超越 Gaia 的二元性：关于专门的双星光谱调查

星系多性是星体天体物理学中的基本要素，但银河系的双星统计数据仍然受到较好的约束。Gaia 任务通过提供高精度的光学、光度和全局径向速度，以及数十万个非单星解（DR3），彻底改变了双星天体物理学。然而，RVS 亮度限制、任务时间范围和扫描律 impose strong selection effects in period, mass ratio, inclination and semi-amplitude, leaving large regions of the binary parameter space either sparsely sampled or effectively inaccessible. In this white paper we outline the case for a dedicated, wide-field, multi-epoch spectroscopic survey explicitly optimised for binary science: deeper than the Gaia RVS limit, with flexible cadence from hours to years, and with moderate to high spectral resolution. Using a simplified forward model of Gaia DR5-like performance, we highlight the populations for which robust orbital solutions will be rare (ultra short period, very long period, low-amplitude and compact-object binaries), and show how a "Binarity Beyond Gaia" survey would fill these gaps. Such a programme would deliver a bias correctable census of stellar multiplicity across the Milky Way and provide the spectroscopic backbone needed to exploit binary samples from Rubin/LSST, Roman and LISA.