The Kinematic Properties of BHB and RR Lyrae stars towards the Anticentre and the North Galactic Pole: The Transition between the Inner and the Outer Halo

TL;DR

This study analyzes the kinematic properties of BHB and RR Lyrae stars towards the Anticentre and North Galactic Pole, revealing a transition from inner to outer halo with increasing galactocentric distance and a dominance of retrograde orbits.

Contribution

It provides a comprehensive kinematic and distance analysis of BHB and RR Lyrae stars in these regions, highlighting the orbital characteristics and possible common origins of outlier groups.

Findings

Retrograde orbits dominate beyond 12.5 kpc galactocentric distance.

Halo BHB and RR Lyrae stars become more spherical with distance.

Identification of RR Lyrae groups possibly originating from a single globular cluster.

Abstract

We identify 51 blue horizontal branch (BHB) stars, 12 possible BHB stars and 58 RR Lyrae stars in Anticentre fields. Their selection does not depend on their kinematics. Light curves and ephemerides are given for 7 previously unknown RR Lyrae stars. All but 4 of the RR Lyrae stars are of Oosterhoff type I. Our selection criteria for BHB stars give results that agree with those used by Smith et al. (2010) and Ruhland et al. (2011). We use 5 methods to determine distances for the BHB stars and 3 methods for the RR Lyrae stars to get distances on a uniform scale. Absolute proper motions (largely derived from the GSCII and SDSS (DR7) databases) are given for these stars; radial velocities are given for 31 of the BHB stars and 37 of the RR Lyrae stars. Combining these data for BHB and RR Lyrae stars with those previously found in fields at the North Galactic Pole, we find that retrograde orbits dominate for galactocentric distances greater than 12.5 kpc. The majority of metal-poor stars in the solar neighbourhood are known to be concentrated in a Lperp vs. Lz angular momentum plot. We show that the ratio of the number of outliers to the number in the main concentration increases with galactocentric distance. The location of these outliers with Lperp and Lz shows that the halo BHB and RR Lyrae stars have more retrograde orbits and a more spherical distribution with increasing galactocentric distance. Six RR Lyrae stars are identified in the H99 group of outliers; the small spread in their [Fe/H] suggests that they could have come from a single globular cluster. Another group of outliers contains two pairs of RR Lyrae stars; the stars in each pair have similar properties.