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

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 Vía Láctea (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 ZYJHKs near-infrared (IR) passbands. A total of 12,554 BHB stars were detected, in a region within −10.°0 ≤ ℓ ≤ 10.°2 and −10.°2 ≤ b ≤ −8.°0. We provide accurate coordinates and near-IR photometry for this sample of BHB stars. We searched for overdensities 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 overdensities detected are of low significance. We also constructed Ks-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.

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