Characteristics of the two sequences seen in the high-velocity Hertzsprung-Russell diagram in Gaia DR2

Abstract

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 ($V_{\mathrm{T}}>200$ 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.

9 pages, 9 figures, published in ApJ

References in corpus (2)

• A First Constraint on the Thick Disk Scale Length: Differential Radial Abundances in K Giants at Galactocentric Radii 4, 8, and 12 kpc
• The Gaia-ESO Survey: a quiescent Milky Way with no significant dark/stellar accreted disc

Cited by in corpus (20)

• Evidence from the H3 Survey that the Stellar Halo is Entirely Comprised of Substructure
• Age dissection of the Milky Way discs: red giants in the Kepler field
• The SkyMapper-Gaia RVS view of the Gaia-Enceladus-Sausage -- an investigation of the metallicity and mass of the Milky Way's last major merger
• Selecting accreted populations: metallicity, elemental abundances, and ages of the Gaia-Sausage-Enceladus and Sequoia populations
• Characterizing epochs of star formation across the Milky Way disc using age-metallicity distributions of GALAH stars
• The tale of the tail -- disentangling the high transverse velocity stars in Gaia DR2
• The Photometric Metallicity and Carbon Distributions of the Milky Way's Halo and Solar Neighborhood from S-PLUS Observations of SDSS Stripe 82
• A bottom-heavy initial mass function for the likely-accreted blue-halo stars of the Milky Way
• Deciphering the evolution of the Milky Way discs: Gaia APOGEE Kepler giant stars and the Besançon Galaxy Model
• Contribution of Gaia Sausage to the Galactic Stellar Halo Revealed by K Giants and Blue Horizontal Branch Stars from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope, Sloan Digital Sky Survey, and Gaia
• A Blueprint for the Milky Way's Stellar Populations. III. Spatial Distributions and Population Fractions of Local Halo Stars
• Explaining the chemical trajectories of accreted and in-situ halo stars of the Milky Way
• A Blueprint for the Milky Way's Stellar Populations. II. Improved Isochrone Calibration in the SDSS and Pan-STARRS Photometric Systems
• The Reduced Proper Motion selected halo: methods and description of the catalogue
• Influence of the Gaia-Sausage-Enceladus on the density shape of the Galactic stellar halo revealed by halo K giants from the LAMOST survey
• Connecting the local stellar halo and its dark matter density to dwarf galaxies via blue stragglers
• A Blueprint for the Milky Way's Stellar Populations. IV. A String of Pearls $-$ the Galactic Starburst Sequence
• Estimating the age-metallicity distribution of a stellar sample from the probability distributions of the individual stars
• Dynamical masses across the Hertzsprung-Russell diagram
• Deciphering the Kinematic Substructure of Local Dark Matter with LAMOST K Giants