磁场在约3000万年内衰减，与磁通量守恒和恒星膨胀一致；而低质量Ap星在数亿年内未表现出显著的磁场减弱，暗示其磁场演化机制存在质量依赖性。

ABSTRACT

The evolution of magnetic fields in Ap stars during the main sequence phase is presently mostly unconstrained by observation because of the difficulty of assigning accurate ages to known field Ap stars. We are carrying out a large survey of magnetic fields in cluster Ap stars with the goal of obtaining a sample of these stars with well-determined ages. In this paper we analyse the information available from the survey as it currently stands. We select from the available observational sample the stars that are probably (1) cluster or association members and (2) magnetic Ap stars. For the stars in this subsample we determine the fundamental parameters T_eff, log(L/L_o), and M/M_o. With these data and the cluster ages we assign both absolute age and fractional age (the fraction of the main sequence lifetime completed). For this purpose we have derived new bolometric corrections for Ap stars. Magnetic fields are present at the surfaces of Ap stars from the ZAMS to the TAMS. Statistically for the stars with M > 3 M_o the fields decline with advancing age approximately as expected from flux conservation together with increased stellar radius, or perhaps even faster than this rate, on a time scale of about 3 10^7 yr. In contrast, lower mass stars show no compelling evidence for field decrease even on a timescale of several times 10^8 yr. Study of magnetic cluster stars is now a powerful tool for obtaining constraints on evolution of Ap stars through the main sequence. Enlarging the sample of known cluster magnetic stars, and obtaining more precise RMS fields, will help to clarify the results obtained so far. Further field observations are in progress.

研究动机与目标

• 约束主序阶段Ap星磁场的演化，该阶段因场星年龄不确定而研究不足。
• 通过研究年龄确定良好的疏散星团和星协中的Ap星，克服年龄不确定性。
• 利用观测到的磁场强度和基本参数，确定磁场强度随年龄和恒星质量的变化。
• 推导适用于Ap星的新绝对星等修正，以提高光度和温度估计的准确性。
• 检验磁场演化是否遵循磁通量守恒或更快衰减，特别是与恒星半径增长的关系。

提出的方法

• 从一项调查中选取被确认为星团或星协成员的Ap星，依据运动学和光度成员资格标准。
• 通过圆偏振观测测量磁场强度，主要使用加拿大-法国-夏威夷望远镜上的ESPaDOnS光谱仪。
• 从能量分布函数和星团成员资格约束中确定恒星参数（$T_{\mathrm{eff}}$，$L/L_\odot$，$M/M_\odot$）。
• 利用星团年龄估计和演化模型，计算绝对年龄和相对年龄（已完成主序寿命的比例）。
• 推导专用于Ap星的新绝对星等修正，以提高从观测星等推算光度的准确性。
• 分别对大质量与低质量Ap星，分析磁场强度与恒星年龄的相关性。

实验结果

研究问题

• RQ1 Ap星的磁场是否在主序寿命内衰减？若然，衰减速率如何？
• RQ2 磁场衰减速率如何随恒星质量变化，特别是$M > 3M_\odot$与低质量恒星之间？
• RQ3 观测到的磁场衰减是否与恒星演化过程中膨胀导致的磁通量守恒一致？
• RQ4 年轻星团与较老星协中的Ap星在磁场演化上是否存在显著差异？
• RQ5 低质量系统中的Ap星是否在超过$10^8$年的时标上显示出磁场强度演化的证据？

主要发现

• 从零龄主序（ZAMS）到终止龄主序（TAMS），Ap星表面的磁场可被探测到。
• 对于$M > 3M_\odot$的Ap星，磁场强度的衰减速率与磁通量守恒结合恒星半径膨胀的预期大致一致，或可能更快，时间尺度约为$3 \times 10^7$年。
• 在数倍于$10^8$年的时标内，未观测到低质量Ap星磁场显著减弱的有力证据。
• 观测到的磁场演化表明，磁场衰减机制可能具有质量依赖性，大质量恒星中衰减更快。
• 本研究证实，星团中的Ap星是约束主序阶段磁场演化的重要观测工具。
• 未来需要更高信噪比的RMS磁场测量，以更清晰地揭示磁场演化趋势并减小不确定性。