Investigation of Galactic open cluster remnants: the case of NGC 7193

TL;DR

This study confirms NGC 7193 as a genuine open cluster remnant, providing detailed physical parameters and demonstrating its advanced dynamical evolution through spectroscopic and photometric analysis.

Contribution

It offers a comprehensive multi-method approach to confirm the nature of an open cluster remnant, including spectroscopy, photometry, and statistical analysis, which is novel for this object.

Findings

NGC 7193 is a genuine open cluster remnant.

Derived parameters: distance 501 pc, age 2.5 Gyr, metallicity -0.17.

Luminosity and mass functions show low-mass star depletion.

Abstract

Galactic open clusters (OCs) that survive the early gas-expulsion phase are gradually destroyed over time by the action of disruptive dynamical processes. Their final evolutionary stages are characterized by a poorly populated concentration of stars called open cluster remnant (OCR). This study is devoted to assess the real physical nature of the OCR candidate NGC 7193. GMOS/Gemini spectroscopy of 53 stars in the inner target region were obtained to derive radial velocities and atmospheric parameters. We also employed photometric and proper motion data. The analysis method consists of the following steps: (i) analysis of the statistical resemblance between the cluster and a set of field samples with respect to the sequences defined in colour-magnitude diagrams (CMDs); (ii) a 5-dimensional iteractive exclusion routine was employed to identify outliers from kinematical and positional data; (iii) isochrone fitting to the $K_{s}\times(J-K_{s})$ CMD of the remaining stars and the dispersion of spectral types along empirical sequences in the $(J-H)\times(H-K_{s})$ diagram was checked. A group of stars was identified for which the mean heliocentric distance is compatible with that obtained via isochrone fitting and whose metallicities are compatible with each other. Fifteen member stars observed spectroscopically were identified together with other 19 probable members. Our results indicate that NGC 7193 is a genuine OCR, of an once very populous OC, for which the following parameters were derived: $d=501\,\pm\,46\,$pc, $t=2.5\,\pm\,1.2\,$Gyr, $\langle\,[Fe/H]\,\rangle=-0.17\,\pm\,0.23$ and $E(B-V)=0.05\,\pm\,0.05$. Its luminosity and mass functions show depletion of low mass stars, confirming the OCR dynamically evolved state.