# Chemical Cartography with APOGEE: Mapping Disk Populations with a Two-Process Model and Residual Abundances

**Authors:** David H. Weinberg, Jon A. Holtzman, Jennifer A. Johnson, Christian Hayes, Sten Hasselquist, Matthew Shetrone, Yuan-Sen Ting, Rachael L. Beaton, Timothy C. Beers, Jonathan C. Bird, Dmitry Bizyaev, Michael R. Blanton, Katia Cunha, Jose G. Fernandez-Trincado, Peter M. Frinchaboy, D. A. Garcia-Hernandez, Emily Griffith, James W. Johnson, Henrik Jonsson, Richard R. Lane  
**arXiv:** [2108.08860](https://arxiv.org/abs/2108.08860)  
**Date:** 2021-08-19  
**Summary page:** https://www.summarizepaper.com/en/paper/2108.08860v1/

## Key points

- Statistical analysis of elemental abundances in Milky Way disk stars using data from APOGEE-2 DR17
- Focus on 16 elemental abundances measured in 34,410 stars
- Two-process model used to decompose abundance patterns into "prompt" and "delayed" components
- Residuals (\Delta[X/H]) computed for each star based on fit, ranging from ~0.01-0.03 dex to ~0.1 dex
- Correlations of residuals reveal complex underlying structure with two distinct correlated element groups
- Identification of physical outliers and potential measurement errors by selecting poorly fit stars
- Use of residual abundances to compare populations while controlling for differences in metallicity and [\alpha/Fe]
- Nearly identical abundance patterns in outer disk compared to main disk region (R=3-13 kpc; |Z|<2 kpc)
- Slight depressions in multiple elements for LMC and Gaia Sausage/Enceladus stars (0.05-0.2 dex)
- Significant deviations in multiple elements for \omega Cen (0.4-1 dex)
- Opportunities for discovering chemically distinctive stars and stellar populations, constraining nucleosynthetic yields, and testing chemical evolution models
- Introduction to the 2-process model provides a good starting point for readers seeking a broad overview
- Figures 4–7 illustrate median trends, Figures 12 & 15 depict distributions & covariance of residual abundances, Figure 20 showcases examples of high–χ² stars, and Figure 22 displays residual patterns of selected populations

## Summary

This paper presents a novel statistical analysis of elemental abundances in Milky Way disk stars using data from the final data release (DR17) of APOGEE-2. The study focuses on 16 elemental abundances measured in 34,410 stars and applies a 2-process model to decompose abundance patterns into a "prompt" component associated with core collapse supernovae and a "delayed" component associated with Type Ia supernovae. The authors fit median abundance ratio trends ([X/Mg] vs. [Mg/H]) with the 2-process model and compute residuals (\Delta[X/H]) for each star based on this fit. The rms residuals range from ~0.01-0.03 dex for the most precisely measured abundances to ~0.1 dex for certain elements like Na, V, and Ce. The correlations of these residuals reveal a complex underlying structure, including two distinct correlated element groups: one comprised of Ca, Na, Al, K, Cr, and Ce; and another comprised of Ni, V, Mn, and Co. 

By selecting stars that are poorly fit by the 2-process model, the authors identify various physical outliers and potential measurement errors. They also demonstrate how residual abundances can be used to compare populations while controlling for differences in metallicity and [\alpha/Fe]. The study finds nearly identical abundance patterns in the outer disk compared to the main disk region (R=3-13 kpc; |Z|<2 kpc), slight depressions (0.05-0.2 dex) in multiple elements for LMC and Gaia Sausage/Enceladus stars; and significant deviations (0.4-1 dex) in multiple elements for \omega Cen. Overall, this analysis of residual abundances provides new opportunities for discovering chemically distinctive stars and stellar populations; constraining nucleosynthetic yields; and testing chemical evolution models that incorporate stochasticity in element production and redistribution. For readers seeking a broad overview of this paper's content, the introduction to the 2-process model in §2 provides a good starting point. Figures 4–7 illustrate median trends; while Figures 12 & 15 depict distributions & covariance of residual abundances; Figure 20 showcases examples of high–χ² stars; & Figure 22 displays residual patterns of selected populations—allowing readers to loop back to earlier sections as needed depending on their interests & needs throughout their reading experience.. Readers interested in overall abundance trends & their interpretation should focus on §4; those intrigued by unusual stars & stellar populations will find relevant information in §§6 & 7; & those interested in dimensionality of stellar distribution in abundance space—& its connection to nucleosynthesis physics & chemical evolution—should focus on §§5 & 8 respectively.. Throughout the paper is an emphasis placed upon accurately determining abundances for multiple elements within large stellar samples as an ongoing challenge faced by researchers today..