迈向整体磁制动模型——II：解释类太阳恒星与太阳的若干长期内部自转与表面自转特性
Towards a holistic magnetic braking model -- II: explaining several long-term internal- and surface-spin properties of solar-like stars and the Sun
AI总结：
本研究拓展α-Ω发电机驱动的磁制动模型，解释类太阳恒星与太阳的多类自转演化特性，并否定年轻大质量太阳假说对黯淡年轻太阳问题的解释效力。
摘要：
我们将由α-Ω发电机机制驱动的磁制动（MB）模型从完全对流M型矮星（FCMDs）拓展，以解释部分对流矮星（PCDs）从星盘消散阶段到主序拐点的表面与内部自转周期P_spin演化。在我们的模型中，核心自转由核包层边界处的剪切作用主导，而包层自转由磁制动（MB）与剪切作用共同主导。我们发现：（1）质量最大的完全对流M型矮星（FCMDs）的自转减速效应强于部分对流矮星（PCDs）和质量更小的完全对流M型矮星（FCMDs）；（2）K型矮星的自转减速停滞与活动性增强，以及年龄超过数吉年的G型矮星的堆积现象，与恒星结构和磁制动（MB）相关，而与核包层耦合效应的关联较弱；（3）我们给出的核包层收敛时标τ_converge(M_*, P_spin)表达式取值在几十到100百万年之间，该时标强烈依赖恒星结构，但对磁制动（MB）强度和剪切作用的依赖较弱，因此快速自转的大质量恒星更早达到共转状态；（4）我们对表面磁场的估算结果与观测总体一致，且我们的风质量损失演化解释了类太阳恒星π¹ UMa的弱风现象；（5）借助我们的模型，“年轻大质量太阳假说”作为黯淡年轻太阳问题的解决方案很可能可以被排除，因为根据我们的模型，太阳通过星风损失的最大质量比解决该问题所需的质量小约一个数量级。
英文摘要：
We extend our model of magnetic braking (MB), driven by an $α-Ω$ dynamo mechanism, from fully convective M-dwarfs (FCMDs) to explain the surface and internal spin $P_\mathrm{spin}$ evolution of partly convective dwarfs (PCDs) starting from the disc-dispersal stage to the main-sequence turnoff. In our model, the spin of the core is governed by shear at the core-envelope boundary while the spin of the envelope is governed by MB and shear. We show that (1) the most massive FCMDs experience a stronger spin-down than PCDs and less massive FCMDs, (2) the stalled spin-down and enhanced activity of K-dwarfs and the pileup of G-dwarfs older than a few Gyr are stellar-structure- and MB-dependent, and weakly dependent on core-envelope coupling effects, (3) our expression of the core-envelope convergence time-scale $τ_\mathrm{converge}(M_\ast,\,P_\mathrm{spin})$ between a few 10 to 100~Myr strongly depends on stellar structure but weakly on MB strength and shear, such that fast and massive rotators achieve corotation earlier, (4) our estimates of the surface magnetic fields are in general agreement with observations and our wind mass loss evolution explains the weak winds from the solar analog $π^1$ UMa and (5) with our model the massive young Sun hypothesis as a solution to the faint young Sun problem can likely be ruled out, because the maximum mass lost by winds from our Sun with our model is about an order of magnitude smaller than required to solve the problem.
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