标题： 赫尔米流的精确星震年龄 显示英文标题

标题： Precise Asteroseismic Ages for the Helmi Streams

摘要： 赫尔米流是银河系吸积的一个矮星系的遗迹，其恒星现在在银河晕中形成了独特的运动学和化学子结构。 精确测定这些通常很暗淡的河外起源恒星的年龄一直非常困难，这是由于仅使用光谱数据、干涉测量或粗略的星震学测量存在局限性。 利用美国宇航局凌日系外行星勘测卫星的观测数据，我们报告了赫尔米流中两颗最明亮的红巨星HD 175305和HD 128279的详细星震学建模。 通过建模这两颗恒星的个体振荡模式频率和光谱特性，我们确定了它们的基本性质，包括质量、半径和年龄（$\tau$）。 我们报告HD 175305的年龄为$\tau = 11.16 \pm 0.91$Gyr，HD 128279的年龄为$\tau = 12.52 \pm 1.05$Gyr，这与之前对赫尔米流的恒星形成历史以及两颗恒星之间的差异化学丰度一致。 有了单个流成员的精确年龄，我们的结果加强了赫尔米流的前身星系至少存在于120亿年前的假设。 我们的结果还强调，当使用全局星震学参数而不是个体模式频率来推断金属贫乏、$\alpha$增强的红巨星年龄时，其年龄可能会被严重低估。 显示英文摘要

摘要： The Helmi streams are remnants of a dwarf galaxy that was accreted by the Milky Way and whose stars now form a distinct kinematic and chemical substructure in the Galactic halo. Precisely age-dating these typically faint stars of extragalactic origin has been notoriously difficult due to the limitations of using only spectroscopic data, interferometry, or coarse asteroseismic measurements. Using observations from NASA's Transiting Exoplanet Survey Satellite, we report the detailed asteroseismic modeling of two of the brightest red giants within the Helmi streams, HD 175305 and HD 128279. By modeling the individual oscillation mode frequencies and the spectroscopic properties of both stars, we determine their fundamental properties including mass, radius, and age ($\tau$). We report $\tau = 11.16 \pm 0.91$ Gyr for HD 175305 and $\tau = 12.52 \pm 1.05$ Gyr for HD 128279, consistent with previously inferred star-formation histories for the Helmi streams and the differential chemical abundances between the two stars. With precise ages for individual stream members, our results reinforce the hypothesis that the Helmi streams' progenitor must have existed at least 12 Gyr ago. Our results also highlight that the ages of metal-poor, $\alpha$-enhanced red giants can be severely underestimated when inferred using global asteroseismic parameters instead of individual mode frequencies.