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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 ($V_{rm T} > 200$ km/s) 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.
As the opening review to the focus meeting ``Stellar Behemoths: Red Supergiants across the Local Universe, I here provide a brief introduction to red supergiants, setting the stage for subsequent contributions. I highlight some recent activity in the
The distribution of stars in the Hertzsprung-Russell diagram narrates their evolutionary history and directly assesses their properties. Placing stars in this diagram however requires the knowledge of their distances and interstellar extinctions, whi
We present a comprehensive stellar atmosphere analysis of 329 O- and B-type stars in the Small Magellanic Cloud (SMC) from the RIOTS4 survey. Using spectroscopically derived effective temperature (Teff) and surface gravities, we find that classical B
We highlight the power of the Gaia DR2 in studying many fine structures of the Hertzsprung-Russell diagram (HRD). Gaia allows us to present many different HRDs, depending in particular on stellar population selections. We do not aim here for complete
We use an improved wavelet analysis technique to reconstruct the $(U,V,W)$ velocity distribution for $sim 250000$ stars from Gaia DR2, residing in the solar neighborhood of $200$~pc. The 2D wavelet transforms for three bivariate distributions $(U,V)$