Through Thick and Thin: Kinematic and Chemical Components in the Solar Neighbourhood

Abstract

We search for the existence of chemically-distinct stellar components in the solar neighbourhood using published data. Extending earlier work, we show that when the abundances of Fe, alpha elements, and the r-process element Eu are considered, stars separate neatly into two groups that delineate the traditional thin and thick disk of the Milky Way. The group akin to the thin disk is traced by stars with [Fe/H]>-0.7 and alpha/Fe<0.2. The thick disk-like group overlaps the thin disk in [Fe/H] but has higher abundances of alpha elements and Eu. Stars in the range -1.5<[Fe/H]<-0.7 with low [alpha/Fe] ratios, however, seem to belong to a separate, dynamically-cold, non-rotating component that we associate with tidal debris. The kinematically-hot stellar halo dominates the sample for [Fe/H]<-1.5. These results suggest that it may be possible to define the main dynamical components of the solar neighbourhood using only their chemistry, an approach with a number of interesting consequences. The kinematics of thin disk stars is then independent of metallicity: their average rotation speed remains roughly constant in the range -0.7<[Fe/H]<+0.4, a result that argues against radial migration having played a substantial role in the evolution of the thin disk. The velocity dispersion of stars assigned to the thin disk is also independent of [Fe/H], implying that the familiar increase in velocity dispersion with decreasing metallicity is the result of the increased prevalence of the thick disk at lower metallicities, rather than of the sustained operation of a dynamical heating mechanism. The substantial overlap in [Fe/H] and, probably, stellar age, of the various components might affect other reported trends in the properties of stars in the solar neighbourhood.

8 pages, 5 figures, some clarifications after referee report. Conclusions unchanged

References in corpus (7)

• Two Stellar Components in the Halo of the Milky Way
• alpha-, r-, and s-process element trends in the Galactic thin and thick disks
• Chemical Abundances for 855 Giants in the Globular Cluster Omega Centauri (NGC 5139)
• Radial mixing and the transition between the thick and thin Galactic discs
• Chemical similarities between Galactic bulge and local thick disk red giant stars
• Vertical distribution of Galactic disk stars IV - AMR and AVR from clump giants
• Fashionably Late? Building up the Milky Way's Inner Halo

Cited by in corpus (31)

• Footprints of the Sagittarius dwarf galaxy in the $Gaia$ data set
• The Milky Way has no thick disk
• Inside Out and Upside Down: Tracing the Assembly of a Simulated Disk Galaxy Using Mono-Age Stellar Populations
• A Spectroscopic Catalog of the Brightest (J<9) M Dwarfs in the Northern Sky
• The Genesis of the Milky Way's Thick Disk via Stellar Migration
• Formation and Evolution of the Disk System of the Milky Way: [alpha/Fe] Ratios and Kinematics of the SEGUE G-Dwarf Sample
• Star formation, supernovae, iron, and alpha: consistent cosmic and Galactic histories
• Disentangling the Galactic Halo with APOGEE: I. Chemical and Kinematical Investigation of Distinct Metal-Poor Populations
• The origin of galactic metal-rich stellar halo components with highly eccentric orbits
• The effect of radial migration on galactic disks
• Binary stars in the Galactic thick disc
• A radial age gradient in the geometrically thick disk of the Milky Way
• Slicing and dicing the Milky Way disk in SDSS
• The GALAH Survey: Chemical tagging and chrono-chemodynamics of accreted halo stars with GALAH+ DR3 and $Gaia$ eDR3
• Instantaneous starburst of the massive clusters Westerlund 1 and NGC 3603 YC
• Chemo-dynamics of outer halo dwarf stars, including \textit{Gaia}-Sausage and \textit{Gaia}-Sequoia candidates
• Exploring the origin of low-metallicity stars in Milky Way-like galaxies with the NIHAO-UHD simulations
• On the origin of the Galactic thin and thick discs, their abundance gradients and the diagnostic potential of their abundance ratios
• Impacts of a Flaring Star-forming Disc and Stellar Radial Mixing on the Vertical Metallicity Gradient
• The stellar velocity distribution function in the Milky Way galaxy
• Mapping the Galactic Disk with the LAMOST and Gaia Red Clump Sample VII: the Stellar Disk Structure Revealed by the Mono-abundance Populations
• Yule-Simpson's paradox in Galactic Archaeology
• Milky Way's Thick and Thin disk: Is there distinct thick disk?
• Measuring the vertical response of the Galactic disc to an infalling satellite
• The imprint of radial migration on the vertical structure of galaxy disks
• Multiwavelength Absolute Magnitudes and Colours of Red Clump Stars in {\it Gaia} Era
• Variations in $\alpha$-element ratios trace the chemical evolution of the disk
• On the Flaring of Thick Disc of Galaxies: Insights from Simulations
• On the Chemical and Structural Evolution of the Galactic Disk
• The Implications of Local Fluctuations in the Galactic Midplane for Dynamical Analysis in the Gaia Era
• Bimodal Distribution of Galactic Disk Stars on the [alpha/Fe]-[Fe/H] Plane as a Possible Evidence of Discontinuous Radial Migration History