# An old, metal-rich accreted stellar component in the Milky Way stellar disk

**Authors:** Diane K. Feuillet, Sofia Feltzing, Christian Sahlholdt, Thomas Bensby  
**arXiv:** [2206.03875](https://arxiv.org/abs/2206.03875)  
**Date:** 2022-06-08  
**Summary page:** https://www.summarizepaper.com/en/paper/2206.03875v1/

## Key points

- Our study explores the possibility of ancient star mergers in the cool stellar disk of the Milky Way
- Galaxies evolve through a hierarchical process involving mergers of smaller galaxies into larger ones over time
- Analysis of motions and elemental abundances using advancements like Gaia can reveal traces of past events and distinguish between stars from mergers and those originating within the main galaxy
- The merger history of our galaxy is accessible for study in the stellar halo, revealing structures like Gaia-Enceladus-Sausage
- Simulations suggest mergers play a significant role in shaping cool stellar disks
- Data from APOGEE DR17 and Gaia have identified an old stellar component within our galaxy's cool disk
- RR Lyrae variables with similar chemical signatures provide insights into dating this ancient accreted component exclusively to be ancient
- Kinematic data and elemental abundance ratios help identify stars with chemical signatures indicative of an accreted origin and kinematics resembling our galaxy's stellar disk
- Discovery of RR Lyrae variables sharing properties with this accreted component offers independent age estimates for this newfound stellar population

## Summary

Our study explores the possibility that the cool stellar disk of the Milky Way has incorporated mergers with ancient stars. Galaxies evolve through a hierarchical process where smaller galaxies merge into larger ones over time. By analyzing motions and elemental abundances of stars in the Milky Way and using advancements like the ESA astrometric satellite Gaia, we can uncover traces of past events and distinguish between stars that originated within the main galaxy and those from mergers. The merger history of our galaxy is particularly accessible for study in the stellar halo, which has revealed new structures like the Gaia-Enceladus-Sausage. Simulations also suggest that mergers play a significant role in shaping cool stellar disks. Through a combination of data from 100 giant branch stars in APOGEE DR17 and astrometric information from Gaia, we have identified an old stellar component within the cool disk of our galaxy. We have also pinpointed RR Lyrae variables with similar chemical signatures to these accreted red giant stars in this disk's kinematics. These RR Lyrae variables provide valuable insights into dating this accreted component and reveal it to be exclusively ancient. By leveraging kinematic data and elemental abundance ratios, we have successfully identified a sample of stars with chemical signatures indicative of an accreted origin and kinematics resembling those of our galaxy's stellar disk. Furthermore, our discovery of RR Lyrae variables sharing properties with this potentially accreted component offers an avenue for obtaining independent age estimates for this newfound stellar population. Our findings strongly suggest that we are observing an ancient stellar population that originated outside our galaxy and later became integrated into its structure.