Hipparcos2 Neighbourhood

Table <code>hipparcos2_neighbourhood</code> includes all good neighbours for each matched Hipparcos2 object. A good neighbour for a given Hipparcos2 source is a nearby Gaia object whose position is compatible (within position errors) with the Hipparcos2 target. The cross-match algorithm is not symmetric and searches Hipparcos2 sources counterparts in Gaia. The cross-match algorithm is positional and exploits the full 5 parameter covariance matrix of the Gaia astrometric solution when available and the external catalogue positions and position errors. In addition it takes into account the Gaia environment using the local density.
Please note that the cross-match algorithm is a trade-off between multiple requirements, in particular between completeness and correctness. It is thus not limited to a simple cone search.
Reference papers:

Marrese et al. (2017)
Marrese et al. (2019)

`hipparcos2_neighbourhood` 表收录了各匹配后依巴谷2号（Hipparcos2）天体的全部优质邻源。针对某一依巴谷2号源而言，其优质邻源指的是位置与该依巴谷2号目标天体相容（处于位置误差范围内）的近距盖亚（Gaia）天体。本交叉匹配算法具备非对称性，仅会在盖亚星表中搜寻依巴谷2号源的对应天体。该算法基于位置信息开展计算，在可用时会充分利用盖亚天体测量解的全5参数协方差矩阵，并结合外部星表的位置与位置误差进行处理。此外，算法还会通过局部密度来考量盖亚星表的局部环境特征。

请注意，本交叉匹配算法是多项需求间的权衡结果，尤其需要在完备性与正确性之间进行平衡，因此其并不局限于简单的锥型搜索。

参考文献：
Marrese 等（2017）
Marrese 等（2019）