Chemical abundance analysis of the old, rich open cluster Trumpler 20

Aims: Trumpler 20 is an open cluster located at low Galactic longitude, just beyond the great Carina spiral arm, and whose metallicity and fundamental parameters were very poorly known until now. As it is most likely a rare example of an old, rich open cluster - possibly a twin of NGC 7789 - it is useful to characterize it. To this end, we determine here the abundance of several elements and their ratios in a sample of stars in the clump of Trumpler 20. Methods: We present high-resolution spectroscopy of eight clump stars. Based on their radial velocities, we identify six bona fide cluster members, and for five of them (the sixth being a fast rotator) we perform a detailed abundance analysis. Results: We find that Trumpler 20 is slightly more metal-rich than the Sun, having [Fe/H] = +0.09 ± 0.10. The abundance ratios of α-elements are generally solar. In line with recent studies of clusters as old as Trumpler 20, Ba is overabundant compared to the Sun. Our analysis of the iron-peak elements (Cr and Ni) does not reveal anything anomalous. Based on these results, we re-estimate the cluster age to be 1.5-0.1+0.2 Gyr. Its distance to the Galactic centre turns out to be 7.3 kpc. With this distance and metallicity, Trumpler 20 fits fairly well in the metallicity gradient for the Galactic inner disc. Conclusions: With this new study, the characterization of Trumpler 20 is now on much more solid ground. Further studies should focus on the estimate of the binary fraction and on its main sequence membership.