表面化石磁场对大质量恒星演化的影响：III。$τ$ Sco的案例
The effects of surface fossil magnetic fields on massive star evolution: III. The case of $τ$ Sco
摘要：
$τ$ Sco是上Sco星协中一颗被充分研究的磁性B型星，它有许多令人惊讶的特征。它旋转得很慢，显示氮过剩。它的表面磁场比纯粹的偶极结构复杂得多，这在大质量磁性恒星中是不寻常的。我们采用CMFGEN辐射转移代码来确定的基本参数和表面CNO和氦丰度。然后，我们采用MESA和GENEC恒星演化模型解释表面磁场的影响。为了使$τ$ Sco的性质与单星模型相一致，旋转混合的效率需要增加3到10倍，磁制动的效率需要增加10倍。自旋下降可以通过假设一个磁场衰减的情况来解释。然而，同时进行的化学浓缩对单星设想提出了挑战。以前的研究确实提出了$τ$ Sco的恒星合并起源。然而，合并方案也面临着类似的挑战，我们的磁性单星模型来解释$τ$天蝎座的同时缓慢旋转和氮过剩。总之，单星通道似乎不太可能和通用的解释这些差异，而合并的情况和其他潜在的双演化通道仍然需要进一步评估，以确定它们是否可以自洽地解释观测到的$τ$ Sco。
英文摘要：
$τ$ Sco, a well-studied magnetic B-type star in the Upper Sco association, has a number of surprising characteristics. It rotates very slowly and shows nitrogen excess. Its surface magnetic field is much more complex than a purely dipolar configuration which is unusual for a magnetic massive star. We employ the CMFGEN radiative transfer code to determine the fundamental parameters and surface CNO and helium abundances. Then, we employ MESA and GENEC stellar evolution models accounting for the effects of surface magnetic fields. To reconcile $τ$ Sco's properties with single-star models, an increase is necessary in the efficiency of rotational mixing by a factor of 3 to 10 and in the efficiency of magnetic braking by a factor of 10. The spin down could be explained by assuming a magnetic field decay scenario. However, the simultaneous chemical enrichment challenges the single-star scenario. Previous works indeed suggested a stellar merger origin for $τ$ Sco. However, the merger scenario also faces similar challenges as our magnetic single-star models to explain $τ$ Sco's simultaneous slow rotation and nitrogen excess. In conclusion, the single-star channel seems less likely and versatile to explain these discrepancies, while the merger scenario and other potential binary-evolution channels still require further assessment as to whether they may self-consistently explain the observables of $τ$ Sco.
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