The Optical Gravitational Lensing Experiment: Analysis of the bulge RR Lyrae population from the OGLE-III data

TL;DR

This study analyzes 16,836 RR Lyrae stars from OGLE-III data to understand the bulge's structure, metallicity, and distance, providing new insights into the Galactic center's properties and RR Lyrae population distribution.

Contribution

It presents the first comprehensive analysis of bulge RR Lyrae stars from OGLE-III, including extinction, metallicity, spatial distribution, and implications for the Galactic center distance.

Findings

The ratio of total to selective extinction R_I=1.080±0.007 is independent of color.

Bulge RR Lyrae stars form a metal-uniform population with [Fe/H]=-1.02±0.18.

Distance to the Galactic center is R_0=8.54±0.42 kpc.

Abstract

We have analyzed the data on 16,836 RR Lyrae (RR Lyr) variables observed toward the Galactic bulge during the third phase of the Optical Gravitational Lensing Experiment (OGLE-III), which took place in 2001-2009. Using these standard candles, we show that the ratio of total to selective extinction toward the bulge is given by R_I=A_I/E(V-I)=1.080+/-0.007 and is independent of color. We demonstrate that the bulge RR Lyr stars form a metal-uniform population, slightly elongated in its inner part. The photometrically derived metallicity distribution is sharply peaked at [Fe/H]=-1.02+/-0.18, with a dispersion of 0.25 dex. In the inner regions (|l|<3, |b|<4) the RR Lyr tend to follow the barred distribution of the bulge red clump giants. The distance to the Milky Way center inferred from the bulge RR Lyr is R_0=8.54+/-0.42 kpc. We report a break in the mean density distribution at a distance of ~0.5 kpc from the center indicating its likely flattening. Using the OGLE-III data, we assess that (4-7)x10^4 type ab RR Lyr variables should be detected toward the bulge area of the on going near-IR VISTA Variables in the Via Lactea (VVV) survey, where the uncertainty partially results from the unknown RR Lyr spatial density distribution within 0.2 kpc from the Galactic center.