Mapping the Galactic disk with the LAMOST and $Gaia$ Red clump sample: V: On the origin of the "young" [$\alpha$/Fe]-enhanced stars

Abstract

Using a sample of nearly 140,000 primary red clump stars selected from the LAMOST and $Gaia$ surveys, we have identified a large sample of "young" [$\alpha$/Fe]-enhanced stars with stellar ages younger than 6.0 Gyr and [$\alpha$/Fe] ratios greater than 0.15 dex. The stellar ages and [$\alpha$/Fe] ratios are measured from LAMOST spectra, using a machine learning method trained with common stars in the LAMOST-APOGEE fields (for [$\alpha$/Fe]) and in the LAMOST-$Kepler$ fields (for stellar age). The existence of these "young" [$\alpha$/Fe]-enhanced stars is not expected from the classical Galactic chemical evolution models. To explore their possible origins, we have analyzed the spatial distribution, and the chemical and kinematic properties of those stars and compared the results with those of the chemically thin and thick disk populations. We find that those "young" [$\alpha$/Fe]-enhanced stars have distributions in number density, metallicity, [C/N] abundance ratio, velocity dispersion and orbital eccentricity that are essentially the same as those of the chemically thick disk population. Our results clearly show those so-called "young" [$\alpha$/Fe]-enhanced stars are not really young but $genuinely$ $old$. Although other alternative explanations can not be fully ruled out, our results suggest that the most possible origin of these old stars is the result of stellar mergers or mass transfer.

10 pages, 10 figures, accepted for publication in ApJ

References in corpus (13)

• galpy: A Python Library for Galactic Dynamics
• The Dawes Review 2: Nucleosynthesis and stellar yields of low and intermediate-mass single stars
• A First Constraint on the Thick Disk Scale Length: Differential Radial Abundances in K Giants at Galactocentric Radii 4, 8, and 12 kpc
• Young [$\alpha$/Fe]-enhanced stars discovered by CoRoT and APOGEE: What is their origin?
• The age-metallicity structure of the Milky Way disk
• The LAMOST Stellar Parameter Pipeline at Peking University --- LSP3
• Chemical Abundances of Luminous Cool Stars in the Galactic Center from High-Resolution Infrared Spectroscopy
• The AMBRE Project: chemical evolution models for the Milky Way thick and thin discs
• Binary stars in the Galactic thick disc
• LAMOST Spectroscopic Survey of the Galactic Anticentre (LSS-GAC): the second release of value-added catalogues
• Determination of the Local Standard of Rest using the LSS-GAC DR1
• Mapping the Galactic disk with the LAMOST and Gaia Red clump sample: I: precise distances, masses, ages and 3D velocities of $\sim$ 140000 red clump stars
• Origin of $\alpha$-rich young stars: clues from C, N and O

Cited by in corpus (20)

• Overview of the LAMOST survey in the first decade
• Spectroscopic age estimates for APOGEE red-giant stars: Precise spatial and kinematic trends with age in the Galactic disc
• The K2 Galactic Archaeology Program Data Release 3: Age-abundance patterns in C1-C8 and C10-C18
• Mapping the Galactic Disk with the LAMOST and Gaia Red Clump Sample VII: the Stellar Disk Structure Revealed by the Mono-abundance Populations
• Most "young" $\alpha$-rich stars have high masses but are actually old
• Cannibals in the thick disk II -- Radial-velocity monitoring of the young alpha-rich stars
• [Mg/Fe] ratios in the solar neighbourhood: stellar yields and chemical evolution scenarios
• The Flattening Metallicity Gradient in the Milky Way's Thin Disk
• Universal properties of the high- and low-α disk: small intrinsic abundance scatter and migrating stars
• Stragglers of the thick disc
• Discovering Strongly-lensed QSOs From Unresolved Light Curves
• The Gaia-ESO survey: placing constraints on the origin of r-process elements
• Characterising abundance-age relations of GALAH stars using oxygen-enhanced stellar models
• A radiation transfer model for the Milky Way. II The global properties and large scale structure
• Rediscovering the Milky Way with orbit superposition approach and APOGEE data II. Chrono-chemo-kinematics of the disc
• The tilt of the velocity ellipsoid of different Galactic disk populations
• Stellar Population Astrophysics (SPA) with the TNG. alpha-elements, lithium, sodium and aluminum in 16 open clusters
• The fractions of post-binary-interaction stars and evolved blue straggler stars on the red giant branch of globular clusters
• Exploring the History of Stellar Mergers with Chemistry: Examining the Origins of Massive $\alpha$-Enriched Stars using Carbon Isotope Ratios
• Turning Points in the Age-Metallicity Relations -- Created by Late Satellite Infall and Enhanced by Radial Migration