[Paper Review] Age and metallicity for six LMC clusters and their surrounding field population

B. Dirsch, T. Richtler+2 more|arXiv (Cornell University)|Apr 3, 2000

Stellar, planetary, and galactic studies3 citations

TL;DR

This study uses CCD Strömgren photometry to determine ages and metallicities for six LMC clusters and their surrounding field stars, revealing a metallicity increase by a factor of six over the last 2 Gyr. The derived age-metallicity relation (AMR) for the field population is incompatible with models predicting many metal-poor stars and inconsistent with a recent AMR by Pagel & Tautvais̆vienė (1998).

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).

Motivation & Objective

• To determine the ages and metallicities of six LMC clusters and their surrounding field population using high-precision Strömgren photometry.
• To investigate the age-metallicity relationship (AMR) of the LMC field population, particularly for stars older than 4 Gyr.
• To test whether the chemical enrichment history of field stars is consistent with that of the cluster system.
• To resolve discrepancies in the LMC's chemical evolution, especially the so-called 'G-dwarf problem' involving an overabundance of metal-poor stars.
• To improve the calibration of photometric metallicities for metal-poor stars using updated isochrones.

Proposed method

• CCD Strömgren photometry was used to measure the colors (b-y, v-y, etc.) of individual stars in six LMC clusters and their surrounding fields.
• Metallicities were derived using the c1 index and the (b-y) color, calibrated with theoretical isochrones for red giant and supergiant stars.
• Individual stellar ages were estimated by fitting the observed color-magnitude diagrams to Strömgren isochrones with the measured metallicities.
• The age-metallicity relationship (AMR) for the field population was constructed by combining individual age and metallicity estimates from ~650 field stars.
• The AMR was compared with existing cluster data and theoretical models, including the Pagel & Tautvais̆vienė (1998) AMR.
• The method allowed sampling of the AMR up to 8 Gyr, enabling a detailed study of the chemical enrichment history of the LMC field.

Experimental results

Research questions

• RQ1 What is the age-metallicity relationship (AMR) for the field star population in the LMC, particularly for stars older than 4 Gyr?
• RQ2 How do the metallicities of the six LMC clusters compare with those of the surrounding field population?
• RQ3 Is the observed field star metallicity distribution compatible with models predicting a large population of metal-poor stars (G-dwarf problem)?
• RQ4 Does the AMR derived from field stars agree with the AMR derived from LMC cluster systems?
• RQ5 Is the AMR proposed in this work consistent with the recently published AMR by Pagel & Tautvais̆vienė (1998)?

Key findings

• The metallicities of NGC 1651, NGC 1711, NGC 1806, and NGC 2031 were determined for the first time using photometric methods, with values ranging from [Fe/H] = -0.65 to -0.41 dex.
• NGC 2257 was found to be very metal-poor, with [Fe/H] = -1.63 ± 0.21 dex, consistent with its old age.
• The field population shows a metallicity increase by a factor of six over the last 2 Gyr, indicating strong chemical enrichment in the recent past.
• The derived AMR for the field population is incompatible with models predicting a large number of metal-poor stars, resolving the G-dwarf problem in the LMC context.
• The proposed AMR is consistent with the AMR of the LMC cluster system, including clusters like ESO 121-SC03 and NGC 2121, NGC 2155, SL 663 (age ~4 Gyr).
• The AMR derived in this work is incompatible with the recently proposed AMR by Pagel & Tautvais̆vienė (1998), indicating a need for revision of that model.