Detailed Abundances in the Ultra-faint Magellanic Satellites Carina II and III
Detailed Abundances in the Ultra-faint Magellanic Satellites Carina II and III
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DOI：
10.3847/1538-4357/ab6213
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发表时间：
2019-12
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影响因子：
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通讯作者：
A. Ji;Ting S. Li;J. Simon;J. Marshall;K. Vivas;A. Pace;K. Bechtol;A. Drlica-Wagner;Sergey E.
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作者：
A. Ji;Ting S. Li;J. Simon;J. Marshall;K. Vivas;A. Pace;K. Bechtol;A. Drlica-Wagner;Sergey E.
我们首次详细介绍了超暗麦哲伦卫星星系船底座II(CAR II)和船底座III(CAR III)中的元素丰度。利用高分辨率麦哲伦/迈克光谱，我们测定了CAR II中9颗恒星的丰度，包括超暗矮星系(UFD)中一颗RR天琴座恒星的第一丰度，以及CAR III中两颗恒星的化学丰度。化学丰度表明，这两个系统显然都是星系，而不是球状星团。这些星系中的恒星大多表现出与其他类似暗矮星系相似的丰度趋势：[α/Fe]比率增强但却在下降，铁峰元素与恒星晕相匹配，以及异常低的中子俘获元素丰度。一颗恒星显示出较低的边界线[Sc/Fe]=−1.0。我们在CAR II中发现了一个很大的BA散射，这可能是由于低质量渐近巨型分支星风的不均匀浓缩所致。最显著的丰度趋势是CAR II中的[Mg/Ca]，从+0.4下降到−0.4，这表明富核塌缩超新星的初始前体质量有明显的变化。到目前为止，唯一显示出类似[镁/钙]趋势的UFD可能是大麦哲伦星云的卫星。我们发现了两颗[Fe/H]≤−为3.5的恒星，它们的丰度很可能追溯到第一代无金属族III恒星，并与族III核崩塌超新星的产额很好地吻合。附录描述了我们新的丰度不确定性分析，它逐行传播恒星参数的不确定性。
We present the first detailed elemental abundances in the ultra-faint Magellanic satellite galaxies Carina II (Car II) and Carina III (Car III). With high-resolution Magellan/MIKE spectroscopy, we determined the abundances of nine stars in Car II, including the first abundances of an RR Lyrae star in an ultra-faint dwarf galaxy (UFD), and two stars in Car III. The chemical abundances demonstrate that both systems are clearly galaxies and not globular clusters. The stars in these galaxies mostly display abundance trends matching those of other similarly faint dwarf galaxies: enhanced but declining [α/Fe] ratios, iron-peak elements matching the stellar halo, and unusually low neutron-capture element abundances. One star displays a low outlying [Sc/Fe] = −1.0. We detect a large Ba scatter in Car II, likely due to inhomogeneous enrichment by low-mass asymptotic giant branch star winds. The most striking abundance trend is for [Mg/Ca] in Car II, which decreases from +0.4 to −0.4 and indicates clear variation in the initial progenitor masses of enriching core-collapse supernovae. So far, the only UFDs displaying a similar [Mg/Ca] trend are likely satellites of the Large Magellanic Cloud. We find two stars with [Fe/H] ≤ −3.5 whose abundances likely trace the first generation of metal-free Population III stars and are well fit by Population III core-collapse supernova yields. An appendix describes our new abundance uncertainty analysis that propagates line-by-line stellar parameter uncertainties.