Chemical abundances of metal-poor RR Lyrae stars in the Magellanic Clouds

TL;DR

This study provides the first detailed spectroscopic analysis of metal-poor RR Lyrae stars in the Magellanic Clouds, revealing extremely low metallicities and chemical abundance ratios that align with Milky Way halo stars, supporting galaxy formation models.

Contribution

First spectroscopic measurement of chemical abundances in metal-poor RR Lyrae stars in the Magellanic Clouds, extending knowledge of their chemical composition and comparison with Milky Way halo stars.

Findings

Lowest metallicity measured in Magellanic Clouds at [Fe/H] = -2.7

Photometric metallicities are systematically higher than spectroscopic for stars with [Fe/H] > -2.8

Alpha-element overabundance of 0.36 dex consistent with Milky Way halo stars

Abstract

We present for the first time a detailed spectroscopic study of chemical element abundances of metal-poor RR Lyrae stars in the Large and Small Magellanic Cloud (LMC and SMC). Using the MagE echelle spectrograph at the 6.5m Magellan telescopes, we obtain medium resolution (R ~ 2000 - 6000) spectra of six RR Lyrae stars in the LMC and three RR Lyrae stars in the SMC. These stars were chosen because their previously determined photometric metallicities were among the lowest metallicities found for stars belonging to the old populations in the Magellanic Clouds. We find the spectroscopic metallicities of these stars to be as low as [Fe/H]_{spec} = -2.7dex, the lowest metallicity yet measured for any star in the Magellanic Clouds. We confirm that for metal-poor stars, the photometric metallicities from the Fourier decomposition of the lightcurves are systematically too high compared to their spectroscopic counterparts. However, for even more metal-poor stars below [Fe/H]_{phot} < -2.8dex this trend is reversed and the spectroscopic metallicities are systematically higher than the photometric estimates. We are able to determine abundance ratios for ten chemical elements, which extend the abundance measurements of chemical elements for RR Lyrae stars in the Clouds beyond [Fe/H] for the first time. For the overall [alpha/Fe] ratio, we obtain an overabundance of 0.36dex, which is in very good agreement with results from metal-poor stars in the Milky Way halo as well as from the metal-poor tail in dwarf spheroidal galaxies. Comparing the abundances with those of the stars in the Milky Way halo we find that the abundance ratios of stars of both populations are consistent with another. Therefore we conclude that from a chemical point of view early contributions from Magellanic-type galaxies to the formation of the Galactic halo as claimed in cosmological models are plausible.