Common Proper-Motion Halo Stars

Abstract

Using the PPMXL proper-motion catalogue we have attempted to compile our own catalogue of common-proper-motion halo candidates, for use in a study of the difference in the lower end of the metallicity distribution of Globular clusters [𝐹𝑒⁄𝐻] = −2.4 and single field stars [𝐹𝑒⁄𝐻] = −4.1. This was achieved by tailoring several selection criteria based on the expected characteristics of typical halo stars, such as similar proper-motions and reduced proper motion data, and applying them to the chosen proper-motion catalogue. The final selection criteria produced a catalogue of 17,500 common proper-motion (CPM) halo binary candidates, and have been further refined to 2,326 pairs based on conclusions drawn from our analysis of the larger catalogue, and have shown that several pre-existing CPM halo binaries are included in the catalogue. A selection of 528 pairs has been identified from this final catalogue as having an increased likelihood over the larger catalogue of being CPM binary pairs. A large number of issues have been identified with the PPMXL catalogue; chiefly among them is the large proportion of data that has been found to be spurious, caused in part by methods used in the creation of the original catalogue, which are discussed in this study. Additional faults found with the final catalogue lead us to the conclusion during the project that the data was of insufficient quality for the planned study on the metallicity distribution; however the refined catalogue and especially the 528 high-interest pairs are likely good candidates for finding real, wide halo binaries. Instead, identification of numerous problems found in the United States Naval Observatory, USNO-B1, catalogue and discussion of their likely causes has been performed. A sample of 600,000 stars were found that share the characteristics of halo giants, though it is demonstrated that a large number of these are likely foreground disk stars.