Two distinct halo populations in the solar neighborhood. Evidence from stellar abundance ratios and kinematics

Abstract

Precise abundance ratios are determined for 94 dwarf stars with 5200 < Teff < 6300 K, -1.6 < [Fe/H] < -0.4, and distances D < 335 pc. Most of them have halo kinematics, but 16 thick-disk stars are included. Equivalent widths of atomic lines are measured from VLT/UVES and NOT/FIES spectra with resolutions R = 55000 and R = 40000, respectively. An LTE abundance analysis based on MARCS models is applied to derive precise differential abundance ratios of Na, Mg, Si, Ca, Ti, Cr, and Ni with respect to Fe. The halo stars fall into two populations, clearly separated in [alpha/Fe], where alpha refers to the average abundance of Mg, Si, Ca, and Ti. Differences in [Na/Fe] and [Ni/Fe] are also present with a remarkably clear correlation between these two abundance ratios. The `high-alpha' stars may be ancient disk or bulge stars `heated' to halo kinematics by merging satellite galaxies or they could have formed as the first stars during the collapse of a proto-Galactic gas cloud. The kinematics of the `low-alpha' stars suggest that they have been accreted from dwarf galaxies, and that some of them may originate from the omega Cen progenitor galaxy.

Accepted for publication in A&A as a four-page Letter with five pages of online material

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• Effects of secular growth and mergers on the evolution of metallicity gradients and azimuthal variations in a Milky Way-like galaxy
• Exploring the ex-situ components within Gaia DR3
• Euclid: Early Release Observations -- The surface brightness and colour profiles of the far outskirts of galaxies in the Perseus cluster
• Chemodynamics of metal-poor wide binaries in the Galactic halo: Association with the Sequoia event
• Searching For Transiting Planets Around Halo Stars. I. Sample Selection and Validation
• New Lithium Measurements in Metal-Poor Stars
• A chemodynamical analysis of bright metal-poor stars from the HESP-GOMPA survey -- Indications of a non-prevailing site for light r-process elements
• Constraining the Planet Occurrence Rate around Halo Stars of Potentially Extragalactic Origin
• HALO7D III: Chemical Abundances of Milky Way Halo Stars from Medium Resolution Spectra
• Chemodynamically Characterizing the Jhelum Stellar Stream with APOGEE-2
• Globular Clusters of the Galaxy: Chemical Composition vs Kinematics
• Combining Astrometry and Elemental Abundances: The Case of the Candidate Pre-Gaia Halo Moving Groups G03-37, G18-39, and G21-22
• The nature of the KFR08 stellar stream
• AuriDESI: Mock Catalogues for the DESI Milky Way Survey
• The Pristine Survey -- XXVII. Journey to the Galactic outskirts -- Mapping the outer halo red giant stars down to the very metal-poor end
• Magnesium isotope ratios in Milky Way and dwarf galaxy stars
• Medium-resolution Spectroscopy of Red Giant Branch Stars in $\omega$ Centauri
• Different Sodium enhancements among multiple populations of Milky Way globular clusters
• First Star Survivors as Metal-Rich Halo Stars that Experienced Supernova Explosions in Binary Systems
• The Origin of the Metal-Poor Common Proper Motion Pair HD 134439/134440: Insights from New Elemental Abundances
• The Chemodynamical Nature of the Triangulum-Andromeda Overdensity
• Investigating the vertical distribution of the disk as a function of radial action
• Photometric Calibration of the [$\alpha$/Fe] Element: I. Calibration with UBV Photometry
• Satellites in the Local Group and Other Nearby Groups
• Revisited parameters for the twin bulge globular clusters NGC 6528 and NGC 6553
• A slow spin to win -- the gradual evolution of the proto-Galaxy to the old disc
• J01020100-7122208: an accreted evolved blue straggler that wasn't ejected from a supermassive black hole
• The disc origin of the Milky Way bulge: On the high velocity dispersion of metal-rich stars at low latitude
• Unevolved Li-rich stars at low metallicity: a possible formation pathway through novae
• Photometric Calibration of the $[\alpha /$Fe] Element: II. Calibration with SDSS Photometry
• On the Use of Field RR Lyrae as Galactic Probes -- VIII. Early Formation of the Galactic Spheroid
• A theoretical investigation of far-infrared fine structure lines at $z>6$ and of the origin of the [OIII]88/[CII]158 enhancement
• The Spheroidal Bulge of the Milky Way: Chemodynamically Distinct from the Inner-Thick Disc and Bar
• The Nephele ecosystem: stars, globular clusters, and stellar streams associated with the progenitor galaxy of $\omega$ Centauri
• Modelling the chemical evolution of the Milky Way
• A Hint of Three-section Halo as Seen from the APOGEE DR14
• Quantifying chemical and kinematical properties of Galactic disks
• The metal-poorest tail of the Galactic halo: hypothesis on its origin from precise spectral analysis
• Main-sequence Turnoff Stars as Probes of the Ancient Galactic Relic: Chemo-dynamical Analysis of a Pilot Sample
• Disentanglement of the chemodynamical assembly: mapping the Milky Way disks
• Partitioning the Galactic Halo with Gaussian Mixture Models
• Benchmark stars, benchmark spectrographs: Detailed spectroscopic comparison of ESPRESSO, PEPSI, and HARPS data for Gaia benchmark stars
• The accreted Galaxy: An overview of TESS metal-poor accreted stars candidates
• Two faces of Gaia-Sausage-Enceladus: Mining the chemical abundance space with graph attention networks
• Physical and Chemical Properties of Galactic Global Clusters with Various Origins Identified from the Gaia DR2 Data
• The Gaia Ultracool Dwarf Sample -- IV. GTC/OSIRIS optical spectra of Gaia late-M and L dwarfs
• Modeling the near-UV band of GK stars, Paper III: Dependence on abundance pattern
• Disentangling Milky Way halo populations at low metallicities using [Al/Fe]
• New families in our Solar neighborhood: applying Gaussian Mixture models for objective classification of structures in the Milky Way and in simulations
• The GALAH Survey: Neutron-Capture Elemental Abundances for 350,000 Gaia-RVS spectra and the Chemodynamics of Accreted Structures
• On the Galactic chemical evolution of sulphur. Sulphur abundances from the [S i] 1082 nm line in giants
• The chemodynamical memory of a major merger in a NIHAO-UHD Milky Way analogue -- I. A golden thread through time and space
• Demonstration of Plate Analysis, an algorithm for precise relative astrometry
• Gaia-Sausage-Enceladus: Lithium evolution from early red-giant-branch and main-sequence stars
• Searching for and characterizing halo substructures with the GALAH DR4 survey