We derive the effective temperatures and gravities of 461 OB stars in 19 young clusters by fitting the Hγ profile in their spectra. We use synthetic model profiles for rotating stars to develop a method to estimate the polar gravity for these stars, which we argue is a useful indicator of their evolutionary status. We combine these results with projected rotational velocity measurements obtained in a previous paper on these same open clusters. We find that the more massive B stars experience a spin-down as predicted by the theories for the evolution of rotating stars. Furthermore, we find that the members of binary stars also experience a marked spin-down with advanced evolutionary state due to tidal interactions. We also derive non-LTE-corrected helium abundances for most of the sample by fitting the He I λλ4026, 4387, 4471 lines. A large number of helium peculiar stars are found among cooler stars with Teff 23,000 K. The analysis of the high-mass stars (8.5 M☉ M 16 M☉) shows that the helium enrichment process progresses through the main-sequence (MS) phase and is greater among the faster rotators. This discovery supports the theoretical claim that rotationally induced internal mixing is the main cause of surface chemical anomalies that appear during the MS phase. The lower mass stars appear to have slower rotation rates among the low-gravity objects, and they have a large proportion of helium peculiar stars. We suggest that both properties are due to their youth. The low-gravity stars are probably pre-main-sequence objects that will spin up as they contract. These young objects very likely host a remnant magnetic field from their natal cloud, and these strong fields sculpt out surface regions with unusual chemical abundances.
展开▼
机译：通过将Hγ谱拟合到它们的光谱中，我们得出19个年轻星团中461 OB恒星的有效温度和重力。我们使用合成模型轮廓来分析恒星的旋转，以开发一种方法来估计这些恒星的极地引力，我们认为这是它们演化状态的有用指标。我们将这些结果与先前的论文在这些相同的开放星团上获得的预计旋转速度测量结果结合起来。我们发现，质量更大的B恒星经历了自转下降，这是旋转恒星演化的理论所预测的。此外，我们发现由于潮汐相互作用，双星的成员也经历了明显的自旋下降，并具有先进的演化状态。通过拟合He I λλ4026、4387、4471谱线，我们还获得了大多数样品的未经LTE校正的氦气丰度。在Teff <23,000 K的较冷恒星中发现了大量的氦特有恒星。对高质量恒星（8.5M☉展开▼
