We present a method to determine nitrogen abundance ratios with respect to iron ([N/Fe]) from molecular CN-band features observed in low-resolution (R ∼ 2000) stellar spectra obtained by the Sloan Digital Sky Survey (SDSS) and the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). Various tests are carried out to check the systematic and random errors of our technique, and the impact of signal-to-noise (S/N) ratios of stellar spectra on the determined [N/Fe]. We find that the uncertainty of our derived [N/Fe] is less than 0.3 dex for S/N ratios larger than 10 in the ranges Teff = [4000, 6000] K, log g = [0.0, 3.5], [Fe/H] = [−3.0, 0.0], [C/Fe] = [−1.0, +4.5], and [N/Fe] = [−1.0, +4.5], the parameter space that we are interested in to identify N-enhanced stars in the Galactic halo. A star-by-star comparison with a sample of stars with [N/Fe] estimates available from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) also suggests a similar level of uncertainty in our measured [N/Fe], after removing its systematic error. Based on these results, we conclude that our method is able to reproduce [N/Fe] from low-resolution spectroscopic data, with an uncertainty sufficiently small to discover N-rich stars that presumably originated from disrupted Galactic globular clusters.

Abstract
1. INTRODUCTION
2. METHODOLOGY FOR MEASURINGNITROGEN-TO-IRON ABUNDANCE RATIOS
    2.1. Fundamental Atmospheric Parameters
    2.2. Carbon-to-Iron Ratios ([C/Fe])
    2.3. Determination of Nitrogen-to-Iron AbundanceRatios ([N/Fe])
3. CHECKS ON THE ROBUSTNESS OF ESTIMATED[N/FE]
    3.1. Impacts of S/N
    3.2. Impacts of Systematic Errors of Adopted StellarParameters on Determined [N/Fe]
4. CALIBRATION AND VALIDATION WITH AHIGH-RESOLUTION ABUNDANCE ANALYSIS
    4.1. Checks on the Accuracy and Precision of the SSPPStellar Parameters and [C/Fe]
    4.2. Calibration with APOGEE
    4.3. Validation with SAGA
5. SUMMARY AND CONCLUSIONS
ACKNOWLEDGMENTS
REFERENCES

• Changmin Kim(Department of Astronomy, Space Science and Geology, Chungnam National University)
• Young Sun Lee(Department of Astronomy and Space Science, Chungnam National University) Corresponding author
• Timothy C. Beers(Department of Physics and JINA Center for the Evolution of the Elements, University of Notre Dame, IN 46556, USA)
• Thomas Masseron(Instituto de Astrof´ısica de Canarias, E-38205 La Laguna, Tenerife, Spain, Departamento de Astrof´ısica, Universidad de La Laguna, E-38206 La Laguna, Tenerife, Spain)