Tracking Carbon Through the Milky Way: What the Stars Tell Us About How Carbon Forms
Carbon is one of the most important elements in the universe, it forms the backbone of life and contributes to the chemistry of galaxies. In their paper, “The Galactic Chemical Evolution of Carbon: Implications for Stellar Nucleosynthesis,” Daniel Boyea and collaborators use models of the Milky Way’s chemical evolution to determine how much of the galaxy’s carbon is produced by different types of stars. By comparing models to observations of subgiant stars from the APOGEE survey, the authors shed light on a long-standing question: which kinds of stars, massive ones that end their lives in supernovae, or smaller ones in their asymptotic giant branch (AGB) phase, are responsible for enriching the galaxy with carbon?
Observing the Chemical Fingerprints of Stars
The authors begin by assembling a large sample of subgiant stars observed by the APOGEE survey. Subgiants are ideal for this analysis because their surface compositions have not been altered by internal mixing, preserving the chemical makeup of the gas from which they formed. Boyea and collaborators examine how the ratio of carbon to magnesium ([C/Mg]) varies with both metallicity ([Mg/H]) and the ratio of magnesium to iron ([Mg/Fe]). They find that [C/Mg] tends to increase slightly with [Mg/H], and decreases with [Mg/Fe], suggesting that different processes contribute to carbon enrichment over time.
How Carbon is Made in Stars
The paper next reviews the stellar processes that create carbon. In massive stars, carbon forms through nuclear fusion during helium burning and is later released in core-collapse supernovae (CCSN). In lower-mass AGB stars, carbon is brought to the surface by convection and ejected into space through stellar winds. Boyea et al. compare several existing AGB yield models, FRUITY, ATON, Monash, and NuGrid, and find that these models predict a wide range of carbon production, depending on stellar mass, metallicity, and assumptions about stellar physics. Generally, AGB stars produce less carbon as metallicity increases, while massive star yields may rise with metallicity due to stronger stellar winds.
Modeling the Milky Way’s Carbon History
To test these ideas, the team employs a “multi-zone” galactic chemical evolution model that divides the Milky Way into hundreds of regions, each with its own star formation history. Using the open-source code vice, they simulate how carbon, magnesium, and iron build up over time in each part of the Galaxy. The model accounts for delayed contributions from AGB stars and Type Ia supernovae, as well as radial migration, how stars drift across the Galactic disk over billions of years.
Matching Models to Observations
By comparing their models with APOGEE’s observed abundance trends, Boyea and colleagues infer that about 10–40% of the carbon in the Sun likely originated from AGB stars, with the rest coming from massive stars. Their results suggest that carbon production by massive stars must increase with metallicity, consistent with theoretical models that include the effects of stellar rotation. This trend helps explain the gentle rise in [C/Mg] as metallicity increases. The [C/Mg] versus [Mg/Fe] relationship, on the other hand, constrains the amount and timing of delayed carbon enrichment from AGB stars.
Refining the Picture of Stellar Yields
The authors also test alternative scenarios to understand the shape of the observed abundance trends. Shifting AGB yields toward lower-mass progenitors, stars with longer lifetimes, improves agreement between the models and the data, implying that real AGB stars might enrich the galaxy over slightly longer timescales than predicted by standard stellar models. This could mean that existing models underestimate how efficiently low-mass AGB stars dredge up and eject carbon, or that uncertainties in iron production affect the comparison.
Conclusion: A Balanced Contribution
Boyea and collaborators conclude that both massive stars and AGB stars play essential, complementary roles in enriching the Milky Way with carbon. While massive stars dominate the early carbon production through supernovae, AGB stars provide a delayed contribution that continues to shape the galaxy’s chemical evolution. By linking detailed models to precise stellar data, this work refines our understanding of how carbon, and by extension, life’s building blocks, spread through the cosmos.
Source: Boyea
