An unsupervised method for identifying X-enriched stars directly from spectra: Li in LAMOST

Stars with peculiar element abundances are important markers of chemical enrichment mechanisms within stars and the Galaxy. We present a simple method, tangent space projection (TSP), for the detection of X-enriched stars, for arbitrary elements $X$, from blended lines. Our method does not require stellar labels, but instead directly estimates the counterfactual unrenriched spectrum from other unlabelled spectra. As a case study, we apply this method to the 6708 Å Li doublet in LAMOST DR5, identifying 8,428 Li-enriched stars seamlessly across evolutionary state. We comment on the explanation for Li-enrichement for different subpopulations, including planet accretion, nonstandard mixing, and youth.