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Direct Hubble constant measurements rely on a solid extragalactic distance scale comma which is conventionally calibrated with Cepheids and recently the tip of the red giant branch (TRGB). As a different approach comma Mira variables exhibit promising properties as independent distance indicators. Miras are luminous in the NIR comma follow tight period-luminosity relations (PLRs) comma and are present in all types of galaxies. Long-period Miras (400<P<1500 d) are much brighter and on steeper PLRs than the commonly used short-period Miras (100<P<400 d) comma and they would potentially add a new route to check the Hubble tension between the early and late Universe. For JWST and WFIRST they are easier to detect than Cepheids and reach further than the TRGB. We propose to (1) study the long-period Miras in the type Ia supernova host galaxy NGC 2525 and (2) refine the Mira-based Hubble constant with 4 Mira-SN calibrators. To do this we request four epochs of NIR imaging toward this face-on spiral galaxy. We will combine these observations with archival data to form a continuous time series with a 1300 d baseline comma from which we will derive the period and mean intensity of long-period Miras in this system. We will calibrate the PLR for long-period Miras and develop them as a new longer-range distance indicator. For the second task we will measure the NIR colors in NGC 2525 comma NGC 1559 comma NGC 5643 comma and NGC 5861 in order to classify oxygen-rich Miras in these type Ia supernova hosts. The expanded Mira-SN calibrators and removal of carbon-rich Mira contamination will reduce the error in the current Mira-based Hubble constant by a factor of two.

哈勃常数（Hubble constant）的直接测量依赖于可靠的河外距离尺度，传统上该尺度通过造父变星（Cepheids）校准，近期则常采用红巨星分支顶端（tip of the red giant branch, TRGB）。作为一种差异化的研究路径，米拉变星（Mira variables）作为独立距离指示器展现出极具潜力的应用价值。米拉变星在近红外（NIR）波段光度极高，遵循严格的周期-光度关系（period-luminosity relations, PLRs），且存在于各类星系中。长周期米拉变星（周期400<P<1500天）比常用的短周期米拉变星（100<P<400天）亮度更高，且周期-光度关系斜率更陡，有望为验证早期宇宙与晚期宇宙之间的哈勃张力（Hubble tension）提供全新途径。相较于詹姆斯·韦布空间望远镜（James Webb Space Telescope, JWST）与南希·格蕾丝·罗曼空间望远镜（Wide Field Infrared Survey Telescope, WFIRST）所能探测到的造父变星与红巨星分支顶端，米拉变星更易被观测，且可抵达更远的宇宙距离。本研究拟开展两项工作：（1）对Ia型超新星（type Ia supernova）宿主星系NGC 2525中的长周期米拉变星展开研究；（2）依托4个米拉-超新星校准天体（Mira-SN calibrators），优化基于米拉变星的哈勃常数测量。为此，我们申请对该面向正面的旋涡星系开展4个观测历元的近红外成像观测。我们将把本次观测数据与存档数据整合，构建基线时长达1300天的连续时间序列，据此推导该星系内长周期米拉变星的周期与平均光度。我们将校准长周期米拉变星的周期-光度关系，将其开发为新一代长程距离指示器。在第二项工作中，我们将测量NGC 2525、NGC 1559、NGC 5643与NGC 5861的近红外颜色，以对这些Ia型超新星宿主星系内的富氧米拉变星（oxygen-rich Miras）开展分类工作。通过扩充米拉-超新星校准天体样本并剔除碳富米拉变星（carbon-rich Mira）的污染，我们可将当前基于米拉变星的哈勃常数测量误差降低至原有水平的1/2。