The Gaia-ESO Survey: The inner disc, intermediate-age open cluster Pismis 18^⋆

¹ Section of Astrophysics, Astronomy and Mechanics, Department of Physics, National and Kapodistrian University of Athens, 15784 Athens, Greece
e-mail: dh@physics.uoc.gr
² IAASARS, National Observatory of Athens, 15236 Penteli, Greece
³ INAF, Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴ Dipartimento di Fisica “E.Fermi”, Universitá di Pisa, Largo Bruno Pontecorvo 3, 56127 Pisa, Italy
⁵ INFN, Sezione di Pisa, Largo Bruno Pontecorvo 3, 56127 Pisa, Italy
⁶ INAF, Osservatorio di Astrofisica e Fisica dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
⁷ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁸ Institute of Theoretical Physics and Astronomy, Vilnius University, Saulėtekio av. 3, 10257 Vilnius, Lithuania
⁹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹⁰ Astronomy Department, Indiana University Bloomington, Swain West 319, 727 East 3rd Street, Bloomington, IN 47405-7105, USA
¹¹ Institute of Astronomy, Cambridge University, Madingley Road, Cambridge CB3 0HA, UK
¹² Lund Observatory, Department of Astronomy and Theoretical Physics, Box 43, 221 00 Lund, Sweden
¹³ Instituto de Astrofísica de Andalucía, Camino Bajo de Huétor, 50, 18008 Granada, Spain
¹⁴ INAF, Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
¹⁵ Dipartimento di Fisica e Astronomia, Università di Catania, Italy
¹⁶ Nicolaus Copernicus Astronomical Center, Polish Academy of Sciences, ul. Bartycka 18, 00-716 Warsaw, Poland
¹⁷ Instituto de Física y Astronomía, Fac. de Ciencias, Universidad de Valparaíso, Gran Bretana 1111, Playa Ancha, Chile
¹⁸ Max-Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁹ Dipartimento di Fisica e Astronomia Galileo Galilei, Universitá di Padova, Vicolo Osservatorio 3, 35122 Padova, Italy
²⁰ School of Physics and Astronomy, Monash University, Clayton 3800, Victoria, Australia
²¹ Faculty of Information Technology, Monash University, Clayton, 3800, Victoria, Australia
²² Departamento de Didáctica, Universidad de Cśdiz, 11519 Puerto Real, Cádiz, Spain
²³ Núcleo de Astronomía, Facultad de Ingeniería y Ciencias, Universidad Diego Portales (UDP), Santiago, Chile
²⁴ Departamento de Ciencias Fisicas, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago, Chile

Received: 13 November 2018
Accepted: 14 February 2019

Abstract

Context. Pismis 18 is a moderately populated, intermediate-age open cluster located within the solar circle at a Galactocentric distance of about seven kpc. Few open clusters have been studied in detail in the inner disc region before the Gaia-ESO Survey.

Aims. 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.

Methods. Gaia-ESO Survey data for 142 potential members, lying on the upper main sequence and on the red clump, yielded radial velocity measurements, which, together with proper motion measurements from the Gaia Second Data Release (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.

Results. The average radial velocity of 26 high confidence members is −27.5 ± 2.5 (std) km s⁻¹ with an average proper motion of pmra = −5.65 ± 0.08 (std) mas yr⁻¹ and pmdec = −2.29 ± 0.11 (std) mas yr⁻¹. According to the new estimates, based on high confidence members, Pismis 18 has an age of τ = 700⁺⁴⁰₋₅₀ Myr, interstellar reddening of E(B − V) = 0.562^+0.012_−0.026 mag and a de-reddened distance modulus of DM₀ = 11.96^+0.10_−0.24 mag. The median metallicity of the cluster (using the six UVES stars) is [Fe/H] = +0.23 ± 0.05 dex, with [α/Fe] = 0.07 ± 0.13 and a slight enhancement of s- and r-neutron-capture elements.

Conclusions. With the present work, we fully characterized the open cluster Pismis 18. We confirmed its present location in the inner disc. We estimated a younger age than the previous literature values and we gave, for the first time, its metallicity and its detailed abundances. Its [α/Fe] and [s-process/Fe], both slightly super-solar, are in agreement with other inner-disc open clusters observed by the Gaia-ESO survey.