Metallicity Analysis of MACHO Galactic Bulge RR0 Lyrae stars from their Lightcurves

TL;DR

This study analyzes the metallicities of 2690 RR0 Lyrae stars in the Galactic Bulge and Sagittarius dwarf galaxy using lightcurve Fourier coefficients, revealing metallicity distributions, spatial trends, and marginal evidence for a Galactic bar.

Contribution

It provides a large-scale empirical metallicity analysis of RR0 Lyrae stars in the Galactic Bulge and Sagittarius dwarf galaxy, using lightcurve data calibrated with Fourier coefficients.

Findings

Bulge RR0 Lyrae stars have average [Fe/H] = -1.25.

Sagittarius RR0 stars have lower average [Fe/H] = -1.55.

Evidence suggests a metallicity gradient and marginal Galactic bar presence.

Abstract

We present metallicities of 2690 RR0 Lyrae stars observed toward the MACHO Survey fields in the Galactic Bulge. These [Fe/H] values are based upon an empirically calibrated relationship that uses the Fourier coefficients of the light curve and are accurate to ~0.2 dex. The majority of the RR0 Lyrae stars in our sample are located in the Galactic Bulge, but 255 RR0 stars are associated with Sagittarius dwarf galaxy. We find that the RR0 Lyrae stars that belong to the Galactic Bulge have average metallicities [Fe/H] = -1.25, with a broad metallicity range from [Fe/H] = -2.26 to -0.15. The RR0 stars from Sagittarius dwarf galaxy have lower average metallicity of [Fe/H] = -1.55 \pm 0.02, with an intrinsic dispersion of 0.25 dex, similar to that in the bulge. A correlation between metallicity and galactocentric distance is found, in a sense that for the metal-poor RR0 Lyrae stars ([Fe/H]< -1.5 dex), the closer a star is to the Galactic center, on average, the more metal rich it is. However, for the metal-rich RR0 Lyrae stars ([Fe/H] > -1.2 dex), this trend is reversed. Using mean magnitudes of MACHO RR Lyrae stars, we searched for the evidence of the Galactic bar, and found marginal evidence of a bar. The absence of a strong bar indicates that the RR Lyrae in the bulge represent a different population than the majority of the bulge stars, which are metal rich and are part of a bar.