2019ApJ...871...58T

The large amount of chemical and kinematic information available in large spectroscopic surveys has inspired the search for chemically peculiar stars in the field. Though these metal-poor field stars ([Fe/H] < -1) are commonly enriched in nitrogen, their detailed spatial, kinematic, and chemical distributions suggest that various groups may exist, and thus their origin is still a mystery. To study these stars statistically, we increase the sample size by identifying new CN-strong stars with LAMOST DR3 for the first time. We use CN-CH bands around 4000 Å to find CN-strong stars, and further separate them into CH-normal stars (44) and CH-strong (or CH) stars (35). The chemical abundances from our data-driven software and APOGEE DR14 suggest that most CH-normal stars are N-rich, and this cannot be explained by an internal mixing process alone. The kinematics of our CH-normal stars indicate that a substantial fraction of these stars are retrograding, pointing to an extragalactic origin. The chemistry and kinematics of CH-normal stars imply that they may be stars dissolved from globular clusters, or accreted halo stars, or both.