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. We present a simple method, tangent space projection (TSP), for the detection of X-enriched stars, for arbitrary elements X, even from blended lines. Our method does not require stellar labels, but instead directly estimates the counterfactual unenriched spectrum from other unlabeled spectra. As a case study, we apply this method to the 6708 Å Li doublet in LAMOST DR5, identifying 8428 Li-enriched stars seamlessly across evolutionary state. We comment on the explanation for Li-enrichment for different subpopulations, including planet accretion, nonstandard mixing, and youth.