Characteristics of the Two Sequences Seen in the High-velocity Hertzsprung–Russell Diagram in Gaia DR2

In this study we use a sample of about 9 million SkyMapper stars with metallicities to investigate the properties of the two stellar populations seen in the high-velocity (VT > 200 km s−1) Gaia DR2 Hertzsprung–Russell diagram. Based on 10,000 red giant branch (RGB) stars (out of 75,000 with high velocity), we find that the two sequences have different metallicity distribution functions; one peaks at −1.4 dex (blue sequence) and the other at −0.7 dex (red sequence). Isochrones with ages in the range 11–13.5 Gyr, and metallicities chosen to match the observations for each sequence, fit the turnoffs and broad RGBs well, indicating that the two populations formed at comparable times within the uncertainties. We find that the mean tangential velocity of disk stars increases steadily with decreasing metallicity, and that the red sequence is made up of the high-velocity stars at the lowest metallicities of the thick-disk population. Using relative number densities, we further find that the red-sequence stars are more centrally concentrated in the Galaxy, and we estimate the radial scale length of this population to be on the order of 2–3 kpc. The blue-sequence stars, on the other hand, follow a nearly flat radial density profile. These findings tighten the link between the red-sequence stars and the chemically defined thick disk.