Calibrating the BHB Star Distance Scale and the Halo Kinematic Distance to the Galactic Centre

TL;DR

This study uses halo star kinematics to accurately determine the distance to the Galactic centre and calibrate BHB star distances, revealing differences in velocity and dispersion properties compared to RR Lyrae stars.

Contribution

It provides the first distance estimate to the Galactic centre based on halo star kinematics and calibrates BHB star distances using a statistical-parallax method.

Findings

Distance to Galactic centre: 8.2+/-0.6 kpc

BHB star velocity in rotation direction: -240+/-4 km/s

Velocity dispersion tensor less anisotropic than RR Lyrae stars

Abstract

We report the first determination of the distance to the Galactic centre based on the kinematics of halo objects. We apply the statistical-parallax technique to the sample of ~2500 Blue Horizontal Branch (BHB) stars compiled by Xue et al. (2011) to simultaneously constrain the correction factor to the photometric distances of BHB stars as reported by those authors and the distance to the Galactic centre to find R0=8.2+/-0.6 kpc. We also find that the average velocity of our BHB star sample in the direction of Galactic rotation, V0=-240+/-4 km/s, is greater by about 20 km/s in absolute value than the corresponding velocity for halo RR Lyrae type stars (V0=-222+/-4 km/s) in the Galactocentric distance interval from 6 to 18 kpc, whereas the total (sigma V) and radial (sigma r) velocity dispersion of the of the BHB sample are smaller by about 40-45 km/s than the corresponding parameters of the velocity dispersion ellipsoid of halo RR Lyrae type variables. The velocity dispersion tensor of halo BHB stars proved to be markedly less anisotropic than the corresponding tensor for RR Lyrae type variables: the corresponding anisotropy parameter values are equal to beta(BHB)=0.51+/-0.02 and beta(RR)=0.71+/-0.03, respectively.