# Binarity as the solution to the stellar evolution enigma posed by NGC 6791

## TL;DR

This paper proposes that binary star evolution, particularly the common envelope phase, explains the formation of extreme horizontal branch stars in NGC 6791, resolving longstanding questions about stellar evolution in this cluster.

## Contribution

The study demonstrates that binary evolution models can naturally produce EHB stars in NGC 6791 without requiring unusual mass-loss assumptions.

## Key findings

- Binary evolution reproduces observed EHB properties in NGC 6791.
- Common envelope phase is key to EHB formation in the model.
- Model matches observed temperature and gravity distributions.

## Abstract

Binary evolution is investigated as the source for the extreme horizontal branch (EHB) stars in the old and metal rich open cluster NGC~6791. Employing an updated version of our binary stellar evolution code we demonstrate that EHB stars naturally emerge from the common envelope phase. In sum, the binary model reproduces the observed ($T_{\rm{eff}}$, $\log{g}$) and temporal properties of the EHB over-density tied to NGC 6971, without needing an ad-hoc and anomalous mass-loss prescription.