# Stars that Move Together Were Born Together

## TL;DR

This study combines simulations and observational data to identify co-moving star pairs in the Galaxy, providing evidence that many such pairs are likely born together and originate from young, high-mass clusters.

## Contribution

It demonstrates that co-moving pairs identified in Gaia and LAMOST data are consistent with simulation predictions, confirming their common origin and shedding light on star formation processes.

## Key findings

- 111 co-moving pairs identified in the Solar neighborhood.
- Pairs show similar metallicities, indicating common origin.
- Data matches simulation predictions for co-natal pairs.

## Abstract

It is challenging to reliably identify stars that were born together outside of actively star-forming regions and bound stellar systems. However, co-natal stars should be present throughout the Galaxy, and their demographics can shed light on the clustered nature of star formation and the dynamical state of the disk. In previous work we presented a set of simulations of the Galactic disk that followed the clustered formation and dynamical evolution of 4 billion individual stars over the last 5 Gyr. The simulations predict that a high fraction of co-moving stars with physical and 3D velocity separation of $\Delta r < 20$ pc and $\Delta v < 1.5$ km s$^{-1}$ are co-natal. In this \textit{Letter}, we use \textit{Gaia} DR2 and LAMOST DR4 data to identify and study co-moving pairs. We find that the distribution of relative velocities and separations of pairs in the data is in good agreement with the predictions from the simulation. We identify 111 co-moving pairs in the Solar neighborhood with reliable astrometric and spectroscopic measurements. These pairs show a strong preference for having similar metallicities when compared to random field pairs. We therefore conclude that these pairs were very likely born together. The simulations predict that co-natal pairs originate preferentially from high-mass and relatively young ($< 1$ Gyr) star clusters. \textit{Gaia} will eventually deliver well-determined metallicities for the brightest stars, enabling the identification of thousands of co-natal pairs due to disrupting star clusters in the solar neighborhood.