晚型WN星的质量损失及其Z依赖性：接近爱丁顿极限的超大质量星
Mass loss from late-type WN stars and its Z-dependence: very massive stars approaching the Eddington limit
摘要：
Wolf-Rayet（WR）星的质量损失对于大质量恒星的最终命运和化学产额具有重要意义。它的Z-依赖关系的长期伽马射线暴（GRB）的形成和早期恒星世代的产量进行了讨论。然而，WR型星风的形成机制仍在争论中。我们提出了第一个完全自洽的大气/风模型晚型WN星。我们研究了导致它们强烈质量损失的机制，并研究了对恒星参数的依赖性，特别是对金属丰度Z的依赖性。我们确定WNL恒星非常巨大的恒星接近爱丁顿极限，可能仍处于中央H燃烧阶段。由于它们的高L/M比，这些恒星发展出光学厚度，辐射驱动的风。这些风显示出与OB星的薄风性质不同的性质。由此产生的质量损失很大程度上取决于Z，但也取决于爱丁顿因子和恒星温度。我们结合我们的结果在一个参数化的质量损失食谱WNL明星。根据我们目前的模型计算，接近爱丁顿极限的恒星倾向于形成强烈的WR型风，即使在非常低的Z。因此，我们的模型预测了低金属丰度恒星的有效质量损失机制。对于极贫金属的恒星，我们发现，与初级氮的自我富集可以驱动WR型质量损失。这些第一批WN星可能在早期ISM的新鲜产生的氮富集中发挥重要作用。
英文摘要：
The mass loss from Wolf-Rayet (WR) stars is of fundamental importance for the final fate of massive stars and their chemical yields. Its Z-dependence is discussed in relation to the formation of long-duration Gamma Ray Bursts (GRBs) and the yields from early stellar generations. However, the mechanism of formation of WR-type stellar winds is still under debate. We present the first fully self-consistent atmosphere/wind models for late-type WN stars. We investigate the mechanisms leading to their strong mass loss, and examine the dependence on stellar parameters, in particular on the metallicity Z. We identify WNL stars as very massive stars close to the Eddington limit, potentially still in the phase of central H-burning. Due to their high L/M ratios, these stars develop optically thick, radiatively driven winds. These winds show qualitatively different properties than the thin winds of OB stars. The resultant mass loss depends strongly on Z, but also on the Eddington factor, and the stellar temperature. We combine our results in a parametrized mass loss recipe for WNL stars. According to our present model computations, stars close to the Eddington limit tend to form strong WR-type winds, even at very low Z. Our models thus predict an efficient mass loss mechanism for low metallicity stars. For extremely metal-poor stars, we find that the self-enrichment with primary nitrogen can drive WR-type mass loss. These first WN stars might play an important role in the enrichment of the early ISM with freshly produced nitrogen.
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