# Spin alignment of stars in old open clusters

## TL;DR

This study reveals that stars within old open clusters exhibit strong spin alignment, indicating that the initial angular momentum of their birth clouds was effectively transferred to individual stars and persisted over billions of years.

## Contribution

It provides observational evidence of stellar spin alignment in old clusters and links it to the initial angular momentum conditions of the cluster-forming molecular clouds.

## Key findings

- Stars in the clusters show strong spin alignment.
- Simulations suggest at least 50% rotational kinetic energy in proto-clusters.
- Angular momentum transfer from cloud to stars is efficient and long-lasting.

## Abstract

Stellar clusters form by gravitational collapse of turbulent molecular clouds, with up to several thousand stars per cluster. They are thought to be the birthplace of most stars and therefore play an important role in our understanding of star formation, a fundamental problem in astrophysics. The initial conditions of the molecular cloud establish its dynamical history until the stellar cluster is born. However, the evolution of the cloud's angular momentum during cluster formation is not well understood. Current observations have suggested that turbulence scrambles the angular momentum of the cluster-forming cloud, preventing spin alignment amongst stars within a cluster. Here we use asteroseismology to measure the inclination angles of spin axes in 48 stars from the two old open clusters NGC~6791 and NGC~6819. The stars within each cluster show strong alignment. Three-dimensional hydrodynamical simulations of proto-cluster formation show that at least 50 % of the initial proto-cluster kinetic energy has to be rotational in order to obtain strong stellar-spin alignment within a cluster. Our result indicates that the global angular momentum of the cluster-forming clouds was efficiently transferred to each star and that its imprint has survived after several gigayears since the clusters formed.