标题： $\rm[Fe/H] \sim -4.8$星在$Gaia$XP 光谱中的发现 显示英文标题

标题： Discovery of an $\rm[Fe/H] \sim -4.8$ Star in $Gaia$ XP Spectra

摘要： 我们报告发现GDR3_526285 ($Gaia$ DR3源ID 5262850721755411072)，一颗具有$\rm[Fe/H] = -4.82 \pm 0.25$的恒星，也是迄今为止发现的金属（$\text{atomic number} > 2$）质量分数最低的恒星之一（$Z_{\rm GDR3\_526585} \lesssim 1.0 \times 10^{-6}$）。 我们首先将其识别为超贫金属（UMP；$\rm[Fe/H] < -4$）红巨星分支（RGB）星候选体，位于$Gaia$BP/RP（XP）光谱测光目录（$Gaia$ $G$ 量度$\approx$15）。 多波段测光和高分辨率光谱分析在局部热力学平衡下的结合确认了GDR3_526285作为银河系外晕中距离（$\approx$24 kpc从太阳）的红巨星（$T_{\rm eff} = 4596\,{\rm K}$, $\log g = 0.88$）的状态。我们仅获得了$\rm[C/H] < -4.32$的碳丰度上限，结果为$\rm[C/Fe] < +0.50$。对演化过程中的碳耗竭（$\Delta \rm[C/Fe] = +0.68$）进行修正后，名义上的碳铁比上限变为$\rm[C/Fe]_{\rm cor} < +1.18$。鉴于其异常低的[C/H]，GDR3_526285很可能形成于通过尘埃颗粒冷却的气体，而不是细结构线冷却。GDR3_526285的动力学特性表明，这颗恒星可能因麦哲伦系统的吸积而受到动态扰动，或者曾经是麦哲伦云的成员，后来被银河系剥离。 我们的结果展示了使用$Gaia$ XP 对低金属含量目标进行全面天空搜索的潜力，并确认了此处描述的方法是一种有用的“藏宝图”，可用于发现更多的UMP恒星。 显示英文摘要

摘要： We report on the discovery of GDR3_526285 ($Gaia$ DR3 Source ID 5262850721755411072), a star with $\rm[Fe/H] = -4.82 \pm 0.25$ and one of the lowest metal ($\text{atomic number} > 2$) mass fractions ever found ($Z_{\rm GDR3\_526585} \lesssim 1.0 \times 10^{-6}$). We first identified it as an ultra metal-poor (UMP; $\rm[Fe/H] < -4$) red giant-branch (RGB) star candidate in the $Gaia$ BP/RP (XP) spectro-photometric catalog ($Gaia$ $G$ magnitude $\approx$15). A combination of multi-band photometry and high-resolution spectroscopic analysis under local thermodynamic equilibrium confirmed the status of GDR3_526285 as a distant ($\approx$24 kpc from the Sun) RGB star ($T_{\rm eff} = 4596\,{\rm K}$, $\log g = 0.88$) in the Milky Way's outer halo. We obtain only an upper limit for the carbon abundance of $\rm[C/H] < -4.32$, resulting in $\rm[C/Fe] < +0.50$. A correction for the evolutionary carbon depletion ($\Delta \rm[C/Fe] = +0.68$) brings the nominal carbon-to-iron ratio upper limit to $\rm[C/Fe]_{\rm cor} < +1.18$. Given its extraordinarily low [C/H], GDR3_526285 likely formed from gas cooled via dust grains rather than fine structure line cooling. The kinematics of GDR3_526285 suggests that this star was either dynamically perturbed by the infall of the Magellanic system or was formerly a member of the Magellanic Clouds and was later stripped by the Milky Way. Our results showcase the potential of an all-sky search for low-metallicity targets with $Gaia$ XP and confirm that the methodology described here is an useful "treasure map" for finding additional UMP stars.