[Paper Review] Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. VI. Age and abundance structure of the stellar populations in the central sub-kpc of the Milky Way

T. Bensby, S. Feltzing+30 more|arXiv (Cornell University)|Feb 9, 2017

Stellar, planetary, and galactic studiesPhysics and Astronomy154 references66 citations

TL;DR

The paper analyzes 90 microlensed bulge dwarf and subgiant stars to derive ages and abundances for 11 elements, revealing a multi-peaked metallicity distribution, young/intermediate ages at high metallicity, discrete star-formation episodes, and a slightly elevated alpha-knee compared to the local thick disk.

ABSTRACT

We present a detailed elemental abundance study of 90 F and G dwarf, turn-off and subgiant stars in the Galactic bulge. Based on high-resolution spectra acquired during gravitational microlensing events, stellar ages and abundances for 11 elements (Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Zn, Y and Ba) have been determined. We find that the Galactic bulge has a wide metallicity distribution with significant peaks at [Fe/H]=-1.09, -0.63, -0.20, +0.12, +0.41. We also find a high fraction of intermediate-age to young stars: at [Fe/H]>0 more than 35 % are younger than 8 Gyr. For [Fe/H]

Motivation & Objective

• Characterize the age and chemical abundance structure of stars within the central sub-kpc of the Milky Way bulge.
• Investigate how the bulge's star formation history relates to the thin and thick disk populations.
• Assess whether bulge abundance trends align with secular evolution scenarios.
• Provide refined stellar parameters and ages for a large microlensed bulge dwarf sample.

Proposed method

• High-resolution spectroscopic analysis of 90 microlensed bulge dwarf and subgiant stars.
• Abundance determination for 11 elements (Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Zn, Y, Ba) using equivalent widths and the Uppsala MARCS model atmospheres.
• Effective temperatures from excitation balance of Fe I, surface gravities from Fe I–Fe II ionization balance, and microturbulence from Fe I line strengths, with NLTE corrections on Fe I (Lind et al. 2012).
• Age estimation from Bayesian isochrone methods and age probability distribution functions using Y2 isochrones (α-enhanced) and a Salpeter IMF.
• Empirical corrections to Teff and log g to align with previous analyses (Bensby et al. 2014) and re-calculation of abundances and ages accordingly.

Experimental results

Research questions

• RQ1 What is the metallicity distribution of bulge stars when sampled via microlensed dwarfs and subgiants?
• RQ2 What are the age distributions and star-formation episodes implied by precise ages in the bulge region?
• RQ3 How do bulge abundance trends, particularly alpha elements, compare to local thick and thin disk trends?
• RQ4 Do the bulge abundance and age structures support a secular evolution origin?
• RQ5 How does the bulge MDF vary with Galactic latitude and relate to bar components?

Key findings

• The bulge metallicity distribution is multi-peaked with peaks at [Fe/H] = -1.09, -0.63, -0.20, +0.12, +0.41.
• More than 35% of stars with [Fe/H] > 0 are younger than 8 Gyr, while most stars with [Fe/H] ≲ -0.5 are ≥10 Gyr old.
• Several significant star-formation episodes occurred at ~3, 6, 8, and 11 Gyr ago.
• Tentatively, the knee in the alpha-element trends of sub-solar bulge metallicities lies at a slightly higher [Fe/H] than in the local thick disk.
• Bulge age and abundance structure show similarities to Galactic thin and thick disks, with some indications of a faster bulge enrichment and bar-related components.
• Overall, the results support a secular origin for the bulge, shaped by disk material and central bar dynamics.