16283

Multiple populations are recognized to be a common trait of nearly all ancient Globular Clusters (GCs) comma but no model or scenario so far proposed for their formation satisfies all the observational constraints comma and observations are still necessary to discriminate among competing models. HST observations have largely contributed to define the properties of multiple populations in particular comma HST observations have clearly revealed the photometric manifestation of the presence chemically distinct groups of stars. A recent spectroscopic investigation of NGC6402 has shown that this massive GC hosts three populations comma including a primordial comma a quasi-primordial and an extreme group comma this latter displaying light elements abundances that in other GCs comma (e.g.NGC2808) comma are associated with a very high helium. This peculiar distribution of abundances has stimulated innovative formation scenarios where the cluster requires two distinct sources and timescales of enrichment. We propose to derive the 'chromosome map. two-color diagram that harnesses the unique UV capabilities of the UVIS channel of WFC3 and investigate the possible abundance gap in these spectro--photometric data comma by separating the different groups. We will then determine the relative helium abundances of the groups comma and discern between two competing scenarios for the formation of the extreme population in this clusterdoublePoint either a scenario in which these stars formed from pristine gas mixed with ejecta of a supermassive star formed and living in the first virgul5-15 Myr in the cluster core or the Asympotic Giant Branch (AGB) scenario comma in which the extreme stars are formed from the pure ejecta of super-- AGB stars.

几乎所有古老球状星团（Globular Clusters, GCs）均被认为普遍具有多星族特征，但迄今为止提出的所有球状星团形成模型或情景均无法满足全部观测约束条件，因此仍需通过观测来区分相互竞争的各类模型。哈勃空间望远镜（Hubble Space Telescope, HST）的观测在定义多星族性质方面发挥了重要作用，具体而言，HST观测清晰地揭示了存在化学组成不同恒星群的光度学特征。近期一项针对NGC 6402的光谱学研究表明，该大质量球状星团拥有三个星族，分别为原初星族、准原初星族与极端星族；其中极端星族的轻元素丰度，在其他球状星团（例如NGC 2808）中通常与极高氦丰度相关联。这种特殊的丰度分布催生了创新性的形成情景，该情景认为该星团需要两种截然不同的物质来源与增丰时间尺度。我们计划借助广角相机3（Wide Field Camera 3, WFC3）的紫外可见通道（Ultraviolet Visual Channel, UVIS）独有的紫外观测能力，获取“染色体图”（即双色图），并通过分离不同恒星群，探究这些光谱光度数据中可能存在的丰度间隙。随后我们将测定各恒星群的相对氦丰度，并区分该星团极端星族形成的两种相互竞争的情景：一种情景认为这些恒星形成于混合了星团核心最初5至15百万年形成的超大质量恒星抛射物的原始气体；另一种为渐近巨星支（Asymptotic Giant Branch, AGB）情景，即极端星族恒星形成于超AGB星的纯抛射物。