We use Gaia DR2 to measure the initial mass function (IMF) of stars within 250 pc and masses in the range of 0.2 < m/M⊙ < 1.0, separated according to kinematics and metallicity, as determined from Gaia transverse velocity, vT, and location on the Hertzsprung–Russell diagram (HRD). The predominant thin-disc population (vT < 40 km s−1) has an IMF similar to traditional (e.g. Kroupa) stellar IMFs, with star numbers per mass interval dN/dm described by a broken power law, m−α, and index $\alpha _\textrm {high}=2.03^{+0.14}_{-0.05}$ above m ∼ 0.5, shallowing to $\alpha _\textrm {low}=1.34^{+0.11}_{-0.22}$ at m ≲ 0.5. Thick-disc stars (60 km s−1 <vT < 150 km s−1) and stars belonging to the ‘high-metallicity’ or ‘red-sequence’ halo (vT > 100 km s−1 or vT > 200 km s−1, and located above the isochrone on the HRD with a metallicity [M/H] > −0.6) have a somewhat steeper high-mass slope, $\alpha _\textrm {high}=2.35^{+0.97}_{-0.19}$ (and a similar low-mass slope $\alpha _\textrm {low}=1.14^{+0.42}_{-0.50}$). Halo stars from the ‘blue sequence’, which are characterized by low metallicity ([M/H] < −0.6), however, have a distinct, bottom-heavy IMF, well described by a single power law with $\alpha =1.82^{+0.17}_{-0.14}$ over most of the mass range probed. The IMF of the low-metallicity halo is reminiscent of the Salpeter-like IMF that has been measured in massive early-type galaxies, a stellar population that, like Milky Way halo stars, has a high ratio of α elements to iron, [α/Fe]. Blue-sequence stars are likely the debris from accretion by the Milky Way, ∼10 Gyr ago, of the Gaia-Enceladus dwarf galaxy, or similar events. These results hint at a distinct mode of star formation common to two ancient stellar populations – elliptical galaxies and galaxies possibly accreted early-on by ours.

中文翻译：

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银河系可能吸积的蓝晕恒星的底重初始质量函数

我们使用 Gaia DR2 来测量 250 pc 以内恒星的初始质量函数（IMF），质量在 0.2 < 范围内。m/M⊙ < 1.0，根据运动学和金属丰度分离，由 Gaia 横向速度、vT 和 Hertzsprung-Russell 图 (HRD) 上的位置确定。占主导地位的薄盘星群（vT < 40 km s-1）具有类似于传统（例如 Kroupa）恒星 IMF 的 IMF，每质量间隔 dN/dm 的恒星数由破幂律 m-α 和指数 $\alpha _\textrm {high}=2.03^{+0.14}_{-0.05}$ 高于 m ∼ 0.5，变浅至 $\alpha _\textrm {low}=1.34^{+0.11}_{-0.22 }$ 在 m ≲ 0.5。厚盘星（60 km s-1 <vT < 150 km s-1）和属于“高金属”或“红色序列”晕的恒星（vT > 100 km s-1 或 vT > ; 200 公里 s−1, 并且位于 HRD 等时线之上，金属丰度 [M/H] > −0.6) 具有稍陡的高质量斜率 $\alpha _\textrm {high}=2.35^{+0.97}_{-0.19}$（和类似的低质量斜率 $\alpha _\textrm {low }=1.14^{+0.42}_{-0.50}$)。来自“蓝色序列”的晕星，其特征是金属丰度低（[M/H] < -0.6），然而，有一个明显的、底部重的 IMF，可以用单一幂律很好地描述，$\alpha =在探测的大部分质量范围内为 1.82^{+0.17}_{-0.14}$。低金属晕的 IMF 让人想起在大质量早期型星系中测量到的类似 Salpeter 的 IMF，这是一个像银河系晕星一样具有高 α 元素与铁的比例的恒星群体，[α /Fe]。蓝色序列恒星可能是银河系吸积的碎片，大约 10 Gyr 之前，Gaia-Enceladus 矮星系，或类似事件。这些结果暗示了两种古老恒星群体共有的一种独特的恒星形成模式——椭圆星系和可能早期被我们吸积的星系。