[Paper Review] The age structure of stellar populations in the solar vicinity. Clues of a two-phase formation history of the Milky Way disk

M. Haywood, P. Di Matteo+3 more|arXiv (Cornell University)|May 20, 2013

Stellar, planetary, and galactic studies113 references260 citations

TL;DR

This paper proposes a two-phase formation history for the Milky Way disk based on high-precision abundance and age data of solar neighborhood stars. It identifies the thick disk as forming over 4–5 Gyr with monotonic chemical enrichment, while the inner thin disk formed later from conditions set by the thick disk’s end. The key result is that radial migration has played a minimal role in shaping the local disk, and the outer thin disk formed concurrently with the thick disk’s inner phase, challenging the inside-out disk formation paradigm.

ABSTRACT

We analyze high quality abundances data of solar neighborhood stars and show that there are two distinct regimes of [alpha/Fe] versus age which we identify as the epochs of the thick and thin disk formation. A tight correlation between metallicity and [alpha/Fe] versus age is clearly identifiable on thick disk stars, implying that this population formed from a well mixed ISM, over a time scale of 4-5 Gyr. Thick disk stars vertical velocity dispersion correlate with age, with the youngest objects having as small scale heights as those of thin disk stars. A natural consequence of these two results is that a vertical metallicity gradient is expected in this population. We suggest that the thick disk set the initial conditions for the formation of the inner thin disk. This provides also an explanation of the apparent coincidence between the step in metallicity at 7-10 kpc in the thin disk and the confinment of the thick disk at about R<10 kpc. We suggest that the outer thin disk developped outside the influence of the thick disk, but also that the high alpha-enrichment of the outer regions may originate from a primordial pollution by the gas expelled from the thick disk. Local metal-poor thin disk stars, whose properties are best explained by an origin in the outer disk, are shown to be as old as the youngest thick disk (9-10 Gyr), implying that the outer thin disk started to form while the thick disk formation was still on-going in the inner Galaxy. We point out that, given the tight age-abundance relations in the thick disk, an inside-out process would give rise to a radial gradient in abundances in this population which is not observed. Finally, we argue that the data discussed here leave little room for radial migration, either to have contaminated the solar vicinity, or, to have redistributed stars in significant proportion across the solar annulus.

Motivation & Objective

• To determine the formation history of the Milky Way disk using high-precision stellar ages and abundances in the solar neighborhood.
• To resolve the long-standing debate on whether the thick and thin disks are distinct populations or part of a continuous formation process.
• To test the validity of the inside-out disk formation model by examining age-metallicity and age-[α/Fe] relations.
• To assess the role of radial migration in redistributing stars across the solar annulus.
• To clarify the nomenclature and physical distinction between thick and thin disk components based on formation conditions.

Proposed method

• Analysis of a sample of 1,111 FGK stars from the Adibekyan et al. (2012) survey with high-precision spectroscopic abundances and age determinations.
• Use of [α/Fe] versus [Fe/H] and age diagrams to identify distinct chemical and dynamical sequences corresponding to thick and thin disk populations.
• Application of stellar evolution models and isochrone fitting to derive individual stellar ages from photometry and metallicity.
• Correlation of vertical velocity dispersion with age in the thick disk to infer structural evolution and scale height changes.
• Comparison of observed age-metallicity and age-[α/Fe] relations with predictions from inside-out formation and radial migration models.
• Use of kinematic and chemical data to infer the origin of metal-poor thin disk stars and their connection to outer disk formation.

Experimental results

Research questions

• RQ1 Does the observed age-metallicity and age-[α/Fe] relation in the solar neighborhood support a two-phase formation history for the Milky Way disk?
• RQ2 To what extent did radial migration (churning) redistribute stars across the solar annulus, affecting the observed local kinematics and chemistry?
• RQ3 What is the origin of the metal-poor thin disk stars in the solar vicinity, and how do their ages relate to the thick disk formation epoch?
• RQ4 Can the observed radial metallicity gradient in the thin disk and the confinement of the thick disk within R < 10 kpc be explained by a common formation mechanism?
• RQ5 Is the conventional distinction between 'thin' and 'thick' disks still valid when considering formation times, chemical conditions, and kinematics?

Key findings

• The thick disk formed over a 4–5 Gyr period with monotonic chemical enrichment, showing a tight correlation between [α/Fe] and age, indicating a well-mixed interstellar medium.
• Vertical velocity dispersion in the thick disk decreased from ~50 km s⁻¹ to ~25 km s⁻¹ over time, indicating progressive thinning of the disk as star formation evolved.
• The inner thin disk formed about 8 Gyr ago, inheriting chemical conditions—specifically [Fe/H] ≈ -0.1 to +0.1 dex and [α/Fe] ≈ 0.1 dex—left by the end of thick disk formation.
• The outer thin disk began forming simultaneously with the thick disk’s inner phase, at 9–10 Gyr ago, likely from a mixture of enriched gas from the thick disk and accreted metal-poor gas.
• The observed age-[α/Fe] and age-metallicity relations in the thick disk rule out a continuous inside-out formation process, as such a model would produce radial gradients not observed.
• Radial migration (churning) has played a minimal role in redistributing stars in the solar vicinity or across the solar annulus, as the observed kinematic and chemical patterns are inconsistent with significant mixing.