Is there a metallicity gradient in the LMC?

TL;DR

This study investigates the presence of a metallicity gradient in the Large Magellanic Cloud by analyzing RR Lyrae variables and AGB star ratios, revealing a slight inner gradient and a significant outer drop likely due to age differences.

Contribution

It provides new evidence for a metallicity gradient in the LMC and reinterprets AGB star ratio data after bias correction, highlighting age effects.

Findings

Radial metallicity gradient in RR Lyrae variables.

Inner slight C/M ratio gradient suggests small metallicity variation.

Outer C/M ratio drop indicates older stellar populations dominate.

Abstract

A small but significant radial gradient in the mean periods of LMC RR Lyrae variables is established from the OGLEIII survey data. This is interpreted as a metallicity gradient but other possibilities are discussed. Data on the ratio of photometrically selected C- and M-type AGB stars in the LMC, kindly provided by M-R. L. Cioni, are reanalysed. Removing the effects of bias leads to conclusions strikingly different to the original ones. There is a slight gradient of the C/M ratio in the inner part of the LMC which might be due to a very small mean metallicity gradient. In the outer part of the LMC the C/M ratio drops dramatically. The most likely reason for this is that the proportion of older stars increases in the outer regions. The mean metallicity of the inner AGB star population estimated from the C/M ratio is lower than for intermediate age LMC clusters and suggest that this population is in the mean older than the clusters and has a mean age which falls in the LMC cluster age gap.