标题： 收缩的红超巨星自转加速 显示英文标题

标题： The spin-up of contracting red supergiants

摘要： 我们报道了一种机制，这种机制可能会导致一颗旋转的单星在离开Hayashi线时表面自转速度增加，其强度远远超过单纯由于恒星收缩所预期的自转加速。 通过分析刚性旋转的对流恒星包层，我们定性地探讨了这些包层如何分别通过下边界或上边界的物质损失而被加速或减速的机制。 我们发现第一种情况描述了后退对流包层的情况，这些包层倾向于保留大部分角动量，同时质量减少，从而增加了对流区以及接近恒星表面的层的比角动量。 我们在一颗旋转12倍太阳质量恒星的恒星演化计算中定量地探索了这一自转加速机制，发现它在离开红超巨星分支后会加速到临界自转状态。 我们讨论了这种自转加速对几种类型恒星周围的星际物质的影响，即后AGB星、B[e]星、前主序星，特别是1987A型超新星的前身。 显示英文摘要

摘要： We report on a mechanism which may lead to a spin-up of the surface of a rotating single star leaving the Hayashi line, which is much stronger than the spin-up expected from the mere contraction of the star. By analyzing rigidly rotating, convective stellar envelopes, we qualitatively work out the mechanism through which these envelopes may be spun up or down by mass loss through their lower or upper boundary, respectively. We find that the first case describes the situation in retreating convective envelopes, which tend to retain most of the angular momentum while becoming less massive, thereby increasing the specific angular momentum in the convection zone and thus in the layers close to the stellar surface. We explore the spin-up mechanism quantitatively in a stellar evolution calculation of a rotating 12 M_sun star, which is found to be spun up to critical rotation after leaving the red supergiant branch. We discuss implications of this spin-up for the circumstellar matter around several types of stars, i.e., post-AGB stars, B[e] stars, pre-main sequence stars, and, in particular, the progenitor of Supernova 1987A.