Journal
ASTROPHYSICAL JOURNAL
Volume 863, Issue 2, Pages -Publisher
IOP Publishing Ltd
DOI: 10.3847/1538-4357/aad235
Keywords
Galaxy: evolution; Galaxy: halo; Galaxy: kinematics and dynamics
Categories
Funding
- Agence Nationale de la Recherche (ANR) [ANR-15-CE31-0007]
- Alfred P. Sloan Foundation
- U.S. Department of Energy Office of Science
- Center for High-Performance Computing at the University of Utah
- Carnegie Institution for Science, Carnegie Mellon University
- Harvard-Smithsonian Center for Astrophysics
- Johns Hopkins University
- Kavli Institute for the Physics and Mathematics of the Universe (IPMU)/University of Tokyo
- Lawrence Berkeley National Laboratory
- Max-Planck-Institut fur Astronomie (MPIA Heidelberg)
- Max-Planck-Institut fur Astrophysik (MPA Garching)
- Max-Planck-Institut fur Extraterrestrische Physik (MPE)
- National Astronomical Observatories of China
- University of Notre Dame
- Observatario Nacional/MCTI
- Ohio State University
- Pennsylvania State University
- Shanghai Astronomical Observatory
- Universidad Nacional Autonoma de Mexico
- University of Arizona
- University of Colorado Boulder
- University of Oxford
- University of Portsmouth
- University of Utah
- University of Virginia
- University of Washington
- University of Wisconsin
- Vanderbilt University
- Yale University
- Brazilian Participation Group
- Chilean Participation Group
- French Participation Group
- United Kingdom Participation Group
- Instituto de Astrofisica de Canarias
- Leibniz Institut fur Astrophysik Potsdam (AIP)
- New Mexico State University
- New York University
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We investigate the nature of the double color-magnitude sequence observed in the Gaia DR2 HR diagram of stars with high transverse velocities. The stars in the reddest-color sequence are likely dominated by the dynamically hot tail of the thick disk population. Information from Nissen & Schuster and from the APOGEE survey suggests that stars in the blue-color sequence have elemental abundance patterns that can be explained by this population having a relatively low star formation efficiency during its formation. In dynamical and orbital spaces, such as the Toomre diagram, the two sequences show a significant overlap, but with a tendency for stars on the blue-color sequence to dominate regions with no or retrograde rotation and high total orbital energy. In the plane defined by the maximal vertical excursion of the orbits versus their apocenters, stars of both sequences redistribute into discrete wedges. We conclude that stars that are typically assigned to the halo in the solar vicinity are actually both accreted stars lying along the blue sequence in the HR diagram, and the low rotational velocity tail of the old Galactic disk, possibly dynamically heated by past accretion events. Our results imply that a halo population formed in situ and responsible for the early chemical enrichment prior to the formation of the thick disk has yet to be robustly identified, and that what has been defined as the stars of the in situ stellar halo of the Galaxy may in fact be fossil records of its last significant merger.
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