Mapping the Milky Way’s Metal: Chemical Clues from Open Star Clusters
In this eighth installment of the Open Cluster Chemical Abundances and Mapping (OCCAM) Survey, Jonah Otto and collaborators utilize data from 164 open clusters to explore how elements are distributed across the Milky Way galaxy. Their goal is to study the Galaxy’s chemical structure using newly expanded observations from SDSS-V’s Milky Way Mapper (MWM) Data Release 19 (DR19). Open clusters, which are groups of stars born from the same cloud of gas and thus share similar properties, are ideal for tracing the chemical history of the Galaxy because their ages and compositions can be determined more accurately than those of isolated stars.
Expanding the Dataset and Methods
The research builds upon previous large spectroscopic surveys, such as APOGEE and Gaia, but this new study includes a 74% increase in high-quality open cluster data compared to past OCCAM analyses. The authors investigate how the amount of metals (elements heavier than helium, typically measured by iron, or [Fe/H]) changes with distance from the Galactic center. Two types of distances are used: the current Galactocentric radius (RGC) and the guiding center radius (RGuide), which accounts for a star cluster's orbit around the Galaxy. Both single-line and two-part (bilinear) fits are applied to model how metallicity changes with distance. Although a bilinear fit allows for a “knee” or bend in the slope, statistical tests show that a simple linear model best describes the data.
Elemental Abundance Trends
When looking at individual elements beyond iron, such as oxygen (O), magnesium (Mg), silicon (Si), and others, the authors find that most do not show significant changes with distance. However, some elements, like titanium (Ti) and cobalt (Co), do exhibit more scatter, likely due to difficulties in measuring them accurately in the data. Notably, neutron-capture elements like cerium (Ce) and neodymium (Nd) reveal opposite trends: Ce increases while Nd decreases with distance from the Galactic center. These patterns contribute to a more nuanced picture of the chemical evolution of the Galactic disk.
Discovering Azimuthal Variations
A key new feature of this work is the ability to look at how these chemical gradients vary in different directions around the Galaxy, what astronomers call “azimuthal variation.” This was not possible in earlier OCCAM papers due to smaller datasets. By dividing the Galaxy into slices both radially (distance from the center) and azimuthally (angle around the center), the authors find that the radial metallicity gradients differ depending on the direction in the Galactic disk. This suggests that the Milky Way’s spiral structure or past dynamical events may have influenced the way metals are spread throughout the Galaxy.
Building a Reliable Sample
To carry out their analysis, the team matched stars from the Gaia catalog with spectra from the MWM survey. They used a combination of position, motion, and chemical information to assign membership probabilities to stars within clusters. Only clusters that passed several quality checks, including their appearance in color-magnitude diagrams and agreement with known theoretical models, were included in the final analysis. This approach ensured that the results were based on the most reliable data, minimizing contamination from non-member stars.
Conclusion: Toward a Deeper Understanding of the Milky Way
This expanded OCCAM sample provides new insight into the Milky Way’s chemical architecture. While some of the results confirm previous findings, others, especially the detection of azimuthal variations, open new avenues for exploring how our Galaxy formed and evolved. Future surveys with deeper data will allow astronomers to refine these gradients further and investigate fainter and older clusters that may hold even more secrets about our cosmic neighborhood.
Source: Otto
