Establishing the Galactic Centre Distance Using VVV Bulge RR Lyrae Variables

TL;DR

This paper uses infrared VVV photometry of RR Lyrae stars to accurately measure the Galactic Centre distance, addressing extinction and contamination issues, and introduces a new distance indicator based on blended star overdensities.

Contribution

It presents a novel method for Galactic Centre distance measurement using high latitude RR Lyrae stars with minimized extinction uncertainties and identifies a new blended star-based distance indicator.

Findings

Galactic Centre distance estimated at 8.30 ± 0.36 kpc.

Derived the $M_{K_s}$ relation for RRab stars from LMC data.

Discovered a new distance indicator from blended RRab and red clump stars.

Abstract

This study's objective was to exploit infrared VVV (VISTA Variables in the Via Lactea) photometry for high latitude RRab stars to establish an accurate Galactic Centre distance. RRab candidates were discovered and reaffirmed ($n=4194$) by matching $K_s$ photometry with templates via $\chi^2$ minimization, and contaminants were reduced by ensuring targets adhered to a strict period-amplitude ($\Delta K_s$) trend and passed the Elorietta et al. classifier. The distance to the Galactic Centre was determined from a high latitude Bulge subsample ($|b|>4^{o}$, $R_{GC}=8.30 \pm 0.36$ kpc, random uncertainty is relatively negligible), and importantly, the comparatively low color-excess and uncrowded location mitigated uncertainties tied to the extinction law, the magnitude-limited nature of the analysis, and photometric contamination. Circumventing those problems resulted in a key uncertainty being the $M_{K_s}$ relation, which was derived using LMC RRab stars ($M_{K_s}=-(2.66\pm0.06) \log{P}-(1.03\pm0.06)$, $(J-K_s)_0=(0.31\pm0.04) \log{P} + (0.35\pm0.02)$, assuming $\mu_{0,LMC}=18.43$). The Galactic Centre distance was not corrected for the cone-effect. Lastly, a new distance indicator emerged as brighter overdensities in the period-magnitude-amplitude diagrams analyzed, which arise from blended RRab and red clump stars. Blending may thrust faint extragalactic variables into the range of detectability.