标题： 氮富集在A型和F型超巨星的大气中 显示英文标题

标题： Nitrogen Enrichment in Atmospheres of A- and F- Type Supergiants

摘要： 利用30颗银河系A型和F型超巨星的新精确基本参数，即它们的有效温度\(T_\{\text\{eff\}\}\)和表面重力\(\log g\)，我们对这些恒星大气中的氮丰度进行了非局域热平衡（non-LTE）分析。 结果显示，非局域热平衡修正值随着\(T_\{\text\{eff\}\}\)的增加而增大。 这种类型恒星普遍存在的氮过量现象得到了证实。 大多数恒星的氮过剩\([N/Fe]\)在0.2到0.9 dex之间，其中恒星分布的[N/Fe]最大位置在0.4到0.7 dex之间。 氮过剩现象在考虑旋转引起的混合效应的B型主序（MS）恒星及其下一演化阶段，即经历过第一次物质下沉的A型和F型超巨星时得到了讨论。 超巨星主序前身中的旋转引起的混合可能是氮过剩的重要原因。 当我们把我们的结果与为具有混合效应的恒星开发的理论预测进行比较时，发现大多数超巨星（30颗中的28颗）显示出的氮富集可以预期（i）要么是在主序阶段之后，对于初始自转速度\(v_0=200-400\)km/s的恒星；（ii）要么是在第一次物质下沉之后，对于初始自转速度\(v_0=50-400\)km/s的恒星。 由于第一次物质下沉后红蓝循环寿命较长，后者更被偏好。 没有明显氮过剩的两颗超巨星，HR 825和HR 7876，可能是具有相对较低初始自转速度约100 km/s的后主序天体。 建议的\(v_0\)范围大致与从观测到的B型主序恒星（A和F型超巨星的前身）投影自转速度推断的结果一致。 显示英文摘要

摘要： Using new accurate fundamental parameters of 30 Galactic A and F supergiants, namely their effective temperatures Teff and surface gravities log g, we implemented a non-LTE analysis of the nitrogen abundance in their atmospheres. It is shown that the non-LTE corrections to the N abundances increase with Teff. The nitrogen overabundance as a general feature of this type of stars is confirmed. A majority of the stars has a nitrogen excess [N/Fe] between 0.2 and 0.9 dex with the maximum position of the star's distribution on [N/Fe] between 0.4 and 0.7 dex. The N excesses are discussed in light of predictions for B-type main sequence (MS) stars with rotationally induced mixing and for their next evolutionary phase, i.e. A- and F-type supergiants that have experienced the first dredge-up. Rotationally induced mixing in the MS progenitors of the supergiants may be a significant cause of the nitrogen excesses. When comparing our results with predictions of the theory developed for stars with the mixing, we find that the bulk of the supergiants (28 of 30) show the N enrichment that can be expected (i) either after the MS phase for stars with the initial rotational velocities v0 = 200-400 km s-1, (ii) or after the first dredge-up for stars with v0 = 50-400 km s-1. The latter possibility is preferred on account of the longer lifetime for stars on red-blue loops following the first dredge-up. Two supergiants without a discernible N enrichment, namely HR 825 and HR 7876, may be post-MS objects with the relatively low initial rotational velocity of about 100 km s-1. The suggested range for v0 is approximately consistent with inferences from the observed projected rotational velocities of B-type MS stars, progenitors of A and F supergiants.