2016MNRAS.463..580B

We present the high-resolution (R ≃ 60 000), high signal-to-noise ratio (S/N ≃ 120) spectroscopic analysis of 12 red giant members of the Galactic open cluster NGC 6940. We applied Yonsei-Yale isochrones to the colour-magnitude diagram, which suggested an age of 1.1 Gyr for the cluster with a turn-off mass of 2 M_☉. Atmospheric parameters (T_eff, log g, [Fe/H], and ξ_t) were determined via equivalent widths of Fe I, Fe II, Ti I, and Ti II lines. Calculated mean metallicity of the cluster is <[Fe/H]> = 0.04 ± 0.02. We derived abundances of α (Mg, Si, Ca), Fe-group (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn), and n-capture (Y, La, Nd, Eu) elements to be about solar. Light odd-Z elements Na and Al are slightly enhanced in MMU 108 and MMU 152 by ∼0.34 and ∼0.16 dex, respectively. Abundances of light elements Li, C, N, O, and ¹²C/¹³C ratios were derived from spectrum syntheses of the Li I resonance doublet at 6707 Å, [O I] line at 6300 Å, C₂ Swan bandheads at 5164 and 5635 Å, and strong ^12,13CN system lines in the 7995-8040 Å region. Most carbon isotopic ratios are similar to those found in other solar-metallicity giants, but MMU 152 has an unusual value of ¹²C/¹³C = 6. Evaluation of the LiCNO abundances and ¹²C/¹³C ratios along with the present theoretical models suggests that all the red giants in our sample are core-helium-burning clump stars.