Relative distances of Omega Centauri and 47 Tucanae

TL;DR

This study uses optical and near-infrared photometry to precisely determine the relative distances of Omega Centauri and 47 Tucanae through multiple methods, finding consistent distance ratios but some discrepancies in absolute distance measurements.

Contribution

It provides new estimates of the RGB tip magnitudes and compares various distance calibration methods for two prominent globular clusters.

Findings

Distance ratios agree within one sigma across methods.

Absolute TRGB and RR Lyrae distances are consistent within 0.10-0.15 mag.

Kinematic distances are 0.2-0.3 mag smaller than other methods.

Abstract

We present precise optical and near-infrared ground-based photometry of two Globular Clusters (GCs): Omega Cen and 47 Tuc. These photometric catalogs are unbiased in the Red Giant Branch (RGB) region close to the tip. We provide new estimates of the RGB tip (TRGB) magnitudes--m_I(TRGB)=9.84+/-0.05, Omega Cen; m_I(TRGB)=9.46+/-0.06, 47 Tuc--and use these to determine the relative distances of the two GCs. We find that distance ratios based on different calibrations of the TRGB, the RR Lyrae stars and kinematic distances agree with each other within one sigma. Absolute TRGB and RR Lyrae distance moduli agree within 0.10--0.15 mag, while absolute kinematic distance moduli are 0.2--0.3 mag smaller. Absolute distances to 47 Tuc based on the Zero-Age-Horizontal-Branch and on the white dwarf fitting agree within 0.1 mag, but they are 0.1--0.3 mag smaller than TRGB and RR Lyrae distances.