Most binaries are undetected. Astrometric reductions of a system using the assumption that the object moves like a single point mass can be biased by unresolved binary stars. The discrepancy between the centre of mass of the system (which moves like a point mass) and the centre of light (which is what we observe) introduces additional motion. We explore the extent to which binary systems affect single object models fit to astrometric data. This tells us how observations are diluted by binaries and which systems cause the largest discrepancies - but also allows us to make inferences about the binarity of populations based on observed astrometric error. By examining a sample of mock observations, we show that binaries with periods close to one year can mimic parallax and thus bias distance measurements, whilst long period binaries can introduce significant apparent proper motion. Whilst these changes can soak up some of the error introduced by the binary, the total deviation from the best fitting model can be translated into a lower limit on the on-sky separation of the pair. Throughout we link these predictions to data from the Gaia satellite, whilst leaving the conclusions generalizable to other surveys.
Hubble Space Telescope Astrometry of the Metal-Poor Visual Binary $μ$ Cassiopeiae: Dynamical Masses, Helium Content, and Age
仙后座$μ$（英语：$μ$ Cassiopeiae）是一颗近距离、高速、贫金属（$\rm[Fe/H]=-0.81$）的目视双星。我们已经使用了高分辨率的成像与哈勃太空望远镜（HST），获得了近二十年，以确定周期（21.568年）和精确的轨道要素。结合已发表的地面和空间天体测量学，我们确定了$μ$ Cas的两个组成部分的动力学质量：$0.7440\pm0.0122\,M_\odot$为G5 V主星，$0.1728\pm0.0035\,M_\odot$为其暗淡的dM伴星。我们没有发现显着的扰动，在HST天体测量由于系统中的第三个机构。我们计划的主要目的是借助恒星模型确定缺金属主星$μ$ Cas A的氦含量和年龄。虽然我们现在有了一个精确的质量，但其他参数仍然存在不确定性，包括其有效温度。此外，重新审查档案干涉观测导致怀疑的角直径被高估了百分之几。在绝对星等与色平面上，$μ$ Cas A比球状星团47的主序星温度稍低，亮度也稍高;这可能意味着它的氦含量较低，和/或较老，和/或金属丰度较高。我们对$μ$ Cas A的氦含量和年龄的最佳估计是$Y=0.255\pm0.014$和$12.7\pm2.7$ Gyr-使$μ$ Cas可能是天空中肉眼可见的最古老的恒星。对系统的绝对视差、$μ$ Cas A的有效温度及其角直径的改进测量将提供更严格的约束。
BE Lyncis is not a Black Hole Binary: Lessons From Gaia and Hipparcos Astrometry
California Institute of Technology（加州理工学院）；Texas Tech University（德克萨斯理工大学）；Universitat de Barcelona（巴塞罗那大学）；Max Planck Institute for Astronomy（马克斯·普朗克天文学研究所）
通过 Gaia 和 Hipparcos 恒星测量数据，研究者发现 BE Lyn 不可能是黑洞双星，其观测到的自行异常与预期不符，且黑洞双星模型与观测数据矛盾。
Mass-dependent multiplicity fraction in low mass stars revealed by Gaia astrometry
Max-Planck-Institut für Astronomie（马克斯·普朗克天文学研究所）；Fakultät für Physik und Astronomie, Universität Heidelberg（海德堡大学物理与天文学院）；Department of Astronomy, California Institute of Technology（加州理工学院天文学系）
