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We propose to calibrate the Population II extragalactic distance ladder to a precision of 1% by acquiring HST-ACS imaging of 34 Milky Way (MW) RR Lyrae with excellent Gaia parallaxes. We will use filters commonly used for extragalactic distances (F475W comma F606W comma F814W) thereby eliminating uncertainties from filter transformations and zero points comma which are a substantial source of systematics as recently demonstrated for Pop I distance calibration (i.e. comma Cepheids). From our imaging comma we will construct the first empirical RR Lyrae period-Wesenheit magnitude-metallicity (PWZ) relationship in native HST filters using well-characterized MW RR Lyrae (existing excellent periods comma metallicities comma parallaxes) to the same precision (1%) as HST+Gaia recently established for Cepheids. With this secure PWZ comma we will measure RR Lyrae distances to 23 nearby galaxies with large RR Lyrae and TRGB populations that have archival HST data in the same filters. This process will produce a new calibration of the TRGB in HST filters that is directly anchored to Gaia. These new distance calibrations will enable broad community science such as a revised Pop II measurement of the local H0 anchored to Gaia comma calibrations of stellar evolution models comma more robust star formation histories comma and provide an anchor for JWST TRGB distances that will extend to virgul50 Mpc. Gaia will continue to provide improved parallaxes comma reducing our PWZ distance calibration below 1%. However comma without our proposed HST observations the error floor will be limited to 2-3% due to filter transformations and zero point uncertainties comma underscoring the importance of obtaining these observations now during HST.s remaining lifetime.

我们提议通过获取34颗具有高精度盖亚视差的银河系（MW）天琴座RR型变星的哈勃空间望远镜-高级巡天相机（HST-ACS）成像数据，将星族II河外距离阶梯的校准精度提升至1%。我们将使用河外距离测量中常用的滤光片（F475W、F606W、F814W），从而消除滤光片转换和零点带来的不确定性——正如近期星族I距离校准（即造父变星）研究所揭示的，这些不确定性是系统误差的重要来源。基于这些成像数据，我们将利用特征明确的银河系天琴座RR型变星（已有的高精度周期、金属丰度和视差数据），构建首个在原生哈勃滤光片下的经验性天琴座RR型变星周期-韦森heit星等-金属丰度（PWZ）关系，其精度将达到哈勃空间望远镜+盖亚近期为造父变星建立的1%水平。借助这一可靠的PWZ关系，我们将对23个拥有大量天琴座RR型变星和红巨星分支末端（TRGB）天体且具有相同滤光片下存档哈勃数据的邻近星系，测量其天琴座RR型变星距离。这一过程将生成红巨星分支末端在哈勃滤光片下的新校准关系，该关系直接锚定于盖亚数据。这些新的距离校准将支持广泛的社区科学研究，例如基于盖亚数据修订的星族II本地哈勃常数（H0）测量、恒星演化模型的校准、更可靠的恒星形成历史研究，并为詹姆斯·韦布空间望远镜（JWST）延伸至约50兆秒差距（Mpc）的红巨星分支末端距离测量提供锚点。盖亚将持续提供精度更高的视差数据，使我们的PWZ距离校准精度降至1%以下。然而，若缺乏我们提议的哈勃观测，由于滤光片转换和零点不确定性，误差下限将限制在2-3%，这凸显了在哈勃剩余寿命期间立即获取这些观测数据的重要性。