[Paper Review] Exploring the Milky Way stellar disk. A detailed elemental abundance study of 714 F and G dwarf stars in the Solar neighbourhood

T. Bensby, S. Feltzing+1 more|arXiv (Cornell University)|Sep 10, 2013

Stellar, planetary, and galactic studiesPhysics and Astronomy111 references458 citations

TL;DR

This study presents a high-resolution spectroscopic analysis of 714 F and G dwarf stars in the Solar neighbourhood, using Fe I/Fe II ionisation and excitation balance to derive stellar parameters and elemental abundances. It reveals a bimodal [α/Fe] distribution indicating two chemically distinct populations: an old, α-enhanced thick disk formed in the inner Galaxy and a younger, less α-enhanced thin disk from the outer disk, with dynamical features like the Hercules stream and Arcturus group likely originating from internal Galactic processes rather than external accretion.

ABSTRACT

ABRIDGED: METHODS: We have conducted a high-resolution spectroscopic study of 714 F and G dwarf and subgiant stars in the Solar neighbourhood. The star sample has been kinematically selected to trace the Galactic thin and thick disks to their extremes...... The determination of stellar parameters and elemental abundances is based on a standard 1-D LTE analysis using equivalent width measurements in high-resolution (R=40000-110000) and high signal-to-noise (S/N=150-300) spectra obtained with..... RESULTS: .... Our data show that there is an old and alpha-enhanced disk population, and a younger and less alpha-enhanced disk population. While they overlap greatly in metallicity between -0.7

Motivation & Objective

• To map the age and abundance structure of the Milky Way's stellar disk using a large sample of nearby F and G dwarf stars.
• To distinguish between the thin and thick disk populations based on chemical and kinematical properties.
• To investigate the origin of kinematic substructures such as the Hercules stream and Arcturus moving group.
• To identify and correct systematic biases in stellar parameter determination, particularly the 'flat main sequence syndrome' in 1D LTE analysis.
• To constrain the formation history of the Galactic disk using elemental abundances and orbital parameters.

Proposed method

• High-resolution (R = 40,000–110,000) and high signal-to-noise (S/N = 150–300) spectroscopy from multiple telescopes, including VLT/UVES, ESO 1.5-m/2.2-m/FEROS, NOT/SOFIN/FIES, and Magellan/MIKE.
• 1D LTE analysis using equivalent width measurements for 714 F and G dwarf stars, with NLTE corrections applied for individual Fe I lines.
• Stellar parameters (Teff, log g, [Fe/H]) derived via ionisation and excitation balance of Fe I and Fe II lines.
• Orbital and kinematic parameters computed using Galactic potential models and stellar positions/velocities.
• Empirical correction applied to surface gravity (log g) for main-sequence stars to resolve the 'flat main sequence syndrome' by comparing to Hipparcos parallaxes.
• Chemical abundance analysis for 13 elements (O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y, Ba) to trace disk population differentiation.

Experimental results

Research questions

• RQ1 What is the chemical and kinematical distinction between the thin and thick disk populations in the Solar neighbourhood?
• RQ2 Do the Hercules stream and Arcturus moving group exhibit chemical or age signatures consistent with being disrupted clusters or extragalactic accretion remnants?
• RQ3 To what extent does the standard 1D LTE analysis with ionisation balance produce systematic errors in log g for low-mass main-sequence stars?
• RQ4 How do the orbital parameters of stars correlate with their [α/Fe] and metallicity, and what do they reveal about their birthplaces in the Galactic disk?
• RQ5 Is the observed bimodality in [α/Fe] a real population distinction or an artifact of parameter uncertainties?

Key findings

• A clear bimodal distribution in [α/Fe] is observed, with an old, α-enhanced thick disk population and a younger, low-α thin disk population, even though they overlap in metallicity between [Fe/H] ≈ −0.7 and +0.1.
• The α-enhanced population has orbital parameters indicating inner Galactic disk birthplaces, supporting a short scale-length for the thick disk, while the low-α stars originate primarily from the outer disk.
• The Hercules stream and Arcturus moving group show no distinct chemical or age patterns; their abundance and age distributions closely match the thick disk, indicating they are likely dynamical features from internal Galactic processes rather than accreted systems.
• The standard 1D LTE analysis using ionisation balance produces a 'flat main sequence' in the HR diagram for low-mass main-sequence stars, which is corrected empirically using Hipparcos parallaxes, while evolved stars (turn-off and beyond) remain unaffected.
• The 'flat main sequence syndrome' is likely an artifact of forcing ionisation balance in low-mass stars, and this issue affects studies relying solely on ionisation/excitation balance, not those using parallax-based log g.
• The epoch of disk population separation (~8 Gyr ago) aligns with evidence for early major merger events, suggesting a possible link between thick disk formation and early accretion, though the thick disk itself is likely not a remnant of such events.