Search for an intrinsic metallicity spread in old globular clusters of the Large Magellanic Cloud

TL;DR

This study investigates the presence of intrinsic metallicity spreads in ten old globular clusters of the Large Magellanic Cloud, finding mostly no significant spreads and suggesting similarities with Galactic GCs.

Contribution

First measurement of intrinsic [Fe/H] spreads in LMC GCs using calibrated Strömgren photometry, revealing mostly null spreads and comparable properties to Milky Way GCs.

Findings

Most GCs show no significant [Fe/H] spread.

NGC1786 shows a possible intrinsic dispersion of 0.07±0.04 dex.

Measurement uncertainties need to be reduced to detect smaller spreads.

Abstract

We report for the first time on the magnitude of the intrinsic [Fe/H] spread among ten old globular clusters (GCs) of the Large Magellanic Cloud (LMC). Such spreads are merely observed in approximately five per cent of the Milky Way GCs and recently gained more attention in theoretical models of GC evolution. We derived metallicities with a typical precision of 0.05 dex < sigma[Fe/H] < 0.20 dex for an average of 14 red giant branch stars per GC from Str\"omgren photometry. The respective, metallicity-sensitive indices have been calibrated to precise and accurate high-dispersion spectroscopy. For all clusters we found null [Fe/H] spreads with a typical uncertainty of 0.04 dex, with the possible exception of NGC1786 that shows an intrinsic dispersion of 0.07+-0.04 dex. The mean, observed standard deviation of the derived metallicities for nearly 40 per cent of our GC sample amounted to smaller than 0.05 dex. At present, we cannot exclude that the remaining GCs also have intrinsic Fe-abundance variations in excess of 0.05 dex, but in order to significantly detect those, the measurement errors on individual [Fe/H]-values would need to be lowered to the 0.03--0.07 dex level. These findings suggest, along with those from ages and light-element abundances, that the LMC GCs studied here are alike to the majority of Galactic GCs.