Radial velocity and chemical composition of evolved stars in the open clusters NGC 6940 and Tombaugh 5

Abstract

We present and discuss medium resolution (R $\sim$ 13000), high signal-to-noise ($\frac{\mathrm{S}}{\mathrm{N}}\sim 100$), spectroscopic observations in the field of the open clusters NGC\,6940 and Tombaugh\,5. Spectra were recorded for seven candidate red giant stars in both clusters. For the latter we present the very first chemical abundance analysis. We derive radial velocities for all the stars in NGC\,6940, confirming membership to the cluster for all of them, while on the same ground we exclude two stars in To\,5. We perform a chemical abundance analysis of different atomic species, in particular FeI, SiI, CaI, TiI and NiI. The mean metallicity of NGC\,6940 is [Fe/H]=+0.09$\pm$0.06\,dex, in good agreement with previous works, while for To\,5 is [Fe/H]=+0.06$\pm$0.11\,dex. Therefore, both clusters exhibit a chemical composition close to the solar value, and do not deviate from the [Fe/H] Galactic radial abundance gradient. With these new values we estimate the fundamental cluster parameters, after having derived clusters' distances from the \textit{Gaia} DR2 database. By adopting these distances, we derive updated estimated for the clusters ages: 1.0$\pm$0.1\,Gyr of NGC\,6940 and 0.25$\pm$0.05 Gyr for Tombaugh\,5.

Accepted for publication in AJ. 13 pages, 7 figures

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