VVV Survey of Blue Horizontal-Branch Stars in the Bulge-Halo Transition Region of the Milky Way

TL;DR

This study uses the VVV survey to identify and analyze over 12,000 blue horizontal-branch stars in the Milky Way's bulge-halo transition, providing their coordinates, photometry, and variability characteristics.

Contribution

It presents a large, near-infrared catalog of BHB stars in the transition region, including their variability and a search for stellar over-densities, with a novel selection method based on globular cluster standards.

Findings

Detected 12,554 BHB stars in the specified region.

Few over-densities found, likely of low significance.

Identified hundreds of eclipsing binaries and a dozen pulsating BHB stars.

Abstract

We characterize the population of blue horizontal-branch (BHB) stars in the bulge-halo transition region of the Milky Way using the VISTA Variables in the V\'ia L\'actea (VVV) ESO Public Survey data. The selection of BHB stars is made using the globular cluster M22 as a reference standard, and constructing color-magnitude and color-color diagrams with specific cuts in the $ZYJHK_s$ near-infrared (IR) passbands. A total of 12,554 BHB stars were detected, in a region within $-10.0^{\circ} \leq \ell \leq 10.2^{\circ}$ and $-10.2^{\circ} \leq b \leq -8.0^{\circ}$. We provide accurate coordinates and near-IR photometry for this sample of BHB stars. We searched for over-densities of stars with sizes similar to those of known globular clusters and stellar streams. By comparing real data with Monte Carlo simulations, we conclude that the few over-densities detected are of low significance. We also constructed $K_s$-band light curves for the BHB stars to study their variability. Taking an average of 52 epochs to calculate periods and amplitudes, we identify hundreds of candidate eclipsing binaries and a dozen pulsating stars. Finally, we made some comparisons with results obtained in a previous study for RR Lyrae variable stars in this same region.