标题： UOCS-IX。 基于AstroSat/UVIT对疏散星团NGC 2818的研究：蓝离散星、黄离散星、行星状星云及其成员资格 显示英文标题

标题： UOCS-IX. AstroSat/UVIT study of the open cluster NGC 2818: Blue Stragglers, Yellow Stragglers, Planetary Nebula, and their membership

摘要： 我们展示了银河系中间年龄疏散星团 NGC 2818 的首幅远紫外（FUV）成像结果，该星团的视野内包含一个行星状星云（PN），这些图像是利用 AstroSat 卫星上的紫外成像望远镜（UVIT）拍摄的。 通过结合 UVIT 探测到的源与 Gaia EDR3 数据，我们识别出星团成员。 基于光学和 FUV-光学颜色-亮度图的位置，我们探测到四颗明亮且炽热的蓝离散星（BSSs）以及两颗黄离散星（YSSs）。 根据使用光谱能量分布（SED）估算的参数，我们认为 BSSs 要么是碰撞产物，要么可能有不可见的白矮星（WD）伴星。 我们的 YSS 光度分析确认了它们的双星性质，这与光谱结果一致。 我们发现 YSSs 是通过质量转移情景形成的，而炽热组分可能是 A 型次矮星。 比较行星状星云与星团的径向速度（RV）、Gaia EDR3 视向运动以及行星状星云和星团的消光，未排除行星状星云属于该星团的可能性。 将中心恒星的位置与理论后渐近巨星支（pAGB）模型进行比较表明，它已进入白矮星冷却阶段，其质量推断约为 0.66 太阳质量。 对应的前身恒星质量约为 2.1 太阳质量，与星团的主序点质量相当，表明前身恒星可能在星团中形成。 我们认为，NGC 2818 可能是少数已知包含行星状星云的星团之一，为检验恒星演化模型提供了独特的机会。 显示英文摘要

摘要： We present the first far-UV (FUV) imaging results of the intermediate-age Galactic open cluster NGC 2818 that has a Planetary nebula (PN) within the field using images taken from the Ultra-violet Imaging Telescope (UVIT) aboard AstroSat. We identify cluster members by combining UVIT-detected sources with Gaia EDR3 data. We detect four bright and hot blue straggler stars (BSSs) and two yellow straggler stars (YSSs) based on their location in the optical and FUV-optical color-magnitude diagrams. Based on the parameters estimated using Spectral Energy Distribution (SED), we infer that BSSs are either collisional products or might have undetectable white dwarf (WD) companions. Our photometric analysis of YSSs confirms their binarity, consistent with the spectroscopic results. We find YSSs to be formed through a mass-transfer scenario and the hot components are likely to be A-type subdwarfs. A comparison of the radial velocity (RV), Gaia EDR3 proper motion of the PN with the cluster, and reddening towards the PN and the cluster does not rule out the membership of the PN. Comparing the central star's position with theoretical pAGB models suggest that it has already entered the WD cooling phase, and its mass is deduced to be ~0.66Msun. The corresponding progenitor mass turns out to be ~2.1Msun, comparable to the turn-off mass of the cluster, implying that the progenitor could have formed in the cluster. We suggest that the NGC 2818 might be one of the few known clusters to host a PN, providing a unique opportunity to test stellar evolution models.