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천문학회지(JKAS) , Vol.55 no.2 (2022)
pp.23~36

DOI : 10.5303/JKAS.2022.55.2.023

ESTIMATION OF NITROGEN-TO-IRON ABUNDANCE RATIOS FROM LOW-RESOLUTION SPECTRA

Changmin Kim

(Department of Astronomy, Space Science and Geology, Chungnam National University)

Young Sun Lee

(Department of Astronomy and Space Science, Chungnam National University)

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)

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.

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