For stars with unresolved companions, motions of the centre of light and that of mass decouple, causing a single-source astrometric model to perform poorly. We show that such stars can be easily detected with the reduced chi2 statistic, or RUWE, provided as part of Gaia DR2. We convert RUWE into the amplitude of the image centroid wobble, which, if scaled by the source distance, is proportional to the physical separation between companions (for periods up to several years). We test this idea on a sample of known spectroscopic binaries and demonstrate that the amplitude of the centroid perturbation scales with the binary period and the mass ratio as expected. We apply this technique to the Gaia DR2 data and show how the binary fraction evolves across the Hertzsprung--Russell diagram. The observed incidence of unresolved companions is high for massive young stars and drops steadily with stellar mass, reaching its lowest levels for white dwarfs. We highlight the elevated binary fraction for the nearby Blue Stragglers and Blue Horizontal Branch stars. We also illustrate how unresolved hierarchical triples inflate the relative velocity signal in wide binaries. Finally, we point out a hint of evidence for the existence of additional companions to the hosts of extrasolar hot jupiters.

中文翻译：

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盖亚 DR2 天体测量未解决的恒星伴星

对于伴星未解析的恒星，光心的运动和质量的运动解耦，导致单源天体测量模型表现不佳。我们表明，使用作为 Gaia DR2 的一部分提供的简化 chi2 统计量或 RUWE，可以轻松检测到此类恒星。我们将 RUWE 转换为图像质心摆动的幅度，如果按源距离缩放，则与伴星之间的物理间隔成正比（长达数年）。我们在已知光谱双星样本上测试了这个想法，并证明了质心扰动的幅度与预期的双星周期和质量比成比例。我们将此技术应用于 Gaia DR2 数据，并展示二进制分数如何在 Hertzsprung--Russell 图上演变。观测到的未解析伴星的发生率对于大质量年轻恒星来说很高，随着恒星质量稳定下降，在白矮星中达到最低水平。我们强调了附近蓝色流浪星和蓝色水平分支星的升高的双星分数。我们还说明了未解决的分层三元组如何夸大宽二进制文件中的相对速度信号。最后，我们指出了一些证据，表明太阳系外热木星的宿主存在额外的伴星。