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The galactic chemical evolution of carbon: Implications for stellar nucleosynthesis

By analyzing Galactic chemical evolution models against APOGEE observations, this study concludes that carbon production in the Milky Way disk is driven by metallicity-dependent yields from core-collapse supernovae that offset declining asymptotic giant branch contributions, implying that AGB stars account for only 10–40% of solar metallicity carbon.

Original authors: Daniel A. Boyea, James W. Johnson, David H. Weinberg

Published 2026-06-17

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CC BY 4.0

Original authors: Daniel A. Boyea, James W. Johnson, David H. Weinberg

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Mystery: Where Does Carbon Come From?

Imagine the universe as a giant, cosmic kitchen. Carbon is the second most common ingredient in this kitchen (after oxygen), essential for making life, planets, and stars. But for a long time, astronomers have been arguing about who the "head chefs" are that cook up this carbon.

There are two main candidates:

The "Fast Chefs" (Massive Stars): These are huge, short-lived stars that explode as supernovae. They cook quickly and release their ingredients almost immediately.
The "Slow Chefs" (AGB Stars): These are aging, lower-mass stars that swell up and gently puff out their material over billions of years. They cook slowly and release their ingredients with a long delay.

The big question this paper asks is: How much carbon does each chef contribute? Is it a 50/50 split? Do the fast chefs do 90% of the work? Or is it the other way around?

The Detective Work: Using "Time Travel" to Check the Receipts

To solve this, the authors didn't just look at the ingredients; they looked at the timing of when they were added to the galactic soup.

They used a massive database of stars (from the APOGEE survey) to look at subgiant stars. Think of these stars as "time capsules." Unlike red giants, which get messy and mix their internal ingredients with their surface (like stirring a pot), subgiants have kept their original "birth recipe" intact. By measuring the carbon in these stars, the scientists can see what the galaxy was made of when those stars were born.

They compared two specific ratios:

Carbon vs. Magnesium ([C/Mg]): Magnesium is a "fast chef" ingredient (mostly from supernovae).
Carbon vs. Iron ([C/Mg] vs. [Mg/Fe]): Iron is a "slow chef" ingredient (mostly from Type Ia supernovae, which happen long after the stars are born).

The Clues: What the Data Revealed

The scientists ran computer simulations of the Milky Way's history, trying different recipes for how much carbon the "Fast Chefs" and "Slow Chefs" produce. Here is what they found:

1. The "Fast Chefs" Need to Get Better at Cooking as the Kitchen Gets Messier
The data showed that as the galaxy got richer in metals (like magnesium), the ratio of Carbon to Magnesium went up slightly.

The Problem: Theoretical models said the "Slow Chefs" (AGB stars) should actually get worse at making carbon as the kitchen gets metal-rich.
The Solution: To match the data, the "Fast Chefs" (massive stars) must get better at making carbon as the metallicity increases. This fits with theories that massive stars spin faster (rotation), which helps them cook up more carbon.

2. The "Slow Chefs" Are the Delayed Ingredient
The relationship between Carbon and Iron is the key to the timing. Because Iron takes a long time to appear (from delayed supernovae), the amount of Carbon relative to Iron tells us how much Carbon was also delayed.

The Finding: The slope of the data line suggests that the "Slow Chefs" (AGB stars) are responsible for about 10% to 40% of the carbon in our solar neighborhood. The rest comes from the "Fast Chefs."

3. The "Slow Chefs" Might Be Even Slower Than We Thought
The data showed a very straight, consistent line. However, standard models predicted the "Slow Chefs" would stop cooking carbon after a few billion years, which would make the line curve.

The Twist: To make the models fit the straight line in the data, the authors had to pretend that the "Slow Chefs" are actually slightly smaller stars that live even longer than we thought. This suggests our current understanding of how these stars age might be slightly off, or that our assumptions about how fast iron is produced need tweaking.

The Bottom Line

This paper is like a forensic audit of the galaxy's kitchen. By looking at the "receipts" (chemical abundances) in ancient stars, the authors concluded:

Massive stars (supernovae) are the primary source of carbon, but their output increases as the galaxy gets richer in metals.
Aging stars (AGB) contribute a significant but smaller portion (roughly 10–40%) of the carbon.
The timing matters: The fact that carbon and iron rise and fall together in a specific way proves that a chunk of carbon is "delayed," coming from stars that live long lives.

The study confirms that while we have a good general recipe, the exact "cooking times" and "ingredient amounts" for these stellar chefs still need a little fine-tuning to perfectly match the cosmic kitchen we see today.

Technical Summary: The Galactic Chemical Evolution of Carbon: Implications for Stellar Nucleosynthesis

Problem Statement
Despite carbon (C) being the second most abundant metal in the Universe, its nucleosynthetic origin remains uncertain. While it is established that both core-collapse supernovae (CCSN) from massive stars and asymptotic giant branch (AGB) stars produce carbon, the relative contributions of these sources are disputed. Theoretical yield predictions are hindered by uncertainties in stellar evolution physics, including mass loss rates, rotational mixing, nuclear reaction rates, and convection. Previous empirical constraints have often relied on [C/Fe] versus [Fe/H] trends, which the authors argue conflate metallicity-dependent yields with delayed enrichment effects. This paper aims to disentangle these effects to constrain the relative yields of CCSN and AGB stars using Galactic Chemical Evolution (GCE) models benchmarked against high-precision abundance data.

Methodology
The authors employ multi-zone GCE models of the Milky Way disk using the vice (Versatile Integrator for Chemical Evolution) framework. The models consist of 200 radial zones incorporating star formation histories, gas inflows, outflows, and radial stellar migration.

Observational Benchmark: The study utilizes a sample of 14,066 subgiant stars from the APOGEE DR17 survey (Roberts et al. 2024). Subgiants are selected because their photospheric carbon abundances closely reflect their birth composition, avoiding the internal mixing processes (e.g., first dredge-up) that alter abundances in red giants. The analysis focuses on the trends of [C/Mg] versus [Mg/H] and [C/Mg] versus [Mg/Fe].
Yield Modeling:
- AGB Stars: The authors test four literature yield tables (FRUITY, ATON, Monash, NuGrid). They introduce a scaling factor $\beta_{C}^{AGB}$ to adjust the yields and define $f_{C}^{AGB}$ as the fraction of solar carbon produced by AGB stars.
- Massive Stars (CCSN): Instead of adopting a single literature table, the authors parameterize the CCSN carbon yield ( $y_{C}^{CC}$ ) as a linear function of metallicity ( $Z$ ). This allows them to empirically constrain the metallicity dependence ( $\zeta_{C}^{CC}$ ) required to match observations.
- Other Sources: Type Ia supernovae (SNe Ia) and novae are assumed to contribute negligibly to $^{12}$ C production.
Analysis Strategy: The authors compare model predictions against the observed median trends in [C/Mg]-[Mg/H] and [C/Mg]-[Mg/Fe]. They utilize the [C/Mg]-[Mg/Fe] trend specifically to isolate the contribution of delayed enrichment (AGB stars) from the metallicity dependence of instantaneous enrichment (CCSN).

Key Contributions and Results

Decoupling Metallicity Dependence and Delayed Enrichment: The study demonstrates that the [C/Mg]-[Mg/H] trend is primarily set by the total carbon yield as a function of metallicity, whereas the [C/Mg]-[Mg/Fe] trend is sensitive to the delay time distribution of carbon production. This distinction allows for the separation of AGB contributions from CCSN metallicity dependence, a task difficult with [C/Fe] trends alone.
CCSN Yield Evolution: Observations show a gently rising [C/Mg] with increasing [Mg/H]. However, standard AGB models predict declining carbon yields with increasing metallicity. To reproduce the observed rise in [C/Mg], the authors find that CCSN yields must increase with metallicity to offset the declining AGB contribution. This result is consistent with massive star nucleosynthesis models that incorporate rotation (e.g., Limongi & Chieffi 2018), where rotational mixing enhances carbon production at higher metallicities.
Quantifying AGB Contribution: Based on the slope of the [C/Mg]-[Mg/Fe] relation, the authors constrain the AGB contribution to solar carbon production ( $f_{C}^{AGB}$ ) to be between 10% and 40%. In their fiducial model, they adopt $f_{C}^{AGB} \approx 0.25$ .
Discrepancies in Delay Times: The authors note that standard AGB yield tables predict a concave-down [C/Mg]-[Mg/Fe] trend due to the rapid cessation of carbon production in single stellar populations after a few Gyr, whereas SNe Ia iron production continues longer. The observed data does not show this level of concavity. The authors suggest that artificially shifting AGB carbon production toward lower-mass progenitors (longer lifetimes) improves the fit to the linear observed trend. This may indicate a discrepancy with current stellar evolution predictions regarding AGB delay times or assumptions about iron production rates.
Robustness: The results regarding the relative contribution of AGB stars are robust across different AGB yield tables (FRUITY, Monash, NuGrid) and different survey datasets (GALAH, Gaia-ESO, LAMOST), despite global systematic offsets in absolute abundance scales.

Significance
The paper provides a robust empirical constraint on the origin of galactic carbon, resolving a long-standing debate regarding the dominance of massive stars versus AGB stars. By utilizing [C/Mg] ratios and subgiant stars, the authors demonstrate that massive stars likely dominate carbon production at solar metallicity, with AGB stars contributing approximately 10–40%. Furthermore, the study highlights the necessity of metallicity-dependent CCSN yields (driven by rotation) to explain the observed chemical evolution trends. The findings suggest that current stellar evolution models may underestimate the delay times for carbon production in AGB stars or require adjustments to the assumed iron production rates to fully match the observed abundance patterns.

The Big Mystery: Where Does Carbon Come From?

The Detective Work: Using "Time Travel" to Check the Receipts

The Clues: What the Data Revealed

The Bottom Line

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