一、搜索极端贫金属星

Searching for extremely metal-poor stars from Mephisto wide surveys

图 1：SkyMapper测光巡天发现的目前金属丰度最贫的恒星（SMSS J031300.36-670839.3）光谱，取自Keller et al. (2014)；the spectrum of the star SMSS J031300.36-670839.3 with the lowest iron abundance so far found by the SkyMapper photometric survey (Keller et al. 2014).

对极端贫金属星的搜寻与研究可极大促进对宇宙大爆炸本质的深入认识，理解并限制宇宙中第一代恒星的形成及其质量分布函数，约束银河系乃至星系的形成与演化理论（Beers & Chrislieb 2005; Frebel & Norris 2015）。

The discovery and study of extremely metal-poor (EMP) stars are of vital importance (see review by Beers & Chrislieb 2005; Frebel & Norris 2015) for 1) understanding the nature of the big bang and the first stars; 2) probing the first mass function; 3) constraining the supernovae element production model; 4) determining the metallicity distribution function (MDF) of the stellar halo and 5) exploring the astrophysical site(s) of neutron-capture element production.

当前已有多个大型测光巡天项目在开展极端贫金属星的系统搜寻工作，其中南半球的测光巡天在此方面取得了巨大的成功。所采用的滤光片能有效覆盖恒星光谱中的Ca II K 吸收线区域，从而可高效筛选极端贫金属星候选体。尽管SkyMapper所在的台址、巡天图像的质量、定标精度以及极限星等都不甚理想，但其仍然在先导巡天中发现了40多颗金属丰度小于 −3 dex的极端贫金属星，其中一颗为截至目前金属丰度最贫的恒星（[Fe/H]< −7.1 dex, Keller et al. 2014），相关工作引起了巨大反响。

Over the last decades, many photometric and spectroscopic surveys have conducted to search EMP stars. Amongst them, the SkyMapper Southern Survey (SMSS) has achieved great success in this filed. The special design of v filter adopted by SMSS has enabled one to select EMP candidates from photometric colors efficiently. During the commissioning phase of SMSS, over 40 EMP stars are confirmed by the high-resolution spectroscopy (Jacobson et al. 2015; Marino et al. 2019), and one of them is found with the lowest iron abundance ([Fe/H] < − 7.1; Keller et al. 2014). 

Mephisto不仅有着与类似的滤光片设计，而且在观测效率、测光图像质量、星等及颜色测量精度、观测极限深度等方面有明显提高，根据现有晕星金属丰度分布函数粗略估计，Mephisto有望探测到约百万颗r波段亮于23等的贫金属星（−3 < [Fe/H] < −2 dex）以及几万颗极端贫金属星（[Fe/H] < −3 dex）候选体，为精确测量第一代恒星的质量分布函数、限制第一代恒星形成理论、研究银河系乃至宇宙极早期物理过程提供至关重要的约束。

Given the similar filter design and better site of our Mephisto survey, a huge sample of EMP candidates can be constructed from the survey. This sample will provide key constraints on the MDF of the most metal-poor stars in our Galaxy and revolutionize our knowledge about the early universe.

二、利用蓝水平分支星细致描绘银河系恒星晕三维结构

3D Mapping the stellar halo of the Milky Way by the Blue Horizontal-Branch Stars from the Mephisto survey

图 2：银河系结构示意图；the structure of the Milky Way

在浩如烟海的各类星系中，银河系是我们唯一可以将其星族成分解析为单体，并对其在三维空间、三维速度、元素丰度和年龄等多维相空间里进行细致研究的典型漩涡星系。因此，对银河系的细致研究将对理解星系的形成和演化产生深远的影响。而银河系的晕（包括恒星晕和暗物质晕）被证实具有非常复杂的结构，其中所蕴藏的信息使人们有可能揭示银河系的结构、形成和丰富的演化历史。因此对银晕进行研究有着重要的意义。然而迄今为止，我们对银晕的了解还很欠缺，其中一个不可忽视的原因，是缺少大样本的晕星来示踪银晕的结构、化学以及运动学性质。

Probing the Galactic structure is important for understanding the assembly history of the Milky Way as well as galaxies in general. The Milky Way halo contains some of the oldest stars and structures found in our Galaxy, and thus provides information of the earliest stage of the evolution of our Galaxy. Despite its crucial importance, our knowledge of the stellar halo is still far from complete, partly due to the lack of large samples of halo tracers to probe its properties.

蓝水平分支星是年老贫金属晕星，因此是研究银河系恒星晕的理想示踪天体。相较于其他银晕示踪天体，蓝水平分支星拥有如下优势：1）它们比大部分巨星支以及主序拐点星要亮；2）它们很容易从光谱中得到证人（很强的Balmer跳变以及窄而强的Balmer线）；3）它们是近似的标准烛光（本征亮度分布很窄）。通常，蓝水平分支星样本通过大规模光谱巡天数据来构建（如Sirko et al. 2004; Xue et al. 2018; 2014）。然而，这样构建的样本数目有限且遭受很大的选择效应。为克服光谱巡天构建蓝水平分支星样本的局限性，之前的研究尝试基于测光颜色挑选蓝水平分支星，但所使用的颜色来自宽波段巡天（如SDSS ugriz；Smith et al. 2010）不能很有效地将蓝水平分支星从蓝离散星中分离出来。综上，当前的分光/测光构建的蓝水平分支星样本很难清晰的描绘银晕的结构。

The blue horizontal branch (BHB) stars are old, metal-poor halo stars. They are ideal tracers for mapping the structure of Galactic stellar halo. Compared to other halo tracers, BHB has the following advantages: 1) they are more luminous than most giant branch and main-sequence turn-off stars; 2) they are easily identified given their spectral features (strong Balmer jump and narrow strong Balmer lines); 3) they are good standard candles given their narrow range of intrinsic luminosities (hence “horizontal branch”). Traditionally, BHB sample stars are built from large-scale spectroscopic surveys using their spectral features (e.g., Sirko et al. 2004; Xue et al. 2018; 2014). However, the number of sample stars selected spectroscopically is very limited (no more than ten thousand) and the sample also suffers strong selection effects. Other efforts on building large sample of BHB stars are also tried based on stellar colors (e.g., Smith et al. 2010) but the traditional broad photometric bands (e.g., SDSS ugriz) can not select BHB stars from blue straggler (BS) stars efficiently. The purity and completeness of the build BHB sample is quite low and thus the sample is hard to map the clear structure of the stellar halo.

图 3：上图：利用 SkyMapper DR2 (Onken et al. 2020) 银纬10<|b|<20◦数据展示的利用u-v v.s. g-i 区分 BHB 和 BS 星的效果图。图中蓝色和红色点分别表示利用 SEGUE 巡天光谱数据证认的 BHB 和 BS 星 (Xue et al. 2014)。下图：取上图中−0.31<g−i <−0.24恒星的u-v 颜色分布图。红色和蓝色线表示对该分布的拟合，其中红色代表BS贡献而蓝色代表BHB贡献。虚线标记了两个高丝拟合的交点，被用作区分BHB和BS的颜色判据。Upper panel: u−v versus g−i for stars with Galactic latitude 10<|b|<20◦ from SkyMapper DR2 (Onken et al. 2020). Blue and red dots represent, respectively, the BHB and BS stars classified from the SEGUE spectra (Xue et al. 2014). The grey lines represent the color bins we further to draw the number distribution diagram along u−v. Lower panel: number distribution of stars with Galactic latitude 10<|b|<20◦ and−0.31<g−i <−0.24. The red and blue lines represent two Gaussian fits to the number distribution, with one represents the BS stars (red line) and another represents the BHB stars. The cross point (see the dashed grey lines) of the two gaussian functions is then adopted as the color cut to separate BHB and BS stars.

Mephisto巡天采用的类SkyMapper的uv滤光片对恒星的Balmer跳变特征很敏感，因此可以利用u-v颜色高效的挑选蓝水平分支星并构建大样本。通过SkyMapper DR2（Onken et al. 2020）数据的测试，利用测光u-v与g-i颜色挑选构建的蓝水平分支星样本纯净度和完备度都超过了95%，与分光数据构建的样本相当或更好。

By adopting SkyMapper-like uv filters, a large sample of BHB stars down to a limiting magnitude of r ~ 23.0 mag can be constructed, given the strong sensitivity of u-v color on the Balmer jump. According to the tests on SkyMapper DR2 data (Onken et al. 2020, see Fig. 1), the purity and completeness of the BHB sample constructed from uv photometry, as well as other optical colors (e.g., g-i), are greater than 95 per cent, comparable or even better than those achieved spectroscopically.

未来，利用Mephisto的W巡天数据，我们期待可以构建一个：1）数目超过数万颗，2）高纯净度和完备度，3）选择效应清楚，4）距离准确以及5）较为粗略的金属丰度和年龄的估计的蓝水平分支星大样本。此样本可用于细致刻画银河系银晕的三维结构，及进一步为理解银河系的形成和演化提供至关重要的约束。

With the data from Mephisto shallow/main wide surveys, we expect to build a sample of BHB stars with 1) number exceeding hundred thousand, 2) very high purity and completeness, 3) clear selection function, 4) very accurate distance estimates, 5) possibly rough metallicity and age estimates. The built BHB sample will certainly allow one to map the stellar halo of the Milky Way in exquisite detail, and further provide strong clues to understanding the formation and evolution of the Galaxy.