The chemical compositions and evolutionary status of red giants in the open cluster NGC 6940

Abstract

We present the high resolution (R $\approx$ 60000), high signal-to-noise (S/N $\simeq$ 120) spectroscopic analysis of 12 red giant members of the Galactic open cluster NGC 6940. We applied Yonsei-Yale isochrones to the color-magnitude diagram, which suggested an age of 1.1 Gyr for the cluster with a turn-off mass of 2 M${}_{\odot }$. Atmospheric parameters ($T_{\mathrm{eff}}$, $logg$, [Fe/H] and ${\xi }_t$) were determined via equivalent widths of Fe I, Fe II, Ti I, and Ti II lines. Calculated mean metallicity of the cluster is $⟨[Fe/H]⟩=0.04\pm 0.02$. We derived abundances of $\alpha$ (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 $\sim$0.34 dex and $\sim$0.16 dex, respectively. Abundances of light elements Li, C, N, O, and ${}^{12}$C/${}^{13}$C ratios were derived from spectrum syntheses of the Li I resonance doublet at 6707 $\overset{\lower.5em\circ}{\mathrm{A}}$, [O I] line at 6300 $\overset{\lower.5em\circ}{\mathrm{A}}$, C${}_2$ Swan bandheads at 5164 $\overset{\lower.5em\circ}{\mathrm{A}}$ and 5635 $\overset{\lower.5em\circ}{\mathrm{A}}$, and strong ${}^{12,13}$CN system lines in the 7995$-$8040 $\overset{\lower.5em\circ}{\mathrm{A}}$ region. Most carbon isotopic ratios are similar to those found in other solar-metallicity giants, but MMU 152 has an unusual value of ${}^{12}$C/${}^{13}$C=6. Evaluation of the LiCNO abundances and ${}^{12}$C/${}^{13}$C ratios along with the present theoretical models suggests that all the red giants in our sample are core-helium-burning clump stars.

20 pages, 15 figures

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