Untangling the Milky Way’s Halo with Aluminum
The outer parts of our galaxy, known as the halo, contain stars that formed in two different ways. Some were born inside the Milky Way (in-situ), while others were brought in by smaller galaxies that merged with ours (accreted). Astronomers have long tried to separate these groups using the motions of stars or their chemical fingerprints. However, at very low metallicities (stars with few heavy elements), the usual chemical tracers like magnesium ([Mg/Fe]) fail to clearly separate the two populations. In this new study, Ernandes and collaborators explore how aluminum abundances ([Al/Fe]) can provide a sharper distinction.
The Data: Halo Stars in the Solar Neighborhood
The team studied 56 halo stars that pass through the region around the Sun. Many of these stars were part of a benchmark dataset first examined by Nissen & Schuster in 2010. Using the Very Large Telescope in Chile, Ernandes and colleagues obtained high-resolution spectra that allowed them to measure aluminum through its resonance lines at 3944 and 3961 angstroms. To get accurate results, they accounted for “NLTE effects,” which correct for how atoms absorb light in conditions that differ from ideal laboratory assumptions. These corrections proved crucial to reliably separating stellar populations.
Measuring Aluminum in Halo Stars
Aluminum turned out to be a powerful chemical tracer. The stars fell into two groups: high-aluminum and low-aluminum. Those with [Al/Fe] higher than –0.3 were mostly in-situ stars, while those below that threshold matched accreted stars. This division remained clear even at very low metallicities, where α-elements like magnesium no longer distinguish populations. The authors also compared their findings to earlier classifications and found three stars that had previously been misidentified, showing that aluminum helps correct past ambiguities.
What Aluminum Tells Us About Stellar Origins
The discriminating power of aluminum lies in its nucleosynthetic origins. Aluminum is mainly forged in massive stars, but the efficiency of its production depends on the history of star formation and chemical enrichment in a galaxy. Dwarf galaxies, which are typically less efficient at producing aluminum, host stars with lower [Al/Fe]. This explains why accreted stars brought into the Milky Way by mergers stand out chemically from in-situ stars. Still, the authors caution that interpreting aluminum abundances requires care, as the exact yields vary across different environments.
Beyond Chemistry: The Role of Kinematics
The team also examined how their chemical classifications matched up with orbital properties derived from Gaia data. While some kinematic selection methods can successfully isolate stars from the Gaia-Sausage-Enceladus merger, they miss a substantial fraction of accreted stars at lower orbital energies. Aluminum abundances provide an independent and more complete way of identifying these populations, complementing but not replacing orbital information.
A Cleaner Picture of the Halo
By combining detailed chemical analysis with stellar motions, Ernandes and collaborators show that aluminum abundances are an especially reliable way to trace the origin of halo stars. Their results reveal that aluminum not only separates in-situ from accreted stars but also helps highlight substructures within the accreted halo itself. This offers astronomers a sharper tool for unraveling the complex assembly history of the Milky Way.
Source: Ernandes
