两个极暗矮星系中的碳测量：Grus II和Tucana IV
Carbon measurements in two ultra-faint dwarf galaxies: Grus II and Tucana IV
AI总结：
研究极暗矮星系Grus II和Tucana IV中的碳增强金属贫乏星，以探索宇宙早期恒星的化学演化和原始化学丰度。
摘要：
极暗矮星系（UFDs）是局部群中一些最古老且金属含量最低的环境。特别是，它们被预测会宿主最早恒星（仅H和He），这些恒星照亮了我们的宇宙。截至目前尚未发现无金属恒星，但它们的化学产物可以在古老第二代恒星的表面观测到，如碳增强金属贫乏恒星（CEMP-no，[C/Fe]>+0.7）。然而，在每个UFD中只有少数恒星亮度足够进行光谱后续研究，因此研究尽可能多的这些系统至关重要。本文对两个最近发现的UFDs，Grus II和Tucana IV中的恒星进行了后续研究。分析的光谱是通过甚大望远镜（VLT）的多目标光谱仪FLAMES/Giraffe获得的，包括两个波长范围：红色光谱围绕CaII三重线（8498 Å，8542 Å，8662 Å）用于推导径向速度和[Fe/H]，以及蓝色光谱覆盖约4300 Å的CH带。总共分析了21个Grus II成员候选人的光谱和17个Tucana IV成员候选人的光谱，包括红巨星分支（RGB）和水平分支（HB）恒星。我们确定了Grus II中有13个成员（其中8个为RGB恒星）和Tucana IV中有7个成员（其中3个为RGB恒星）。在Grus II的RGB恒星中，我们发现了3个CEMP-no恒星，[Fe/H]~ -3和[C/Fe]>+1，以及2个CEMP-no恒星，[Fe/H]略高且[C/Fe]>+0.7。在Tucana IV中，我们发现了一个CEMP-no恒星（[Fe/H]=-2.75和[C/Fe]=+0.83）。本项目与未来对UFDs中CEMP恒星的研究相结合，使我们能够研究这些古老原始系统中最早恒星的影响，从而研究宇宙中首次化学丰度的过程。
英文摘要：
The ultra-faint dwarf galaxies (UFDs) are some of the oldest and most metal-poor environments in the Local Group. In particular, they are predicted to host the first stars (only H and He) that lit up in our Universe. No metal-free stars have been found to date, but their chemical products can be observed on the surfaces of the ancient second-generation stars such as the carbon-enhanced metal-poor stars (CEMP-no, [C/Fe]>+0.7).} However, in each UFD there are only a few stars bright enough for spectroscopic follow-up, therefore it is crucial to study as many of these systems as possible. Here we follow up stars belonging to two recently discovered UFDs, Grus II and Tucana IV. The spectra analyzed were obtained with the multi-object spectrograph FLAMES/Giraffe at the Very Large Telescope (VLT). This includes spectra in two wavelength ranges: red spectra around the CaII triplet (8498 Å, 8542 Å, 8662 Å) used to derive radial velocity and [Fe/H], and blue spectra covering the CH band at ~ 4300 Å. In total, we analyzed 21 spectra of member candidates for Grus II and 17 for Tucana IV, including both Red Giant Branch (RGB) and Horizontal Branch (HB) stars. We identified 13 members in Grus II (thereof 8 RGB stars) and 7 members in Tucana IV (thereof 3 RGB stars). Among the RGB stars in Grus II, we found three CEMP-no stars at [Fe/H]~-3 and [C/Fe]>+1 and two CEMP-no stars at slightly higher [Fe/H] and [C/Fe]>+0.7. In Tucana IV, we found one CEMP-no star ([Fe/H]=-2.75 and [C/Fe] = +0.83). This project, along with future investigations of CEMP stars in UFDs, allows us to study the impact of the first stars in these ancient and primitive systems and consequently the first chemical enrichment that occurred in the Universe.
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