具有大angular momentum含量的单个B型星的演化
The evolution of single B-type stars with a large angular momentum content
摘要：
日内瓦恒星演化小组在过去几年中提出的中等质量旋转恒星模型数据库可以用来建立综合的恒星星族，充分考虑恒星旋转的影响。然而，到目前为止，我们仍然缺乏恒星的演化轨道，旋转接近临界极限在整个MS阶段。这是因为在零年龄MS（ZAMS）施加的平面内部旋转轮廓被ZAMS之后立即发生的纬向电流的作用所修改，导致表面旋转速度突然降低，直到它达到准静止状态。我们计算恒星模型与非固体旋转在ZAMS获得恒星的演化轨道具有较大的内容的角动量，达到旋转赤道速度接近临界极限，在整个MS阶段。这些模型具有更长的MS寿命和更高的表面化学富集已经在MS结束时，特别是在Z=0.002。在ZAMS的固体旋转的恒星模型充分代表的整体特征和演化的差分旋转模型相同的角动量内容，但具有较低的初始表面旋转速度。对于这些模型，我们建议使用的初始旋转速率的值推导出一旦达到准稳态，在表面速度突然下降。有趣的是，初始赤道旋转速度几乎是独立的恒星模型，我们计算相同的质量和角动量内容在ZAMS。如果像一些观测证据所表明的那样，Z=0.002的B型恒星在ZAMS中以比Z=0.014的恒星更高的赤道速度旋转，我们的发现将表明SMC中B型恒星的角动量含量高于银河系对应物。
英文摘要：
The database of intermediate mass rotating stellar models presented in the past years by the Geneva Stellar Evolution Group can be used to build synthetic stellar populations that fully account for the effects of stellar rotation. However, up to now we still lacked stellar evolutionary tracks that rotate close to the critical limit during the whole MS phase. This occurs because the flat internal profile of rotation imposed at the Zero-Age MS (ZAMS) is modified by the action of meridional currents immediately after the ZAMS, causing the surface rotational velocity to decrease abruptly until it reaches a quasi-stationary state. We compute stellar models with non-solid rotation at the ZAMS to obtain stellar evolutionary tracks with a larger content of angular momentum, that attain rotational equatorial velocities close to the critical limit throughout their MS phase. These models have a longer MS lifetime and a higher surface chemical enrichment already at the end of the MS, particularly at Z=0.002. Stellar models with solid rotation at the ZAMS adequately represent the overall characteristics and evolution of differentially rotating models of identical angular momentum content, but with a lower initial surface rotational velocity. For these models we recommend to use as the initial rotational rate the values derived once the quasi-stationary state is reached, after the abrupt decrease in surface velocity. Interestingly, the initial equatorial rotational velocities are virtually metallicity independent for the stellar models we computed with the same mass and angular momentum content at the ZAMS. If, as some observational evidence indicates, B-type stars at Z=0.002 rotate with a higher equatorial velocity at the ZAMS than stars with Z=0.014, our finding would indicate that the angular momentum content of B-type stars in the SMC is higher than their Galactic counterparts.
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