Age and metallicity for six LMC clusters and their surrounding field population

TL;DR

This study uses CCD Stroemgren photometry to determine ages and metallicities of six LMC clusters and their surrounding field stars, revealing an age-metallicity relation and challenging existing models predicting many metal-poor stars.

Contribution

It introduces a method to estimate individual stellar ages and metallicities, enabling detailed analysis of the LMC's field star population and its evolution over 8 Gyr.

Findings

Field metallicity increased sixfold over 2 Gyr.

Metallicity data contradict G-dwarf problem models.

Field star AMR aligns with cluster system AMR.

Abstract

We investigate, on the basis of CCD Stroemgren photometry, the ages and metallicities of six LMC clusters together with their surrounding field population. The clusters and metallicities are: NGC1651 (in the range [Fe/H]=-0.65 dex to -0.41 dex), NGC1711 (-0.57+/-0.17 dex), NGC 1806 (-0.71+/-0.23 dex), NGC2031 (-0.52+/-0.21 dex) and NGC2136/37 (-0.55+/-0.23 dex) and NGC2257 (-1.63+/-0.21 dex). In the cluster surroundings, we found about 650 field stars that were suitable to be used for a determination of an age-metallicity relation (AMR). Our method is to estimate ages for individual stars on the basis of Stroemgren isochrones with individually measured metallicities. With this method we are able to sample the AMR of the field population up to 8 Gyr. Our metallicity data are incompatible with models predicting many metal-poor stars (G-dwarf problem). The metallicity of the field population increased by a factor of six, starting around 2 Gyr ago. The proposed AMR is consistent with the AMR of the LMC cluster system (including ESO 121 SC03 and three clusters with an age of 4 Gyr).