Title: Probing the early Milky Way with GHOST spectra of an extremely metal-poor star in the Galactic disk Show Chinese title

Title: 用GHOST光谱研究银河系早期的银河系盘中一颗极端贫金属恒星

Abstract: Pristine_183.6849+04.8619 (P1836849) is an extremely metal-poor ([Fe/H]$=-3.3\pm0.1$) star on a prograde orbit confined to the Galactic disk. Such stars are rare and may have their origins in protogalactic fragments that formed the early Milky Way, in low mass satellites accreted later, or forming in situ in the Galactic plane. Here we present a chemo-dynamical analysis of the spectral features between $3700-11000$\r{A} from a high-resolution spectrum taken during Science Verification of the new Gemini High-resolution Optical SpecTrograph (GHOST). Spectral features for many chemical elements are analysed (Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Fe, Ni), and valuable upper limits are determined for others (C, Na, Sr, Ba). This main sequence star exhibits several rare chemical signatures, including (i) extremely low metallicity for a star in the Galactic disk, (ii) very low abundances of the light $\alpha$-elements (Na, Mg, Si) compared to other metal-poor stars, and (iii) unusually large abundances of Cr and Mn, where [Cr, Mn/Fe]$_{\rm NLTE}>+0.5$. A comparison to theoretical yields from supernova models suggests that two low mass Population III objects (one 10 M$_\odot$ supernova and one 17 M$_\odot$ hypernova) can reproduce the abundance pattern well (reduced $\chi^2<1$). When this star is compared to other extremely metal-poor stars on quasi-circular, prograde planar orbits, differences in both chemistry and kinematics imply there is little evidence for a common origin. The unique chemistry of P1836849 is discussed in terms of the earliest stages in the formation of the Milky Way. Show Chinese abstract

Abstract: Pristine_183.6849+04.8619 (P1836849) 是一个极端贫金属的 ([Fe/H]$=-3.3\pm0.1$) 恒星，其轨道是顺行的，并局限于银河系盘中。这样的恒星很少见，它们的起源可能来自形成早期银河系的原星系碎片，或者后来被吸积的低质量卫星，或在银河系平面内原地形成的。在这里，我们对新 Gemini 高分辨率光学光谱仪 (GHOST) 在科学验证期间获取的高分辨率光谱中 $3700-11000$Å 范围内的光谱特征进行了化学动力学分析。许多化学元素的光谱特征得到了分析 (Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Fe, Ni)，并对其他元素 (C, Na, Sr, Ba) 确定了有价值的上限。这颗主序星表现出几种罕见的化学特征，包括 (i) 在银河系盘中的恒星中极低的金属量，(ii) 相比其他贫金属恒星，轻元素 $\alpha$的丰度非常低 (Na, Mg, Si)，以及 (iii) Cr 和 Mn 的丰度异常大，其中 [Cr, Mn/Fe]$_{\rm NLTE}>+0.5$。与超新星模型的理论产出进行比较表明，两个低质量的第三星族天体 (一个 10 M$_\odot$超新星和一个 17 M$_\odot$超新星) 可以很好地再现丰度模式 (减少的$\chi^2<1$)。当将该恒星与其他处于准圆形、顺行平面轨道上的极端贫金属恒星进行比较时，化学和运动学方面的差异表明几乎没有共同起源的证据。P1836849 的独特化学特性被讨论为银河系形成最早阶段的情况。