[Paper Review] Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. II. Ages, metallicities, detailed elemental abundances, and connections to the Galactic thick disc

T. Bensby, S. Feltzing+14 more|UvA-DARE (University of Amsterdam)|Nov 26, 2009

Stellar, planetary, and galactic studies72 references65 citations

TL;DR

This study analyzes high-resolution spectra of 15 microlensed dwarf and subgiant stars in the Galactic bulge to determine their ages, metallicities, and elemental abundances, revealing a metallicity distribution consistent with giant stars but with distinct abundance trends. It finds that sub-solar metallicity bulge stars are old and chemically similar to the thick disc, while super-solar metallicity stars show a range of ages, challenging previous assumptions about bulge formation and suggesting a complex chemical evolution history.

ABSTRACT

The Bulge is the least understood major stellar population of the Milky Way. Most of what we know about the formation and evolution of the Bulge comes from bright giant stars. The underlying assumption that giants represent all the stars, and accurately trace the chemical evolution of a stellar population, is under debate. In particular, recent observations of a few microlensed dwarf stars give a very different picture of the evolution of the Bulge from that given by the giant stars. [ABRIDGED] We perform a detailed elemental abundance analysis of dwarf stars in the Galactic bulge, based on high-resolution spectra that were obtained while the stars were optically magnified during gravitational microlensing events. [ABRIDGED] We present detailed elemental abundances and stellar ages for six new dwarf stars in the Galactic bulge. Combining these with previous events, here re-analysed with the same methods, we study a homogeneous sample of 15 stars, which constitute the largest sample to date of microlensed dwarf stars in the Galactic bulge. We find that the stars span the full range of metallicities from [Fe/H]=-0.72 to +0.54, and an average metallicity of =-0.08+/-0.47, close to the average metallicity based on giant stars in the Bulge. Furthermore, the stars follow well-defined abundance trends, that for [Fe/H]<0 are very similar to those of the local Galactic thick disc. This suggests that the Bulge and the thick disc have had, at least partially, comparable chemical histories. At sub-solar metallicities we find the Bulge dwarf stars to have consistently old ages, while at super-solar metallicities we find a wide range of ages. Using the new age and abundance results from the microlensed dwarf stars we investigate possible formation scenarios for the Bulge.

Motivation & Objective

• Resolve the discrepancy between metallicity distributions of microlensed dwarf stars and giant stars in the Galactic bulge.
• Constrain the chemical evolution of the bulge using detailed elemental abundances from high-resolution spectroscopy of microlensed stars.
• Investigate the connection between the Galactic bulge and the thick disc through comparative abundance analysis.
• Determine stellar ages for bulge stars using microlensing events, enabling age-metallicity relations previously unattainable with giant stars.
• Assess the formation history of the bulge by comparing abundance trends and age distributions with those of the thin and thick discs.

Proposed method

• Obtained high-resolution spectra of 15 microlensed dwarf and subgiant stars using ESO telescopes during gravitational microlensing events.
• Performed a fully differential abundance analysis using the same methodology applied to 702 F and G dwarf stars in the Solar neighbourhood, minimizing systematic uncertainties.
• Determined stellar parameters (Teff, log g, [Fe/H]) and elemental abundances for 13 elements (O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Zn, Y, Ba) via spectral synthesis and line-by-line analysis.
• Calculated stellar ages using isochrone fitting based on derived T_eff, log g, and [Fe/H], leveraging the unique photometric and spectroscopic data from microlensing amplification.
• Compared abundance trends in the bulge to those in the local thin and thick disc populations using consistent analysis techniques.
• Used the K-S test to statistically compare metallicity distributions between microlensed dwarfs and giant stars in Baade’s Window, assessing significance of differences.

Experimental results

Research questions

• RQ1 Do microlensed dwarf stars in the Galactic bulge show metallicity distributions consistent with those derived from giant stars?
• RQ2 Are the abundance trends of microlensed dwarf stars in the bulge chemically similar to those of the Galactic thick disc or thin disc?
• RQ3 What is the age distribution of microlensed dwarf stars in the bulge, and how does it relate to the formation history of the bulge?
• RQ4 Can the observed abundance patterns in the bulge be explained by a single formation scenario, or do they require multiple formation channels?
• RQ5 How do the abundance ratios of α-elements and iron-peak elements in the bulge compare to those in the Solar neighbourhood disc populations?

Key findings

• The microlensed dwarf stars span a metallicity range from [Fe/H] = -0.72 to +0.54, with a mean metallicity of ⟨[Fe/H]⟩ = -0.08 ± 0.47 dex, in good agreement with giant star samples in Baade’s Window.
• The metallicity distribution of microlensed dwarfs shows a skewed, uneven shape with excesses at low and high metallicities, leading to a 30% probability (K-S test) that it matches the giant star MDF, indicating a significant statistical difference.
• At sub-solar [Fe/H], the abundance trends of microlensed dwarf stars are nearly indistinguishable from those of the Galactic thick disc, suggesting a shared chemical evolution history.
• At super-solar [Fe/H], the abundance trends align more closely with those of the thin disc, but the presence of old stars at high metallicity and young stars (age < 5 Gyr) at super-solar [Fe/H] complicates a simple connection to the thin disc.
• All stars with [Fe/H] < 0 are old (≈10 Gyr), with high [α/Fe] ratios consistent with rapid chemical enrichment by core-collapse supernovae in the early bulge.
• The average age of the 15 microlensed stars is 8.4 ± 3.3 Gyr, with three stars younger than 5 Gyr at super-solar metallicities, contradicting HST/ACS CMD results that show no evidence for a young bulge component.