Chemical abundance analysis of the old, rich open cluster Trumpler 20
  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. The
primary goal is to measure Trumpler 20 metallicity, so far very poorly
constrained, and revise the cluster's fundamental parameters. 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.
  We find that Trumpler 20 is slightly more metal-rich than the Sun, having
[Fe/H]=+0.09$\pm$0.10. The abundance ratios of alpha-elements are generally
solar. In line with recent studies of clusters as old as Trumpler 20, Ba is
overabundant compared to the Sun. Our analysis of the iron-peak elements (Cr
and Ni) does not reveal anything anomalous. Based on these results, we
re-estimate the cluster age to be 1.5$^{+0.2}_{-0.1}$ 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. 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.
