Binarity as the solution to the Stellar Evolution Enigma Posed by NGC 6791

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_eff, log g) and temporal properties of the EHB overdensity tied to NGC 6971, without needing an ad hoc and anomalous mass-loss prescription.