[Paper Review] The COMBS Survey -- III. The Chemodynamical Origins of Metal-Poor Bulge Stars

Madeline Lucey, Keith Hawkins+7 more|arXiv (Cornell University)|Jul 6, 2021

Stellar, planetary, and galactic studiesPhysics and Astronomy219 references27 citations

TL;DR

This study combines high-resolution VLT/GIRAFFE spectroscopy (R ~ 20,000) of 319 metal-poor stars in the Galactic bulge with precise kinematic and dynamical analysis to disentangle chemically and dynamically distinct populations. It reveals that the inner and outer bulge populations exhibit higher chemical complexity and lower abundance correlations than the halo, indicating a more diverse nucleosynthetic enrichment history in the early bulge, and identifies one star with potential pair-instability supernova signatures and two with globular cluster-like Mg-Al anti-correlations.

ABSTRACT

The characteristics of the stellar populations in the Galactic Bulge inform and constrain the Milky Way's formation and evolution. The metal-poor population is particularly important in light of cosmological simulations, which predict that some of the oldest stars in the Galaxy now reside in its center. The metal-poor bulge appears to consist of multiple stellar populations that require dynamical analyses to disentangle. In this work, we undertake a detailed chemodynamical study of the metal-poor stars in the inner Galaxy. Using R$\sim$ 20,000 VLT/GIRAFFE spectra of 319 metal-poor (-2.55 dex$\leq$[Fe/H]$\leq$0.83 dex, with $\overline{ m{[Fe/H]}}$=-0.84 dex) stars, we perform stellar parameter analysis and report 12 elemental abundances (C, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Zn, Ba, and Ce) with precisions of $\approx$0.10 dex. Based on kinematic and spatial properties, we categorise the stars into four groups, associated with the following Galactic structures: the inner bulge, the outer bulge, the halo, and the disk. We find evidence that the inner and outer bulge population is more chemically complex (i.e., higher chemical dimensionality and less correlated abundances) than the halo population. This result suggests that the older bulge population was enriched by a larger diversity of nucleosynthetic events. We also find one inner bulge star with a [Ca/Mg] ratio consistent with theoretical pair-instability supernova yields and two stars that have chemistry consistent with globular cluster stars.

Motivation & Objective

• To disentangle the chemodynamical origins of metal-poor stars in the Galactic bulge, which are critical for understanding the Milky Way's early formation.
• To resolve the ambiguity in the bulge's stellar populations by separating contributions from the inner/outer bulge, halo, and disk using kinematic and chemical data.
• To investigate whether metal-poor bulge stars show signatures of primordial enrichment processes, such as pair-instability supernovae or globular cluster self-enrichment.
• To determine the relative ages and formation histories of metal-poor populations in the bulge by comparing their chemical complexity and abundance patterns.

Proposed method

• Acquired high-resolution (R ~ 20,000) VLT/GIRAFFE spectra for 319 metal-poor stars in the Galactic bulge, covering [Fe/H] from -2.55 to +0.83 dex.
• Performed detailed stellar parameter analysis and measured 12 elemental abundances (C, Na, Mg, Al, Si, Ca, Sc, Ti, Cr, Mn, Zn, Ba, Ce) with ~0.10 dex precision.
• Classified stars into four kinematic and spatial groups: inner bulge, outer bulge, halo, and disk, based on orbital properties and spatial distribution.
• Quantified chemical complexity using correlation coefficients between elemental abundances and principal component analysis to assess dimensionality of abundance space.
• Compared observed abundance ratios (e.g., [Ca/Mg], [Na/Mg]) with theoretical yields from pair-instability supernovae and globular cluster self-enrichment models.
• Used Gaia data and radial velocities to derive orbital properties and assess contamination from halo interlopers.

Experimental results

Research questions

• RQ1 What are the chemodynamical origins of metal-poor stars in the Galactic bulge, and how do they relate to different Galactic components?
• RQ2 How does the chemical complexity of metal-poor bulge stars compare to that of the halo and disk populations?
• RQ3 Do any of the metal-poor bulge stars show chemical signatures of enrichment by pair-instability supernovae?
• RQ4 Are there stars in the bulge with chemistry consistent with second-generation globular cluster stars?
• RQ5 How does the metallicity-dependent abundance pattern in the bulge reflect the relative ages and formation histories of its stellar populations?

Key findings

• The inner and outer bulge populations exhibit significantly lower abundance correlations (mean correlation coefficient of 0.38 and 0.53, respectively) and higher chemical dimensionality than the halo population, which has a higher correlation coefficient (0.57) and explains 98.0% of variance with only four components.
• The inner bulge population shows higher chemical complexity than the outer bulge, suggesting it is older and enriched by a more diverse set of nucleosynthetic events, consistent with simulations predicting older, tightly bound stars in the inner region.
• One inner bulge star with [Fe/H] = -1.07 dex shows a [Ca/Mg] ratio of 0.83 dex, consistent with theoretical pair-instability supernova yields, though its [Al/Mg] ratio deviates from expected PISN patterns.
• Two stars—one in the inner bulge and one in the outer bulge—exhibit the Mg-Al anti-correlation characteristic of second-generation globular cluster stars, with one showing a [Al/Mg] ratio similar to NGC 2808.
• In low-metallicity ([Fe/H] ≤ -1 dex) bulge populations, Ba abundances are positively correlated with α-elements, unlike in the halo where the correlation is negative, suggesting different neutron-capture nucleosynthesis origins.
• The study confirms that halo interlopers significantly affect kinematic and chemical interpretations of the bulge, and that removing them reduces velocity dispersion and eliminates evidence for a classical metal-poor bulge component.