The Gaia-ESO Survey: The inner disc, intermediate-age open cluster
  Pismis 18
  Pismis 18 is a moderately populated, intermediate-age open cluster located
within the solar circle at a Galactocentric distance of about 7 kpc. Few open
clusters have been studied in detail in the inner disc region before the
Gaia-ESO Survey. New data from the Gaia-ESO Survey allowed us to conduct an
extended radial velocity membership study as well as spectroscopic metallicity
and detailed chemical abundance measurements for this cluster. Gaia-ESO Survey
data for 142 potential members, lying on the upper MS and on the red clump,
yielded radial velocity measurements, which, together with proper motion
measurements from the Gaia DR2, were used to determine the systemic velocity of
the cluster and membership of individual stars. Photometry from Gaia DR2 was
used to re-determine cluster parameters based on high confidence member stars
only. Cluster abundance measurements of six radial-velocity member stars with
UVES high-resolution spectroscopy are presented for 23 elements. According to
the new estimates, based on high confidence members, Pismis 18 has an age of
$700^{+40}_{-50}$ Myr, interstellar reddening of E(B-V) =
$0.562^{+0.012}_{-0.026}$ mag and a de-reddened distance modulus of $DM_0 =
11.96^{+0.10}_{-0.24}$ mag. The median metallicity of the cluster (using the
six UVES stars) is [Fe/H] = $+0.23 \pm 0.05$ dex, with [$\alpha$/Fe]= $0.07 \pm
0.13$ and a slight enhancement of s- and r- neutron-capture elements. With the
present work, we fully characterized the open cluster Pismis 18, confirming its
present location in the inner disc. We estimated a younger age than the
previous literature values and gave, for the first time, its metallicity and
its detailed abundances. Its [$\alpha$/Fe] and [s-process/Fe], both slightly
super-solar, are in agreement with other inner-disc open clusters observed by
the Gaia-ESO survey. [abridged]
