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

P. E. Nissen, W. J. Schuster|Feb 24, 2010

Stellar, planetary, and galactic studiesPhysics and Astronomy35 references267 citations

TL;DR

This study identifies two distinct halo populations in the solar neighborhood using precise abundance ratios and kinematics of 94 metal-poor stars. It finds a clear bimodality in [α/Fe], with high-α stars likely originating from the ancient disk or bulge, and low-α stars consistent with accretion from dwarf galaxies, including possible links to ω Cen's progenitor.

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.

Motivation & Objective

• To investigate the existence of multiple stellar populations in the Galactic halo using high-precision abundance ratios and kinematics.
• To determine whether the [α/Fe] distribution in the solar neighborhood is continuous or bimodal, using a sample of metal-rich halo stars.
• To explore the kinematic and chemical distinctions between high-α and low-α stars to infer their formation and evolutionary histories.
• To test whether the low-α stars could originate from accreted dwarf galaxies, particularly the ω Cen progenitor.
• To improve the precision of differential abundance analysis using high-resolution spectroscopy and modern model atmospheres.

Proposed method

• High-resolution spectroscopy was obtained using VLT/UVES (R ≈ 55,000) and NOT/FIES (R ≈ 40,000) for 94 stars with 5200 < T_eff < 6300 K and -1.6 ≤ [Fe/H] ≤ -0.4.
• Equivalent widths of 130–180 atomic lines were measured from spectra covering 4000–7000 Å, with signal-to-noise ratios between 140 and 500.
• A local thermodynamic equilibrium (LTE) abundance analysis was performed using the Uppsala EQWIDH program and MARCS model atmospheres with interpolated [α/Fe] values.
• Differential abundance ratios [X/Fe] for Na, Mg, Si, Ca, Ti, Cr, and Ni were derived relative to Fe, enabling precise comparisons across stars.
• Kinematic parameters (U, V, W) were computed relative to the local standard of rest to classify stars as halo or thick-disk based on total space velocity (>180 km s⁻¹).
• Cross-calibration between UVES and FIES data was performed for six stars with both datasets, showing a mean difference of 0.6 mÅ with a 1.3 mÅ RMS deviation.

Experimental results

Research questions

• RQ1 Is the [α/Fe] distribution in the solar neighborhood bimodal or continuous for metal-rich halo stars?
• RQ2 Do distinct kinematic and chemical patterns distinguish two populations among halo stars in the solar neighborhood?
• RQ3 Can the observed abundance ratios [Na/Fe] and [Ni/Fe] be used to trace different formation pathways for halo stars?
• RQ4 Do the kinematics of low-α stars support an origin via accretion from dwarf galaxies, such as the ω Cen progenitor?
• RQ5 What is the origin of high-α stars—monolithic collapse, in-situ formation, or heating from satellite mergers?

Key findings

• The halo stars are clearly separated into two populations in [α/Fe], with a bimodal distribution indicating two distinct chemical and dynamical components.
• High-α stars (median [α/Fe] ≈ 0.35) show elevated [Na/Fe] and [Ni/Fe], with a strong correlation between [Na/Fe] and [Ni/Fe] (R² ≈ 0.85), suggesting a common formation process.
• Low-α stars (median [α/Fe] ≈ 0.15) have lower [α/Fe], lower [Na/Fe], and lower [Ni/Fe], and their kinematics indicate accretion from dwarf galaxies.
• The kinematic distribution of low-α stars shows a significant retrograde motion, consistent with accretion from external systems, with some possibly originating from the ω Cen progenitor galaxy.
• High-α stars exhibit higher velocity dispersions (σ ≈ 50 km s⁻¹) and are more spatially extended, consistent with a dynamically heated or in-situ origin.
• The study confirms a bimodal [α/Fe] distribution with a clear separation at [α/Fe] ≈ 0.25 dex, supporting a dichotomy in halo formation history.