Stellar Loci X: Selection of Nitrogen-Rich Giants Using GALEX NUV and Synthetic SDSS Photometry
Nitrogen-rich (N-rich) field stars (likely second-generation escapees from globular clusters) provide a powerful, but currently sample-limited, probe of globular-cluster disruption in the Galactic halo. We exploit the fact that enhanced NH, CN absorption depresses optical u band flux, while the near-ultraviolet (NUV) band is largely insensitive to nitrogen enhancement while remaining sensitive to overall metallicity. Combining GALEX NUV photometry with (synthetic) SDSS optical photometry, we construct a [Fe/H]-dependent stellar-locus model for giants and select significant redder u-g outliers as N-rich candidates. For a relaxed cut ([Fe/H] <= -0.8, 2 sigma) we identify 970 candidates, with an estimated completeness of 64-65% and purity of 41-62% from multiple validations. A stricter selection ([Fe/H] <= -1.0, 3 sigma) yields 220 candidates, with lower completeness (30-38%) but substantially higher purity (60-91%). The dominant contaminants show anomalous spectral energy distributions (e.g., UV excess), and are themselves useful targets for follow-up. The diverse kinematic and dynamical properties of our candidates suggest multiple origins. This scalable photometric approach enables systematic searches for N-rich halo stars and is well suited to forthcoming wide-field surveys such as CSST.