标题： 关于尘埃和质量损失在星团扩展主序转折点的作用：NGC 1783案例 显示英文标题

标题： On the role of dust and mass loss in the extended main sequence turnoff of star clusters: the case of NGC 1783

摘要： 色星等图（CMD）形态显示了年龄为“$\lesssim 2$Gyr”的大麦哲伦云星团NGC 1783的“扩展”主序转折点（eMSTO）和上主序（MS）存在一小群紫外光较暗的恒星，在哈勃空间望远镜的紫外滤光片中，这些恒星的颜色位于典型的“扇形”分布的红色一侧。 我们通过假设一些原本应位于eMSTO左侧的恒星被因外围物质盘蒸发时高速旋转的恒星外围凝聚的尘埃环遮挡来模拟这些紫外光较暗的恒星。 需要一个合理的低光学深度（10$\mu$）来模拟这个紫外光较暗的群体。 在解释eMSTO时引入尘埃可能需要重新评估先前关于年龄和/或旋转范围在麦哲伦云星团中的作用的结论：整个eMSTO可以由尘埃星占据，最红的紫外光较暗星只是分布的尾端，具有最大的遮蔽和视线上的尘埃环。 模型预测具有较高投影自转速度（$v \sin$i）的恒星比慢速旋转的恒星更偏红色。 造成尘埃的质量损失也可能导致上主序中恒星的非单调分布，在紫外星等图中显示出两个峰值和间隙。 显示英文摘要

摘要： The Color Magnitude Diagram (CMD) morphology of the "extended" main sequence turnoff (eMSTO) and upper main sequence (MS) of the intermediate age ($\lesssim 2$ Gyr) Large Magellanic Cloud Cluster NGC 1783 shows the presence of a small group of UV-dim stars, that, in the ultraviolet Hubble Space Telescope filters, are located at colors on the red side of the typical "fan" shape displayed by the eMSTO. We model the UV-dim stars by assuming that some of the stars which would intrinsically be located on the left side of the eMSTO are obscured by a ring of dust due to grain condensation at the periphery of the excretion disc expelled when they spin at the high rotation rates typical of stars in the Be stage. A reasonably low optical depth at 10$\mu$ is necessary to model the UV-dim group. Introduction of dust in the interpretation of the eMSTO may require a substantial re-evaluation of previous conclusions concerning the role of age and/or rotation spreads in the MC clusters: the entire eMSTO can be populated by dusty stars, and the reddest UV-dim stars simply represents the tail of the distribution with both maximum obscuration and the dust ring seen along the line of sight. The model stars having higher rotational projected velocity ($v \sin$ i) are predicted to be preferentially redder than the slowly-rotating stars. The mass loss responsible for the dust may also cause the non-monotonic distribution of stars in the upper main sequence, with two peaks and gaps showing up in the UV CMD.