Evolution towards the critical limit and the origin of Be stars
Evolution towards the critical limit and the origin of Be stars
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DOI：
10.1051/0004-6361:20078095
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发表时间：
2007-11
影响因子：
6.5
通讯作者：
S. Ekström;G. Meynet;A. Maeder;F. Barblan
中科院分区：
文献类型：
--
作者：
S. Ekström;G. Meynet;A. Maeder;F. Barblan
上下文越来越多的证据表明经典Be星的自转速度接近临界速度。如果是这样的话，那么出现的问题是这种高表面速度的起源。目标。我们确定哪些机制加速主序星演化过程中的表面。我们研究了它们对金属丰度的依赖性，并导出了不同年龄和金属丰度的星团中具有不同表面速度的恒星的频率。方法.我们已经计算了112个恒星模型，它们的初始质量在3到60 M�之间，金属丰度在0到0.020之间，比值在0.1到0.99之间，有7个不同的值。对于所有的模型，进行计算，直到主层序演化结束或直到达到临界极限。结果在主序星寿命期间，表面速度的演化是由纬向环流（将角动量带到表面）和恒星风造成的质量损失（将其移除）之间的相互作用造成的。这两种机制对金属丰度的依赖在确定每种金属丰度时起着关键作用，对于能够达到或至少接近临界极限的恒星，初始质量（光谱类型）的限制范围。目前的模型预测，在年龄在1000万年到2500万年之间的星团中，快速旋转恒星的频率更高。这是大多数Be星被观测到的年龄范围。为了重现Be星的观测频率，首先必须假设Be星星现象已经发生在β/β临界值≥ 0.7的恒星上，其次，在零龄主序星上，快速旋转星的比例在金属丰度较低时较高。根据星星成为Be星星的阶段，它可能在表面呈现出更大或更少的氮富集。
Context. More and more evidence leads to considering classical Be stars as rotating close to the critical velocity. If so, then the question that arises is the origin of this high surface velocity. Aims. We determine which mechanisms accelerate the surface of single stars during the main sequence evolution. We study their dependence on the metallicity and derive the frequency of stars with different surface velocities in clusters of various ages and metallicities. Methods. We have computed 112 stellar models of four different initial masses between 3 and 60 M� , at four different metallicities between 0 and 0.020, and with seven different values of the ratio Ω/Ωcrit between 0.1 and 0.99. For all the models, computations were performed until either the end of the main sequence evolution or until the critical limit was reached. Results. The evolution of surface velocities during the main sequence lifetime results from an interplay between meridional circulation (bringing angular momentum to the surface) and mass loss by stellar winds (removing it). The dependence on metallicity of these two mechanisms plays a key role in determining, for each metallicity, a limiting range of initial masses (spectral types) for stars able to reach or at least approach the critical limit. Present models predict a higher frequency of fast rotating stars in clusters with ages between 10 and 25 Myr. This is the range of ages where most of Be stars are observed. To reproduce the observed frequencies of Be stars, it is necessary to first assume that the Be star phenomenon already occurs for stars with υ/υcrit ≥ 0.7 and, second, that the fraction of fast rotators on the zero-age main sequence is higher at lower metallicities. Depending on the stage at which the star becomes a Be star, it may present either larger or less enrichments in nitrogen at the surface.