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Chemical abundances of open clusters from high-resolution infrared spectra - I. NGC 6940

2019    Topcu, G. Bocek; Afsar, M.; Sneden, C.; Pilachowski, C. A.; Denissenkov, P. A.; VandenBerg, D. A.; Strickland, E.; Ozdemir, S.; Mace, G. N.; Kim, H.; Jaffe, D. T.

We present near-infrared spectroscopic analysis of 12 red giant members of the Galactic open cluster NGC 6940. High-resolution (R similar or equal to 45 000) and high-signal-to-noise ratio (S/N > 100) near-infrared H-and K-band spectra were gathered with the Immersion Grating Infrared Spectrograph (IGRINS) on the 2.7-m Smith Telescope at McDonald Observatory. We obtained abundances of H-burning (C, N, O), alpha (Mg, Si, S, Ca), light odd-Z (Na, Al, P, K), Fe-group (Sc, Ti, Cr, Fe, Co, Ni), and neutron-capture (Ce, Nd, Yb) elements. We report the abundances of S, P, K, Ce, and Yb in NGC 6940 for the first time. Many OH and CN features in the H band were used to obtain O and N abundances. C abundances were measured from four different features: CO molecular lines in the K band, high excitation CI lines present in both near-infrared and optical, CH and C-2 bands in the optical region. We have also determined C-12/C-13 ratios from the R-branch band heads of first overtone (2-0) and (3-1) (CO)-C-12 and (2-0) (CO)-C-13 lines near 23 440 angstrom and (3-1) (CO)-C-13 lines at about 23 730 angstrom. We have also investigated the HF feature at 23 358.3 angstrom, finding solar fluorine abundances without ruling out a slight enhancement. For some elements (such as the alpha group), IGRINS data yield more internally self-consistent abundances. We also revisited the CMD of NGC 6940 by determining the most probable cluster members using Gaia DR2. Finally, we applied Victoria isochrones and MESA models in order to refine our estimates of the evolutionary stages of our targets.